ty_module_resolver 0.0.10

/*!
This module principally provides several routines for resolving a particular module
name to a `Module`:

* [`file_to_module`][]: resolves the module `.<self>` (often as the first step in resolving `.`)
* [`resolve_module`][]: resolves an absolute module name

You may notice that we actually provide `resolve_(real)_(shadowable)_module_(confident)`.
You almost certainly just want [`resolve_module`][]. The other variations represent
restrictions to answer specific kinds of questions, usually to empower IDE features.

* The `real` variation disallows all stub files, including the vendored typeshed.
  This enables the goto-definition ("real") vs goto-declaration ("stub or real") distinction.

* The `confident` variation disallows "desperate resolution", which is a fallback
  mode where we start trying to use ancestor directories of the importing file
  as search-paths, but only if we failed to resolve it with the normal search-paths.
  This is mostly just a convenience for cases where we don't want to try to define
  the importing file (resolving a `KnownModule` and tests).

* The `shadowable` variation disables some guards that prevents third-party code
  from shadowing any vendored non-stdlib `KnownModule`. In particular `typing_extensions`,
  which we vendor and heavily assume the contents of (and so don't ever want to shadow).
  This enables checking if the user *actually* has `typing_extensions` installed,
  in which case it's ok to suggest it in features like auto-imports.

There is some awkwardness to the structure of the code to specifically enable caching
of queries, as module resolution happens a lot and involves a lot of disk access.

For implementors, see `import-resolution-diagram.svg` for a flow diagram that
specifies ty's implementation of Python's import resolution algorithm.
*/

use std::borrow::Cow;
use std::iter::FusedIterator;

use rustc_hash::{FxBuildHasher, FxHashSet};

use ruff_db::PythonFile;
use ruff_db::files::{File, FilePath, FileRootKind, directory_listing, system_path_to_file};
use ruff_db::source::source_text;
use ruff_db::system::{System, SystemPath, SystemPathBuf};
use ruff_db::vendored::VendoredFileSystem;
use ruff_python_ast::{
    self as ast, PySourceType,
    visitor::{Visitor, walk_body},
};

use crate::db::Db;
use crate::module::{Module, ModuleKind};
use crate::module_name::{ImportingFile, ModuleName};
use crate::path::{ModulePath, SearchPath, SystemOrVendoredPathRef};
use crate::strategy::MisconfigurationStrategy;
use crate::typeshed::{TypeshedVersions, vendored_typeshed_versions};
use crate::{ResolverEnvironment, ResolverFile, SearchPathSettings, SearchPathSettingsError};

/// Resolves a module name to a module.
pub fn resolve_module<'db>(
    db: &'db dyn Db,
    importing_file: ImportingFile<'db>,
    module_name: &ModuleName,
) -> Option<Module<'db>> {
    let resolver_environment = importing_file.resolver_environment(db);
    let interned_name = ModuleNameIngredient::new(
        db,
        module_name,
        ModuleResolveMode::Typing,
        resolver_environment,
    );

    resolve_module_query(db, interned_name)
        .or_else(|| desperately_resolve_module(db, importing_file.file(db), interned_name))
}

/// Resolves the module referenced by a `from` import statement.
///
/// Returns `None` if the statement does not name a valid module or the module cannot be resolved.
pub fn resolve_module_for_import_from<'db>(
    db: &'db dyn Db,
    importing_file: ImportingFile<'db>,
    import: &ast::StmtImportFrom,
) -> Option<Module<'db>> {
    let module_name = ModuleName::from_import_statement(db, importing_file, import).ok()?;
    resolve_module(db, importing_file, &module_name)
}

/// Resolves a module name to a module, without desperate resolution available.
///
/// This is appropriate for resolving a `KnownModule`, or cases where for whatever reason
/// we don't have a well-defined importing file.
pub fn resolve_module_confident<'db>(
    db: &'db dyn Db,
    resolver_environment: ResolverEnvironment<'db>,
    module_name: &ModuleName,
) -> Option<Module<'db>> {
    let interned_name = ModuleNameIngredient::new(
        db,
        module_name,
        ModuleResolveMode::Typing,
        resolver_environment,
    );

    resolve_module_query(db, interned_name)
}

/// Resolves a module name to a module (stubs not allowed).
pub fn resolve_real_module<'db>(
    db: &'db dyn Db,
    importing_file: ImportingFile<'db>,
    module_name: &ModuleName,
) -> Option<Module<'db>> {
    let resolver_environment = importing_file.resolver_environment(db);
    let interned_name = ModuleNameIngredient::new(
        db,
        module_name,
        ModuleResolveMode::Runtime,
        resolver_environment,
    );

    resolve_module_query(db, interned_name)
        .or_else(|| desperately_resolve_module(db, importing_file.file(db), interned_name))
}

/// Resolves a module name to a module, without desperate resolution available (stubs not allowed).
///
/// This is appropriate for resolving a `KnownModule`, or cases where for whatever reason
/// we don't have a well-defined importing file.
pub fn resolve_real_module_confident<'db>(
    db: &'db dyn Db,
    resolver_environment: ResolverEnvironment<'db>,
    module_name: &ModuleName,
) -> Option<Module<'db>> {
    let interned_name = ModuleNameIngredient::new(
        db,
        module_name,
        ModuleResolveMode::Runtime,
        resolver_environment,
    );

    resolve_module_query(db, interned_name)
}

/// Resolves a module name to a module (stubs not allowed, some shadowing is
/// allowed).
///
/// In particular, this allows `typing_extensions` to be shadowed by a
/// non-standard library module. This is useful in the context of the LSP
/// where we don't want to pretend as if these modules are always available at
/// runtime.
///
/// This should generally only be used within the context of the LSP. Using it
/// within ty proper risks being unable to resolve builtin modules since they
/// are involved in an import cycle with `builtins`.
pub fn resolve_real_shadowable_module<'db>(
    db: &'db dyn Db,
    importing_file: ImportingFile<'db>,
    module_name: &ModuleName,
) -> Option<Module<'db>> {
    let resolver_environment = importing_file.resolver_environment(db);
    let interned_name = ModuleNameIngredient::new(
        db,
        module_name,
        ModuleResolveMode::RuntimeSomeShadowingAllowed,
        resolver_environment,
    );

    resolve_module_query(db, interned_name)
        .or_else(|| desperately_resolve_module(db, importing_file.file(db), interned_name))
}

/// Selects typing or runtime module-resolution semantics.
#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, get_size2::GetSize)]
pub enum ModuleResolveMode {
    /// Resolve modules for type checking, preferring stubs over runtime implementations.
    ///
    /// This is the "normal" mode almost everything uses, as type checkers are in fact supposed
    /// to *prefer* stubs over the actual implementations.
    Typing,

    /// Resolve modules to their runtime implementations without considering stubs.
    ///
    /// This is the "goto definition" mode, where we need to ignore the typing spec and find actual
    /// implementations. When querying searchpaths this also notably replaces typeshed with
    /// the "real" stdlib.
    Runtime,

    /// Like [`ModuleResolveMode::Runtime`], but permits some modules to be shadowed.
    ///
    /// In particular, this allows `typing_extensions` to be shadowed by a
    /// non-standard library module. This is useful in the context of the LSP
    /// where we don't want to pretend as if these modules are always available
    /// at runtime.
    RuntimeSomeShadowingAllowed,
}

#[salsa::interned(heap_size=ruff_memory_usage::heap_size)]
#[derive(Debug)]
pub(crate) struct ModuleResolveModeIngredient<'db> {
    #[returns(copy)]
    resolver_environment: ResolverEnvironment<'db>,
    #[returns(copy)]
    mode: ModuleResolveMode,
}

impl ModuleResolveMode {
    fn is_typing(self) -> bool {
        matches!(self, Self::Typing)
    }

    /// Returns `true` if the module name refers to a standard library module
    /// which can't be shadowed by a first-party module.
    ///
    /// This includes "builtin" modules, which can never be shadowed at runtime
    /// either. Additionally, certain other modules that are involved in an
    /// import cycle with `builtins` (`types`, `typing_extensions`, etc.) are
    /// also considered non-shadowable, unless the module resolution mode
    /// specifically opts into allowing some of them to be shadowed. This
    /// latter set of modules cannot be allowed to be shadowed by first-party
    /// or "extra-path" modules in ty proper, or we risk panics in unexpected
    /// places due to being unable to resolve builtin symbols. This is similar
    /// behaviour to other type checkers such as mypy:
    /// <https://github.com/python/mypy/blob/3807423e9d98e678bf16b13ec8b4f909fe181908/mypy/build.py#L104-L117>
    pub(super) fn is_non_shadowable(self, minor_version: u8, module_name: &str) -> bool {
        // Builtin modules are never shadowable, no matter what.
        if ruff_python_stdlib::sys::is_builtin_module(minor_version, module_name) {
            return true;
        }
        // Similarly for `types`, which is always available at runtime.
        if module_name == "types" {
            return true;
        }

        // Otherwise, some modules should only be conditionally allowed
        // to be shadowed, depending on the module resolution mode.
        match self {
            ModuleResolveMode::Typing | ModuleResolveMode::Runtime => {
                module_name == "typing_extensions"
            }
            ModuleResolveMode::RuntimeSomeShadowingAllowed => false,
        }
    }
}

/// Salsa query that resolves an interned [`ModuleNameIngredient`] to a module.
///
/// This query should not be called directly. Instead, use [`resolve_module`]. It only exists
/// because Salsa requires the module name to be an ingredient.
#[salsa::tracked(returns(copy), heap_size=ruff_memory_usage::heap_size)]
fn resolve_module_query<'db>(
    db: &'db dyn Db,
    module_name: ModuleNameIngredient<'db>,
) -> Option<Module<'db>> {
    let name = module_name.name(db);
    let mode = module_name.mode(db);
    let resolver_environment = module_name.resolver_environment(db);
    let _span = tracing::trace_span!("resolve_module", %name).entered();

    let Some(resolved) = resolve_name(db, resolver_environment, name, mode) else {
        tracing::debug!("Module `{name}` not found in search paths");
        return None;
    };

    resolved
        .into_iter()
        .next()
        .map(|candidate| candidate.into_module(db, resolver_environment, name))
}

/// Like `resolve_module_query` but for cases where it failed to resolve the module
/// and we are now Getting Desperate and willing to try the ancestor directories of
/// the `importing_file` as potential temporary search paths that are private
/// to this import.
///
/// The reason this is split out is because in 99.9% of cases `resolve_module_query`
/// will find the right answer (or no valid answer exists), and we want it to be
/// aggressively cached. Including the `importing_file` as part of that query would
/// trash the caching of import resolution between files.
///
/// Cache desperate resolution because repeated unresolved imports in a project can otherwise
/// re-walk the same importing-file-relative search paths many times.
#[salsa::tracked(returns(copy))]
fn desperately_resolve_module<'db>(
    db: &'db dyn Db,
    importing_file: File,
    module_name: ModuleNameIngredient<'db>,
) -> Option<Module<'db>> {
    let name = module_name.name(db);
    let mode = module_name.mode(db);
    let resolver_environment = module_name.resolver_environment(db);
    let _span = tracing::trace_span!("desperately_resolve_module", %name).entered();

    let Some(resolved) =
        desperately_resolve_name(db, importing_file, resolver_environment, name, mode)
    else {
        let mode = match mode {
            ModuleResolveMode::Typing => "typing mode",
            ModuleResolveMode::Runtime => "runtime mode",
            ModuleResolveMode::RuntimeSomeShadowingAllowed => {
                "runtime mode with some shadowing allowed"
            }
        };
        tracing::debug!("Module `{name}` not found while looking in parent dirs ({mode})");
        return None;
    };

    resolved
        .into_iter()
        .next()
        .map(|candidate| candidate.into_module(db, resolver_environment, name))
}

/// Resolves the module for the given path.
///
/// Returns `None` if the path is not a module locatable via any of the known search paths.
#[allow(unused)]
pub(crate) fn path_to_module<'db>(
    db: &'db dyn Db,
    resolver_environment: ResolverEnvironment<'db>,
    path: &FilePath,
) -> Option<Module<'db>> {
    // It's not entirely clear on first sight why this method calls `file_to_module` instead of
    // it being the other way round, considering that the first thing that `file_to_module` does
    // is to retrieve the file's path.
    //
    // The reason is that `file_to_module` is a tracked Salsa query and salsa queries require that
    // all arguments are Salsa ingredients (something stored in Salsa). `Path`s aren't salsa ingredients but
    // `VfsFile` is. So what we do here is to retrieve the `path`'s `VfsFile` so that we can make
    // use of Salsa's caching and invalidation.
    let file = path.to_file(db)?;
    file_to_module(db, ResolverFile::new(db, file, resolver_environment))
}

/// Resolves the module for the file with the given id.
///
/// Returns `None` if the file is not a module locatable via any of the known search paths.
///
/// This function can be understood as essentially resolving `import .<self>` in the file itself,
/// and indeed, one of its primary jobs is resolving `.<self>` to derive the module name of `.`.
/// This intuition is particularly useful for understanding why it's correct that we pass
/// the file itself as `importing_file` to various subroutines.
#[salsa::tracked(returns(copy), heap_size=ruff_memory_usage::heap_size)]
pub fn file_to_module<'db>(
    db: &'db dyn Db,
    resolver_file: ResolverFile<'db>,
) -> Option<Module<'db>> {
    let resolver_environment = resolver_file.environment(db);
    let file = resolver_file.file(db);
    let _span = tracing::trace_span!("file_to_module", ?file).entered();

    let path = SystemOrVendoredPathRef::try_from_file(db, file)?;

    file_to_module_impl(
        db,
        resolver_file,
        path,
        search_paths(db, resolver_environment, ModuleResolveMode::Typing),
    )
    .or_else(|| {
        file_to_module_impl(
            db,
            resolver_file,
            path,
            relative_desperate_search_paths(db, resolver_file).iter(),
        )
    })
}

fn file_to_module_impl<'db, 'a>(
    db: &'db dyn Db,
    resolver_file: ResolverFile<'db>,
    path: SystemOrVendoredPathRef<'a>,
    mut search_paths: impl Iterator<Item = &'a SearchPath>,
) -> Option<Module<'db>> {
    let module_name = search_paths.find_map(|candidate: &SearchPath| {
        let relative_path = match path {
            SystemOrVendoredPathRef::System(path) => candidate.relativize_system_path(path),
            SystemOrVendoredPathRef::Vendored(path) => candidate.relativize_vendored_path(path),
        }?;
        relative_path.to_module_name()
    })?;

    // Resolve the module name to see if Python would resolve the name to the same path.
    // If it doesn't, then that means that multiple modules have the same name in different
    // root paths, but that the module corresponding to `path` is in a lower priority search path,
    // in which case we ignore it.
    let module = resolve_module(db, ImportingFile::ResolverFile(resolver_file), &module_name)?;
    let module_file = module.file(db)?;

    let file: File = resolver_file.file(db);
    let file_path = file.path(db);
    if file_path == module_file.path(db) {
        return Some(module);
    } else if file.source_type(db) == PySourceType::Python
        && module_file.source_type(db) == PySourceType::Stub
    {
        // If a .py and .pyi are both defined, the .pyi will be the one returned by `resolve_module().file`,
        // which would make us erroneously believe the `.py` is *not* also this module (breaking things
        // like relative imports). So here we try `resolve_real_module().file` to cover both cases.
        let module =
            resolve_real_module(db, ImportingFile::ResolverFile(resolver_file), &module_name)?;
        let module_file = module.file(db)?;
        if file_path == module_file.path(db) {
            return Some(module);
        }
    }
    // This path is for a module with the same name but with a different precedence. For example:
    // ```
    // src/foo.py
    // src/foo/__init__.py
    // ```
    // The module name of `src/foo.py` is `foo`, but the module loaded by Python is `src/foo/__init__.py`.
    // That means we need to ignore `src/foo.py` even though it resolves to the same module name.
    None
}

pub fn search_paths<'db>(
    db: &'db dyn Db,
    resolver_environment: ResolverEnvironment<'db>,
    resolve_mode: ModuleResolveMode,
) -> SearchPathIterator<'db> {
    let search_paths = resolver_environment.search_paths(db);

    SearchPathIterator {
        db,
        static_paths: search_paths.static_paths.iter(),
        stdlib_path: search_paths.stdlib(resolve_mode),
        dynamic_paths: None,
        mode: ModuleResolveModeIngredient::new(db, resolver_environment, resolve_mode),
    }
}

#[derive(Debug, Clone, Copy, Default)]
struct StubPackagePaths<'a> {
    before_stdlib: &'a [SearchPath],
    after_stdlib: &'a [SearchPath],
}

impl StubPackagePaths<'_> {
    fn is_empty(self) -> bool {
        self.before_stdlib.is_empty() && self.after_stdlib.is_empty()
    }
}

#[derive(Clone, Debug, Eq, PartialEq, get_size2::GetSize)]
struct StubPackageIndex {
    paths: Box<[SearchPath]>,
    stdlib_offset: usize,
}

impl StubPackageIndex {
    /// Indexes search paths that may contain a stub package, preserving their position relative to
    /// the standard library.
    fn from_search_paths<'a>(
        db: &dyn Db,
        search_paths: impl Iterator<Item = &'a SearchPath>,
    ) -> Self {
        let mut paths = Vec::new();
        let mut stdlib_offset = None;

        for search_path in search_paths {
            if search_path.is_standard_library() {
                stdlib_offset = Some(paths.len());
            } else if search_path_may_contain_stub_package(db, search_path) {
                paths.push(search_path.clone());
            }
        }

        let stdlib_offset = stdlib_offset.unwrap_or(paths.len());
        Self {
            paths: paths.into_boxed_slice(),
            stdlib_offset,
        }
    }

    /// Returns all indexed paths in normal typing-resolution order.
    fn all(&self) -> StubPackagePaths<'_> {
        StubPackagePaths {
            before_stdlib: self.before_stdlib(),
            after_stdlib: self.after_stdlib(),
        }
    }

    /// Splits the indexed paths between the stub-overlay pass and its normal fallback.
    ///
    /// The overlay contains only extra paths, which all precede stdlib. The fallback retains the
    /// remaining paths' positions relative to stdlib.
    fn split_overlay(&self) -> (StubPackagePaths<'_>, StubPackagePaths<'_>) {
        let before_stdlib = self.before_stdlib();
        let (extra, remaining) =
            before_stdlib.split_at(before_stdlib.partition_point(SearchPath::is_extra));

        (
            StubPackagePaths {
                before_stdlib: extra,
                after_stdlib: &[],
            },
            StubPackagePaths {
                before_stdlib: remaining,
                after_stdlib: self.after_stdlib(),
            },
        )
    }

    /// Returns indexed paths that precede stdlib in normal typing resolution.
    fn before_stdlib(&self) -> &[SearchPath] {
        &self.paths[..self.stdlib_offset]
    }

    /// Returns indexed paths that follow stdlib in normal typing resolution.
    fn after_stdlib(&self) -> &[SearchPath] {
        &self.paths[self.stdlib_offset..]
    }
}

/// Returns an index of search paths that may contain a top-level stub package, preserving their
/// resolution order relative to stdlib.
#[salsa::tracked(returns(ref), heap_size=ruff_memory_usage::heap_size)]
fn stub_package_index(
    db: &dyn Db,
    resolver_environment: ResolverEnvironment<'_>,
) -> StubPackageIndex {
    StubPackageIndex::from_search_paths(
        db,
        search_paths(db, resolver_environment, ModuleResolveMode::Typing),
    )
}

fn search_path_may_contain_stub_package(db: &dyn Db, search_path: &SearchPath) -> bool {
    let Some(path) = search_path.as_system_path() else {
        return false;
    };

    directory_listing(db, path)
        .is_ok_and(|listing| listing.iter().any(|(name, _)| name.ends_with("-stubs")))
}

/// Get the search-paths for desperate resolution of absolute imports in this file.
///
/// Currently this is "all ancestor directories that don't contain an `__init__.py(i)`"
/// (from closest-to-importing-file to farthest).
///
/// (For paranoia purposes, all relative desperate search-paths are also absolute
/// valid desperate search-paths, but don't worry about that.)
///
/// We exclude `__init__.py(i)` dirs to avoid truncating packages.
#[salsa::tracked(returns(as_deref), heap_size=ruff_memory_usage::heap_size)]
fn absolute_desperate_search_paths(
    db: &dyn Db,
    importing_file: ResolverFile<'_>,
) -> Option<Box<[SearchPath]>> {
    let resolver_environment = importing_file.environment(db);
    let importing_file = importing_file.file(db);
    let system = db.system();
    let importing_path = importing_file.path(db).as_system_path()?;

    // Only allow this if the importing_file is under the first-party search path
    let (base_path, rel_path) = search_paths(db, resolver_environment, ModuleResolveMode::Typing)
        .find_map(|search_path| {
        if !search_path.is_first_party() {
            return None;
        }
        Some((
            search_path.as_system_path()?,
            search_path.relativize_system_path_only(importing_path)?,
        ))
    })?;

    // Only allow searching up to the first-party path's root
    let mut search_paths = Vec::new();
    for rel_dir in rel_path.ancestors() {
        let candidate_path = base_path.join(rel_dir);
        let Ok(listing) = directory_listing(db, &candidate_path) else {
            continue;
        };
        // Any dir that isn't a proper package is plausibly some test/script dir that could be
        // added as a search-path at runtime. Notably this reflects pytest's default mode where
        // it adds every dir with a .py to the search-paths (making all test files root modules),
        // unless they see an `__init__.py`, in which case they assume you don't want that.
        let isnt_regular_package = !listing.entry_is_file(db, &candidate_path, "__init__.py")
            && !listing.entry_is_file(db, &candidate_path, "__init__.pyi");
        // Any dir with a pyproject.toml or ty.toml is a valid relative desperate search-path and
        // we want all of those to also be valid absolute desperate search-paths. It doesn't
        // make any sense for a folder to have `pyproject.toml` and `__init__.py` but let's
        // not let something cursed and spooky happen, ok? d
        if isnt_regular_package
            || listing.entry_is_file(db, &candidate_path, "pyproject.toml")
            || listing.entry_is_file(db, &candidate_path, "ty.toml")
        {
            let search_path = SearchPath::first_party(system, candidate_path).ok()?;
            search_paths.push(search_path);
        }
    }

    if search_paths.is_empty() {
        None
    } else {
        Some(search_paths.into_boxed_slice())
    }
}

/// Get the search-paths for desperate resolution of relative imports in this file.
///
/// Currently this is "the closest ancestor dir that contains a pyproject.toml (or ty.toml)",
/// which is a completely arbitrary decision. However it's fairly important that relative
/// desperate search-paths pick a single "best" answer because every one is *valid* but one
/// that's too long or too short may cause problems.
///
/// For now this works well in common cases where we have some larger workspace that contains
/// one or more python projects in sub-directories, and those python projects assume that
/// absolute imports resolve relative to the pyproject.toml they live under.
///
/// Being so strict minimizes concerns about this going off a lot and doing random
/// chaotic things. In particular, all files under a given pyproject.toml will currently
/// agree on this being their desperate search-path, which is really nice.
#[salsa::tracked(returns(clone), heap_size=ruff_memory_usage::heap_size)]
fn relative_desperate_search_paths(
    db: &dyn Db,
    importing_file: ResolverFile<'_>,
) -> Option<SearchPath> {
    let resolver_environment = importing_file.environment(db);
    let importing_file = importing_file.file(db);
    let system = db.system();
    let importing_path = importing_file.path(db).as_system_path()?;

    // Only allow this if the importing_file is under the first-party search path
    let (base_path, rel_path) = search_paths(db, resolver_environment, ModuleResolveMode::Typing)
        .find_map(|search_path| {
        if !search_path.is_first_party() {
            return None;
        }
        Some((
            search_path.as_system_path()?,
            search_path.relativize_system_path_only(importing_path)?,
        ))
    })?;

    // Only allow searching up to the first-party path's root
    for rel_dir in rel_path.ancestors() {
        let candidate_path = base_path.join(rel_dir);
        let Ok(listing) = directory_listing(db, &candidate_path) else {
            continue;
        };
        // Any dir with a pyproject.toml or ty.toml might be a project root
        if listing.entry_is_file(db, &candidate_path, "pyproject.toml")
            || listing.entry_is_file(db, &candidate_path, "ty.toml")
        {
            let search_path = SearchPath::first_party(system, candidate_path).ok()?;
            return Some(search_path);
        }
    }

    None
}
#[derive(Clone, Debug, PartialEq, Eq, Hash, get_size2::GetSize)]
pub struct SearchPaths {
    /// Search paths that have been statically determined purely from reading
    /// ty's configuration settings. These shouldn't ever change unless the
    /// config settings themselves change.
    static_paths: Vec<SearchPath>,

    /// Path to typeshed, which should come immediately after static paths.
    ///
    /// This can currently only be None if the `SystemPath` this points to is already in `static_paths`.
    stdlib_path: Option<SearchPath>,

    /// Path to the real stdlib, this replaces typeshed (`stdlib_path`) for goto-definition searches
    /// ([`ModuleResolveMode::Runtime`]).
    real_stdlib_path: Option<SearchPath>,

    /// site-packages paths are not included in the above fields:
    /// if there are multiple site-packages paths, editable installations can appear
    /// *between* the site-packages paths on `sys.path` at runtime.
    /// That means we can't know where a second or third `site-packages` path should sit
    /// in terms of module-resolution priority until we've discovered the editable installs
    /// for the first `site-packages` path
    site_packages: Vec<SearchPath>,

    typeshed_versions: TypeshedVersions,
}

impl SearchPaths {
    /// Validate and normalize the raw settings given by the user
    /// into settings we can use for module resolution
    ///
    /// This method also implements the typing spec's [module resolution order].
    ///
    /// [module resolution order]: https://typing.python.org/en/latest/spec/distributing.html#import-resolution-ordering
    pub(crate) fn from_settings<Strategy: MisconfigurationStrategy>(
        settings: &SearchPathSettings,
        system: &dyn System,
        vendored: &VendoredFileSystem,
        strategy: &Strategy,
    ) -> Result<Self, Strategy::Error<SearchPathSettingsError>> {
        fn canonicalize(path: &SystemPath, system: &dyn System) -> SystemPathBuf {
            system
                .canonicalize_path(path)
                .unwrap_or_else(|_| path.to_path_buf())
        }

        let SearchPathSettings {
            extra_paths,
            src_roots,
            custom_typeshed: typeshed,
            site_packages_paths,
            real_stdlib_path,
        } = settings;

        let mut static_paths = vec![];

        for path in extra_paths {
            let path = canonicalize(path, system);
            tracing::debug!("Adding extra search-path `{path}`");

            let path = strategy.fallback_opt(
                SearchPath::extra(system, path).map_err(SearchPathSettingsError::from),
                |err| {
                    tracing::debug!("Skipping invalid extra search-path: {err}");
                },
            )?;
            static_paths.extend(path);
        }

        for src_root in src_roots {
            tracing::debug!("Adding first-party search path `{src_root}`");
            let path = strategy.fallback_opt(
                SearchPath::first_party(system, src_root.to_path_buf())
                    .map_err(SearchPathSettingsError::from),
                |err| {
                    tracing::debug!("Skipping invalid first-party search-path: {err}");
                },
            )?;
            static_paths.extend(path);
        }

        let (typeshed_versions, stdlib_path) = if let Some(typeshed) = typeshed {
            let typeshed = canonicalize(typeshed, system);
            tracing::debug!("Adding custom-stdlib search path `{typeshed}`");

            let versions_path = typeshed.join("stdlib/VERSIONS");

            let results = system
                .read_to_string(&versions_path)
                .map_err(|error| SearchPathSettingsError::FailedToReadVersionsFile {
                    path: versions_path,
                    error,
                })
                .and_then(|versions_content| Ok(versions_content.parse()?))
                .and_then(|parsed| Ok((parsed, SearchPath::custom_stdlib(system, &typeshed)?)));

            strategy.fallback(results, |err| {
                tracing::debug!("Skipping custom-stdlib search-path: {err}");
                (
                    vendored_typeshed_versions(vendored),
                    SearchPath::vendored_stdlib(),
                )
            })?
        } else {
            tracing::debug!("Using vendored stdlib");
            (
                vendored_typeshed_versions(vendored),
                SearchPath::vendored_stdlib(),
            )
        };

        let real_stdlib_path = if let Some(path) = real_stdlib_path {
            strategy.fallback_opt(
                SearchPath::real_stdlib(system, path.clone())
                    .map_err(SearchPathSettingsError::from),
                |err| {
                    tracing::debug!("Skipping invalid real-stdlib search-path: {err}");
                },
            )?
        } else {
            None
        };

        let mut site_packages: Vec<_> = Vec::with_capacity(site_packages_paths.len());

        for path in site_packages_paths {
            tracing::debug!("Adding site-packages search path `{path}`");
            let path = strategy.fallback_opt(
                SearchPath::site_packages(system, path.clone())
                    .map_err(SearchPathSettingsError::from),
                |err| {
                    tracing::debug!("Skipping invalid site-packages search-path: {err}");
                },
            )?;
            site_packages.extend(path);
        }

        // TODO vendor typeshed's third-party stubs as well as the stdlib and
        // fallback to them as a final step?
        //
        // See: <https://github.com/astral-sh/ruff/pull/19620#discussion_r2240609135>

        // Filter out module resolution paths that point to the same directory
        // on disk (the same invariant maintained by [`sys.path` at runtime]).
        // (Paths may, however, *overlap* -- e.g. you could have both `src/`
        // and `src/foo` as module resolution paths simultaneously.)
        //
        // This code doesn't use an `IndexSet` because the key is the system
        // path and not the search root.
        //
        // [`sys.path` at runtime]: https://docs.python.org/3/library/site.html#module-site
        let mut seen_paths = FxHashSet::with_capacity_and_hasher(static_paths.len(), FxBuildHasher);

        static_paths.retain(|path| {
            if let Some(path) = path.as_system_path() {
                seen_paths.insert(path.to_path_buf())
            } else {
                true
            }
        });

        // Users probably shouldn't do this but... if they've shadowed their stdlib we should deduplicate it away.
        // This notably will mess up anything that checks if a search path "is the standard library" as we won't
        // "remember" that fact for static paths.
        //
        // (We used to shove these into static_paths, so the above retain implicitly did this. I am opting to
        // preserve this behaviour to avoid getting into the weeds of corner cases.)
        let stdlib_path_is_shadowed = stdlib_path
            .as_system_path()
            .is_some_and(|path| seen_paths.contains(path));
        let real_stdlib_path_is_shadowed = real_stdlib_path
            .as_ref()
            .and_then(SearchPath::as_system_path)
            .is_some_and(|path| seen_paths.contains(path));

        let stdlib_path = if stdlib_path_is_shadowed {
            None
        } else {
            Some(stdlib_path)
        };
        let real_stdlib_path = if real_stdlib_path_is_shadowed {
            None
        } else {
            real_stdlib_path
        };

        Ok(SearchPaths {
            static_paths,
            stdlib_path,
            real_stdlib_path,
            site_packages,
            typeshed_versions,
        })
    }

    /// Returns a new `SearchPaths` with no search paths configured.
    ///
    /// The vendored standard library remains available.
    pub fn empty(vendored: &VendoredFileSystem) -> Self {
        Self {
            static_paths: vec![],
            stdlib_path: Some(SearchPath::vendored_stdlib()),
            real_stdlib_path: None,
            site_packages: vec![],
            typeshed_versions: vendored_typeshed_versions(vendored),
        }
    }

    /// Registers file roots for all non-dynamically discovered search paths.
    pub fn try_register_static_roots(&self, db: &dyn Db) {
        let files = db.files();
        for path in self
            .static_paths
            .iter()
            .chain(self.site_packages.iter())
            .chain(&self.stdlib_path)
        {
            if let Some(system_path) = path.as_system_path() {
                // Nested first-party paths reuse the project root. Other nested paths, such as
                // site-packages inside `.venv`, need their own search-path root.
                if !path.is_first_party() || files.root(db, system_path).is_none() {
                    files.try_add_root(db, system_path, FileRootKind::SearchPath);
                }
            }
        }
    }

    fn stdlib(&self, mode: ModuleResolveMode) -> Option<&SearchPath> {
        match mode {
            ModuleResolveMode::Typing => self.stdlib_path.as_ref(),
            ModuleResolveMode::Runtime | ModuleResolveMode::RuntimeSomeShadowingAllowed => {
                self.real_stdlib_path.as_ref()
            }
        }
    }

    pub fn custom_stdlib(&self) -> Option<&SystemPath> {
        self.stdlib_path
            .as_ref()
            .and_then(SearchPath::as_system_path)
    }

    pub fn typeshed_versions(&self) -> &TypeshedVersions {
        &self.typeshed_versions
    }
}

/// Collect all dynamic search paths. For each `site-packages` path:
/// - Collect that `site-packages` path
/// - Collect any search paths listed in `.pth` files in that `site-packages` directory
///   due to editable installations of third-party packages.
///
/// The editable-install search paths for the first `site-packages` directory
/// should come between the two `site-packages` directories when it comes to
/// module-resolution priority.
#[salsa::tracked(returns(deref), heap_size=ruff_memory_usage::heap_size)]
pub(crate) fn dynamic_resolution_paths<'db>(
    db: &'db dyn Db,
    mode: ModuleResolveModeIngredient<'db>,
) -> Vec<SearchPath> {
    tracing::debug!("Resolving dynamic module resolution paths");

    let SearchPaths {
        static_paths,
        stdlib_path,
        site_packages,
        typeshed_versions: _,
        real_stdlib_path,
    } = mode.resolver_environment(db).search_paths(db);

    let mut dynamic_paths = Vec::new();

    if site_packages.is_empty() {
        return dynamic_paths;
    }

    let mut existing_paths: FxHashSet<_> = static_paths
        .iter()
        .filter_map(|path| path.as_system_path())
        .map(Cow::Borrowed)
        .collect();

    // Use the `ModuleResolveMode` to determine which stdlib (if any) to mark as existing
    let stdlib = match mode.mode(db) {
        ModuleResolveMode::Typing => stdlib_path,
        ModuleResolveMode::Runtime | ModuleResolveMode::RuntimeSomeShadowingAllowed => {
            real_stdlib_path
        }
    };
    if let Some(path) = stdlib.as_ref().and_then(SearchPath::as_system_path) {
        existing_paths.insert(Cow::Borrowed(path));
    }

    let files = db.files();
    let system = db.system();

    for site_packages_search_path in site_packages {
        let site_packages_dir = site_packages_search_path
            .as_system_path()
            .expect("Expected site package path to be a system path");

        if !existing_paths.insert(Cow::Borrowed(site_packages_dir)) {
            continue;
        }

        dynamic_paths.push(site_packages_search_path.clone());

        // As well as modules installed directly into `site-packages`,
        // the directory may also contain `.pth` files.
        // Each `.pth` file in `site-packages` may contain one or more lines
        // containing a (relative or absolute) path.
        // Each of these paths may point to an editable install of a package,
        // so should be considered an additional search path.
        let listing = match directory_listing(db, site_packages_dir) {
            Ok(listing) => listing,
            Err(error) => {
                tracing::warn!(
                    "Failed to search for editable installation in {site_packages_dir}: {error}"
                );
                continue;
            }
        };

        // The Python documentation specifies that `.pth` files in `site-packages`
        // are processed in alphabetical order. `DirectoryListing` is already sorted.
        // https://docs.python.org/3/library/site.html#module-site
        let pth_files = listing.iter().filter(|(name, file_type)| {
            !file_type.is_directory() && SystemPath::new(name).extension() == Some("pth")
        });

        for (name, _) in pth_files {
            let path = site_packages_dir.join(name);
            // Track each `.pth` file independently so content changes invalidate this query.
            let Ok(file) = system_path_to_file(db, &path).inspect_err(|error| {
                tracing::warn!("Failed to open .pth file `{path}`: {error}");
            }) else {
                continue;
            };
            let contents = source_text(db, file);
            if let Some(error) = contents.read_error() {
                tracing::warn!("Failed to read .pth file `{path}`: {error}");
                continue;
            }

            let installations = contents.lines().filter_map(|line| {
                let line = line.trim_end();
                if line.is_empty()
                    || line.starts_with('#')
                    || line.starts_with("import ")
                    || line.starts_with("import\t")
                {
                    return None;
                }

                Some(SystemPath::absolute(line, site_packages_dir))
            });

            for installation in installations {
                let installation = system
                    .canonicalize_path(&installation)
                    .unwrap_or(installation);

                if existing_paths.insert(Cow::Owned(installation.clone())) {
                    match SearchPath::editable(system, installation.clone()) {
                        Ok(search_path) => {
                            tracing::debug!(
                                "Adding editable installation to module resolution path {path}",
                                path = installation
                            );

                            // Register a file root for editable installs that are outside any other root
                            // (Most importantly, don't register a root for editable installations from the project
                            // directory as that would change the durability of files within those folders).
                            // Not having an exact file root for editable installs just means that
                            // some queries (like `list_modules_in`) will run slightly more frequently
                            // than they would otherwise.
                            if let Some(dynamic_path) = search_path.as_system_path() {
                                if files.root(db, dynamic_path).is_none() {
                                    files.try_add_root(db, dynamic_path, FileRootKind::SearchPath);
                                }
                            }

                            dynamic_paths.push(search_path);
                        }

                        Err(error) => {
                            tracing::debug!("Skipping editable installation: {error}");
                        }
                    }
                }
            }
        }
    }

    dynamic_paths
}

/// Iterate over the available module-resolution search paths,
/// following the invariants maintained by [`sys.path` at runtime]:
/// "No item is added to `sys.path` more than once."
/// Dynamic search paths (required for editable installs into `site-packages`)
/// are only calculated lazily.
///
/// [`sys.path` at runtime]: https://docs.python.org/3/library/site.html#module-site
pub struct SearchPathIterator<'db> {
    db: &'db dyn Db,
    static_paths: std::slice::Iter<'db, SearchPath>,
    stdlib_path: Option<&'db SearchPath>,
    dynamic_paths: Option<std::slice::Iter<'db, SearchPath>>,
    mode: ModuleResolveModeIngredient<'db>,
}

impl<'db> Iterator for SearchPathIterator<'db> {
    type Item = &'db SearchPath;

    fn next(&mut self) -> Option<Self::Item> {
        let SearchPathIterator {
            db,
            static_paths,
            stdlib_path,
            mode,
            dynamic_paths,
        } = self;

        static_paths
            .next()
            .or_else(|| stdlib_path.take())
            .or_else(|| {
                dynamic_paths
                    .get_or_insert_with(|| dynamic_resolution_paths(*db, *mode).iter())
                    .next()
            })
    }
}

impl FusedIterator for SearchPathIterator<'_> {}

/// A thin wrapper around a module name, resolution mode, and resolver environment to make them a Salsa
/// ingredient.
///
/// This is needed because Salsa requires that all query arguments are salsa ingredients.
#[salsa::interned(debug, heap_size=ruff_memory_usage::heap_size)]
struct ModuleNameIngredient<'db> {
    #[returns(ref)]
    pub(super) name: ModuleName,
    #[returns(copy)]
    pub(super) mode: ModuleResolveMode,
    #[returns(copy)]
    pub(super) resolver_environment: ResolverEnvironment<'db>,
}

/// Given a module name and a list of search paths in which to lookup modules,
/// attempt to resolve the module name
fn resolve_name<'db>(
    db: &'db dyn Db,
    resolver_environment: ResolverEnvironment<'db>,
    name: &ModuleName,
    mode: ModuleResolveMode,
) -> Option<ResolvedNames> {
    let resolver = NameResolver::new(db, resolver_environment, name, mode);

    match mode {
        ModuleResolveMode::Typing => {
            resolver.resolve_typing(stub_package_index(db, resolver_environment))
        }
        ModuleResolveMode::Runtime | ModuleResolveMode::RuntimeSomeShadowingAllowed => {
            resolver.resolve_runtime(search_paths(db, resolver_environment, mode))
        }
    }
}

/// Like `resolve_name` but for cases where it failed to resolve the module
/// and we are now Getting Desperate and willing to try the ancestor directories of
/// the `importing_file` as potential temporary search paths that are private
/// to this import.
fn desperately_resolve_name<'db>(
    db: &'db dyn Db,
    importing_file: File,
    resolver_environment: ResolverEnvironment<'db>,
    name: &ModuleName,
    mode: ModuleResolveMode,
) -> Option<ResolvedNames> {
    let importing_file = ResolverFile::new(db, importing_file, resolver_environment);
    let search_paths = absolute_desperate_search_paths(db, importing_file).unwrap_or_default();
    let resolver = NameResolver::new(db, resolver_environment, name, mode);

    match mode {
        ModuleResolveMode::Typing => resolver.resolve_desperate_typing(search_paths),
        ModuleResolveMode::Runtime | ModuleResolveMode::RuntimeSomeShadowingAllowed => {
            resolver.resolve_runtime(search_paths.iter())
        }
    }
}

#[derive(Debug, Clone, Copy)]
enum ResolvedModule {
    NamespacePackage,
    LegacyNamespacePackage(File),
    RegularPackage(File),
    Module(File),
}

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
enum ComponentFileFilter {
    /// Prefer `.pyi` over `.py` in typing mode, or only accept `.py` in runtime mode.
    ByMode,

    /// Only accept a `.pyi` file.
    StubOnly,
}

/// Where a candidate sits in the typing specification's module-resolution order.
///
/// Variants are declared from highest to lowest precedence so that derived ordering can be used
/// when traversing candidates. This is a precedence tier rather than a total ordering: the stable
/// sorts used by the resolver preserve search-path order between candidates in the same tier.
#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
enum CandidatePrecedence {
    /// A PEP 561 stub-only package named `<package>-stubs`.
    ///
    /// Stub packages take precedence over candidates for `<package>` regardless of where those
    /// candidates appear in the search-path order.
    StubPackage,

    /// A candidate whose precedence is determined by search-path order.
    ///
    /// This includes `.pyi` and `.py` packages and modules from extra paths, first-party code,
    /// editable installs, site-packages, and the standard library.
    SearchPathOrder,
}

#[derive(Debug, Clone)]
struct ModuleResolutionCandidate {
    path: ModulePath,
    module: ResolvedModule,
    py_typed: PyTyped,
    precedence: CandidatePrecedence,
}

impl ModuleResolutionCandidate {
    fn root(search_path: &SearchPath) -> Self {
        Self::with_precedence(search_path, CandidatePrecedence::SearchPathOrder)
    }

    fn stub(search_path: &SearchPath) -> Self {
        Self::with_precedence(search_path, CandidatePrecedence::StubPackage)
    }

    fn with_precedence(search_path: &SearchPath, precedence: CandidatePrecedence) -> Self {
        Self {
            path: search_path.to_module_path(),
            module: ResolvedModule::NamespacePackage,
            py_typed: PyTyped::Untyped,
            precedence,
        }
    }

    // Is this some kind of namespace package?
    fn is_any_namespace_package(&self) -> bool {
        match self.module {
            ResolvedModule::NamespacePackage => true,
            ResolvedModule::LegacyNamespacePackage(_) => true,
            ResolvedModule::RegularPackage(_) => false,
            ResolvedModule::Module(_) => false,
        }
    }

    // This is the module we were actually interested in resolving, complete the resolution
    fn into_module<'db>(
        self,
        db: &'db dyn Db,
        resolver_environment: ResolverEnvironment<'db>,
        name: &ModuleName,
    ) -> Module<'db> {
        match self.module {
            ResolvedModule::NamespacePackage => {
                tracing::trace!("Resolve namespace package `{name}`");
                Module::namespace_package(db, resolver_environment, Cow::Borrowed(name))
            }
            ResolvedModule::LegacyNamespacePackage(file) => {
                // legacy namespace packages behave like regular packages
                // when they're the target of the resolution
                tracing::trace!(
                    "Resolved legacy namespace package `{name}` to `{path}`",
                    path = file.path(db)
                );
                Module::file_module(
                    db,
                    file,
                    resolver_environment,
                    Cow::Borrowed(name),
                    ModuleKind::Package,
                    self.path.into_search_path(),
                )
            }
            ResolvedModule::RegularPackage(file) => {
                tracing::trace!(
                    "Resolved package `{name}` to `{path}`",
                    path = file.path(db)
                );
                Module::file_module(
                    db,
                    file,
                    resolver_environment,
                    Cow::Borrowed(name),
                    ModuleKind::Package,
                    self.path.into_search_path(),
                )
            }
            ResolvedModule::Module(file) => {
                tracing::trace!("Resolved module `{name}` to `{path}`", path = file.path(db));
                Module::file_module(
                    db,
                    file,
                    resolver_environment,
                    Cow::Borrowed(name),
                    ModuleKind::Module,
                    self.path.into_search_path(),
                )
            }
        }
    }

    fn missing_submodule_is_terminal(&self) -> bool {
        if matches!(self.py_typed, PyTyped::Partial) {
            return false;
        }

        // Regular packages and modules are both terminal. A `foo.py`
        // in a higher-priority search path is not shadowed by
        // `foo/__init__.py` in a lower-priority one. Note that both
        // shadow namespace packages.
        matches!(
            self.module,
            ResolvedModule::RegularPackage(_) | ResolvedModule::Module(_)
        )
    }

    fn to_str<'a>(&self, db: &'a dyn Db) -> Cow<'a, str> {
        match self.module {
            ResolvedModule::NamespacePackage => {
                Cow::Owned(self.path.to_system_path().unwrap_or_default().to_string())
            }
            ResolvedModule::LegacyNamespacePackage(file) => Cow::Borrowed(file.path(db).as_str()),
            ResolvedModule::RegularPackage(file) => Cow::Borrowed(file.path(db).as_str()),
            ResolvedModule::Module(file) => Cow::Borrowed(file.path(db).as_str()),
        }
    }
}

struct NameResolver<'db, 'name> {
    context: ResolverContext<'db>,
    name: &'name ModuleName,
    is_non_shadowable: bool,
}

impl<'db, 'name> NameResolver<'db, 'name> {
    fn new(
        db: &'db dyn Db,
        resolver_environment: ResolverEnvironment<'db>,
        name: &'name ModuleName,
        mode: ModuleResolveMode,
    ) -> Self {
        let python_version = resolver_environment.python_version(db);
        Self {
            context: ResolverContext::new(db, resolver_environment, mode),
            name,
            is_non_shadowable: mode.is_non_shadowable(python_version.minor, name.as_str()),
        }
    }

    /// Resolves the name as seen by a type checker.
    ///
    /// This includes PEP 561 stub packages and user-provided stub overlays, with runtime source as
    /// a fallback when no stub provides the requested module. A stub overlay may use runtime
    /// packages as parents, but its final module must come from a stub file.
    fn resolve_typing(&self, stub_packages: &StubPackageIndex) -> Option<ResolvedNames> {
        if self.name.components().nth(1).is_none() {
            let candidates = self.discover_roots(
                search_paths(
                    self.context.db,
                    self.context.resolver_environment,
                    ModuleResolveMode::Typing,
                ),
                stub_packages.all(),
            );
            return self.resolve_remaining(candidates, ComponentFileFilter::ByMode);
        }

        // Only submodules need separate overlay resolution: their extra-path namespace parent can
        // be shadowed before the resolver reaches the requested stub. Reuse those roots for the
        // normal fallback so that each extra path is probed only once.
        let (overlay_stub_packages, remaining_stub_packages) = stub_packages.split_overlay();
        let mut candidates = self.discover_roots(
            search_paths(
                self.context.db,
                self.context.resolver_environment,
                ModuleResolveMode::Typing,
            )
            .take_while(|search_path| search_path.is_extra()),
            overlay_stub_packages,
        );
        if let Some(resolved) =
            self.resolve_remaining(candidates.clone(), ComponentFileFilter::StubOnly)
        {
            return Some(resolved);
        }

        let remaining_candidates = self.discover_roots(
            search_paths(
                self.context.db,
                self.context.resolver_environment,
                ModuleResolveMode::Typing,
            )
            .skip_while(|search_path| search_path.is_extra()),
            remaining_stub_packages,
        );
        candidates.extend(remaining_candidates);

        self.resolve_remaining(candidates, ComponentFileFilter::ByMode)
    }

    /// Resolves the name for type checking against desperate ancestor search paths.
    ///
    /// These paths can contain PEP 561 stub packages, but never user-provided extra paths, so this
    /// indexes them for stub packages without performing a separate stub-overlay pass. Runtime
    /// resolution instead ignores stub packages and `.pyi` files entirely.
    fn resolve_desperate_typing(&self, search_paths: &[SearchPath]) -> Option<ResolvedNames> {
        let stub_packages =
            StubPackageIndex::from_search_paths(self.context.db, search_paths.iter());
        let candidates = self.discover_roots(search_paths.iter(), stub_packages.all());
        self.resolve_remaining(candidates, ComponentFileFilter::ByMode)
    }

    /// Resolves the name to the implementation that is available at runtime.
    ///
    /// The runtime resolver ignores stub packages and `.pyi` files. Its search paths also use the
    /// real standard library instead of typeshed.
    fn resolve_runtime<'a>(
        &self,
        search_paths: impl Iterator<Item = &'a SearchPath>,
    ) -> Option<ResolvedNames> {
        let candidates = self.discover_roots(search_paths, StubPackagePaths::default());
        self.resolve_remaining(candidates, ComponentFileFilter::ByMode)
    }

    fn discover_roots<'a>(
        &self,
        search_paths: impl Iterator<Item = &'a SearchPath>,
        stub_paths: StubPackagePaths<'_>,
    ) -> ResolvedNames {
        let root_component = self.name.first_component();
        let mut cur_candidates = Vec::new();
        let stub_name = (!stub_paths.is_empty() && !self.is_non_shadowable)
            .then(|| format!("{root_component}-stubs"));
        let mut pending_stub_paths = Vec::new();

        if let Some(stub_name) = &stub_name {
            cur_candidates.extend(stub_paths.before_stdlib.iter().filter_map(|search_path| {
                resolve_stub_package_in_search_path(&self.context, search_path, stub_name)
            }));
            // Defer file probes after stdlib until we know that stdlib does not win.
            pending_stub_paths.extend(stub_paths.after_stdlib.iter().filter(|search_path| {
                candidate_may_exist(
                    &self.context,
                    &ModuleResolutionCandidate::stub(search_path),
                    stub_name,
                )
            }));
        }

        for search_path in search_paths {
            // When a builtin module is imported, standard module resolution is bypassed:
            // the module name always resolves to the stdlib module,
            // even if there's a module of the same name in the first-party root
            // (which would normally result in the stdlib module being overridden).
            // TODO: offer a diagnostic if there is a first-party module of the same name
            if self.is_non_shadowable && !search_path.is_standard_library() {
                continue;
            }

            let is_stdlib = search_path.is_standard_library();
            // A terminal candidate can stop the search unless a matching post-stdlib stub package
            // could still override it. A terminal stdlib candidate always stops the search.
            let can_stop = is_stdlib || pending_stub_paths.is_empty();
            let mut candidate = ModuleResolutionCandidate::root(search_path);
            let resolved = resolve_component(
                &self.context,
                &mut candidate,
                root_component,
                ComponentFileFilter::ByMode,
            )
            .is_ok();
            let terminal = candidate.missing_submodule_is_terminal();
            if resolved {
                cur_candidates.push(candidate);
            }
            // A terminal candidate shadows all later search paths. Earlier candidates remain in
            // play because they already shadow this candidate.
            if terminal && can_stop {
                break;
            }

            // Reaching this point for stdlib means that it did not provide a terminal candidate.
            // The deferred post-stdlib stub packages are therefore eligible, so resolve them now.
            if is_stdlib && let Some(stub_name) = &stub_name {
                cur_candidates.extend(pending_stub_paths.drain(..).filter_map(|search_path| {
                    resolve_stub_package_in_search_path(&self.context, search_path, stub_name)
                }));
            }
        }

        cur_candidates
    }

    fn resolve_remaining(
        &self,
        mut cur_candidates: ResolvedNames,
        final_filter: ComponentFileFilter,
    ) -> Option<ResolvedNames> {
        if cur_candidates.is_empty() {
            return None;
        }

        let mut components = self.name.components().skip(1).peekable();

        loop {
            // Keep a partial stub package's namespace while resolving the next part of the module
            // name. Once the complete name is resolved, a concrete package or module shadows that
            // namespace.
            let has_remaining_components = components.peek().is_some();
            cur_candidates =
                normalize_candidates(self.context.db, cur_candidates, has_remaining_components);

            let Some(component) = components.next() else {
                return Some(cur_candidates);
            };
            let file_filter = if components.peek().is_some() {
                ComponentFileFilter::ByMode
            } else {
                final_filter
            };

            let mut remaining_are_shadowed = false;
            cur_candidates.retain_mut(|candidate| {
                if remaining_are_shadowed {
                    return false;
                }

                let resolved =
                    resolve_component(&self.context, candidate, component, file_filter).is_ok();

                // A terminal candidate shadows every lower-priority candidate, even if resolving
                // this component fails. Higher-priority candidates remain in play.
                remaining_are_shadowed = candidate.missing_submodule_is_terminal();

                resolved
            });

            if cur_candidates.is_empty() {
                return None;
            }
        }
    }
}

fn resolve_stub_package_in_search_path(
    context: &ResolverContext,
    search_path: &SearchPath,
    stub_name: &str,
) -> Option<ModuleResolutionCandidate> {
    let mut candidate = ModuleResolutionCandidate::stub(search_path);
    resolve_component(
        context,
        &mut candidate,
        stub_name,
        ComponentFileFilter::ByMode,
    )
    .ok()?;

    // `mypackage-stubs.py(i)` is not a valid result.
    if matches!(candidate.module, ResolvedModule::Module(_)) {
        tracing::debug!(
            "Search path `{search_path}` contains a module named `{stub_name}` but a standalone \
             module isn't a valid stub."
        );
        None
    } else {
        Some(candidate)
    }
}

fn normalize_candidates(
    db: &dyn Db,
    mut candidates: ResolvedNames,
    has_remaining_components: bool,
) -> ResolvedNames {
    let best_concrete_precedence = candidates
        .iter()
        .filter(|candidate| !candidate.is_any_namespace_package())
        .map(|candidate| candidate.precedence)
        .min();

    candidates.sort_by_key(|candidate| candidate.precedence);

    // Note that we intentionally do *not* filter out ordinary search-path candidates when a stub
    // package is found. Even when a non-namespace, non-partial stub package exists, we keep the
    // other candidates as fallbacks because sub-packages within the stubs may override py.typed to
    // partial. The stub-package candidate is ordered first so it takes priority. Other candidates
    // are only used when the stub package fails to find a submodule in a partial sub-package.
    candidates.retain(|candidate| {
        if !candidate.is_any_namespace_package() {
            return true;
        }

        // A higher-precedence partial namespace remains available while resolving its descendants.
        // At the final component, a concrete package or module shadows it.
        let preserved_for_descendants = best_concrete_precedence.is_none_or(|precedence| {
            has_remaining_components
                && candidate.py_typed == PyTyped::Partial
                && candidate.precedence < precedence
        });

        if preserved_for_descendants {
            return true;
        }

        // TODO: It might be useful to warn when a concrete package or module shadows a legacy
        // namespace package. If we only find legacy and non-legacy namespace packages, this logic
        // retains both.

        tracing::trace!(
            "Discarding namespace package `{}` because a non-namespace entry of the same name \
             was found",
            candidate.to_str(db),
        );
        false
    });

    candidates
}

/// Resolves one component relative to the candidate's current package.
fn resolve_component(
    context: &ResolverContext,
    candidate: &mut ModuleResolutionCandidate,
    module_name: &str,
    file_filter: ComponentFileFilter,
) -> Result<(), ()> {
    if matches!(candidate.module, ResolvedModule::Module(_)) {
        tracing::trace!(
            "Non-package module {} cannot have a child",
            candidate.to_str(context.db)
        );
        return Err(());
    }

    if !candidate_may_exist(context, candidate, module_name) {
        return Err(());
    }

    let package_path = &mut candidate.path;
    package_path.push(module_name);

    // Check for a regular package first (highest priority)
    package_path.push("__init__");
    if let Some(init) = resolve_file_module_with_filter(package_path, context, file_filter) {
        // Remove the `__init__` component for any potential next step
        package_path.pop();
        candidate.py_typed = package_path
            .py_typed(context)
            .inherit_parent(candidate.py_typed);
        if is_legacy_namespace_package(package_path, context, init) {
            candidate.module = ResolvedModule::LegacyNamespacePackage(init);
        } else {
            candidate.module = ResolvedModule::RegularPackage(init);
        }
        return Ok(());
    }

    // Check for a file module next
    package_path.pop();

    if let Some(file_module) = resolve_file_module_with_filter(package_path, context, file_filter) {
        candidate.module = ResolvedModule::Module(file_module);
        return Ok(());
    }

    // Last resort, check if a folder with the given name exists. If so,
    // then this is a namespace package. We need to skip this check for
    // typeshed because the `resolve_file_module` can also return `None` if the
    // `__init__.py` exists but isn't available for the current Python version.
    // Let's assume that the `xml` module is only available on Python 3.11+ and
    // we're resolving for Python 3.10:
    //
    // * `resolve_file_module("xml/__init__.pyi")` returns `None` even though
    //   the file exists but the module isn't available for the current Python
    //   version.
    // * The check here would now return `true` because the `xml` directory
    //   exists, resulting in a false positive for a namespace package.
    //
    // Since typeshed doesn't use any namespace packages today (May 2025),
    // simply skip this check which also helps performance. If typeshed
    // ever uses namespace packages, ensure that this check also takes the
    // `VERSIONS` file into consideration.
    // A namespace package is not backed by a file, so it cannot satisfy a stub-only lookup.
    if file_filter != ComponentFileFilter::StubOnly
        && !package_path.search_path().is_standard_library()
        && package_path.is_directory(context)
    {
        candidate.py_typed = package_path
            .py_typed(context)
            .inherit_parent(candidate.py_typed);
        candidate.module = ResolvedModule::NamespacePackage;
        return Ok(());
    }

    Err(())
}

/// Uses the parent directory's entries to reject candidates that cannot exist without performing
/// individual file-system probes for every supported module layout.
fn candidate_may_exist(
    context: &ResolverContext,
    candidate: &ModuleResolutionCandidate,
    module_name: &str,
) -> bool {
    let Some(parent) = candidate.path.to_system_path() else {
        return true;
    };

    let Ok(listing) = directory_listing(context.db, &parent) else {
        return false;
    };

    // Other suffixes are harmless false positives; the normal probes still determine whether the
    // module exists.
    listing.contains_name_with_prefix(module_name)
}

type ResolvedNames = Vec<ModuleResolutionCandidate>;

/// If `module` exists on disk with an extension permitted by the resolver's mode, return its
/// [`File`].
///
/// Typing resolution prefers `.pyi` over `.py`; runtime resolution only considers `.py`.
pub(super) fn resolve_file_module(
    module: &ModulePath,
    resolver_state: &ResolverContext,
) -> Option<File> {
    resolve_file_module_with_filter(module, resolver_state, ComponentFileFilter::ByMode)
}

fn resolve_file_module_with_filter(
    module: &ModulePath,
    resolver_state: &ResolverContext,
    filter: ComponentFileFilter,
) -> Option<File> {
    let stub_file = if resolver_state.mode.is_typing() {
        module.with_pyi_extension().to_file(resolver_state)
    } else {
        None
    };
    if filter == ComponentFileFilter::StubOnly {
        return stub_file;
    }

    stub_file.or_else(|| {
        module
            .with_py_extension()
            .and_then(|path| path.to_file(resolver_state))
    })
}

/// Determines whether a package is a legacy namespace package.
///
/// Before PEP 420 introduced implicit namespace packages, the ecosystem developed
/// its own form of namespace packages. These legacy namespace packages continue to persist
/// in modern codebases because they work with ancient Pythons and if it ain't broke, don't fix it.
///
/// A legacy namespace package is distinguished by having an `__init__.py` that contains an
/// expression to the effect of:
///
/// ```python
/// __path__ = __import__("pkgutil").extend_path(__path__, __name__)
/// ```
///
/// The resulting package simultaneously has properties of both regular packages and namespace ones:
///
/// * Like regular packages, `__init__.py` is defined and can contain items other than submodules
/// * Like implicit namespace packages, multiple copies of the package may exist with different
///   submodules, and they will be merged into one namespace at runtime by the interpreter
///
/// Now, you may rightly wonder: "What if the `__init__.py` files have different contents?"
/// The apparent official answer is: "Don't do that!"
/// And the reality is: "Of course people do that!"
///
/// In practice we think it's fine to, just like with regular packages, use the first one
/// we find on the search paths. To the extent that the different copies "need" to have the same
/// contents, they all "need" to have the legacy namespace idiom (we do nothing to enforce that,
/// we will just get confused if you mess it up).
fn is_legacy_namespace_package(
    package_path: &ModulePath,
    context: &ResolverContext,
    init: File,
) -> bool {
    // Just an optimization, the stdlib and typeshed are never legacy namespace packages
    if package_path.search_path().is_standard_library() {
        return false;
    }

    // This is all syntax-only analysis so it *could* be fooled but it's really unlikely.
    //
    // The benefit of being syntax-only is speed and avoiding circular dependencies
    // between module resolution and semantic analysis.
    //
    // The downside is if you write slightly different syntax we will fail to detect the idiom,
    // but hey, this is better than nothing!
    let parsed = ruff_db::parsed::parsed_module(
        context.db,
        PythonFile::new(
            context.db,
            init,
            context.resolver_environment.python_version(context.db),
        ),
    );
    let mut visitor = LegacyNamespacePackageVisitor::default();
    visitor.visit_body(parsed.load(context.db).suite());

    visitor.is_legacy_namespace_package
}

/// Info about the `py.typed` file for this package
#[derive(Copy, Clone, Eq, PartialEq, Debug)]
pub(crate) enum PyTyped {
    /// No `py.typed` was found
    Untyped,
    /// A `py.typed` was found containing "partial"
    Partial,
    /// A `py.typed` was found (not partial)
    Full,
}

impl PyTyped {
    /// Inherit py.typed info from the parent package
    ///
    /// > This marker applies recursively: if a top-level package includes it,
    /// > all its sub-packages MUST support type checking as well.
    ///
    /// This implementation implies that once a `py.typed` is specified
    /// all child packages inherit it, so they can never become Untyped.
    /// However they can override whether that's Full or Partial by
    /// redeclaring a `py.typed` file of their own.
    fn inherit_parent(self, parent: Self) -> Self {
        if self == Self::Untyped { parent } else { self }
    }
}

pub(super) struct ResolverContext<'db> {
    pub(super) db: &'db dyn Db,
    pub(super) resolver_environment: ResolverEnvironment<'db>,
    pub(super) mode: ModuleResolveMode,
}

impl<'db> ResolverContext<'db> {
    pub(super) fn new(
        db: &'db dyn Db,
        resolver_environment: ResolverEnvironment<'db>,
        mode: ModuleResolveMode,
    ) -> Self {
        Self {
            db,
            resolver_environment,
            mode,
        }
    }

    pub(super) fn vendored(&self) -> &VendoredFileSystem {
        self.db.vendored()
    }
}

/// Detects if a module contains a statement of the form:
/// ```python
/// __path__ = pkgutil.extend_path(__path__, __name__)
/// ```
/// or
/// ```python
/// __path__ = __import__("pkgutil").extend_path(__path__, __name__)
/// ```
/// or
/// ```python
/// __import__('pkg_resources').declare_namespace(__name__)
/// ```
#[derive(Default)]
struct LegacyNamespacePackageVisitor {
    is_legacy_namespace_package: bool,
    in_body: bool,
}

impl Visitor<'_> for LegacyNamespacePackageVisitor {
    fn visit_body(&mut self, body: &[ruff_python_ast::Stmt]) {
        if self.is_legacy_namespace_package {
            return;
        }

        // Don't traverse into nested bodies.
        if self.in_body {
            return;
        }

        self.in_body = true;

        walk_body(self, body);
    }

    fn visit_stmt(&mut self, stmt: &ast::Stmt) {
        if self.is_legacy_namespace_package {
            return;
        }

        match stmt {
            // __path__ = pkgutil.extend_path(__path__, __name__)
            // __path__ = __import__("pkgutil").extend_path(__path__, __name__)
            ast::Stmt::Assign(ast::StmtAssign { value, targets, .. }) => {
                self.check_pkgutil_extend_path(targets, value);
            }
            // __import__('pkg_resources').declare_namespace(__name__)
            ast::Stmt::Expr(ast::StmtExpr { value, .. }) => {
                self.check_pkg_resources_declare_namespace(value);
            }
            _ => {}
        }
    }
}

impl LegacyNamespacePackageVisitor {
    /// Check for `__path__ = pkgutil.extend_path(__path__, __name__)` or
    /// `__path__ = __import__("pkgutil").extend_path(__path__, __name__)`
    fn check_pkgutil_extend_path(&mut self, targets: &[ast::Expr], value: &ast::Expr) {
        let [ast::Expr::Name(maybe_path)] = targets else {
            return;
        };

        if &*maybe_path.id != "__path__" {
            return;
        }

        let ast::Expr::Call(ast::ExprCall {
            func: extend_func,
            arguments: extend_arguments,
            ..
        }) = value
        else {
            return;
        };

        let ast::Expr::Attribute(ast::ExprAttribute {
            value: maybe_pkg_util,
            attr: maybe_extend_path,
            ..
        }) = &**extend_func
        else {
            return;
        };

        // Match if the left side of the attribute access is either `__import__("pkgutil")` or `pkgutil`
        match &**maybe_pkg_util {
            // __import__("pkgutil").extend_path(__path__, __name__)
            ast::Expr::Call(ruff_python_ast::ExprCall {
                func: maybe_import,
                arguments: import_arguments,
                ..
            }) => {
                let ast::Expr::Name(maybe_import) = &**maybe_import else {
                    return;
                };

                if maybe_import.id() != "__import__" {
                    return;
                }

                let Some(ast::Expr::StringLiteral(name)) =
                    import_arguments.find_argument_value("name", 0)
                else {
                    return;
                };

                if name.value.to_str() != "pkgutil" {
                    return;
                }
            }
            // "pkgutil.extend_path(__path__, __name__)"
            ast::Expr::Name(name) => {
                if name.id() != "pkgutil" {
                    return;
                }
            }
            _ => {
                return;
            }
        }

        // Test that this is an `extend_path(__path__, __name__)` call
        if maybe_extend_path != "extend_path" {
            return;
        }

        let Some(ast::Expr::Name(path)) = extend_arguments.find_argument_value("path", 0) else {
            return;
        };
        let Some(ast::Expr::Name(name)) = extend_arguments.find_argument_value("name", 1) else {
            return;
        };

        self.is_legacy_namespace_package = path.id() == "__path__" && name.id() == "__name__";
    }

    /// Check for `__import__('pkg_resources').declare_namespace(__name__)`
    fn check_pkg_resources_declare_namespace(&mut self, value: &ast::Expr) {
        let ast::Expr::Call(ast::ExprCall {
            func,
            arguments: declare_arguments,
            ..
        }) = value
        else {
            return;
        };

        let ast::Expr::Attribute(ast::ExprAttribute {
            value: maybe_pkg_resources,
            attr: maybe_declare_namespace,
            ..
        }) = &**func
        else {
            return;
        };

        if maybe_declare_namespace != "declare_namespace" {
            return;
        }

        // Match `__import__("pkg_resources")`
        let ast::Expr::Call(ast::ExprCall {
            func: maybe_import,
            arguments: import_arguments,
            ..
        }) = &**maybe_pkg_resources
        else {
            return;
        };

        let ast::Expr::Name(maybe_import) = &**maybe_import else {
            return;
        };

        if maybe_import.id() != "__import__" {
            return;
        }

        let Some(ast::Expr::StringLiteral(name)) = import_arguments.find_argument_value("name", 0)
        else {
            return;
        };

        if name.value.to_str() != "pkg_resources" {
            return;
        }

        // Check that the argument is `__name__`
        let Some(ast::Expr::Name(name_arg)) = declare_arguments.find_argument_value("name", 0)
        else {
            return;
        };

        self.is_legacy_namespace_package = name_arg.id() == "__name__";
    }
}

#[cfg(test)]
mod tests {
    #![expect(
        clippy::disallowed_methods,
        reason = "These are tests, so it's fine to do I/O by-passing System."
    )]
    use ruff_db::Db;
    use ruff_db::files::{File, FilePath, system_path_to_file};
    use ruff_db::system::{DbWithTestSystem as _, DbWithWritableSystem as _};
    use ruff_db::testing::assert_function_query_was_not_run;
    use ruff_python_ast::PythonVersion;

    use crate::db::tests::TestDb;
    use crate::module::ModuleKind;
    use crate::module_name::ModuleName;
    use crate::strategy::FallibleStrategy;
    use crate::testing::{FileSpec, MockedTypeshed, TestCase, TestCaseBuilder};

    use super::*;

    fn resolve_module_confident<'db>(
        db: &'db TestDb,
        module_name: &ModuleName,
    ) -> Option<Module<'db>> {
        super::resolve_module_confident(db, db.resolver_environment(), module_name)
    }

    fn resolve_real_module_confident<'db>(
        db: &'db TestDb,
        module_name: &ModuleName,
    ) -> Option<Module<'db>> {
        super::resolve_real_module_confident(db, db.resolver_environment(), module_name)
    }

    fn path_to_module<'db>(db: &'db TestDb, path: &FilePath) -> Option<Module<'db>> {
        super::path_to_module(db, db.resolver_environment(), path)
    }

    #[test]
    fn first_party_module() {
        let TestCase { db, src, .. } = TestCaseBuilder::new()
            .with_src_files(&[("foo.py", "print('Hello, world!')")])
            .build();

        let foo_module_name = ModuleName::new_static("foo").unwrap();
        let foo_module = resolve_module_confident(&db, &foo_module_name).unwrap();

        assert_eq!(
            Some(&foo_module),
            resolve_module_confident(&db, &foo_module_name).as_ref()
        );

        assert_eq!("foo", foo_module.name(&db));
        assert_eq!(&src, foo_module.search_path(&db).unwrap());
        assert_eq!(ModuleKind::Module, foo_module.kind(&db));

        let expected_foo_path = src.join("foo.py");
        assert_eq!(&expected_foo_path, foo_module.file(&db).unwrap().path(&db));
        assert_eq!(
            Some(foo_module),
            path_to_module(&db, &FilePath::from(expected_foo_path))
        );
    }

    #[test]
    fn site_packages_stub_overrides_first_party_package_when_stdlib_is_missing() {
        let TestCase {
            db, site_packages, ..
        } = TestCaseBuilder::new()
            .with_src_files(&[("foo/__init__.py", "")])
            .with_site_packages_files(&[("foo-stubs/__init__.pyi", "")])
            .build();

        let foo = resolve_module_confident(&db, &ModuleName::new_static("foo").unwrap()).unwrap();
        assert_eq!(
            foo.file(&db).unwrap().path(&db),
            &site_packages.join("foo-stubs/__init__.pyi")
        );
    }

    #[test]
    fn first_party_stub_package_precedes_stdlib() {
        const TYPESHED: MockedTypeshed = MockedTypeshed {
            stdlib_files: &[("foo.pyi", "")],
            versions: "foo: 3.8-",
        };

        let TestCase { db, src, .. } = TestCaseBuilder::new()
            .with_mocked_typeshed(TYPESHED)
            .with_src_files(&[("foo-stubs/__init__.pyi", "")])
            .build();

        let foo = resolve_module_confident(&db, &ModuleName::new_static("foo").unwrap()).unwrap();
        assert_eq!(
            foo.file(&db).unwrap().path(&db),
            &src.join("foo-stubs/__init__.pyi")
        );
    }

    #[test]
    fn desperate_resolution_finds_stub_package() {
        let TestCase { db, src, .. } = TestCaseBuilder::new()
            .with_src_files(&[
                ("nested/main.py", ""),
                ("nested/foo/__init__.py", ""),
                ("nested/foo-stubs/__init__.pyi", ""),
            ])
            .build();
        let importing_file = system_path_to_file(&db, src.join("nested/main.py")).unwrap();

        let foo = resolve_module(
            &db,
            ImportingFile::File(importing_file, db.resolver_environment()),
            &ModuleName::new_static("foo").unwrap(),
        )
        .unwrap();
        assert_eq!(
            foo.file(&db).unwrap().path(&db),
            &src.join("nested/foo-stubs/__init__.pyi")
        );
    }

    #[test]
    fn missing_modules_do_not_create_file_inputs() {
        let TestCase { db, src, .. } = TestCaseBuilder::new()
            .with_src_files(&[("other.py", ""), ("package/__init__.py", "")])
            .build();

        for name in ["missing", "package.missing"] {
            assert!(
                resolve_module_confident(&db, &ModuleName::new_static(name).unwrap()).is_none()
            );
        }

        for relative_path in [
            "missing-stubs/__init__.pyi",
            "missing-stubs/__init__.py",
            "missing/__init__.pyi",
            "missing/__init__.py",
            "missing.pyi",
            "missing.py",
            "package/missing/__init__.pyi",
            "package/missing/__init__.py",
            "package/missing.pyi",
            "package/missing.py",
        ] {
            assert_eq!(
                db.files().try_system(&db, &src.join(relative_path)),
                None,
                "unexpected point probe for {relative_path}"
            );
        }
    }

    #[test]
    fn stdlib_precedes_stub_package_in_site_packages() {
        const TYPESHED: MockedTypeshed = MockedTypeshed {
            stdlib_files: &[("foo.pyi", "")],
            versions: "foo: 3.8-",
        };

        let TestCase { db, stdlib, .. } = TestCaseBuilder::new()
            .with_mocked_typeshed(TYPESHED)
            .with_site_packages_files(&[("foo-stubs/__init__.pyi", "")])
            .build();

        let foo = resolve_module_confident(&db, &ModuleName::new_static("foo").unwrap()).unwrap();
        assert_eq!(foo.file(&db).unwrap().path(&db), &stdlib.join("foo.pyi"));
    }

    #[test]
    fn stubs_over_module_source() {
        let TestCase { db, src, .. } = TestCaseBuilder::new()
            .with_src_files(&[("foo.py", ""), ("foo.pyi", "")])
            .build();

        let foo_module_name = ModuleName::new_static("foo").unwrap();
        let foo_module = resolve_module_confident(&db, &foo_module_name).unwrap();

        assert_eq!(
            Some(&foo_module),
            resolve_module_confident(&db, &foo_module_name).as_ref()
        );

        assert_eq!("foo", foo_module.name(&db));
        assert_eq!(&src, foo_module.search_path(&db).unwrap());
        assert_eq!(ModuleKind::Module, foo_module.kind(&db));

        let expected_foo_path = src.join("foo.pyi");
        assert_eq!(&expected_foo_path, foo_module.file(&db).unwrap().path(&db));
        assert_eq!(
            Some(foo_module),
            path_to_module(&db, &FilePath::from(expected_foo_path))
        );
    }

    /// Tests precedence when there is a package and a sibling stub file.
    ///
    /// NOTE: I am unsure if this is correct. I wrote this test to match
    /// behavior while implementing "list modules." Notably, in this case, the
    /// regular source file gets priority. But in `stubs_over_module_source`
    /// above, the stub file gets priority.
    #[test]
    fn stubs_over_package_source() {
        let TestCase { db, src, .. } = TestCaseBuilder::new()
            .with_src_files(&[("foo/__init__.py", ""), ("foo.pyi", "")])
            .build();

        let foo_module_name = ModuleName::new_static("foo").unwrap();
        let foo_module = resolve_module_confident(&db, &foo_module_name).unwrap();

        assert_eq!(
            Some(&foo_module),
            resolve_module_confident(&db, &foo_module_name).as_ref()
        );

        assert_eq!("foo", foo_module.name(&db));
        assert_eq!(&src, foo_module.search_path(&db).unwrap());
        assert_eq!(ModuleKind::Package, foo_module.kind(&db));

        let expected_foo_path = src.join("foo/__init__.py");
        assert_eq!(&expected_foo_path, foo_module.file(&db).unwrap().path(&db));
        assert_eq!(
            Some(foo_module),
            path_to_module(&db, &FilePath::from(expected_foo_path))
        );
    }

    #[test]
    fn builtins_vendored() {
        let TestCase { db, stdlib, .. } = TestCaseBuilder::new()
            .with_vendored_typeshed()
            .with_src_files(&[("builtins.py", "FOOOO = 42")])
            .build();

        let builtins_module_name = ModuleName::new_static("builtins").unwrap();
        let builtins =
            resolve_module_confident(&db, &builtins_module_name).expect("builtins to resolve");

        assert_eq!(
            builtins.file(&db).unwrap().path(&db),
            &stdlib.join("builtins.pyi")
        );
    }

    #[test]
    fn builtins_custom() {
        const TYPESHED: MockedTypeshed = MockedTypeshed {
            stdlib_files: &[("builtins.pyi", "def min(a, b): ...")],
            versions: "builtins: 3.8-",
        };

        const SRC: &[FileSpec] = &[("builtins.py", "FOOOO = 42")];

        let TestCase { db, stdlib, .. } = TestCaseBuilder::new()
            .with_src_files(SRC)
            .with_mocked_typeshed(TYPESHED)
            .with_python_version(PythonVersion::PY38)
            .build();

        let builtins_module_name = ModuleName::new_static("builtins").unwrap();
        let builtins =
            resolve_module_confident(&db, &builtins_module_name).expect("builtins to resolve");

        assert_eq!(
            builtins.file(&db).unwrap().path(&db),
            &stdlib.join("builtins.pyi")
        );
    }

    #[test]
    fn stdlib() {
        const TYPESHED: MockedTypeshed = MockedTypeshed {
            stdlib_files: &[("functools.pyi", "def update_wrapper(): ...")],
            versions: "functools: 3.8-",
        };

        let TestCase { db, stdlib, .. } = TestCaseBuilder::new()
            .with_mocked_typeshed(TYPESHED)
            .with_python_version(PythonVersion::PY38)
            .build();

        let functools_module_name = ModuleName::new_static("functools").unwrap();
        let functools_module = resolve_module_confident(&db, &functools_module_name).unwrap();

        assert_eq!(
            Some(&functools_module),
            resolve_module_confident(&db, &functools_module_name).as_ref()
        );

        assert_eq!(&stdlib, functools_module.search_path(&db).unwrap());
        assert_eq!(ModuleKind::Module, functools_module.kind(&db));

        let expected_functools_path = stdlib.join("functools.pyi");
        assert_eq!(
            &expected_functools_path,
            functools_module.file(&db).unwrap().path(&db)
        );

        assert_eq!(
            Some(functools_module),
            path_to_module(&db, &FilePath::from(expected_functools_path))
        );
    }

    fn create_module_names(raw_names: &[&str]) -> Vec<ModuleName> {
        raw_names
            .iter()
            .map(|raw| ModuleName::new(raw).unwrap())
            .collect()
    }

    #[test]
    fn resolve_module_uses_resolver_environment_python_version() {
        const TYPESHED: MockedTypeshed = MockedTypeshed {
            stdlib_files: &[("_sha256.pyi", ""), ("py312_only.pyi", "")],
            versions: "_sha256: 3.11-\npy312_only: 3.12-",
        };

        let TestCase {
            db, src, stdlib, ..
        } = TestCaseBuilder::new()
            .with_src_files(&[
                ("main.py", ""),
                ("_sha256.py", ""),
                ("namespace/module.py", ""),
            ])
            .with_mocked_typeshed(TYPESHED)
            .with_python_version(PythonVersion::PY311)
            .build();
        let importing_file = system_path_to_file(&db, src.join("main.py")).unwrap();
        let py311 = ResolverEnvironment::new(&db, PythonVersion::PY311, db.search_paths());
        let py312 = ResolverEnvironment::new(&db, PythonVersion::PY312, db.search_paths());
        let sha256 = ModuleName::new_static("_sha256").unwrap();
        let py311_module =
            resolve_module(&db, ImportingFile::File(importing_file, py311), &sha256).unwrap();
        let py312_module =
            resolve_module(&db, ImportingFile::File(importing_file, py312), &sha256).unwrap();
        assert_eq!(
            py311_module.file(&db).unwrap().path(&db),
            &stdlib.join("_sha256.pyi")
        );
        assert_eq!(
            py312_module.file(&db).unwrap().path(&db),
            &src.join("_sha256.py")
        );
        assert_eq!(py311_module.python_version(&db), PythonVersion::PY311);
        assert_eq!(py312_module.python_version(&db), PythonVersion::PY312);

        let namespace = ModuleName::new_static("namespace").unwrap();
        let py311_namespace =
            resolve_module(&db, ImportingFile::File(importing_file, py311), &namespace).unwrap();
        let py312_namespace =
            resolve_module(&db, ImportingFile::File(importing_file, py312), &namespace).unwrap();
        assert!(matches!(py311_namespace, Module::Namespace(_)));
        assert!(matches!(py312_namespace, Module::Namespace(_)));
        assert_eq!(py311_namespace.python_version(&db), PythonVersion::PY311);
        assert_eq!(py312_namespace.python_version(&db), PythonVersion::PY312);
        assert_ne!(py311_namespace, py312_namespace);

        let py312_only = ModuleName::new_static("py312_only").unwrap();
        assert!(
            resolve_module(&db, ImportingFile::File(importing_file, py311), &py312_only).is_none()
        );
        assert_eq!(
            resolve_module(&db, ImportingFile::File(importing_file, py312), &py312_only)
                .and_then(|module| module.file(&db))
                .unwrap()
                .path(&db),
            &stdlib.join("py312_only.pyi")
        );
    }

    #[test]
    fn resolve_module_uses_resolver_environment_search_paths() {
        let TestCase { mut db, src, .. } = TestCaseBuilder::new()
            .with_src_files(&[("main.py", ""), ("shared.py", "from_src = True")])
            .with_vendored_typeshed()
            .build();
        db.write_file("/alternate/shared.py", "from_alternate = True")
            .unwrap();

        let alternate_paths = SearchPathSettings {
            src_roots: vec![SystemPathBuf::from("/alternate")],
            ..SearchPathSettings::empty()
        }
        .to_search_paths(db.system(), db.vendored(), &FallibleStrategy)
        .unwrap();
        alternate_paths.try_register_static_roots(&db);

        let primary = db.resolver_environment();
        let alternate = ResolverEnvironment::new(&db, PythonVersion::default(), &alternate_paths);
        let importing_file = system_path_to_file(&db, src.join("main.py")).unwrap();
        let name = ModuleName::new_static("shared").unwrap();

        let primary_module =
            resolve_module(&db, ImportingFile::File(importing_file, primary), &name).unwrap();
        let alternate_module =
            resolve_module(&db, ImportingFile::File(importing_file, alternate), &name).unwrap();

        assert_eq!(
            primary_module.file(&db).unwrap().path(&db),
            &src.join("shared.py")
        );
        assert_eq!(
            alternate_module.file(&db).unwrap().path(&db),
            &SystemPathBuf::from("/alternate/shared.py")
        );
        assert_ne!(primary_module, alternate_module);
    }

    #[test]
    fn stdlib_resolution_respects_versions_file_py38_existing_modules() {
        const VERSIONS: &str = "\
            asyncio: 3.8-               # 'Regular' package on py38+
            asyncio.tasks: 3.9-3.11     # Submodule on py39+ only
            functools: 3.8-             # Top-level single-file module
        ";

        const STDLIB: &[FileSpec] = &[
            ("asyncio/__init__.pyi", ""),
            ("asyncio/tasks.pyi", ""),
            ("functools.pyi", ""),
        ];

        const TYPESHED: MockedTypeshed = MockedTypeshed {
            stdlib_files: STDLIB,
            versions: VERSIONS,
        };

        let TestCase { db, stdlib, .. } = TestCaseBuilder::new()
            .with_mocked_typeshed(TYPESHED)
            .with_python_version(PythonVersion::PY38)
            .build();

        let existing_modules = create_module_names(&["asyncio", "functools"]);
        for module_name in existing_modules {
            let resolved_module =
                resolve_module_confident(&db, &module_name).unwrap_or_else(|| {
                    panic!("Expected module {module_name} to exist in the mock stdlib")
                });
            let search_path = resolved_module.search_path(&db).unwrap();
            assert_eq!(
                &stdlib, search_path,
                "Search path for {module_name} was unexpectedly {search_path:?}"
            );
            assert!(
                search_path.is_standard_library(),
                "Expected a stdlib search path, but got {search_path:?}"
            );
        }
    }

    #[test]
    fn stdlib_resolution_respects_versions_file_py38_nonexisting_modules() {
        const VERSIONS: &str = "\
            asyncio: 3.8-               # 'Regular' package on py38+
            asyncio.tasks: 3.9-3.11     # Submodule on py39+ only
            collections: 3.9-           # 'Regular' package on py39+
        ";

        const STDLIB: &[FileSpec] = &[
            ("collections/__init__.pyi", ""),
            ("asyncio/__init__.pyi", ""),
            ("asyncio/tasks.pyi", ""),
        ];

        const TYPESHED: MockedTypeshed = MockedTypeshed {
            stdlib_files: STDLIB,
            versions: VERSIONS,
        };

        let TestCase { db, .. } = TestCaseBuilder::new()
            .with_mocked_typeshed(TYPESHED)
            .with_python_version(PythonVersion::PY38)
            .build();

        let nonexisting_modules = create_module_names(&["collections", "asyncio.tasks"]);

        for module_name in nonexisting_modules {
            assert!(
                resolve_module_confident(&db, &module_name).is_none(),
                "Unexpectedly resolved a module for {module_name}"
            );
        }
    }

    #[test]
    fn stdlib_resolution_respects_versions_file_py39_existing_modules() {
        const VERSIONS: &str = "\
            asyncio: 3.8-               # 'Regular' package on py38+
            asyncio.tasks: 3.9-3.11     # Submodule on py39+ only
            collections: 3.9-           # 'Regular' package on py39+
            functools: 3.8-             # Top-level single-file module
        ";

        const STDLIB: &[FileSpec] = &[
            ("asyncio/__init__.pyi", ""),
            ("asyncio/tasks.pyi", ""),
            ("collections/__init__.pyi", ""),
            ("functools.pyi", ""),
        ];

        const TYPESHED: MockedTypeshed = MockedTypeshed {
            stdlib_files: STDLIB,
            versions: VERSIONS,
        };

        let TestCase { db, stdlib, .. } = TestCaseBuilder::new()
            .with_mocked_typeshed(TYPESHED)
            .with_python_version(PythonVersion::PY39)
            .build();

        let existing_modules =
            create_module_names(&["asyncio", "functools", "collections", "asyncio.tasks"]);

        for module_name in existing_modules {
            let resolved_module =
                resolve_module_confident(&db, &module_name).unwrap_or_else(|| {
                    panic!("Expected module {module_name} to exist in the mock stdlib")
                });
            let search_path = resolved_module.search_path(&db).unwrap();
            assert_eq!(
                &stdlib, search_path,
                "Search path for {module_name} was unexpectedly {search_path:?}"
            );
            assert!(
                search_path.is_standard_library(),
                "Expected a stdlib search path, but got {search_path:?}"
            );
        }
    }
    #[test]
    fn stdlib_resolution_respects_versions_file_py39_nonexisting_modules() {
        const VERSIONS: &str = "\
            importlib: 3.9-   # Namespace package on py39+
            xml: 3.8-3.8      # Namespace package on 3.8 only
        ";

        const STDLIB: &[FileSpec] = &[("importlib/abc.pyi", ""), ("xml/etree.pyi", "")];

        const TYPESHED: MockedTypeshed = MockedTypeshed {
            stdlib_files: STDLIB,
            versions: VERSIONS,
        };

        let TestCase { db, .. } = TestCaseBuilder::new()
            .with_mocked_typeshed(TYPESHED)
            .with_python_version(PythonVersion::PY39)
            .build();

        let nonexisting_modules = create_module_names(&["importlib", "xml", "xml.etree"]);
        for module_name in nonexisting_modules {
            assert!(
                resolve_module_confident(&db, &module_name).is_none(),
                "Unexpectedly resolved a module for {module_name}"
            );
        }
    }

    #[test]
    fn first_party_precedence_over_stdlib() {
        const SRC: &[FileSpec] = &[("functools.py", "def update_wrapper(): ...")];

        const TYPESHED: MockedTypeshed = MockedTypeshed {
            stdlib_files: &[("functools.pyi", "def update_wrapper(): ...")],
            versions: "functools: 3.8-",
        };

        let TestCase { db, src, .. } = TestCaseBuilder::new()
            .with_src_files(SRC)
            .with_mocked_typeshed(TYPESHED)
            .with_python_version(PythonVersion::PY38)
            .build();

        let functools_module_name = ModuleName::new_static("functools").unwrap();
        let functools_module = resolve_module_confident(&db, &functools_module_name).unwrap();

        assert_eq!(
            Some(&functools_module),
            resolve_module_confident(&db, &functools_module_name).as_ref()
        );
        assert_eq!(&src, functools_module.search_path(&db).unwrap());
        assert_eq!(ModuleKind::Module, functools_module.kind(&db));
        assert_eq!(
            &src.join("functools.py"),
            functools_module.file(&db).unwrap().path(&db)
        );

        assert_eq!(
            Some(functools_module),
            path_to_module(&db, &FilePath::from(src.join("functools.py")))
        );
    }

    #[test]
    fn stdlib_uses_vendored_typeshed_when_no_custom_typeshed_supplied() {
        let TestCase { db, stdlib, .. } = TestCaseBuilder::new()
            .with_vendored_typeshed()
            .with_python_version(PythonVersion::default())
            .build();

        let pydoc_data_topics_name = ModuleName::new_static("pydoc_data.topics").unwrap();
        let pydoc_data_topics = resolve_module_confident(&db, &pydoc_data_topics_name).unwrap();

        assert_eq!("pydoc_data.topics", pydoc_data_topics.name(&db));
        assert_eq!(pydoc_data_topics.search_path(&db).unwrap(), &stdlib);
        assert_eq!(
            pydoc_data_topics.file(&db).unwrap().path(&db),
            &stdlib.join("pydoc_data/topics.pyi")
        );
    }

    #[test]
    fn resolve_package() {
        let TestCase { src, db, .. } = TestCaseBuilder::new()
            .with_src_files(&[("foo/__init__.py", "print('Hello, world!'")])
            .build();

        let foo_path = src.join("foo/__init__.py");
        let foo_module =
            resolve_module_confident(&db, &ModuleName::new_static("foo").unwrap()).unwrap();

        assert_eq!("foo", foo_module.name(&db));
        assert_eq!(&src, foo_module.search_path(&db).unwrap());
        assert_eq!(&foo_path, foo_module.file(&db).unwrap().path(&db));

        assert_eq!(
            Some(&foo_module),
            path_to_module(&db, &FilePath::from(foo_path)).as_ref()
        );

        // Resolving by directory doesn't resolve to the init file.
        assert_eq!(None, path_to_module(&db, &FilePath::from(src.join("foo"))));
    }

    #[test]
    fn package_priority_over_module() {
        const SRC: &[FileSpec] = &[
            ("foo/__init__.py", "print('Hello, world!')"),
            ("foo.py", "print('Hello, world!')"),
        ];

        let TestCase { db, src, .. } = TestCaseBuilder::new().with_src_files(SRC).build();

        let foo_module =
            resolve_module_confident(&db, &ModuleName::new_static("foo").unwrap()).unwrap();
        let foo_init_path = src.join("foo/__init__.py");

        assert_eq!(&src, foo_module.search_path(&db).unwrap());
        assert_eq!(&foo_init_path, foo_module.file(&db).unwrap().path(&db));
        assert_eq!(ModuleKind::Package, foo_module.kind(&db));

        assert_eq!(
            Some(foo_module),
            path_to_module(&db, &FilePath::from(foo_init_path))
        );
        assert_eq!(
            None,
            path_to_module(&db, &FilePath::from(src.join("foo.py")))
        );
    }

    #[test]
    fn typing_stub_over_module() {
        const SRC: &[FileSpec] = &[("foo.py", "print('Hello, world!')"), ("foo.pyi", "x: int")];

        let TestCase { db, src, .. } = TestCaseBuilder::new().with_src_files(SRC).build();

        let foo = resolve_module_confident(&db, &ModuleName::new_static("foo").unwrap()).unwrap();
        let foo_real =
            resolve_real_module_confident(&db, &ModuleName::new_static("foo").unwrap()).unwrap();
        let foo_stub = src.join("foo.pyi");

        assert_eq!(&src, foo.search_path(&db).unwrap());
        assert_eq!(&foo_stub, foo.file(&db).unwrap().path(&db));

        assert_eq!(Some(foo), path_to_module(&db, &FilePath::from(foo_stub)));
        assert_eq!(
            Some(foo_real),
            path_to_module(&db, &FilePath::from(src.join("foo.py")))
        );
        assert_ne!(foo_real, foo);
    }

    #[test]
    fn sub_packages() {
        const SRC: &[FileSpec] = &[
            ("foo/__init__.py", ""),
            ("foo/bar/__init__.py", ""),
            ("foo/bar/baz.py", "print('Hello, world!)'"),
        ];

        let TestCase { db, src, .. } = TestCaseBuilder::new().with_src_files(SRC).build();

        let baz_module =
            resolve_module_confident(&db, &ModuleName::new_static("foo.bar.baz").unwrap()).unwrap();
        let baz_path = src.join("foo/bar/baz.py");

        assert_eq!(&src, baz_module.search_path(&db).unwrap());
        assert_eq!(&baz_path, baz_module.file(&db).unwrap().path(&db));

        assert_eq!(
            Some(baz_module),
            path_to_module(&db, &FilePath::from(baz_path))
        );
    }

    #[test]
    fn module_search_path_priority() {
        let TestCase {
            db,
            src,
            site_packages,
            ..
        } = TestCaseBuilder::new()
            .with_src_files(&[("foo.py", "")])
            .with_site_packages_files(&[("foo.py", "")])
            .build();

        let foo_module =
            resolve_module_confident(&db, &ModuleName::new_static("foo").unwrap()).unwrap();
        let foo_src_path = src.join("foo.py");

        assert_eq!(&src, foo_module.search_path(&db).unwrap());
        assert_eq!(&foo_src_path, foo_module.file(&db).unwrap().path(&db));
        assert_eq!(
            Some(foo_module),
            path_to_module(&db, &FilePath::from(foo_src_path))
        );

        assert_eq!(
            None,
            path_to_module(&db, &FilePath::from(site_packages.join("foo.py")))
        );
    }

    #[test]
    #[cfg(target_family = "unix")]
    fn symlink() -> anyhow::Result<()> {
        use anyhow::Context;
        use ruff_db::system::{OsSystem, SystemPath};

        use crate::db::tests::TestDb;

        let mut db = TestDb::new().with_python_version(PythonVersion::PY38);

        let temp_dir = tempfile::tempdir()?;
        let root = temp_dir
            .path()
            .canonicalize()
            .context("Failed to canonicalize temp dir")?;
        let root = SystemPath::from_std_path(&root).unwrap();
        db.use_system(OsSystem::new(root));

        let src = root.join("src");
        let site_packages = root.join("site-packages");
        let custom_typeshed = root.join("typeshed");

        let foo = src.join("foo.py");
        let bar = src.join("bar.py");

        std::fs::create_dir_all(src.as_std_path())?;
        std::fs::create_dir_all(site_packages.as_std_path())?;
        std::fs::create_dir_all(custom_typeshed.join("stdlib").as_std_path())?;
        std::fs::File::create(custom_typeshed.join("stdlib/VERSIONS").as_std_path())?;

        std::fs::write(foo.as_std_path(), "")?;
        std::os::unix::fs::symlink(foo.as_std_path(), bar.as_std_path())?;

        db.set_search_paths(
            SearchPathSettings {
                src_roots: vec![src.clone()],
                custom_typeshed: Some(custom_typeshed),
                site_packages_paths: vec![site_packages],
                ..SearchPathSettings::empty()
            }
            .to_search_paths(db.system(), db.vendored(), &FallibleStrategy)
            .expect("Valid search path settings"),
        );

        let foo_module =
            resolve_module_confident(&db, &ModuleName::new_static("foo").unwrap()).unwrap();
        let bar_module =
            resolve_module_confident(&db, &ModuleName::new_static("bar").unwrap()).unwrap();

        assert_ne!(foo_module, bar_module);

        assert_eq!(&src, foo_module.search_path(&db).unwrap());
        assert_eq!(&foo, foo_module.file(&db).unwrap().path(&db));

        // `foo` and `bar` shouldn't resolve to the same file

        assert_eq!(&src, bar_module.search_path(&db).unwrap());
        assert_eq!(&bar, bar_module.file(&db).unwrap().path(&db));
        assert_eq!(&foo, foo_module.file(&db).unwrap().path(&db));

        assert_ne!(&foo_module, &bar_module);

        assert_eq!(Some(foo_module), path_to_module(&db, &FilePath::from(foo)));
        assert_eq!(Some(bar_module), path_to_module(&db, &FilePath::from(bar)));

        Ok(())
    }

    #[test]
    fn deleting_file_from_different_directory_doesnt_change_module_resolution() {
        let TestCase { mut db, src, .. } = TestCaseBuilder::new()
            .with_src_files(&[("foo.py", "x = 1"), ("other/bar.py", "x = 2")])
            .with_python_version(PythonVersion::PY38)
            .build();

        let foo_module_name = ModuleName::new_static("foo").unwrap();
        let foo_module = resolve_module_confident(&db, &foo_module_name).unwrap();
        let foo_pieces = (
            foo_module.name(&db).clone(),
            foo_module.file(&db),
            foo_module.known(&db),
            foo_module.search_path(&db).cloned(),
            foo_module.kind(&db),
        );

        let bar_path = src.join("other/bar.py");
        let bar = system_path_to_file(&db, &bar_path).expect("bar.py to exist");

        db.clear_salsa_events();

        // Delete `bar.py`
        db.memory_file_system().remove_file(&bar_path).unwrap();
        bar.sync(&mut db);

        // Re-query the foo module. The foo module should still be cached
        // because `bar.py` isn't relevant for resolving `foo`.

        let foo_module2 = resolve_module_confident(&db, &foo_module_name);
        let foo_pieces2 = foo_module2.map(|foo_module2| {
            (
                foo_module2.name(&db).clone(),
                foo_module2.file(&db),
                foo_module2.known(&db),
                foo_module2.search_path(&db).cloned(),
                foo_module2.kind(&db),
            )
        });

        assert!(
            !db.take_salsa_events()
                .iter()
                .any(|event| { matches!(event.kind, salsa::EventKind::WillExecute { .. }) })
        );

        assert_eq!(Some(foo_pieces), foo_pieces2);
    }

    #[test]
    fn adding_file_on_which_module_resolution_depends_invalidates_previously_failing_query_that_now_succeeds()
    -> anyhow::Result<()> {
        let TestCase { mut db, src, .. } = TestCaseBuilder::new().build();
        let foo_path = src.join("foo.py");

        let foo_module_name = ModuleName::new_static("foo").unwrap();
        assert_eq!(resolve_module_confident(&db, &foo_module_name), None);

        // Now write the foo file
        db.write_file(&foo_path, "x = 1")?;

        let foo_file = system_path_to_file(&db, &foo_path).expect("foo.py to exist");

        let foo_module =
            resolve_module_confident(&db, &foo_module_name).expect("Foo module to resolve");
        assert_eq!(foo_file, foo_module.file(&db).unwrap());

        Ok(())
    }

    #[test]
    fn removing_file_on_which_module_resolution_depends_invalidates_previously_successful_query_that_now_fails()
    -> anyhow::Result<()> {
        const SRC: &[FileSpec] = &[("foo.py", "x = 1"), ("foo/__init__.py", "x = 2")];

        let TestCase { mut db, src, .. } = TestCaseBuilder::new().with_src_files(SRC).build();

        let foo_module_name = ModuleName::new_static("foo").unwrap();
        let foo_module =
            resolve_module_confident(&db, &foo_module_name).expect("foo module to exist");
        let foo_init_path = src.join("foo/__init__.py");

        assert_eq!(&foo_init_path, foo_module.file(&db).unwrap().path(&db));

        // Delete `foo/__init__.py` and the `foo` folder. `foo` should now resolve to `foo.py`
        db.memory_file_system().remove_file(&foo_init_path)?;
        db.memory_file_system()
            .remove_directory(foo_init_path.parent().unwrap())?;
        File::sync_path(&mut db, &foo_init_path);
        File::sync_path(&mut db, foo_init_path.parent().unwrap());

        let foo_module =
            resolve_module_confident(&db, &foo_module_name).expect("Foo module to resolve");
        assert_eq!(&src.join("foo.py"), foo_module.file(&db).unwrap().path(&db));

        Ok(())
    }

    #[test]
    fn adding_file_to_search_path_with_lower_priority_does_not_invalidate_query() {
        const TYPESHED: MockedTypeshed = MockedTypeshed {
            versions: "functools: 3.8-",
            stdlib_files: &[("functools.pyi", "def update_wrapper(): ...")],
        };

        let TestCase {
            mut db,
            stdlib,
            site_packages,
            ..
        } = TestCaseBuilder::new()
            .with_mocked_typeshed(TYPESHED)
            .with_python_version(PythonVersion::PY38)
            .build();

        let functools_module_name = ModuleName::new_static("functools").unwrap();
        let stdlib_functools_path = stdlib.join("functools.pyi");

        let functools_module = resolve_module_confident(&db, &functools_module_name).unwrap();
        assert_eq!(functools_module.search_path(&db).unwrap(), &stdlib);
        assert_eq!(
            Ok(functools_module.file(&db).unwrap()),
            system_path_to_file(&db, &stdlib_functools_path)
        );

        // Adding a file to site-packages does not invalidate the query,
        // since site-packages takes lower priority in the module resolution
        db.clear_salsa_events();
        let site_packages_functools_path = site_packages.join("functools.py");
        db.write_file(&site_packages_functools_path, "f: int")
            .unwrap();
        let functools_module = resolve_module_confident(&db, &functools_module_name).unwrap();
        let functools_file = functools_module.file(&db).unwrap();
        let functools_search_path = functools_module.search_path(&db).unwrap().clone();
        let events = db.take_salsa_events();
        assert_function_query_was_not_run(
            &db,
            resolve_module_query,
            ModuleNameIngredient::new(
                &db,
                functools_module_name,
                ModuleResolveMode::Typing,
                db.resolver_environment(),
            ),
            &events,
        );
        assert_eq!(&functools_search_path, &stdlib);
        assert_eq!(
            Ok(functools_file),
            system_path_to_file(&db, &stdlib_functools_path)
        );
    }

    #[test]
    fn adding_file_to_search_path_with_higher_priority_invalidates_the_query() {
        const TYPESHED: MockedTypeshed = MockedTypeshed {
            versions: "functools: 3.8-",
            stdlib_files: &[("functools.pyi", "def update_wrapper(): ...")],
        };

        let TestCase {
            mut db,
            stdlib,
            src,
            ..
        } = TestCaseBuilder::new()
            .with_mocked_typeshed(TYPESHED)
            .with_python_version(PythonVersion::PY38)
            .build();

        let functools_module_name = ModuleName::new_static("functools").unwrap();
        let functools_module = resolve_module_confident(&db, &functools_module_name).unwrap();
        assert_eq!(functools_module.search_path(&db).unwrap(), &stdlib);
        assert_eq!(
            Ok(functools_module.file(&db).unwrap()),
            system_path_to_file(&db, stdlib.join("functools.pyi"))
        );

        // Adding a first-party file invalidates the query,
        // since first-party files take higher priority in module resolution:
        let src_functools_path = src.join("functools.py");
        db.write_file(&src_functools_path, "FOO: int").unwrap();
        let functools_module = resolve_module_confident(&db, &functools_module_name).unwrap();
        assert_eq!(functools_module.search_path(&db).unwrap(), &src);
        assert_eq!(
            Ok(functools_module.file(&db).unwrap()),
            system_path_to_file(&db, &src_functools_path)
        );
    }

    #[test]
    fn deleting_file_from_higher_priority_search_path_invalidates_the_query() {
        const SRC: &[FileSpec] = &[("functools.py", "FOO: int")];

        const TYPESHED: MockedTypeshed = MockedTypeshed {
            versions: "functools: 3.8-",
            stdlib_files: &[("functools.pyi", "def update_wrapper(): ...")],
        };

        let TestCase {
            mut db,
            stdlib,
            src,
            ..
        } = TestCaseBuilder::new()
            .with_src_files(SRC)
            .with_mocked_typeshed(TYPESHED)
            .with_python_version(PythonVersion::PY38)
            .build();

        let functools_module_name = ModuleName::new_static("functools").unwrap();
        let src_functools_path = src.join("functools.py");

        let functools_module = resolve_module_confident(&db, &functools_module_name).unwrap();
        assert_eq!(functools_module.search_path(&db).unwrap(), &src);
        assert_eq!(
            Ok(functools_module.file(&db).unwrap()),
            system_path_to_file(&db, &src_functools_path)
        );

        // If we now delete the first-party file,
        // it should resolve to the stdlib:
        db.memory_file_system()
            .remove_file(&src_functools_path)
            .unwrap();
        File::sync_path(&mut db, &src_functools_path);
        let functools_module = resolve_module_confident(&db, &functools_module_name).unwrap();
        assert_eq!(functools_module.search_path(&db).unwrap(), &stdlib);
        assert_eq!(
            Ok(functools_module.file(&db).unwrap()),
            system_path_to_file(&db, stdlib.join("functools.pyi"))
        );
    }

    #[test]
    fn editable_install_absolute_path() {
        const SITE_PACKAGES: &[FileSpec] = &[("_foo.pth", "/x/src")];
        let x_directory = [("/x/src/foo/__init__.py", ""), ("/x/src/foo/bar.py", "")];

        let TestCase { mut db, .. } = TestCaseBuilder::new()
            .with_site_packages_files(SITE_PACKAGES)
            .build();

        db.write_files(x_directory).unwrap();

        let foo_module_name = ModuleName::new_static("foo").unwrap();
        let foo_bar_module_name = ModuleName::new_static("foo.bar").unwrap();

        let foo_module = resolve_module_confident(&db, &foo_module_name).unwrap();
        let foo_bar_module = resolve_module_confident(&db, &foo_bar_module_name).unwrap();

        assert_eq!(
            foo_module.file(&db).unwrap().path(&db),
            &FilePath::system("/x/src/foo/__init__.py")
        );
        assert_eq!(
            foo_bar_module.file(&db).unwrap().path(&db),
            &FilePath::system("/x/src/foo/bar.py")
        );
    }

    #[test]
    fn editable_install_pth_file_with_whitespace() {
        const SITE_PACKAGES: &[FileSpec] = &[
            ("_foo.pth", "        /x/src"),
            ("_bar.pth", "/y/src        "),
        ];
        let external_files = [("/x/src/foo.py", ""), ("/y/src/bar.py", "")];

        let TestCase { mut db, .. } = TestCaseBuilder::new()
            .with_site_packages_files(SITE_PACKAGES)
            .build();

        db.write_files(external_files).unwrap();

        // Lines with leading whitespace in `.pth` files do not parse:
        let foo_module_name = ModuleName::new_static("foo").unwrap();
        assert_eq!(resolve_module_confident(&db, &foo_module_name), None);

        // Lines with trailing whitespace in `.pth` files do:
        let bar_module_name = ModuleName::new_static("bar").unwrap();
        let bar_module = resolve_module_confident(&db, &bar_module_name).unwrap();
        assert_eq!(
            bar_module.file(&db).unwrap().path(&db),
            &FilePath::system("/y/src/bar.py")
        );
    }

    #[test]
    fn editable_install_relative_path() {
        const SITE_PACKAGES: &[FileSpec] = &[
            ("_foo.pth", "../../x/../x/y/src"),
            ("../x/y/src/foo.pyi", ""),
        ];

        let TestCase { db, .. } = TestCaseBuilder::new()
            .with_site_packages_files(SITE_PACKAGES)
            .build();

        let foo_module_name = ModuleName::new_static("foo").unwrap();
        let foo_module = resolve_module_confident(&db, &foo_module_name).unwrap();

        assert_eq!(
            foo_module.file(&db).unwrap().path(&db),
            &FilePath::system("/x/y/src/foo.pyi")
        );
    }

    #[test]
    fn editable_install_multiple_pth_files_with_multiple_paths() {
        const COMPLEX_PTH_FILE: &str = "\
/

# a comment
/baz

import not_an_editable_install; do_something_else_crazy_dynamic()

# another comment
spam

not_a_directory
";

        const SITE_PACKAGES: &[FileSpec] = &[
            ("_foo.pth", "../../x/../x/y/src"),
            ("_lots_of_others.pth", COMPLEX_PTH_FILE),
            ("../x/y/src/foo.pyi", ""),
            ("spam/spam.py", ""),
        ];

        let root_files = [("/a.py", ""), ("/baz/b.py", "")];

        let TestCase {
            mut db,
            site_packages,
            ..
        } = TestCaseBuilder::new()
            .with_site_packages_files(SITE_PACKAGES)
            .build();

        db.write_files(root_files).unwrap();

        let foo_module_name = ModuleName::new_static("foo").unwrap();
        let a_module_name = ModuleName::new_static("a").unwrap();
        let b_module_name = ModuleName::new_static("b").unwrap();
        let spam_module_name = ModuleName::new_static("spam").unwrap();

        let foo_module = resolve_module_confident(&db, &foo_module_name).unwrap();
        let a_module = resolve_module_confident(&db, &a_module_name).unwrap();
        let b_module = resolve_module_confident(&db, &b_module_name).unwrap();
        let spam_module = resolve_module_confident(&db, &spam_module_name).unwrap();

        assert_eq!(
            foo_module.file(&db).unwrap().path(&db),
            &FilePath::system("/x/y/src/foo.pyi")
        );
        assert_eq!(
            a_module.file(&db).unwrap().path(&db),
            &FilePath::system("/a.py")
        );
        assert_eq!(
            b_module.file(&db).unwrap().path(&db),
            &FilePath::system("/baz/b.py")
        );
        assert_eq!(
            spam_module.file(&db).unwrap().path(&db),
            &FilePath::from(site_packages.join("spam/spam.py"))
        );
    }

    #[test]
    fn module_resolution_paths_cached_between_different_module_resolutions() {
        const SITE_PACKAGES: &[FileSpec] = &[("_foo.pth", "/x/src"), ("_bar.pth", "/y/src")];
        let external_directories = [("/x/src/foo.py", ""), ("/y/src/bar.py", "")];

        let TestCase { mut db, .. } = TestCaseBuilder::new()
            .with_site_packages_files(SITE_PACKAGES)
            .build();

        db.write_files(external_directories).unwrap();

        let foo_module_name = ModuleName::new_static("foo").unwrap();
        let bar_module_name = ModuleName::new_static("bar").unwrap();

        let foo_module = resolve_module_confident(&db, &foo_module_name).unwrap();
        assert_eq!(
            foo_module.file(&db).unwrap().path(&db),
            &FilePath::system("/x/src/foo.py")
        );

        db.clear_salsa_events();
        let bar_module = resolve_module_confident(&db, &bar_module_name).unwrap();
        assert_eq!(
            bar_module.file(&db).unwrap().path(&db),
            &FilePath::system("/y/src/bar.py")
        );
        let events = db.take_salsa_events();
        assert_function_query_was_not_run(
            &db,
            dynamic_resolution_paths,
            ModuleResolveModeIngredient::new(
                &db,
                db.resolver_environment(),
                ModuleResolveMode::Typing,
            ),
            &events,
        );
    }

    #[test]
    fn nested_site_packages_change_does_not_invalidate_dynamic_resolution_paths() {
        const SITE_PACKAGES: &[FileSpec] = &[("_foo.pth", "/x/src"), ("package/__init__.py", "")];

        let TestCase {
            mut db,
            site_packages,
            ..
        } = TestCaseBuilder::new()
            .with_site_packages_files(SITE_PACKAGES)
            .build();

        dynamic_resolution_paths(
            &db,
            ModuleResolveModeIngredient::new(
                &db,
                db.resolver_environment(),
                ModuleResolveMode::Typing,
            ),
        );
        db.clear_salsa_events();

        db.write_file(site_packages.join("package/nested.py"), "")
            .unwrap();
        dynamic_resolution_paths(
            &db,
            ModuleResolveModeIngredient::new(
                &db,
                db.resolver_environment(),
                ModuleResolveMode::Typing,
            ),
        );

        let events = db.take_salsa_events();
        assert_function_query_was_not_run(
            &db,
            dynamic_resolution_paths,
            ModuleResolveModeIngredient::new(
                &db,
                db.resolver_environment(),
                ModuleResolveMode::Typing,
            ),
            &events,
        );
    }

    #[test]
    fn modifying_pth_file_invalidates_dynamic_resolution_paths() {
        const SITE_PACKAGES: &[FileSpec] = &[("_editable.pth", "/x/src")];

        let TestCase {
            mut db,
            site_packages,
            ..
        } = TestCaseBuilder::new()
            .with_site_packages_files(SITE_PACKAGES)
            .build();
        db.write_files([("/x/src/foo.py", ""), ("/y/src/bar.py", "")])
            .unwrap();

        assert!(resolve_module_confident(&db, &ModuleName::new_static("foo").unwrap()).is_some());

        let pth_path = site_packages.join("_editable.pth");
        db.memory_file_system()
            .write_file(&pth_path, "/y/src")
            .unwrap();
        File::sync_path_only(&mut db, &pth_path);

        assert!(resolve_module_confident(&db, &ModuleName::new_static("foo").unwrap()).is_none());
        assert!(resolve_module_confident(&db, &ModuleName::new_static("bar").unwrap()).is_some());
    }

    #[test]
    fn deleting_pth_file_on_which_module_resolution_depends_invalidates_cache() {
        const SITE_PACKAGES: &[FileSpec] = &[("_foo.pth", "/x/src")];
        let x_directory = [("/x/src/foo.py", "")];

        let TestCase {
            mut db,
            site_packages,
            ..
        } = TestCaseBuilder::new()
            .with_site_packages_files(SITE_PACKAGES)
            .build();

        db.write_files(x_directory).unwrap();

        let foo_module_name = ModuleName::new_static("foo").unwrap();
        let foo_module = resolve_module_confident(&db, &foo_module_name).unwrap();
        assert_eq!(
            foo_module.file(&db).unwrap().path(&db),
            &FilePath::system("/x/src/foo.py")
        );

        db.memory_file_system()
            .remove_file(site_packages.join("_foo.pth"))
            .unwrap();

        File::sync_path(&mut db, &site_packages.join("_foo.pth"));

        assert_eq!(resolve_module_confident(&db, &foo_module_name), None);
    }

    #[test]
    fn deleting_editable_install_on_which_module_resolution_depends_invalidates_cache() {
        const SITE_PACKAGES: &[FileSpec] = &[("_foo.pth", "/x/src")];
        let x_directory = [("/x/src/foo.py", "")];

        let TestCase { mut db, .. } = TestCaseBuilder::new()
            .with_site_packages_files(SITE_PACKAGES)
            .build();

        db.write_files(x_directory).unwrap();

        let foo_module_name = ModuleName::new_static("foo").unwrap();
        let foo_module = resolve_module_confident(&db, &foo_module_name).unwrap();
        let src_path = SystemPathBuf::from("/x/src");
        assert_eq!(
            foo_module.file(&db).unwrap().path(&db),
            &FilePath::from(src_path.join("foo.py"))
        );

        db.memory_file_system()
            .remove_file(src_path.join("foo.py"))
            .unwrap();
        db.memory_file_system().remove_directory(&src_path).unwrap();
        File::sync_path(&mut db, &src_path.join("foo.py"));
        File::sync_path(&mut db, &src_path);
        assert_eq!(resolve_module_confident(&db, &foo_module_name), None);
    }

    #[test]
    fn no_duplicate_search_paths_added() {
        let TestCase { db, .. } = TestCaseBuilder::new()
            .with_src_files(&[("foo.py", "")])
            .with_site_packages_files(&[("_foo.pth", "/src")])
            .build();

        let search_paths: Vec<&SearchPath> =
            search_paths(&db, db.resolver_environment(), ModuleResolveMode::Typing).collect();

        assert!(search_paths.contains(
            &&SearchPath::first_party(db.system(), SystemPathBuf::from("/src")).unwrap()
        ));
        assert!(
            !search_paths.contains(
                &&SearchPath::editable(db.system(), SystemPathBuf::from("/src")).unwrap()
            )
        );
    }

    #[test]
    fn multiple_site_packages_with_editables() {
        let mut db = TestDb::new();

        let venv_site_packages = SystemPathBuf::from("/venv-site-packages");
        let site_packages_pth = venv_site_packages.join("foo.pth");
        let system_site_packages = SystemPathBuf::from("/system-site-packages");
        let editable_install_location = SystemPathBuf::from("/x/y/a.py");
        let system_site_packages_location = system_site_packages.join("a.py");

        db.memory_file_system()
            .create_directory_all("/src")
            .unwrap();
        db.write_files([
            (&site_packages_pth, "/x/y"),
            (&editable_install_location, ""),
            (&system_site_packages_location, ""),
        ])
        .unwrap();

        db.set_search_paths(
            SearchPathSettings {
                site_packages_paths: vec![venv_site_packages, system_site_packages],
                ..SearchPathSettings::new(vec![SystemPathBuf::from("/src")])
            }
            .to_search_paths(db.system(), db.vendored(), &FallibleStrategy)
            .expect("Valid search path settings"),
        );

        // The editable installs discovered from the `.pth` file in the first `site-packages` directory
        // take precedence over the second `site-packages` directory...
        let a_module_name = ModuleName::new_static("a").unwrap();
        let a_module = resolve_module_confident(&db, &a_module_name).unwrap();
        assert_eq!(
            a_module.file(&db).unwrap().path(&db),
            &editable_install_location
        );

        db.memory_file_system()
            .remove_file(&site_packages_pth)
            .unwrap();
        File::sync_path(&mut db, &site_packages_pth);

        // ...But now that the `.pth` file in the first `site-packages` directory has been deleted,
        // the editable install no longer exists, so the module now resolves to the file in the
        // second `site-packages` directory
        let a_module = resolve_module_confident(&db, &a_module_name).unwrap();
        assert_eq!(
            a_module.file(&db).unwrap().path(&db),
            &system_site_packages_location
        );
    }

    #[test]
    #[cfg(unix)]
    fn case_sensitive_resolution_with_symlinked_directory() -> anyhow::Result<()> {
        use anyhow::Context;
        use ruff_db::system::OsSystem;

        let temp_dir = tempfile::TempDir::new()?;
        let root = SystemPathBuf::from_path_buf(
            temp_dir
                .path()
                .canonicalize()
                .context("Failed to canonicalized path")?,
        )
        .expect("UTF8 path for temp dir");

        let mut db = TestDb::new();

        let src = root.join("src");
        let a_package_target = root.join("a-package");
        let a_src = src.join("a");

        db.use_system(OsSystem::new(&root));

        db.write_file(
            a_package_target.join("__init__.py"),
            "class Foo: x: int = 4",
        )
        .context("Failed to write `a-package/__init__.py`")?;

        db.write_file(src.join("main.py"), "print('Hy')")
            .context("Failed to write `main.py`")?;

        // The lexical directory listing must accept the symlink named `a` while rejecting `A`.
        std::os::unix::fs::symlink(a_package_target.as_std_path(), a_src.as_std_path())
            .context("Failed to symlink `src/a` to `a-package`")?;

        db.set_search_paths(
            SearchPathSettings::new(vec![src])
                .to_search_paths(db.system(), db.vendored(), &FallibleStrategy)
                .expect("Valid search path settings"),
        );

        // Now try to resolve the module `A` (note the capital `A` instead of `a`).
        let a_module_name = ModuleName::new_static("A").unwrap();
        assert_eq!(resolve_module_confident(&db, &a_module_name), None);

        // Now lookup the same module using the lowercase `a` and it should
        // resolve to the file in the system site-packages
        let a_module_name = ModuleName::new_static("a").unwrap();
        let a_module = resolve_module_confident(&db, &a_module_name).expect("a.py to resolve");
        assert!(
            a_module
                .file(&db)
                .unwrap()
                .path(&db)
                .as_str()
                .ends_with("src/a/__init__.py"),
        );

        Ok(())
    }

    #[test]
    fn file_to_module_where_one_search_path_is_subdirectory_of_other() {
        let project_directory = SystemPathBuf::from("/project");
        let site_packages = project_directory.join(".venv/lib/python3.13/site-packages");
        let installed_foo_module = site_packages.join("foo/__init__.py");

        let mut db = TestDb::new();
        db.write_file(&installed_foo_module, "").unwrap();

        let search_paths = SearchPathSettings {
            src_roots: vec![project_directory],
            site_packages_paths: vec![site_packages.clone()],
            ..SearchPathSettings::empty()
        }
        .to_search_paths(db.system(), db.vendored(), &FallibleStrategy)
        .expect("Valid search path settings");
        db.set_search_paths(search_paths);

        let foo_module_file = File::new(&db, FilePath::from(installed_foo_module));
        let module = file_to_module(
            &db,
            ResolverFile::new(&db, foo_module_file, db.resolver_environment()),
        )
        .unwrap();
        assert_eq!(module.search_path(&db).unwrap(), &site_packages);
    }
}