testing_conventions/

lint.rs

//! Integration-test lints — the `integration lint`
//! command.
//!
//! A *lint* here is a deterministic style/mechanism check on test code, as
//! opposed to the structural `colocated-test` / `coverage` rules. This module hosts
//! the mocking mechanism & style lints; more lints will join them under the
//! same command.
//!
//! Detection is AST-based: each Python test file is parsed with
//! `rustpython_parser` and the tree is walked with a [`Visitor`].
//!
//! Implemented lints:
//! - **`no-monkeypatch`**: a test/fixture function that declares the
//!   `monkeypatch` parameter (pytest's fixture). Patch with `unittest.mock`
//!   wrapped in a `pytest.fixture` instead.
//! - **`no-inline-patch`**: a `patch(...)` / `patch.object(...)` /
//!   `patch.dict(...)` call inside a test body — the `with patch(...)` form or a
//!   bare call. Patches belong in a `pytest.fixture`; a patch *inside* a fixture
//!   is allowed.
//! - **`no-environ-mutation`**: direct mutation of `os.environ` —
//!   `os.environ[...] = …`, `del os.environ[...]`, or a mutating method
//!   (`update` / `pop` / `setdefault` / `clear` / `popitem`). Set env via
//!   `patch.dict(os.environ, {...})` instead.
//! - **`no-constant-patch`**: patching a module-global UPPER_CASE constant,
//!   e.g. `patch("pkg.config.CACHE_DIR", …)`. Inject config explicitly. Waivable
//!   per file via the config `exempt` list.

use std::path::{Path, PathBuf};

use anyhow::{anyhow, Context, Result};
use rustpython_ast::Visitor;
use rustpython_parser::ast::{
    self, Arg, Arguments, Constant, Expr, ExprCall, StmtAssign, StmtAsyncFunctionDef,
    StmtAugAssign, StmtDelete, StmtFunctionDef, StmtIf, StmtImport, StmtImportFrom, WithItem,
};
use rustpython_parser::text_size::{TextRange, TextSize};
use rustpython_parser::Parse;

// `Violation` is shared with the Rust `isolation` lint; it lives in `violation`
// and is re-exported here so `testing_conventions::lint::Violation` still resolves.
pub use crate::violation::Violation;

/// Scan the Python test files under `root` and return every lint violation,
/// sorted by `(file, line)` for deterministic output.
///
/// A *Python test file* is `*_test.py` or `conftest.py` (where fixtures live); a
/// legacy `test_*.py` is ordinary source. Each is parsed and walked. A file
/// that cannot be read or parsed is an error.
pub fn find_violations(root: impl AsRef<Path>) -> Result<Vec<Violation>> {
    let root = root.as_ref();
    // The dist's own top-level package, for `no-first-party-patch`. Resolved
    // once for the whole tree; `None` (no declared package) means that rule flags
    // nothing.
    let first_party = first_party_package(root);
    let mut files = Vec::new();
    collect_python_files(root, &mut files, is_python_test_file)?;
    files.sort();

    let mut violations = Vec::new();
    for file in &files {
        let source = std::fs::read_to_string(file)
            .with_context(|| format!("reading test file `{}`", file.display()))?;
        let suite = ast::Suite::parse(&source, &file.to_string_lossy())
            .map_err(|err| anyhow!("parsing `{}`: {err}", file.display()))?;
        let mut visitor = LintVisitor {
            file,
            source: &source,
            fixture_depth: 0,
            first_party: first_party.as_deref(),
            violations: Vec::new(),
        };
        for stmt in suite {
            visitor.visit_stmt(stmt);
        }
        violations.append(&mut visitor.violations);
    }

    violations.sort_by(|a, b| a.file.cmp(&b.file).then(a.line.cmp(&b.line)));
    Ok(violations)
}

/// Scan the colocated Python unit tests under `root` and return every
/// `unmocked-collaborator` violation: a first-party collaborator a
/// unit test imports without mocking it. The Python arm of `unit lint`
/// ([`crate::isolation::Language::Python`]).
///
/// A *unit test* here is `*_test.py` (not `conftest.py`); a legacy `test_*.py` is
/// ordinary source. First-party is the dist's own package
/// ([`first_party_package`]); a tree with no declared package has no first-party
/// collaborators and so reports nothing.
pub fn find_unit_isolation_violations(root: impl AsRef<Path>) -> Result<Vec<Violation>> {
    let root = root.as_ref();
    // First-party is the dist's own package; with none declared there are no
    // first-party collaborators to flag.
    let Some(first_party) = first_party_package(root) else {
        return Ok(Vec::new());
    };
    let mut files = Vec::new();
    collect_python_files(root, &mut files, is_python_unit_test_file)?;
    files.sort();

    let mut violations = Vec::new();
    for file in &files {
        let source = std::fs::read_to_string(file)
            .with_context(|| format!("reading test file `{}`", file.display()))?;
        let suite = ast::Suite::parse(&source, &file.to_string_lossy())
            .map_err(|err| anyhow!("parsing `{}`: {err}", file.display()))?;
        let base = unit_under_test_base(file);
        let mut visitor = UnitIsolationVisitor {
            source: &source,
            first_party: &first_party,
            base: &base,
            type_checking_depth: 0,
            imports: Vec::new(),
            patch_targets: Vec::new(),
        };
        for stmt in suite {
            visitor.visit_stmt(stmt);
        }
        // A first-party import that is neither the unit under test nor mocked by
        // some `patch(...)` in the file is an un-mocked collaborator.
        for import in &visitor.imports {
            if import.is_uut || import.is_mocked(&visitor.patch_targets) {
                continue;
            }
            violations.push(Violation {
                file: file.to_path_buf(),
                line: import.line,
                rule: "unmocked-collaborator",
                message: format!(
                    "unit test imports `{}` without mocking it — a unit test isolates the \
                     unit under test, so mock every collaborator (patch it by string in a \
                     fixture)",
                    import.display
                ),
            });
        }
    }

    violations.sort_by(|a, b| a.file.cmp(&b.file).then(a.line.cmp(&b.line)));
    Ok(violations)
}

/// One first-party import seen in a unit test, with what it takes to decide
/// whether it's the unit under test or mocked.
struct ImportRecord {
    /// The module path to name in the message (`myproject.ledger`, `.ledger`).
    display: String,
    line: usize,
    /// `true` when this import *is* the unit under test (never a collaborator).
    is_uut: bool,
    /// For `from X import a, b` — the bound symbols. Each must be individually
    /// mocked. Empty for a plain module import.
    symbols: Vec<String>,
    /// For an **absolute** `from X import a, b` — the source module `X`, which a
    /// mocking patch's target must name (`patch("X.a")`). `None` for a relative
    /// `from`-import (no absolute module to compare a patch against) and for a plain
    /// `import X.Y` (which mocks via [`module`](Self::module)).
    source: Option<String>,
    /// For `import X.Y` — the module path (a patch reaching into it counts as a mock).
    module: Option<String>,
}

impl ImportRecord {
    /// `true` when some `patch("…")` target mocks this import.
    ///
    /// - A plain `import X.Y` is mocked by a patch that reaches into the module —
    ///   `patch("X.Y")` or `patch("X.Y.attr")`.
    /// - A `from <module> import a, b` is mocked only when **every** bound symbol is
    ///   individually mocked. A symbol `s` is mocked by a target whose last dotted
    ///   segment is `s` **and**, for an absolute import, whose module path is the
    ///   import's own [`source`](Self::source): `from pkg.ledger import record`
    ///   needs `patch("pkg.ledger.record")`, not merely any `"….record"`. A relative
    ///   import has no absolute module to compare, so a matching last segment alone
    ///   is accepted (name resolution stays a documented non-goal).
    fn is_mocked(&self, patch_targets: &[String]) -> bool {
        if let Some(module) = &self.module {
            let prefix = format!("{module}.");
            return patch_targets
                .iter()
                .any(|target| target == module || target.starts_with(&prefix));
        }
        if self.symbols.is_empty() {
            return false;
        }
        self.symbols.iter().all(|symbol| {
            patch_targets
                .iter()
                .any(|target| self.symbol_is_mocked(target, symbol))
        })
    }

    /// `true` when `target` patches `symbol` for this `from`-import: the target's
    /// last dotted segment is `symbol` and — for an absolute import — its module
    /// path equals the import's own [`source`](Self::source).
    fn symbol_is_mocked(&self, target: &str, symbol: &str) -> bool {
        let Some(module) = target.strip_suffix(&format!(".{symbol}")) else {
            return false;
        };
        match &self.source {
            Some(source) => module == source,
            None => true,
        }
    }
}

/// Walks one parsed unit test, collecting its first-party imports and every
/// `patch("…")` string target so [`find_unit_isolation_violations`] can pair them.
/// Imports guarded by `if TYPE_CHECKING:` are type-only (erased at runtime) and
/// skipped.
struct UnitIsolationVisitor<'a> {
    source: &'a str,
    first_party: &'a str,
    base: &'a str,
    type_checking_depth: usize,
    imports: Vec<ImportRecord>,
    patch_targets: Vec<String>,
}

impl Visitor for UnitIsolationVisitor<'_> {
    fn visit_stmt_import(&mut self, node: StmtImport) {
        if self.type_checking_depth == 0 {
            let line = line_of(self.source, node.range.start());
            for alias in &node.names {
                let module = alias.name.as_str();
                if is_checked_import(import_head(module), self.first_party) {
                    self.imports.push(ImportRecord {
                        display: module.to_string(),
                        line,
                        is_uut: last_segment(module) == self.base,
                        symbols: Vec::new(),
                        source: None,
                        module: Some(module.to_string()),
                    });
                }
            }
        }
        self.generic_visit_stmt_import(node);
    }

    fn visit_stmt_import_from(&mut self, node: StmtImportFrom) {
        if self.type_checking_depth == 0 {
            let level = relative_level(&node);
            let module = node.module.as_ref().map(|m| m.as_str());
            // Relative imports are first-party; an absolute import is checked when
            // its head is first-party or external (third-party / effectful stdlib).
            let should_check = level > 0
                || module.is_some_and(|m| is_checked_import(import_head(m), self.first_party));
            if should_check {
                let line = line_of(self.source, node.range.start());
                let dots = ".".repeat(level);
                match module {
                    // `from <module> import a, b` — the bound symbols are collaborators.
                    // An absolute import (`level == 0`) records its source module so a
                    // mocking patch must name it; a relative import has no absolute
                    // module to compare (`source: None`).
                    Some(module) => self.imports.push(ImportRecord {
                        display: format!("{dots}{module}"),
                        line,
                        is_uut: last_segment(module) == self.base,
                        symbols: node.names.iter().map(|a| a.name.to_string()).collect(),
                        source: (level == 0).then(|| module.to_string()),
                        module: None,
                    }),
                    // `from . import sub` — each name is a submodule.
                    None => {
                        // A re-export barrel is tested by importing its public surface: in
                        // `__init___test.py` (base `__init__`), a bare `from . import …`
                        // (`level == 1`) names the package's own `__init__.py` — the unit
                        // under test — so every bound name is the SUT, never a collaborator
                        // (`__all__` / `__version__` included, since they live in the SUT).
                        // Parity with TS's `index.test.ts` / `import … from './index.js'`.
                        // Scoped exactly: a `from .. import …` (`level == 2`) reaches
                        // the *parent* package, and a `from .core import …` takes the `Some`
                        // branch above — both stay collaborators.
                        let barrel_sut = self.base == "__init__" && level == 1;
                        for alias in &node.names {
                            let name = alias.name.as_str();
                            self.imports.push(ImportRecord {
                                display: format!("{dots}{name}"),
                                line,
                                is_uut: barrel_sut || name == self.base,
                                symbols: vec![name.to_string()],
                                source: None,
                                module: None,
                            });
                        }
                    }
                }
            }
        }
        self.generic_visit_stmt_import_from(node);
    }

    fn visit_expr_call(&mut self, node: ExprCall) {
        if is_patch_call(&node) {
            if let Some(target) = patch_string_target(&node) {
                self.patch_targets.push(target.to_string());
            }
        }
        self.generic_visit_expr_call(node);
    }

    fn visit_stmt_if(&mut self, node: StmtIf) {
        // Imports under `if TYPE_CHECKING:` are type-only — skip the body (the
        // runtime `else` is still walked). Other `if`s recurse normally.
        if is_type_checking(node.test.as_ref()) {
            self.type_checking_depth += 1;
            for stmt in node.body {
                self.visit_stmt(stmt);
            }
            self.type_checking_depth -= 1;
            for stmt in node.orelse {
                self.visit_stmt(stmt);
            }
        } else {
            self.generic_visit_stmt_if(node);
        }
    }
}

/// The leading dotted segment of a module path (`myproject.db` → `myproject`).
fn import_head(module: &str) -> &str {
    module.split('.').next().unwrap_or(module)
}

/// `true` when an import head names a collaborator the unit-isolation rule checks:
/// **first-party** (the dist package) or **external** — a third-party package, or an
/// effectful-stdlib module. The test framework and **pure** stdlib are not
/// collaborators.
fn is_checked_import(head: &str, first_party: &str) -> bool {
    if head == first_party {
        return true; // first-party
    }
    if TEST_FRAMEWORK.contains(&head) {
        return false; // pytest et al. — the harness, never a collaborator
    }
    if EFFECTFUL_STDLIB.contains(&head) {
        return true; // external — effectful stdlib
    }
    if STDLIB_MODULES.contains(&head) {
        return false; // pure stdlib
    }
    true // external — a third-party package
}

/// The test harness — never a collaborator to mock. `unittest` / `unittest.mock`
/// are stdlib (handled by [`STDLIB_MODULES`]); these are the rest.
const TEST_FRAMEWORK: &[&str] = &["pytest", "_pytest", "mock"];

/// Standard-library modules that are **effectful at the head** — the README's
/// External Dependencies (network / subprocess / process & IPC / randomness /
/// database / low-level OS). **Dual-nature** heads (`os`, `pathlib`, `datetime`,
/// `time`, `io`, `logging`, `threading`) are deliberately excluded: a pure use
/// (`os.path.join`, `datetime(2020, 1, 1)`) can't be told from an effectful one at
/// the import, so the clock / filesystem stay caught by the patch convention, not
/// here (a documented non-goal). A tunable heuristic, not an exhaustive map.
const EFFECTFUL_STDLIB: &[&str] = &[
    "asynchat",
    "asyncore",
    "ctypes",
    "curses",
    "dbm",
    "fcntl",
    "ftplib",
    "imaplib",
    "mmap",
    "msvcrt",
    "multiprocessing",
    "nis",
    "nntplib",
    "ossaudiodev",
    "poplib",
    "pty",
    "random",
    "secrets",
    "select",
    "selectors",
    "signal",
    "smtpd",
    "smtplib",
    "socket",
    "socketserver",
    "spwd",
    "sqlite3",
    "ssl",
    "subprocess",
    "syslog",
    "telnetlib",
    "termios",
    "tty",
    "webbrowser",
    "winreg",
    "winsound",
];

/// Top-level standard-library module names (Python's `sys.stdlib_module_names`).
/// Used to tell **pure** stdlib (allowed) from a **third-party** package (checked);
/// the [`EFFECTFUL_STDLIB`] subset is what's actually flagged.
const STDLIB_MODULES: &[&str] = &[
    "abc",
    "aifc",
    "antigravity",
    "argparse",
    "array",
    "ast",
    "asynchat",
    "asyncio",
    "asyncore",
    "atexit",
    "audioop",
    "base64",
    "bdb",
    "binascii",
    "bisect",
    "builtins",
    "bz2",
    "cProfile",
    "calendar",
    "cgi",
    "cgitb",
    "chunk",
    "cmath",
    "cmd",
    "code",
    "codecs",
    "codeop",
    "collections",
    "colorsys",
    "compileall",
    "concurrent",
    "configparser",
    "contextlib",
    "contextvars",
    "copy",
    "copyreg",
    "crypt",
    "csv",
    "ctypes",
    "curses",
    "dataclasses",
    "datetime",
    "dbm",
    "decimal",
    "difflib",
    "dis",
    "distutils",
    "doctest",
    "email",
    "encodings",
    "ensurepip",
    "enum",
    "errno",
    "faulthandler",
    "fcntl",
    "filecmp",
    "fileinput",
    "fnmatch",
    "fractions",
    "ftplib",
    "functools",
    "gc",
    "genericpath",
    "getopt",
    "getpass",
    "gettext",
    "glob",
    "graphlib",
    "grp",
    "gzip",
    "hashlib",
    "heapq",
    "hmac",
    "html",
    "http",
    "idlelib",
    "imaplib",
    "imghdr",
    "imp",
    "importlib",
    "inspect",
    "io",
    "ipaddress",
    "itertools",
    "json",
    "keyword",
    "lib2to3",
    "linecache",
    "locale",
    "logging",
    "lzma",
    "mailbox",
    "mailcap",
    "marshal",
    "math",
    "mimetypes",
    "mmap",
    "modulefinder",
    "msilib",
    "msvcrt",
    "multiprocessing",
    "netrc",
    "nis",
    "nntplib",
    "nt",
    "ntpath",
    "nturl2path",
    "numbers",
    "opcode",
    "operator",
    "optparse",
    "os",
    "ossaudiodev",
    "pathlib",
    "pdb",
    "pickle",
    "pickletools",
    "pipes",
    "pkgutil",
    "platform",
    "plistlib",
    "poplib",
    "posix",
    "posixpath",
    "pprint",
    "profile",
    "pstats",
    "pty",
    "pwd",
    "py_compile",
    "pyclbr",
    "pydoc",
    "pydoc_data",
    "pyexpat",
    "queue",
    "quopri",
    "random",
    "re",
    "readline",
    "reprlib",
    "resource",
    "rlcompleter",
    "runpy",
    "sched",
    "secrets",
    "select",
    "selectors",
    "shelve",
    "shlex",
    "shutil",
    "signal",
    "site",
    "smtpd",
    "smtplib",
    "sndhdr",
    "socket",
    "socketserver",
    "spwd",
    "sqlite3",
    "sre_compile",
    "sre_constants",
    "sre_parse",
    "ssl",
    "stat",
    "statistics",
    "string",
    "stringprep",
    "struct",
    "subprocess",
    "sunau",
    "symtable",
    "sys",
    "sysconfig",
    "syslog",
    "tabnanny",
    "tarfile",
    "telnetlib",
    "tempfile",
    "termios",
    "textwrap",
    "this",
    "threading",
    "time",
    "timeit",
    "tkinter",
    "token",
    "tokenize",
    "tomllib",
    "trace",
    "traceback",
    "tracemalloc",
    "tty",
    "turtle",
    "turtledemo",
    "types",
    "typing",
    "unicodedata",
    "unittest",
    "urllib",
    "uu",
    "uuid",
    "venv",
    "warnings",
    "wave",
    "weakref",
    "webbrowser",
    "winreg",
    "winsound",
    "wsgiref",
    "xdrlib",
    "xml",
    "xmlrpc",
    "zipapp",
    "zipfile",
    "zipimport",
    "zlib",
    "zoneinfo",
];

/// The trailing dotted segment of a module path (`myproject.db` → `db`).
fn last_segment(module: &str) -> &str {
    module.rsplit('.').next().unwrap_or(module)
}

/// The number of leading dots on a `from`-import (`from ..pkg import x` → 2; an
/// absolute import → 0).
fn relative_level(node: &StmtImportFrom) -> usize {
    node.level.map_or(0, |level| level.to_usize())
}

/// `true` for `TYPE_CHECKING` / `typing.TYPE_CHECKING` — the guard whose body holds
/// type-only imports.
fn is_type_checking(test: &Expr) -> bool {
    match test {
        Expr::Name(name) => name.id.as_str() == "TYPE_CHECKING",
        Expr::Attribute(attr) => attr.attr.as_str() == "TYPE_CHECKING",
        _ => false,
    }
}

/// The unit-under-test base name for a test file: `widget_test.py` → `widget`.
/// Only `*_test.py` reaches here (the unit-isolation scan no longer recognizes a
/// legacy `test_*.py`), so stripping the `_test` suffix is all it takes.
fn unit_under_test_base(file: &Path) -> String {
    let name = file
        .file_name()
        .and_then(|n| n.to_str())
        .unwrap_or_default();
    let stem = name.strip_suffix(".py").unwrap_or(name);
    stem.strip_suffix("_test").unwrap_or(stem).to_string()
}

/// Walks one parsed test file, collecting lint violations. Tracks how deep we
/// are inside `@pytest.fixture` functions so `no-inline-patch` can allow patches
/// there while flagging them in test bodies.
struct LintVisitor<'a> {
    file: &'a Path,
    source: &'a str,
    fixture_depth: usize,
    /// The dist's own top-level package, or `None` when undiscoverable.
    first_party: Option<&'a str>,
    violations: Vec<Violation>,
}

impl LintVisitor<'_> {
    fn report(&mut self, range: TextRange, rule: &'static str, message: &str) {
        self.violations.push(Violation {
            file: self.file.to_path_buf(),
            line: line_of(self.source, range.start()),
            rule,
            message: message.to_string(),
        });
    }

    /// Shared entry for both function kinds: run the parameter lint, then return
    /// whether this function is a fixture (so the caller bumps `fixture_depth`).
    fn enter_function(&mut self, args: &Arguments, decorators: &[Expr], range: TextRange) -> bool {
        // `no-monkeypatch`: the `monkeypatch` parameter is the signal.
        let takes_monkeypatch = args
            .posonlyargs
            .iter()
            .chain(&args.args)
            .chain(&args.kwonlyargs)
            .any(|arg| arg.def.arg.as_str() == "monkeypatch")
            || arg_named(&args.vararg, "monkeypatch")
            || arg_named(&args.kwarg, "monkeypatch");
        if takes_monkeypatch {
            self.report(
                range,
                "no-monkeypatch",
                "test takes pytest's `monkeypatch` fixture; patch with `unittest.mock` wrapped in a `pytest.fixture` instead",
            );
        }

        decorators.iter().any(is_fixture_decorator)
    }
}

impl Visitor for LintVisitor<'_> {
    fn visit_stmt_function_def(&mut self, node: StmtFunctionDef) {
        let is_fixture = self.enter_function(&node.args, &node.decorator_list, node.range);
        if is_fixture {
            self.fixture_depth += 1;
        }
        self.generic_visit_stmt_function_def(node);
        if is_fixture {
            self.fixture_depth -= 1;
        }
    }

    fn visit_stmt_async_function_def(&mut self, node: StmtAsyncFunctionDef) {
        let is_fixture = self.enter_function(&node.args, &node.decorator_list, node.range);
        if is_fixture {
            self.fixture_depth += 1;
        }
        self.generic_visit_stmt_async_function_def(node);
        if is_fixture {
            self.fixture_depth -= 1;
        }
    }

    fn visit_expr_call(&mut self, node: ExprCall) {
        let is_patch = is_patch_call(&node);
        // `no-inline-patch`: a patch(...) call outside any fixture is a
        // patch in a test body. Inside a fixture it is the right place.
        if is_patch && self.fixture_depth == 0 {
            self.report(
                node.range,
                "no-inline-patch",
                "patch is called inline in a test body; move it into a `pytest.fixture`",
            );
        }
        // `no-constant-patch`: patching a module-global UPPER_CASE constant.
        // Fires regardless of fixture — config constants are usually patched in one.
        if is_patch && patches_constant(&node) {
            self.report(node.range, "no-constant-patch", CONSTANT_PATCH_MSG);
        }
        // `no-first-party-patch`: in an integration test, patching a
        // first-party target — `patch("ourpkg.mod.fn")` — is forbidden; an
        // integration test runs first-party code for real. Fires regardless of
        // fixture (the patch belongs in one); only when the dist's own package is
        // known (`first_party`) and the target's head segment names it.
        if is_patch {
            if let Some(pkg) = self.first_party {
                if patch_string_target(&node).is_some_and(|target| patches_first_party(target, pkg))
                {
                    self.report(node.range, "no-first-party-patch", FIRST_PARTY_PATCH_MSG);
                }
            }
        }
        // `no-environ-mutation`: `os.environ.update(...)` and friends.
        if is_environ_mutation_call(&node) {
            self.report(node.range, "no-environ-mutation", ENVIRON_MUTATION_MSG);
        }
        self.generic_visit_expr_call(node);
    }

    // The generated `generic_visit_withitem` is a no-op, so a `with patch(...)`
    // context expression is never walked unless we descend into it here.
    fn visit_withitem(&mut self, node: WithItem) {
        self.visit_expr(node.context_expr);
        if let Some(optional_vars) = node.optional_vars {
            self.visit_expr(*optional_vars);
        }
    }

    // `no-environ-mutation`: `os.environ[...] = …`, augmented assignment,
    // and `del os.environ[...]`.
    fn visit_stmt_assign(&mut self, node: StmtAssign) {
        if node.targets.iter().any(is_os_environ_subscript) {
            self.report(node.range, "no-environ-mutation", ENVIRON_MUTATION_MSG);
        }
        self.generic_visit_stmt_assign(node);
    }

    fn visit_stmt_aug_assign(&mut self, node: StmtAugAssign) {
        if is_os_environ_subscript(node.target.as_ref()) {
            self.report(node.range, "no-environ-mutation", ENVIRON_MUTATION_MSG);
        }
        self.generic_visit_stmt_aug_assign(node);
    }

    fn visit_stmt_delete(&mut self, node: StmtDelete) {
        if node.targets.iter().any(is_os_environ_subscript) {
            self.report(node.range, "no-environ-mutation", ENVIRON_MUTATION_MSG);
        }
        self.generic_visit_stmt_delete(node);
    }
}

/// `true` when a `*args` / `**kwargs` arg is named `name`.
fn arg_named(arg: &Option<Box<Arg>>, name: &str) -> bool {
    arg.as_ref().is_some_and(|arg| arg.arg.as_str() == name)
}

/// `true` for an `@pytest.fixture` / `@fixture` decorator, with or without a
/// call (`@pytest.fixture(autouse=True)`).
fn is_fixture_decorator(decorator: &Expr) -> bool {
    let target = match decorator {
        Expr::Call(call) => call.func.as_ref(),
        other => other,
    };
    match target {
        Expr::Name(name) => name.id.as_str() == "fixture",
        Expr::Attribute(attr) => attr.attr.as_str() == "fixture",
        _ => false,
    }
}

/// `true` when a call is `patch(...)`, `patch.object(...)`, `patch.dict(...)`, or
/// the same reached through a module (`mock.patch(...)`, `unittest.mock.patch`).
fn is_patch_call(call: &ExprCall) -> bool {
    match call.func.as_ref() {
        Expr::Name(name) => name.id.as_str() == "patch",
        Expr::Attribute(attr) => {
            let name = attr.attr.as_str();
            name == "patch"
                || ((name == "object" || name == "dict") && attr_base_is_patch(attr.value.as_ref()))
        }
        _ => false,
    }
}

/// `true` when an attribute's base resolves to `patch` — the receiver of
/// `patch.object` / `patch.dict`.
fn attr_base_is_patch(expr: &Expr) -> bool {
    match expr {
        Expr::Name(name) => name.id.as_str() == "patch",
        Expr::Attribute(attr) => attr.attr.as_str() == "patch",
        _ => false,
    }
}

/// Message for the `no-constant-patch` lint.
const CONSTANT_PATCH_MSG: &str = "patches a module-global config constant; inject config explicitly (a consumer that did `from pkg import CONSTANT` snapshots the value at import time and ignores the patch)";

/// Message for the `no-first-party-patch` lint.
const FIRST_PARTY_PATCH_MSG: &str = "patches a first-party target; an integration test must run first-party code for real — only third-party packages and effectful stdlib may be patched";

/// The string-literal first argument of a `patch(...)` call — the dotted target
/// like `"pkg.mod.attr"`. `None` when the first argument isn't a string literal
/// (a non-literal target can't be classified deterministically).
fn patch_string_target(call: &ExprCall) -> Option<&str> {
    if let Some(Expr::Constant(constant)) = call.args.first() {
        if let Constant::Str(target) = &constant.value {
            return Some(target.as_str());
        }
    }
    None
}

/// `true` when a `patch(...)` call's first string argument names a module-global
/// UPPER_CASE constant, e.g. `patch("pkg.config.CACHE_DIR", …)`.
fn patches_constant(call: &ExprCall) -> bool {
    patch_string_target(call)
        .and_then(|target| target.rsplit('.').next())
        .is_some_and(is_upper_constant)
}

/// `true` when a patch `target`'s head dotted segment names the first-party
/// package `pkg`, e.g. `target = "ourpkg.mod.fn"`, `pkg = "ourpkg"`.
fn patches_first_party(target: &str, pkg: &str) -> bool {
    target
        .split('.')
        .next()
        .is_some_and(|head| !head.is_empty() && head == pkg)
}

/// `true` for an ALL-CAPS constant name — letters uppercase, digits and
/// underscores allowed, at least one letter (`CACHE_DIR`, `DEBUG`, `MAX_SIZE`).
fn is_upper_constant(name: &str) -> bool {
    !name.is_empty()
        && name
            .chars()
            .all(|c| c.is_ascii_uppercase() || c.is_ascii_digit() || c == '_')
        && name.chars().any(|c| c.is_ascii_uppercase())
}

/// Message for the `no-environ-mutation` lint.
const ENVIRON_MUTATION_MSG: &str =
    "os.environ is mutated directly; set env via `patch.dict(os.environ, {...})` instead";

/// `true` for the expression `os.environ`.
fn is_os_environ(expr: &Expr) -> bool {
    matches!(
        expr,
        Expr::Attribute(attr)
            if attr.attr.as_str() == "environ"
                && matches!(attr.value.as_ref(), Expr::Name(name) if name.id.as_str() == "os")
    )
}

/// `true` for `os.environ[...]` — a subscript of `os.environ`, the form used as
/// an assignment or `del` target.
fn is_os_environ_subscript(expr: &Expr) -> bool {
    matches!(expr, Expr::Subscript(sub) if is_os_environ(sub.value.as_ref()))
}

/// `true` for a mutating method call on `os.environ` (`os.environ.update(...)`
/// and friends).
fn is_environ_mutation_call(call: &ExprCall) -> bool {
    matches!(
        call.func.as_ref(),
        Expr::Attribute(attr)
            if is_os_environ(attr.value.as_ref()) && is_environ_mutator(attr.attr.as_str())
    )
}

/// `true` for a `dict` method that mutates in place.
fn is_environ_mutator(method: &str) -> bool {
    matches!(
        method,
        "update" | "pop" | "setdefault" | "clear" | "popitem"
    )
}

/// The 1-based line containing byte `offset` in `source`.
fn line_of(source: &str, offset: TextSize) -> usize {
    let offset = (u32::from(offset) as usize).min(source.len());
    source.as_bytes()[..offset]
        .iter()
        .filter(|&&byte| byte == b'\n')
        .count()
        + 1
}

/// The dist's own top-level import package — the first-party root for
/// `no-first-party-patch`.
///
/// Walk up from `root` to the nearest `pyproject.toml`, read its `[project].name`,
/// and [normalize](normalize_dist_name) it to an import name. Returns `None` when
/// no `pyproject.toml` (with a `[project].name`) is found, so a tree with no
/// declared package flags nothing rather than guess. The walk stops at a `.git`
/// boundary so it can't escape the project into an unrelated `pyproject.toml`.
fn first_party_package(root: &Path) -> Option<String> {
    for dir in root.ancestors() {
        let candidate = dir.join("pyproject.toml");
        if candidate.is_file() {
            return read_project_name(&candidate).map(|name| normalize_dist_name(&name));
        }
        if dir.join(".git").exists() {
            break;
        }
    }
    None
}

/// `[project].name` from a `pyproject.toml`, if present and a string.
fn read_project_name(path: &Path) -> Option<String> {
    let contents = std::fs::read_to_string(path).ok()?;
    let value: toml::Value = toml::from_str(&contents).ok()?;
    value
        .get("project")?
        .get("name")?
        .as_str()
        .map(str::to_owned)
}

/// Normalize a distribution name to its import package name: lower-cased, with
/// `-` and `.` mapped to `_` (PEP 503-flavoured — `My-Project` → `my_project`).
fn normalize_dist_name(name: &str) -> String {
    name.trim().to_ascii_lowercase().replace(['-', '.'], "_")
}

/// Recursively collect every Python file under `dir` matching `is_match` into `out`.
fn collect_python_files(
    dir: &Path,
    out: &mut Vec<PathBuf>,
    is_match: fn(&Path) -> bool,
) -> Result<()> {
    let entries =
        std::fs::read_dir(dir).with_context(|| format!("reading directory `{}`", dir.display()))?;
    for entry in entries {
        let path = entry
            .with_context(|| format!("reading an entry under `{}`", dir.display()))?
            .path();
        if path.is_dir() {
            collect_python_files(&path, out, is_match)?;
        } else if is_match(&path) {
            out.push(path);
        }
    }
    Ok(())
}

/// `true` for a file the integration lints scan: `*_test.py` or `conftest.py`
/// (where fixtures live). A legacy `test_*.py` is ordinary source, so
/// it is not scanned.
fn is_python_test_file(path: &Path) -> bool {
    let name = path
        .file_name()
        .and_then(|n| n.to_str())
        .unwrap_or_default();
    name == "conftest.py" || name.ends_with("_test.py")
}

/// `true` for a colocated *unit* test the isolation rule scans: `*_test.py`. A
/// legacy `test_*.py` is ordinary source, and `conftest.py` holds
/// fixtures (not a unit) — neither is scanned.
fn is_python_unit_test_file(path: &Path) -> bool {
    let name = path
        .file_name()
        .and_then(|n| n.to_str())
        .unwrap_or_default();
    name.ends_with("_test.py")
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::sync::atomic::{AtomicU64, Ordering};

    /// A throwaway directory, removed on drop — for the `pyproject.toml` discovery.
    struct TempDir(PathBuf);

    impl TempDir {
        fn new() -> Self {
            static COUNTER: AtomicU64 = AtomicU64::new(0);
            let dir = std::env::temp_dir().join(format!(
                "tc-lint-{}-{}",
                std::process::id(),
                COUNTER.fetch_add(1, Ordering::Relaxed),
            ));
            std::fs::create_dir_all(&dir).unwrap();
            TempDir(dir)
        }

        fn write(&self, name: &str, contents: &str) {
            let path = self.0.join(name);
            if let Some(parent) = path.parent() {
                std::fs::create_dir_all(parent).unwrap();
            }
            std::fs::write(path, contents).unwrap();
        }
    }

    impl Drop for TempDir {
        fn drop(&mut self) {
            let _ = std::fs::remove_dir_all(&self.0);
        }
    }

    #[test]
    fn normalize_dist_name_maps_to_import_name() {
        assert_eq!(normalize_dist_name("My-Project"), "my_project");
        assert_eq!(normalize_dist_name("ns.pkg"), "ns_pkg");
        assert_eq!(normalize_dist_name("  myproject  "), "myproject");
        assert_eq!(normalize_dist_name("myproject"), "myproject");
    }

    /// Parse `src` (a single expression statement) and return its call.
    fn parse_call(src: &str) -> ExprCall {
        let suite = ast::Suite::parse(src, "t.py").expect("snippet should parse");
        match suite.into_iter().next().expect("one statement") {
            ast::Stmt::Expr(stmt) => match *stmt.value {
                Expr::Call(call) => call,
                other => panic!("expected a call, got {other:?}"),
            },
            other => panic!("expected an expression statement, got {other:?}"),
        }
    }

    #[test]
    fn patch_string_target_only_reads_string_literals() {
        let str_call = parse_call("patch(\"pkg.mod.attr\")\n");
        assert_eq!(patch_string_target(&str_call), Some("pkg.mod.attr"));
        // A non-string literal (`patch(42)`), a name (`patch(target)`), and no args
        // all yield `None` — a non-literal target can't be classified.
        let int_call = parse_call("patch(42)\n");
        assert_eq!(patch_string_target(&int_call), None);
        let name_call = parse_call("patch(target)\n");
        assert_eq!(patch_string_target(&name_call), None);
        let empty_call = parse_call("patch()\n");
        assert_eq!(patch_string_target(&empty_call), None);
    }

    /// Build a `from <source> import <symbols>` record (`source: None` → relative).
    fn from_import(source: Option<&str>, symbols: &[&str]) -> ImportRecord {
        ImportRecord {
            display: source.unwrap_or(".rel").to_string(),
            line: 1,
            is_uut: false,
            symbols: symbols.iter().map(|s| (*s).to_string()).collect(),
            source: source.map(str::to_string),
            module: None,
        }
    }

    fn targets(list: &[&str]) -> Vec<String> {
        list.iter().map(|s| (*s).to_string()).collect()
    }

    #[test]
    fn is_mocked_requires_every_symbol_at_the_import_module() {
        let rec = from_import(Some("pkg.ledger"), &["record", "erase"]);
        // Only `record` patched → the un-mocked `erase` leaves the import un-mocked (1a).
        assert!(!rec.is_mocked(&targets(&["pkg.ledger.record"])));
        // Both symbols patched at the import's own module → mocked.
        assert!(rec.is_mocked(&targets(&["pkg.ledger.record", "pkg.ledger.erase"])));
    }

    #[test]
    fn is_mocked_rejects_a_last_segment_match_in_another_module() {
        let rec = from_import(Some("pkg.ledger"), &["record"]);
        // Same last segment, different module → not mocked (1b).
        assert!(!rec.is_mocked(&targets(&["otherpkg.unrelated.record"])));
        // A stdlib target sharing only the last segment likewise (`json.dumps`).
        let dumps = from_import(Some("pkg.formatter"), &["dumps"]);
        assert!(!dumps.is_mocked(&targets(&["json.dumps"])));
        // The exact module path clears it.
        assert!(rec.is_mocked(&targets(&["pkg.ledger.record"])));
    }

    #[test]
    fn is_mocked_relative_import_accepts_a_last_segment_match() {
        // A relative import has no absolute module to compare, so a matching last
        // segment alone is accepted (documented non-goal).
        let rec = from_import(None, &["record"]);
        assert!(rec.is_mocked(&targets(&["pkg.ledger.record"])));
        assert!(!rec.is_mocked(&targets(&["pkg.ledger.other"])));
    }

    #[test]
    fn is_mocked_module_import_matches_a_patch_reaching_in() {
        let rec = ImportRecord {
            display: "pkg.db".to_string(),
            line: 1,
            is_uut: false,
            symbols: Vec::new(),
            source: None,
            module: Some("pkg.db".to_string()),
        };
        assert!(rec.is_mocked(&targets(&["pkg.db.connect"])));
        assert!(rec.is_mocked(&targets(&["pkg.db"])));
        assert!(!rec.is_mocked(&targets(&["pkg.other.connect"])));
        // A `from`-import with no symbols and no module is never mocked.
        let empty = from_import(Some("pkg.mod"), &[]);
        assert!(!empty.is_mocked(&targets(&["pkg.mod.thing"])));
    }

    #[test]
    fn patches_first_party_matches_head_segment() {
        assert!(patches_first_party("myproject.ledger.record", "myproject"));
        assert!(patches_first_party("myproject", "myproject"));
        assert!(!patches_first_party("requests.get", "myproject"));
        assert!(!patches_first_party("myproject_extra.x", "myproject"));
        assert!(!patches_first_party("", "myproject"));
        assert!(!patches_first_party(".leading", "myproject"));
    }

    #[test]
    fn first_party_package_reads_pyproject_name() {
        let tree = TempDir::new();
        tree.write(
            "pyproject.toml",
            "[project]\nname = \"My-Project\"\nversion = \"0.0.0\"\n",
        );
        // Normalized to the import name.
        assert_eq!(first_party_package(&tree.0).as_deref(), Some("my_project"));
    }

    #[test]
    fn first_party_package_is_none_without_a_project_name() {
        let tree = TempDir::new();
        // A pyproject with no `[project].name` — found, but no usable package.
        tree.write("pyproject.toml", "[build-system]\nrequires = []\n");
        tree.write(".git", "");
        assert_eq!(first_party_package(&tree.0), None);
    }

    #[test]
    fn first_party_package_is_none_when_absent() {
        // No pyproject.toml anywhere up the (temp) tree → nothing first-party.
        let tree = TempDir::new();
        assert_eq!(first_party_package(&tree.0), None);
    }

    /// Run the unit-isolation visitor over `source` and return the flagged
    /// (un-mocked, non-UUT first-party) import displays.
    fn unmocked(base: &str, first_party: &str, source: &str) -> Vec<String> {
        let suite = ast::Suite::parse(source, "t.py").expect("snippet should parse");
        let mut visitor = UnitIsolationVisitor {
            source,
            first_party,
            base,
            type_checking_depth: 0,
            imports: Vec::new(),
            patch_targets: Vec::new(),
        };
        for stmt in suite {
            visitor.visit_stmt(stmt);
        }
        visitor
            .imports
            .iter()
            .filter(|i| !i.is_uut && !i.is_mocked(&visitor.patch_targets))
            .map(|i| i.display.clone())
            .collect()
    }

    #[test]
    fn import_head_and_last_segment() {
        assert_eq!(import_head("myproject.db.conn"), "myproject");
        assert_eq!(import_head("requests"), "requests");
        assert_eq!(last_segment("myproject.db.conn"), "conn");
        assert_eq!(last_segment("widget"), "widget");
    }

    #[test]
    fn unit_under_test_base_strips_test_suffix() {
        assert_eq!(
            unit_under_test_base(Path::new("pkg/widget_test.py")),
            "widget"
        );
        // Only `*_test.py` reaches here, so a legacy `test_*.py` keeps its
        // `test_` prefix, and a name without the `_test` suffix is its own stem.
        assert_eq!(
            unit_under_test_base(Path::new("test_widget.py")),
            "test_widget"
        );
        assert_eq!(unit_under_test_base(Path::new("plain.py")), "plain");
    }

    #[test]
    fn recognizes_python_unit_test_files() {
        assert!(is_python_unit_test_file(Path::new("widget_test.py")));
        assert!(is_python_unit_test_file(Path::new("pkg/widget_test.py")));
        // A legacy `test_*.py` is ordinary source, not a unit test.
        assert!(!is_python_unit_test_file(Path::new("test_widget.py")));
        // conftest holds fixtures, not a unit — excluded from unit lint.
        assert!(!is_python_unit_test_file(Path::new("conftest.py")));
        assert!(!is_python_unit_test_file(Path::new("widget.py")));
    }

    #[test]
    fn visitor_flags_first_party_and_external_collaborators() {
        // The UUT is left alone; the first-party collaborator and the third-party
        // import are both flagged (slice 3 broadened the rule to external deps).
        let found = unmocked(
            "widget",
            "myproject",
            "from myproject.widget import build\n\
             from myproject.ledger import record\n\
             import requests\n",
        );
        assert_eq!(
            found,
            vec!["myproject.ledger".to_string(), "requests".to_string()]
        );
    }

    #[test]
    fn visitor_clears_a_mocked_collaborator() {
        // The imported `record` is patched at its own source module → not flagged.
        let found = unmocked(
            "widget",
            "myproject",
            "from myproject.ledger import record\npatch(\"myproject.ledger.record\")\n",
        );
        assert!(found.is_empty(), "got: {found:?}");
    }

    #[test]
    fn visitor_flags_a_wrong_module_patch() {
        // A patch that shares only the last segment but names a different module does
        // not mock the import (#393, 1b): `record` stays an un-mocked collaborator.
        let found = unmocked(
            "widget",
            "myproject",
            "from myproject.ledger import record\npatch(\"otherpkg.unrelated.record\")\n",
        );
        assert_eq!(found, vec!["myproject.ledger".to_string()]);
    }

    #[test]
    fn visitor_flags_a_partly_mocked_multi_symbol_import() {
        // Every imported symbol must be mocked (#393, 1a): patching only `record`
        // leaves the sibling `erase` a real collaborator, so the import is flagged.
        let found = unmocked(
            "widget",
            "myproject",
            "from myproject.ledger import record, erase\npatch(\"myproject.ledger.record\")\n",
        );
        assert_eq!(found, vec!["myproject.ledger".to_string()]);
        // Patching both symbols at their module clears it.
        let both = unmocked(
            "widget",
            "myproject",
            "from myproject.ledger import record, erase\n\
             patch(\"myproject.ledger.record\")\npatch(\"myproject.ledger.erase\")\n",
        );
        assert!(both.is_empty(), "got: {both:?}");
    }

    #[test]
    fn visitor_handles_module_and_relative_imports() {
        // A first-party module import, not the UUT, un-mocked → flagged.
        assert_eq!(
            unmocked("widget", "myproject", "import myproject.db\n"),
            vec!["myproject.db".to_string()]
        );
        // `import myproject.db` reached by a patch → mocked.
        assert!(unmocked(
            "widget",
            "myproject",
            "import myproject.db\npatch(\"myproject.db.connect\")\n"
        )
        .is_empty());
        // Relative imports are first-party; `from . import widget` is the UUT.
        assert_eq!(
            unmocked("widget", "myproject", "from .ledger import record\n"),
            vec![".ledger".to_string()]
        );
        assert_eq!(
            unmocked(
                "widget",
                "myproject",
                "from . import ledger\nfrom . import widget\n"
            ),
            vec![".ledger".to_string()]
        );
    }

    #[test]
    fn visitor_treats_barrel_reexport_import_as_the_unit_under_test() {
        // In `__init___test.py` (base `__init__`), a bare `from . import …`
        // imports the package's own re-export surface — the SUT — so no name is a
        // collaborator, `__all__` / `__version__` included (they live in the SUT).
        assert!(unmocked(
            "__init__",
            "myproject",
            "from . import Thing, __all__, __version__\n"
        )
        .is_empty());
        // Reaching AROUND the barrel into a sibling module directly is still a
        // collaborator (the `module: Some("core")` branch, `core != __init__`).
        assert_eq!(
            unmocked("__init__", "myproject", "from .core import Thing\n"),
            vec![".core".to_string()]
        );
        // `from .. import x` (level 2) resolves to the PARENT package, not the SUT
        // file — still a collaborator even inside a barrel test.
        assert_eq!(
            unmocked("__init__", "myproject", "from .. import sibling\n"),
            vec!["..sibling".to_string()]
        );
        // The barrel shortcut is scoped to the `__init__` base: an ordinary
        // `widget_test.py` doing `from . import ledger` is still a collaborator.
        assert_eq!(
            unmocked("widget", "myproject", "from . import ledger\n"),
            vec![".ledger".to_string()]
        );
    }

    #[test]
    fn visitor_skips_type_checking_imports() {
        // A first-party import guarded by TYPE_CHECKING is type-only — not a runtime
        // collaborator; the runtime `else` import is still seen.
        let found = unmocked(
            "widget",
            "myproject",
            "if TYPE_CHECKING:\n    from myproject.models import Widget\nelse:\n    from myproject.ledger import record\n",
        );
        assert_eq!(found, vec!["myproject.ledger".to_string()]);
    }

    #[test]
    fn is_checked_import_classifies_origins() {
        assert!(is_checked_import("myproject", "myproject")); // first-party
        assert!(!is_checked_import("pytest", "myproject")); // test framework
        assert!(!is_checked_import("_pytest", "myproject"));
        assert!(is_checked_import("subprocess", "myproject")); // effectful stdlib
        assert!(is_checked_import("socket", "myproject"));
        assert!(!is_checked_import("json", "myproject")); // pure stdlib
        assert!(!is_checked_import("dataclasses", "myproject"));
        assert!(is_checked_import("requests", "myproject")); // third-party
        assert!(is_checked_import("stripe", "myproject"));
        // dual-nature stdlib heads stay pure (not flagged) — caught by patching, not import
        assert!(!is_checked_import("os", "myproject"));
        assert!(!is_checked_import("pathlib", "myproject"));
        assert!(!is_checked_import("datetime", "myproject"));
    }

    #[test]
    fn visitor_flags_external_collaborators() {
        // Third-party + effectful stdlib are flagged; pure stdlib and the framework aren't.
        let found = unmocked(
            "widget",
            "myproject",
            "import requests\nimport subprocess\nimport json\nimport pytest\n",
        );
        assert_eq!(found.len(), 2, "got: {found:?}");
        assert!(found.contains(&"requests".to_string()));
        assert!(found.contains(&"subprocess".to_string()));
    }

    #[test]
    fn visitor_type_checking_variants_and_plain_if() {
        // `typing.TYPE_CHECKING` (attribute form) guards type-only imports — both
        // the `from`-import and the plain module import are skipped.
        assert!(unmocked(
            "widget",
            "myproject",
            "if typing.TYPE_CHECKING:\n    from myproject.models import W\n    import myproject.db\n"
        )
        .is_empty());
        // A plain `if` (not TYPE_CHECKING) is walked normally — its first-party
        // import is still a collaborator.
        assert_eq!(
            unmocked(
                "widget",
                "myproject",
                "if ready == 1:\n    from myproject.ledger import record\n"
            ),
            vec!["myproject.ledger".to_string()]
        );
    }

    #[test]
    fn find_unit_isolation_without_pyproject_reports_nothing() {
        // No declared package → no first-party collaborators (the early return).
        let tree = TempDir::new();
        tree.write("widget_test.py", "from myproject.ledger import record\n");
        tree.write(".git", "");
        assert!(find_unit_isolation_violations(&tree.0)
            .expect("a readable tree should succeed")
            .is_empty());
    }

    #[test]
    fn find_unit_isolation_walks_subdirs_and_flags() {
        let tree = TempDir::new();
        tree.write("pyproject.toml", "[project]\nname = \"myproject\"\n");
        // A nested unit test — exercises the directory recursion.
        tree.write("pkg/thing_test.py", "from myproject.ledger import record\n");
        let found =
            find_unit_isolation_violations(&tree.0).expect("a readable tree should succeed");
        assert_eq!(found.len(), 1, "got: {found:?}");
        assert_eq!(found[0].rule, "unmocked-collaborator");
        assert!(found[0].message.contains("myproject.ledger"));
    }

    #[test]
    fn recognizes_python_test_files() {
        assert!(is_python_test_file(Path::new("widget_test.py")));
        assert!(is_python_test_file(Path::new("pkg/widget_test.py")));
        assert!(is_python_test_file(Path::new("conftest.py")));
        // A legacy `test_*.py` is ordinary source, not a test file.
        assert!(!is_python_test_file(Path::new("test_widget.py")));
    }

    #[test]
    fn ignores_non_test_files() {
        assert!(!is_python_test_file(Path::new("widget.py")));
        assert!(!is_python_test_file(Path::new("conftest.pyi")));
        assert!(!is_python_test_file(Path::new("README.md")));
        assert!(!is_python_test_file(Path::new("testing.py")));
    }

    #[test]
    fn line_of_counts_newlines() {
        let src = "a\nb\nc\n";
        assert_eq!(line_of(src, TextSize::from(0)), 1);
        assert_eq!(line_of(src, TextSize::from(2)), 2);
        assert_eq!(line_of(src, TextSize::from(4)), 3);
    }

    #[test]
    fn recognizes_environ_mutators() {
        assert!(is_environ_mutator("update"));
        assert!(is_environ_mutator("pop"));
        assert!(is_environ_mutator("clear"));
        assert!(!is_environ_mutator("get"));
        assert!(!is_environ_mutator("keys"));
    }

    #[test]
    fn recognizes_upper_constants() {
        assert!(is_upper_constant("CACHE_DIR"));
        assert!(is_upper_constant("DEBUG"));
        assert!(is_upper_constant("MAX_2"));
        assert!(!is_upper_constant("cache_dir"));
        assert!(!is_upper_constant("CacheDir"));
        assert!(!is_upper_constant("fetch"));
        assert!(!is_upper_constant(""));
        assert!(!is_upper_constant("_"));
        assert!(!is_upper_constant("123"));
    }
}