codehelion-backend-clang 0.7.0

//! The compilation database, which is the only thing that says how a C or C++
//! file is compiled.
//!
//! A C++ source file on its own says almost nothing about the program it
//! becomes. The macros defined on the command line decide which branches of
//! every header it includes exist at all, and the include path decides which
//! file a quoted name even resolves to. Two translation units can include one
//! header and get two different programs out of it — the fixtures in this
//! repository are exactly that — so a helper that analysed a file without its
//! command would be reporting one of the possible readings and calling it the
//! reading.
//!
//! # Nothing here runs anything
//!
//! The database is read where it already is. This program never runs the
//! commands it lists, and never runs the generator that would produce one:
//! configuring a build is running the project's code, and a C++ project's
//! configure step is a program the project ships. A tree with no database is a
//! tree this helper cannot answer about, which it says rather than fixes.

use std::collections::{BTreeMap, HashMap};
use std::path::{Component, Path, PathBuf};

use codehelion_helper::CompileCommandSelector;
use codehelion_helper_protocol::compile_commands::RecordedCommand;

/// Where a database can sit relative to the directory a project is rooted at.
///
/// The build directory is listed because that is where the generator writes it
/// and it is the usual arrangement; the root is listed because that is where
/// people symlink it to so their editor finds it.
const LOCATIONS: [&str; 2] = ["compile_commands.json", "build/compile_commands.json"];

/// One translation unit, as the database describes it.
pub(crate) struct Entry {
    /// The source it is compiled from.
    pub(crate) file: PathBuf,
    /// The validated arguments to parse it with, or the reason the recorded
    /// invocation cannot safely be used by either compiler frontend.
    arguments: Result<ValidatedArguments, String>,
    /// The macros defined on the command line, as the flags spelled them.
    pub(crate) definitions: Vec<String>,
    /// The complete database identity used to select this entry.
    pub(crate) selector: CompileCommandSelector,
}

impl Entry {
    /// Arguments safe for both libclang and the helper-owned CFG frontend.
    ///
    /// Build commands can ask a compiler to load code, re-expand response
    /// files, or write an output. The CFG reader neither needs nor permits
    /// any of those. Refusing the entire auxiliary reading is deliberate: a
    /// command whose nested arguments are not known safe must not become safe
    /// by selectively guessing which pieces to retain.
    pub(crate) fn arguments(&self) -> Result<&ValidatedArguments, &str> {
        self.arguments.as_ref().map_err(String::as_str)
    }
}

/// Compiler arguments that the helper may give to either frontend.
///
/// Construction is private so an unchecked compilation-database argument
/// cannot accidentally reach libclang or the subprocess. What is forwarded is
/// an allow list: an option added by a future compiler is not passed on until
/// its operand shape and read-only behaviour are reviewed here.
///
/// An unrecognized argument is refused only where letting it through would
/// change what is read rather than what is produced. A build's ordinary code
/// generation, diagnostics and target selection — the `-f`, `-m`, `-O`, `-g`
/// and `-W` families a generator writes by default — is dropped where it is not
/// on the allow list, because dropping it costs at most a predefined macro
/// while refusing it costs the whole translation unit. That is not a licence to
/// drop anything: an option that names a file to read, loads code, re-expands a
/// command line or redirects an output is refused by name, and an unrecognized
/// word from no such family is refused because it may carry a separate operand
/// that would be read as a second input file.
pub(crate) struct ValidatedArguments(Vec<String>);

impl ValidatedArguments {
    fn parse(arguments: &[String]) -> Result<Self, String> {
        let mut accepted = Vec::with_capacity(arguments.len());
        let mut index = 0;
        while index < arguments.len() {
            let argument = &arguments[index];
            if explicitly_forbidden(argument) {
                return Err(format!("compiler argument is not allowed: {argument}"));
            }
            if SAFE_FLAGS.contains(&argument.as_str()) {
                accepted.push(argument.clone());
                index += 1;
                continue;
            }
            if let Some(value) = joined_short_value(argument) {
                require_nonempty(argument, value)?;
                accepted.push(argument.clone());
                index += 1;
                continue;
            }
            if let Some(value) = joined_long_value(argument) {
                require_nonempty(argument, value)?;
                accepted.push(argument.clone());
                index += 1;
                continue;
            }
            if SAFE_WITH_VALUE.contains(&argument.as_str()) {
                let Some(value) = arguments.get(index + 1) else {
                    return Err(format!("compiler argument requires a value: {argument}"));
                };
                if value.is_empty() {
                    return Err(format!("compiler argument has an empty value: {argument}"));
                }
                accepted.push(argument.clone());
                accepted.push(value.clone());
                index += 2;
                continue;
            }
            if discard_without_value(argument) {
                index += 1;
                continue;
            }
            return Err(format!("compiler argument is not allowed: {argument}"));
        }
        Ok(Self(accepted))
    }

    /// The exact arguments whose option/value boundaries were validated.
    pub(crate) fn as_slice(&self) -> &[String] {
        &self.0
    }

    /// Whether every direct source-file read stays below `boundary`.
    ///
    /// Only options that make Clang open a named file are checked here. Include
    /// search paths remain available: they select headers as part of normal
    /// compilation, while `-include` and `-imacros` blindly open a caller
    /// supplied file before parsing the translation unit.
    pub(crate) fn reads_within(&self, boundary: &Path) -> bool {
        let boundary = canonical(boundary);
        let mut directory = boundary.clone();
        let mut index = 0;
        while index < self.0.len() {
            let argument = &self.0[index];
            let (option, value, consumed) = match argument.as_str() {
                "-include" | "-imacros" | "-working-directory" => {
                    let Some(value) = self.0.get(index + 1) else {
                        return false;
                    };
                    (argument.as_str(), value.as_str(), 2)
                }
                _ => {
                    if let Some(value) = argument.strip_prefix("-working-directory=") {
                        ("-working-directory", value, 1)
                    } else {
                        index += 1;
                        continue;
                    }
                }
            };
            let path = canonical(&resolve(Some(&directory), Path::new(value)));
            if !path.starts_with(&boundary) {
                return false;
            }
            if option == "-working-directory" {
                directory = path;
            }
            index += consumed;
        }
        true
    }
}

/// Read-only switches whose meaning does not consume another argument.
///
/// Several of these decide nothing about the code that is generated and
/// everything about the code that is read: `-fPIC` and its relatives define a
/// predefined macro, and a header that asks about it declares different things
/// with and without. Those are forwarded rather than dropped for that reason.
const SAFE_FLAGS: &[&str] = &[
    "-ansi",
    "-fPIC",
    "-fPIE",
    "-fasynchronous-unwind-tables",
    "-fblocks",
    "-fborland-extensions",
    "-fdeclspec",
    "-fdelayed-template-parsing",
    "-fexceptions",
    "-ffreestanding",
    "-fms-compatibility",
    "-fms-extensions",
    "-fno-blocks",
    "-fno-builtin",
    "-fno-exceptions",
    "-fno-pic",
    "-fno-pie",
    "-fno-rtti",
    "-fno-signed-char",
    "-fno-threadsafe-statics",
    "-fno-unsigned-char",
    "-fno-use-cxa-atexit",
    "-fno-wchar",
    "-fobjc-arc",
    "-fobjc-weak",
    "-fopenmp",
    "-fpic",
    "-fpie",
    "-frtti",
    "-fshort-enums",
    "-fshort-wchar",
    "-fsigned-char",
    "-fstack-protector",
    "-fstack-protector-all",
    "-fstack-protector-strong",
    "-fsyntax-only",
    "-fthreadsafe-statics",
    "-funsigned-char",
    "-funwind-tables",
    "-fuse-cxa-atexit",
    "-fwchar",
    "-m32",
    "-m64",
    "-malign-double",
    "-mno-align-double",
    "-mno-red-zone",
    "-mred-zone",
    "-nobuiltininc",
    "-nostdinc",
    "-nostdinc++",
    "-nostdsysteminc",
    "-pthread",
    "-undef",
];

/// Read-only switches that consume their following argument.
const SAFE_WITH_VALUE: &[&str] = &[
    "--sysroot",
    "--target",
    "-D",
    "-F",
    "-I",
    "-U",
    "-arch",
    "-idirafter",
    "-iframework",
    "-iframeworkwithsysroot",
    "-imacros",
    "-include",
    "-iprefix",
    "-iquote",
    "-isystem",
    "-isysroot",
    "-iwithprefix",
    "-iwithprefixbefore",
    "-std",
    "-target",
    "-working-directory",
    "-x",
];

/// Short options for which Clang accepts the value in the same word.
fn joined_short_value(argument: &str) -> Option<&str> {
    ["-D", "-U", "-I", "-F"].into_iter().find_map(|option| {
        argument
            .strip_prefix(option)
            .filter(|value| !value.is_empty())
    })
}

/// Long options whose joined spelling has an explicit `=` boundary.
fn joined_long_value(argument: &str) -> Option<&str> {
    [
        "--sysroot=",
        "--target=",
        "-fclang-abi-compat=",
        "-fdebug-prefix-map=",
        "-ffile-prefix-map=",
        "-fmacro-prefix-map=",
        "-fms-compatibility-version=",
        "-fpack-struct=",
        "-fvisibility=",
        "-isysroot=",
        "-mabi=",
        "-march=",
        "-mcpu=",
        "-mfloat-abi=",
        "-mfpu=",
        // The deployment target, which decides which declarations an Apple SDK
        // header makes available at all. Written by every Xcode build.
        "-miphoneos-version-min=",
        "-mios-simulator-version-min=",
        "-mmacosx-version-min=",
        "-mtargetos=",
        "-mtune=",
        "-mtvos-version-min=",
        "-mwatchos-version-min=",
        "-std=",
        "-stdlib=",
        "-target=",
        "-working-directory=",
        "-x=",
    ]
    .into_iter()
    .find_map(|option| argument.strip_prefix(option))
}

fn require_nonempty(argument: &str, value: &str) -> Result<(), String> {
    if value.is_empty() {
        Err(format!("compiler argument has an empty value: {argument}"))
    } else {
        Ok(())
    }
}

/// Code generation and diagnostics controls that decide what a compiler
/// produces rather than what it reads, and are therefore neither forwarded to a
/// frontend nor grounds for refusing the translation unit.
///
/// The families named here are the ones a build generator writes by default —
/// optimization, debug information, warnings, remarks, and the `-f` and `-m`
/// switches whose meaning is code generation. None of them carries a separate
/// operand in Clang, so dropping the word cannot leave a stray value behind to
/// be read as an input file. The ones among them that do change what is read
/// are on the allow list above and never reach here; the ones that would read
/// or write a file of their own are refused before it.
fn discard_without_value(argument: &str) -> bool {
    matches!(
        argument,
        "-pedantic" | "-pedantic-errors" | "-Qunused-arguments" | "-w"
    ) || argument == "--coverage"
        || argument == "-pipe"
        || argument.starts_with("-O")
        || argument.starts_with("-R")
        || argument.starts_with("-W")
        || argument.starts_with("-f")
        || argument.starts_with("-g")
        || argument.starts_with("-m")
}

/// Options that make a compiler load code, read a file the source does not
/// name, re-expand a command line, or write something.
///
/// Refused rather than dropped, and refused first. Dropping one of these would
/// answer about a program the project does not build — a unit compiled against
/// a prebuilt module carries declarations that arrive from nowhere else — and
/// forwarding one would let a compilation database decide what this process
/// runs and where it writes. This list is what makes the code-generation
/// families below safe to drop wholesale: those families' file-bearing members
/// are all named here.
fn explicitly_forbidden(argument: &str) -> bool {
    argument.starts_with('@')
        || matches!(
            argument,
            "--config"
                | "--config-user-dir"
                | "--config-system-dir"
                | "-B"
                | "-Wa"
                | "-Wl"
                | "-Wp"
                | "-add-plugin"
                | "-gcc-toolchain"
                | "-load"
                | "-mllvm"
                | "-plugin"
        )
        || [
            "--config=",
            "--config-user-dir=",
            "--config-system-dir=",
            "--gcc-toolchain",
            "-B",
            "-Wa,",
            "-Wl,",
            "-Wp,",
            // Every `-X` option hands its operand to a stage of the compiler
            // unread, which is the whole of what makes it dangerous.
            "-X",
            "-fbuild-session-file",
            "-fcrash-diagnostics-dir",
            "-fcuda-include-gpubinary",
            "-fembed-offload-object",
            "-fimplicit-module",
            "-fmemory-profile-use",
            "-fmodule",
            "-fpass-plugin",
            "-fplugin",
            "-fprebuilt-module-path",
            "-fprofile-instr-use",
            "-fprofile-list",
            "-fprofile-remapping-file",
            "-fprofile-sample-use",
            "-fprofile-use",
            "-fsanitize-blacklist",
            "-fsanitize-coverage-allowlist",
            "-fsanitize-coverage-ignorelist",
            "-fsanitize-ignorelist",
            "-fsanitize-system-ignorelist",
            "-fsave-optimization-record",
            "-fthinlto-index",
            "-ftime-trace",
            "-fxray-attr-list",
            "-gcc-toolchain=",
        ]
        .into_iter()
        .any(|prefix| argument.starts_with(prefix))
}

/// A compilation database, and the directory a project rooted at it spells its
/// files against.
pub(crate) struct Database {
    /// The directory the search found the database from, which is the project
    /// root rather than wherever the file itself landed: a database under
    /// `build/` describes the tree above it, and anchoring the answers at the
    /// build directory would spell every source file as `../src/...`.
    pub(crate) root: PathBuf,
    /// One entry per translation unit, in the order the database lists them.
    pub(crate) entries: Vec<Entry>,
    /// Positions in `entries` by the way this project spells each file.
    by_spelling: HashMap<PathBuf, Vec<usize>>,
    /// Positions in `entries` by where each file is.
    by_file: HashMap<PathBuf, Vec<usize>>,
}

/// Where a search for a compilation database ended.
#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub(crate) struct Location {
    /// The database file itself.
    file: PathBuf,
    /// The directory it was found from, which the project spells its files
    /// against.
    root: PathBuf,
}

/// The compilation databases one helper process has looked for.
///
/// A helper answers about every unit of a project, and every unit of a project
/// is governed by the same database. Searching and parsing it per request would
/// make the cost of analysing a tree grow with the square of its size, all of
/// it inside a process whose caller can see only how long it took.
///
/// Both halves of the answer are kept: where the search ended for a directory,
/// and what reading that file produced. Failures are kept too, but as the
/// sentence that explains them rather than as silence — a unit refused because
/// its database is unreadable has to be told so, whether it is the first unit
/// to ask or the thousandth.
#[derive(Default)]
pub(crate) struct Databases {
    /// Where the search from one directory ended, or that it ended nowhere.
    located: BTreeMap<PathBuf, Option<Location>>,
    /// What reading each located file produced.
    read: BTreeMap<Location, Result<Database, String>>,
}

impl Databases {
    /// The database governing `path`, found by walking up from it.
    ///
    /// `Ok(None)` when there is none, which is a tree this helper has nothing
    /// to say about rather than a failure: a project that is entirely Rust has
    /// no compilation database and is not missing one.
    ///
    /// `Err` carries why a database that is there could not be read. That is
    /// not the same silence as a project with no database: one is fixed by
    /// writing a database and the other by repairing the one that is there, so
    /// the caller says it for every unit that asks and refuses to describe the
    /// tree.
    pub(crate) fn nearest(&mut self, path: &Path) -> Result<Option<&Database>, &str> {
        let Some(start) = (if path.is_dir() {
            Some(path)
        } else {
            path.parent()
        }) else {
            return Ok(None);
        };
        let Some(location) = self.locate(start) else {
            return Ok(None);
        };
        self.read
            .entry(location)
            .or_insert_with_key(|location| Database::read(&location.file, &location.root))
            .as_ref()
            .map(Some)
            .map_err(String::as_str)
    }

    /// Where the search from `start` ends, walking up only the first time.
    fn locate(&mut self, start: &Path) -> Option<Location> {
        if let Some(known) = self.located.get(start) {
            return known.clone();
        }
        let found = search(start);
        self.located.insert(start.to_path_buf(), found.clone());
        found
    }
}

/// Walk up from `start` for the database that governs it.
///
/// A database that is there stops the search rather than letting it walk
/// further up, readable or not: the answer for this project is the file that
/// was found, and finding somebody else's higher up would analyse this tree
/// under another project's commands.
fn search(start: &Path) -> Option<Location> {
    for ancestor in start.ancestors() {
        for location in LOCATIONS {
            let candidate = ancestor.join(location);
            if candidate.is_file() {
                return Some(Location {
                    file: candidate,
                    root: ancestor.to_path_buf(),
                });
            }
        }
    }
    None
}

impl Database {
    /// Read the database at `path`, spelling its answers against `root`.
    fn read(path: &Path, root: &Path) -> Result<Self, String> {
        let text = std::fs::read_to_string(path)
            .map_err(|error| format!("reading {}: {error}", path.display()))?;
        let raw: Vec<RecordedCommand> = serde_json::from_str(&text)
            .map_err(|error| format!("parsing {}: {error}", path.display()))?;
        Ok(Self::new(
            // Resolved for the same reason the entries are: the root is what
            // every answer is spelled against, and one spelled against a root
            // the caller reached another way is a set of relative paths that
            // point somewhere else.
            canonical(root),
            raw.iter().filter_map(Recorded::entry).collect(),
        ))
    }

    /// A database of `entries`, indexed so that finding one costs the entries
    /// that name the same file rather than every entry there is.
    pub(crate) fn new(root: PathBuf, entries: Vec<Entry>) -> Self {
        let mut by_spelling: HashMap<PathBuf, Vec<usize>> = HashMap::new();
        let mut by_file: HashMap<PathBuf, Vec<usize>> = HashMap::new();
        for (position, entry) in entries.iter().enumerate() {
            by_spelling
                .entry(PathBuf::from(codehelion_helper::ir::spell(
                    Some(&root),
                    &entry.file,
                )))
                .or_default()
                .push(position);
            by_file
                .entry(entry.file.clone())
                .or_default()
                .push(position);
        }
        Self {
            root,
            entries,
            by_spelling,
            by_file,
        }
    }

    /// The entry for the translation unit named `unit`.
    ///
    /// Named either the way this project spells its files or by where the file
    /// is, because the two sides of a request need not stand in the same place:
    /// a scan rooted inside a tree spells a file against its own root, which is
    /// not the root the database was found from, and a name matched only one
    /// way would come back unanswerable for every unit of a project scanned
    /// from a subdirectory.
    ///
    /// Both spellings are compared as paths rather than as strings. A generator
    /// writes `src/a.cpp` on every platform while a path rebuilt here carries
    /// the separator the platform uses, and on Windows the two name one file
    /// that no string comparison calls equal.
    pub(crate) fn unit(
        &self,
        unit: &str,
        selector: Option<&CompileCommandSelector>,
    ) -> Option<&Entry> {
        let named = Path::new(unit);
        let absolute = canonical(named);
        // The entries that name this file either way, in database order: the
        // first one the selector accepts is the one a full scan would reach.
        let mut named_by_either: Vec<usize> = self
            .by_spelling
            .get(named)
            .into_iter()
            .chain(self.by_file.get(&absolute))
            .flatten()
            .copied()
            .collect();
        named_by_either.sort_unstable();
        named_by_either.dedup();
        named_by_either
            .into_iter()
            .filter_map(|position| self.entries.get(position))
            .find(|entry| selector.is_none_or(|wanted| entry.selector.names_the_same_entry(wanted)))
    }

    /// Every macro the database defines anywhere, sorted and without repeats.
    ///
    /// A run-level answer to a per-unit question, and deliberately so: this is
    /// what says the tree was read under these conditions somewhere in it,
    /// which is what a scan of the whole tree is. Which unit had which is
    /// carried by the unit's own build variant, where it decides something.
    pub(crate) fn definitions(&self) -> Vec<String> {
        let mut all: Vec<String> = self
            .entries
            .iter()
            .flat_map(|entry| entry.definitions.iter().cloned())
            .collect();
        all.sort();
        all.dedup();
        all
    }
}

/// Reading one recorded entry into what this helper analyses with.
///
/// What an entry is made of is shared with the scanner, because the two sides
/// name one entry by the words it records. What is made of those words is this
/// helper's own: the scanner partitions builds with them and this program hands
/// them to a compiler, which is why only one of the two has an allow list.
trait Recorded {
    /// This entry in the shape the rest of the helper uses, or nothing when it
    /// carries no command to read.
    fn entry(&self) -> Option<Entry>;
}

impl Recorded for RecordedCommand {
    fn entry(&self) -> Option<Entry> {
        let directory = self.directory.as_ref().map(PathBuf::from);
        let written = resolve(directory.as_deref(), Path::new(&self.file));
        let words = self.words()?;
        // Resolved, because a generator run from a build directory writes its
        // sources as `../src/a.cpp` while a caller naming the same file names
        // it as it is. The two are one file and no plain string comparison
        // says so.
        let file = canonical(&written);
        let parsed_arguments = parse_arguments(&words, &written, directory.as_deref());
        let arguments = ValidatedArguments::parse(&parsed_arguments);
        Some(Entry {
            file: file.clone(),
            arguments,
            definitions: definitions(&words),
            selector: CompileCommandSelector {
                file: file.display().to_string(),
                directory: directory
                    .as_ref()
                    .map(|path| canonical(path).display().to_string()),
                arguments: words,
            },
        })
    }
}

/// `path` as the filesystem spells it, or with its `.` and `..` folded away
/// when it names nothing there.
///
/// Both sides of a request name files, and the two need not have arrived at
/// their spelling the same way: a scan resolves the root it was pointed at, a
/// generator writes whatever it was run with, and on a machine where the one is
/// reached through a symbolic link the two strings differ while the file is one
/// file. Asking the filesystem is the only thing that says so.
pub(crate) fn canonical(path: &Path) -> PathBuf {
    path.canonicalize().unwrap_or_else(|_| lexical(path))
}

/// `path` with its `.` and `..` folded away.
///
/// The fallback for a path that names nothing, and the answer for the paths a
/// compiler reports back: those are built from an include search path and reach
/// this program by the thousand, which is too many to ask the filesystem about
/// one at a time.
pub(crate) fn lexical(path: &Path) -> PathBuf {
    let mut folded = PathBuf::new();
    for part in path.components() {
        match part {
            Component::CurDir => {}
            // Only where there is something to fold: a leading `..` names a
            // directory the path does not otherwise mention, and dropping it
            // would name a different one.
            Component::ParentDir
                if folded
                    .components()
                    .next_back()
                    .is_some_and(|last| matches!(last, Component::Normal(_))) =>
            {
                folded.pop();
            }
            other => folded.push(other),
        }
    }
    folded
}

/// `path` made absolute against `directory` when it is not already.
///
/// The shared rule, because the scanner resolves the same paths to decide which
/// build a unit belongs to: a relative path read one way here and another way
/// there would analyse a unit under a command the scanner filed elsewhere.
fn resolve(directory: Option<&Path>, path: &Path) -> PathBuf {
    codehelion_helper_protocol::compile_commands::resolve_in_directory(directory, path)
}

/// The recorded invocation as arguments a parser can be given.
///
/// The compiler, the input file and where the object file was to go are
/// dropped: the first is chosen by this helper rather than the project, and the
/// other two say which unit this is rather than how it is read. Everything else
/// is kept, including flags this helper does not understand — a flag that
/// changes what a header declares is not something to be filtered by taste.
fn parse_arguments(words: &[String], file: &Path, directory: Option<&Path>) -> Vec<String> {
    let mut arguments = Vec::new();
    // Relative include paths in a database are relative to the directory the
    // command was to run in, which is not this process's. Said once here so
    // that every path in the command resolves the way the build resolved it.
    if let Some(directory) = directory {
        arguments.push(format!("-working-directory={}", directory.display()));
    }
    let mut index = compiler_words(words, file, directory);
    while index < words.len() {
        let word = words[index].as_str();
        index += 1;
        if resolve(directory, Path::new(word)) == file {
            continue;
        }
        if DROPPED_WITH_VALUE.contains(&word) {
            index += 1;
            continue;
        }
        if DROPPED.contains(&word) || word.starts_with("-o") && word.len() > 2 {
            continue;
        }
        arguments.push(word.to_string());
    }
    arguments
}

/// How many leading words name the program that was run rather than how it
/// read the unit.
///
/// Usually one. A project that interposes a compiler cache or a distributing
/// wrapper records it in front of the compiler, and every word up to the first
/// option or the unit's own source belongs to that prefix: taking only the
/// first would leave the real compiler behind as a word no allow list can
/// account for, and the whole unit would be refused for naming its own
/// compiler.
/// The prefix ends at the first word that is not a program name: an option, a
/// response file, or the unit's own source. A response file ends it because it
/// is a command line rather than a program, and skipping it as part of the
/// prefix would let a database re-expand this helper's arguments unread.
fn compiler_words(words: &[String], file: &Path, directory: Option<&Path>) -> usize {
    words
        .iter()
        .position(|word| {
            word.starts_with('-')
                || word.starts_with('@')
                || resolve(directory, Path::new(word.as_str())) == file
        })
        .unwrap_or(words.len())
}

/// Flags that say where output goes rather than how input is read.
const DROPPED: [&str; 5] = ["-c", "-MD", "-MMD", "-M", "-MM"];

/// The same, for the ones that take a separate value.
const DROPPED_WITH_VALUE: [&str; 4] = ["-o", "-MF", "-MT", "-MQ"];

/// Every macro the command defines or undefines, in the flag's own spelling.
fn definitions(words: &[String]) -> Vec<String> {
    let mut found = Vec::new();
    let mut index = 0;
    while index < words.len() {
        let word = words[index].as_str();
        index += 1;
        for flag in ["-D", "-U"] {
            if word == flag {
                if let Some(value) = words.get(index) {
                    found.push(format!("{flag}{value}"));
                    index += 1;
                }
            } else if let Some(value) = word.strip_prefix(flag)
                && !value.is_empty()
            {
                found.push(format!("{flag}{value}"));
            }
        }
    }
    found
}

#[cfg(test)]
#[allow(clippy::unwrap_used, clippy::expect_used)]
mod tests;