[red-knot] Add support for editable installs to the module resolver (#12307)

Co-authored-by: Micha Reiser <micha@reiser.io>
Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_module_resolver/src/db.rs

@@ -1,7 +1,7 @@
 use ruff_db::Upcast;
 
 use crate::resolver::{
-    file_to_module,
+    editable_install_resolution_paths, file_to_module,
     internal::{ModuleNameIngredient, ModuleResolverSettings},
     resolve_module_query,
 };
@@ -11,6 +11,7 @@ use crate::typeshed::parse_typeshed_versions;
 pub struct Jar(
     ModuleNameIngredient<'_>,
     ModuleResolverSettings,
+    editable_install_resolution_paths,
     resolve_module_query,
     file_to_module,
     parse_typeshed_versions,

crates/red_knot_module_resolver/src/path.rs

@@ -6,7 +6,7 @@
 use std::fmt;
 
 use ruff_db::files::{system_path_to_file, vendored_path_to_file, File, FilePath};
-use ruff_db::system::{SystemPath, SystemPathBuf};
+use ruff_db::system::{System, SystemPath, SystemPathBuf};
 use ruff_db::vendored::{VendoredPath, VendoredPathBuf};
 
 use crate::db::Db;
@@ -73,6 +73,7 @@ enum ModuleResolutionPathBufInner {
     FirstParty(SystemPathBuf),
     StandardLibrary(FilePath),
     SitePackages(SystemPathBuf),
+    EditableInstall(SystemPathBuf),
 }
 
 impl ModuleResolutionPathBufInner {
@@ -134,6 +135,19 @@ impl ModuleResolutionPathBufInner {
                 );
                 path.push(component);
             }
+            Self::EditableInstall(ref mut path) => {
+                if let Some(extension) = extension {
+                    assert!(
+                        matches!(extension, "pyi" | "py"),
+                        "Extension must be `py` or `pyi`; got `{extension}`"
+                    );
+                }
+                assert!(
+                    path.extension().is_none(),
+                    "Cannot push part {component} to {path}, which already has an extension"
+                );
+                path.push(component);
+            }
         }
     }
 }
@@ -197,6 +211,18 @@ impl ModuleResolutionPathBuf {
             .then_some(Self(ModuleResolutionPathBufInner::SitePackages(path)))
     }
 
+    #[must_use]
+    pub(crate) fn editable_installation_root(
+        system: &dyn System,
+        path: impl Into<SystemPathBuf>,
+    ) -> Option<Self> {
+        let path = path.into();
+        // TODO: Add Salsa invalidation to this system call:
+        system
+            .is_directory(&path)
+            .then_some(Self(ModuleResolutionPathBufInner::EditableInstall(path)))
+    }
+
     #[must_use]
     pub(crate) fn is_regular_package(&self, search_path: &Self, resolver: &ResolverState) -> bool {
         ModuleResolutionPathRef::from(self).is_regular_package(search_path, resolver)
@@ -229,6 +255,16 @@ impl ModuleResolutionPathBuf {
     pub(crate) fn to_file(&self, search_path: &Self, resolver: &ResolverState) -> Option<File> {
         ModuleResolutionPathRef::from(self).to_file(search_path, resolver)
     }
+
+    pub(crate) fn as_system_path(&self) -> Option<&SystemPathBuf> {
+        match &self.0 {
+            ModuleResolutionPathBufInner::Extra(path) => Some(path),
+            ModuleResolutionPathBufInner::FirstParty(path) => Some(path),
+            ModuleResolutionPathBufInner::StandardLibrary(_) => None,
+            ModuleResolutionPathBufInner::SitePackages(path) => Some(path),
+            ModuleResolutionPathBufInner::EditableInstall(path) => Some(path),
+        }
+    }
 }
 
 impl fmt::Debug for ModuleResolutionPathBuf {
@@ -250,6 +286,10 @@ impl fmt::Debug for ModuleResolutionPathBuf {
                 .debug_tuple("ModuleResolutionPathBuf::StandardLibrary")
                 .field(path)
                 .finish(),
+            ModuleResolutionPathBufInner::EditableInstall(path) => f
+                .debug_tuple("ModuleResolutionPathBuf::EditableInstall")
+                .field(path)
+                .finish(),
         }
     }
 }
@@ -272,6 +312,7 @@ enum ModuleResolutionPathRefInner<'a> {
     FirstParty(&'a SystemPath),
     StandardLibrary(FilePathRef<'a>),
     SitePackages(&'a SystemPath),
+    EditableInstall(&'a SystemPath),
 }
 
 impl<'a> ModuleResolutionPathRefInner<'a> {
@@ -306,6 +347,7 @@ impl<'a> ModuleResolutionPathRefInner<'a> {
             (Self::Extra(path), Self::Extra(_)) => resolver.system().is_directory(path),
             (Self::FirstParty(path), Self::FirstParty(_)) => resolver.system().is_directory(path),
             (Self::SitePackages(path), Self::SitePackages(_)) => resolver.system().is_directory(path),
+            (Self::EditableInstall(path), Self::EditableInstall(_)) => resolver.system().is_directory(path),
             (Self::StandardLibrary(path), Self::StandardLibrary(stdlib_root)) => {
                 match Self::query_stdlib_version(path, search_path, &stdlib_root, resolver) {
                     TypeshedVersionsQueryResult::DoesNotExist => false,
@@ -333,6 +375,7 @@ impl<'a> ModuleResolutionPathRefInner<'a> {
             (Self::Extra(path), Self::Extra(_)) => is_non_stdlib_pkg(resolver, path),
             (Self::FirstParty(path), Self::FirstParty(_)) => is_non_stdlib_pkg(resolver, path),
             (Self::SitePackages(path), Self::SitePackages(_)) => is_non_stdlib_pkg(resolver, path),
+            (Self::EditableInstall(path), Self::EditableInstall(_)) => is_non_stdlib_pkg(resolver, path),
             // Unlike the other variants:
             // (1) Account for VERSIONS
             // (2) Only test for `__init__.pyi`, not `__init__.py`
@@ -358,6 +401,7 @@ impl<'a> ModuleResolutionPathRefInner<'a> {
             (Self::SitePackages(path), Self::SitePackages(_)) => {
                 system_path_to_file(resolver.db.upcast(), path)
             }
+            (Self::EditableInstall(path), Self::EditableInstall(_)) => system_path_to_file(resolver.db.upcast(), path),
             (Self::StandardLibrary(path), Self::StandardLibrary(stdlib_root)) => {
                 match Self::query_stdlib_version(&path, search_path, &stdlib_root, resolver) {
                     TypeshedVersionsQueryResult::DoesNotExist => None,
@@ -374,7 +418,10 @@ impl<'a> ModuleResolutionPathRefInner<'a> {
     #[must_use]
     fn to_module_name(self) -> Option<ModuleName> {
         match self {
-            Self::Extra(path) | Self::FirstParty(path) | Self::SitePackages(path) => {
+            Self::Extra(path)
+            | Self::FirstParty(path)
+            | Self::SitePackages(path)
+            | Self::EditableInstall(path) => {
                 let parent = path.parent()?;
                 let parent_components = parent.components().map(|component| component.as_str());
                 let skip_final_part =
@@ -421,6 +468,9 @@ impl<'a> ModuleResolutionPathRefInner<'a> {
             Self::SitePackages(path) => {
                 ModuleResolutionPathBufInner::SitePackages(path.with_extension("pyi"))
             }
+            Self::EditableInstall(path) => {
+                ModuleResolutionPathBufInner::EditableInstall(path.with_extension("pyi"))
+            }
         }
     }
 
@@ -437,6 +487,9 @@ impl<'a> ModuleResolutionPathRefInner<'a> {
             Self::SitePackages(path) => Some(ModuleResolutionPathBufInner::SitePackages(
                 path.with_extension("py"),
             )),
+            Self::EditableInstall(path) => Some(ModuleResolutionPathBufInner::EditableInstall(
+                path.with_extension("py"),
+            )),
         }
     }
 
@@ -474,6 +527,13 @@ impl<'a> ModuleResolutionPathRefInner<'a> {
                         .then_some(Self::SitePackages(path))
                 })
             }
+            (Self::EditableInstall(root), FilePathRef::System(absolute_path)) => {
+                absolute_path.strip_prefix(root).ok().and_then(|path| {
+                    path.extension()
+                        .map_or(true, |ext| matches!(ext, "pyi" | "py"))
+                        .then_some(Self::EditableInstall(path))
+                })
+            }
             (Self::Extra(_), FilePathRef::Vendored(_)) => None,
             (Self::FirstParty(_), FilePathRef::Vendored(_)) => None,
             (Self::StandardLibrary(root), FilePathRef::Vendored(absolute_path)) => match root {
@@ -487,6 +547,7 @@ impl<'a> ModuleResolutionPathRefInner<'a> {
                 }
             },
             (Self::SitePackages(_), FilePathRef::Vendored(_)) => None,
+            (Self::EditableInstall(_), FilePathRef::Vendored(_)) => None,
         }
     }
 }
@@ -562,6 +623,10 @@ impl fmt::Debug for ModuleResolutionPathRef<'_> {
                 .debug_tuple("ModuleResolutionPathRef::StandardLibrary")
                 .field(path)
                 .finish(),
+            ModuleResolutionPathRefInner::EditableInstall(path) => f
+                .debug_tuple("ModuleResolutionPathRef::EditableInstall")
+                .field(path)
+                .finish(),
         }
     }
 }
@@ -582,6 +647,9 @@ impl<'a> From<&'a ModuleResolutionPathBuf> for ModuleResolutionPathRef<'a> {
             ModuleResolutionPathBufInner::SitePackages(path) => {
                 ModuleResolutionPathRefInner::SitePackages(path)
             }
+            ModuleResolutionPathBufInner::EditableInstall(path) => {
+                ModuleResolutionPathRefInner::EditableInstall(path)
+            }
         };
         ModuleResolutionPathRef(inner)
     }
@@ -593,6 +661,7 @@ impl PartialEq<SystemPath> for ModuleResolutionPathRef<'_> {
             ModuleResolutionPathRefInner::Extra(path) => path == other,
             ModuleResolutionPathRefInner::FirstParty(path) => path == other,
             ModuleResolutionPathRefInner::SitePackages(path) => path == other,
+            ModuleResolutionPathRefInner::EditableInstall(path) => path == other,
             ModuleResolutionPathRefInner::StandardLibrary(FilePathRef::System(path)) => {
                 path == other
             }
@@ -625,6 +694,7 @@ impl PartialEq<VendoredPath> for ModuleResolutionPathRef<'_> {
             ModuleResolutionPathRefInner::Extra(_) => false,
             ModuleResolutionPathRefInner::FirstParty(_) => false,
             ModuleResolutionPathRefInner::SitePackages(_) => false,
+            ModuleResolutionPathRefInner::EditableInstall(_) => false,
             ModuleResolutionPathRefInner::StandardLibrary(FilePathRef::System(_)) => false,
             ModuleResolutionPathRefInner::StandardLibrary(FilePathRef::Vendored(path)) => {
                 path == other
@@ -707,6 +777,9 @@ mod tests {
                 ModuleResolutionPathRefInner::SitePackages(path) => {
                     ModuleResolutionPathBufInner::SitePackages(path.to_path_buf())
                 }
+                ModuleResolutionPathRefInner::EditableInstall(path) => {
+                    ModuleResolutionPathBufInner::EditableInstall(path.to_path_buf())
+                }
             };
             ModuleResolutionPathBuf(inner)
         }

crates/red_knot_module_resolver/src/resolver.rs

@@ -1,8 +1,12 @@
-use std::ops::Deref;
+use std::collections;
+use std::hash::BuildHasherDefault;
+use std::iter::FusedIterator;
 use std::sync::Arc;
 
+use rustc_hash::FxHasher;
+
 use ruff_db::files::{File, FilePath};
-use ruff_db::system::SystemPathBuf;
+use ruff_db::system::{DirectoryEntry, System, SystemPath, SystemPathBuf};
 
 use crate::db::Db;
 use crate::module::{Module, ModuleKind};
@@ -12,6 +16,79 @@ use crate::resolver::internal::ModuleResolverSettings;
 use crate::state::ResolverState;
 use crate::supported_py_version::TargetVersion;
 
+type SearchPathRoot = Arc<ModuleResolutionPathBuf>;
+
+/// An ordered sequence of search paths.
+///
+/// The sequence respects the invariant maintained by [`sys.path` at runtime]
+/// where no two module-resolution paths ever point to the same directory on disk.
+/// (Paths may, however, *overlap* -- e.g. you could have both `src/` and `src/foo`
+/// as module resolution paths simultaneously.)
+///
+/// [`sys.path` at runtime]: https://docs.python.org/3/library/site.html#module-site
+#[derive(Debug, PartialEq, Eq, Default, Clone)]
+pub(crate) struct SearchPathSequence {
+    raw_paths: collections::HashSet<SystemPathBuf, BuildHasherDefault<FxHasher>>,
+    search_paths: Vec<SearchPathRoot>,
+}
+
+impl SearchPathSequence {
+    fn insert(&mut self, path: SearchPathRoot) -> bool {
+        // Just assume that all search paths that aren't SystemPaths are unique
+        if let Some(fs_path) = path.as_system_path() {
+            if self.raw_paths.contains(fs_path) {
+                false
+            } else {
+                let raw_path = fs_path.to_owned();
+                self.search_paths.push(path);
+                self.raw_paths.insert(raw_path)
+            }
+        } else {
+            self.search_paths.push(path);
+            true
+        }
+    }
+
+    fn contains(&self, path: &SearchPathRoot) -> bool {
+        if let Some(fs_path) = path.as_system_path() {
+            self.raw_paths.contains(fs_path)
+        } else {
+            self.search_paths.contains(path)
+        }
+    }
+
+    fn iter(&self) -> std::slice::Iter<SearchPathRoot> {
+        self.search_paths.iter()
+    }
+}
+
+impl<'a> IntoIterator for &'a SearchPathSequence {
+    type IntoIter = std::slice::Iter<'a, SearchPathRoot>;
+    type Item = &'a SearchPathRoot;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+impl FromIterator<SearchPathRoot> for SearchPathSequence {
+    fn from_iter<T: IntoIterator<Item = SearchPathRoot>>(iter: T) -> Self {
+        let mut sequence = Self::default();
+        for item in iter {
+            sequence.insert(item);
+        }
+        sequence
+    }
+}
+
+impl Extend<SearchPathRoot> for SearchPathSequence {
+    fn extend<T: IntoIterator<Item = SearchPathRoot>>(&mut self, iter: T) {
+        for item in iter {
+            self.insert(item);
+        }
+    }
+}
+
 /// Configures the module resolver settings.
 ///
 /// Must be called before calling any other module resolution functions.
@@ -19,7 +96,7 @@ pub fn set_module_resolution_settings(db: &mut dyn Db, config: RawModuleResoluti
     // There's no concurrency issue here because we hold a `&mut dyn Db` reference. No other
     // thread can mutate the `Db` while we're in this call, so using `try_get` to test if
     // the settings have already been set is safe.
-    let resolved_settings = config.into_configuration_settings();
+    let resolved_settings = config.into_configuration_settings(db.system().current_directory());
     if let Some(existing) = ModuleResolverSettings::try_get(db) {
         existing.set_settings(db).to(resolved_settings);
     } else {
@@ -82,12 +159,14 @@ pub(crate) fn file_to_module(db: &dyn Db, file: File) -> Option<Module> {
 
     let resolver_settings = module_resolver_settings(db);
 
-    let relative_path = resolver_settings
-        .search_paths()
-        .iter()
-        .find_map(|root| root.relativize_path(path))?;
+    let mut search_paths = resolver_settings.search_paths(db);
 
-    let module_name = relative_path.to_module_name()?;
+    let module_name = loop {
+        let candidate = search_paths.next()?;
+        if let Some(relative_path) = candidate.relativize_path(path) {
+            break 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
@@ -133,7 +212,10 @@ pub struct RawModuleResolutionSettings {
 }
 
 impl RawModuleResolutionSettings {
-    /// Implementation of the typing spec's [module resolution order]
+    /// 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].
     ///
     /// TODO(Alex): this method does multiple `.unwrap()` calls when it should really return an error.
     /// Each `.unwrap()` call is a point where we're validating a setting that the user would pass
@@ -143,67 +225,297 @@ impl RawModuleResolutionSettings {
     /// This validation should probably be done outside of Salsa?
     ///
     /// [module resolution order]: https://typing.readthedocs.io/en/latest/spec/distributing.html#import-resolution-ordering
-    fn into_configuration_settings(self) -> ModuleResolutionSettings {
+    fn into_configuration_settings(
+        self,
+        current_directory: &SystemPath,
+    ) -> ModuleResolutionSettings {
         let RawModuleResolutionSettings {
             target_version,
             extra_paths,
             workspace_root,
-            site_packages,
+            site_packages: site_packages_setting,
             custom_typeshed,
         } = self;
 
-        let mut paths: Vec<ModuleResolutionPathBuf> = extra_paths
+        let mut static_search_paths: SearchPathSequence = extra_paths
             .into_iter()
-            .map(|fs_path| ModuleResolutionPathBuf::extra(fs_path).unwrap())
+            .map(|fs_path| {
+                Arc::new(
+                    ModuleResolutionPathBuf::extra(SystemPath::absolute(
+                        fs_path,
+                        current_directory,
+                    ))
+                    .unwrap(),
+                )
+            })
             .collect();
 
-        paths.push(ModuleResolutionPathBuf::first_party(workspace_root).unwrap());
+        static_search_paths.insert(Arc::new(
+            ModuleResolutionPathBuf::first_party(SystemPath::absolute(
+                workspace_root,
+                current_directory,
+            ))
+            .unwrap(),
+        ));
 
-        paths.push(
-            custom_typeshed.map_or_else(ModuleResolutionPathBuf::vendored_stdlib, |custom| {
-                ModuleResolutionPathBuf::stdlib_from_custom_typeshed_root(&custom).unwrap()
-            }),
-        );
+        static_search_paths.insert(Arc::new(custom_typeshed.map_or_else(
+            ModuleResolutionPathBuf::vendored_stdlib,
+            |custom| {
+                ModuleResolutionPathBuf::stdlib_from_custom_typeshed_root(&SystemPath::absolute(
+                    custom,
+                    current_directory,
+                ))
+                .unwrap()
+            },
+        )));
+
+        let mut site_packages = None;
+
+        if let Some(path) = site_packages_setting {
+            let site_packages_root = Arc::new(
+                ModuleResolutionPathBuf::site_packages(SystemPath::absolute(
+                    path,
+                    current_directory,
+                ))
+                .unwrap(),
+            );
+            site_packages = Some(site_packages_root.clone());
+            static_search_paths.insert(site_packages_root);
+        }
 
         // TODO vendor typeshed's third-party stubs as well as the stdlib and fallback to them as a final step
-        if let Some(site_packages) = site_packages {
-            paths.push(ModuleResolutionPathBuf::site_packages(site_packages).unwrap());
-        }
 
         ModuleResolutionSettings {
             target_version,
-            search_paths: OrderedSearchPaths(paths.into_iter().map(Arc::new).collect()),
+            search_path_settings: ValidatedSearchPathSettings {
+                static_search_paths,
+                site_packages,
+            },
         }
     }
 }
 
-/// A resolved module resolution order as per the [typing spec]
+#[derive(Debug, PartialEq, Eq, Clone)]
+struct ValidatedSearchPathSettings {
+    /// Search paths that have been statically determined purely from reading Ruff's configuration settings.
+    /// These shouldn't ever change unless the config settings themselves change.
+    ///
+    /// Note that `site-packages` *is included* as a search path in this sequence,
+    /// but it is also stored separately so that we're able to find editable installs later.
+    static_search_paths: SearchPathSequence,
+    site_packages: Option<SearchPathRoot>,
+}
+
+/// Collect all dynamic search paths:
+/// search paths listed in `.pth` files in the `site-packages` directory
+/// due to editable installations of third-party packages.
+#[salsa::tracked(return_ref)]
+pub(crate) fn editable_install_resolution_paths(db: &dyn Db) -> SearchPathSequence {
+    // This query needs to be re-executed each time a `.pth` file
+    // is added, modified or removed from the `site-packages` directory.
+    // However, we don't use Salsa queries to read the source text of `.pth` files;
+    // we use the APIs on the `System` trait directly. As such, for now we simply ask
+    // Salsa to recompute this query on each new revision.
+    //
+    // TODO: add some kind of watcher for the `site-packages` directory that looks
+    // for `site-packages/*.pth` files being added/modified/removed; get rid of this.
+    // When doing so, also make the test
+    // `deleting_pth_file_on_which_module_resolution_depends_invalidates_cache()`
+    // more principled!
+    db.report_untracked_read();
+
+    let ValidatedSearchPathSettings {
+        static_search_paths,
+        site_packages,
+    } = &module_resolver_settings(db).search_path_settings;
+
+    let mut dynamic_paths = SearchPathSequence::default();
+
+    if let Some(site_packages) = site_packages {
+        let site_packages = site_packages
+            .as_system_path()
+            .expect("Expected site-packages never to be a VendoredPath!");
+
+        // 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 Ok(pth_file_iterator) = PthFileIterator::new(db, site_packages) else {
+            return dynamic_paths;
+        };
+
+        // The Python documentation specifies that `.pth` files in `site-packages`
+        // are processed in alphabetical order, so collecting and then sorting is necessary.
+        // https://docs.python.org/3/library/site.html#module-site
+        let mut all_pth_files: Vec<PthFile> = pth_file_iterator.collect();
+        all_pth_files.sort_by(|a, b| a.path.cmp(&b.path));
+
+        for pth_file in &all_pth_files {
+            dynamic_paths.extend(
+                pth_file
+                    .editable_installations()
+                    .filter_map(|editable_path| {
+                        let possible_search_path = Arc::new(editable_path);
+                        (!static_search_paths.contains(&possible_search_path))
+                            .then_some(possible_search_path)
+                    }),
+            );
+        }
+    }
+    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.
 ///
-/// [typing spec]: https://typing.readthedocs.io/en/latest/spec/distributing.html#import-resolution-ordering
-#[derive(Clone, Debug, Default, Eq, PartialEq)]
-pub(crate) struct OrderedSearchPaths(Vec<Arc<ModuleResolutionPathBuf>>);
+/// [`sys.path` at runtime]: https://docs.python.org/3/library/site.html#module-site
+struct SearchPathIterator<'db> {
+    db: &'db dyn Db,
+    static_paths: std::slice::Iter<'db, SearchPathRoot>,
+    dynamic_paths: Option<std::slice::Iter<'db, SearchPathRoot>>,
+}
 
-impl Deref for OrderedSearchPaths {
-    type Target = [Arc<ModuleResolutionPathBuf>];
+impl<'db> Iterator for SearchPathIterator<'db> {
+    type Item = &'db SearchPathRoot;
 
-    fn deref(&self) -> &Self::Target {
-        &self.0
+    fn next(&mut self) -> Option<Self::Item> {
+        let SearchPathIterator {
+            db,
+            static_paths,
+            dynamic_paths,
+        } = self;
+
+        static_paths.next().or_else(|| {
+            dynamic_paths
+                .get_or_insert_with(|| editable_install_resolution_paths(*db).into_iter())
+                .next()
+        })
     }
 }
 
+impl<'db> FusedIterator for SearchPathIterator<'db> {}
+
+/// Represents a single `.pth` file in a `site-packages` directory.
+/// One or more lines in a `.pth` file may be a (relative or absolute)
+/// path that represents an editable installation of a package.
+struct PthFile<'db> {
+    system: &'db dyn System,
+    path: SystemPathBuf,
+    contents: String,
+    site_packages: &'db SystemPath,
+}
+
+impl<'db> PthFile<'db> {
+    /// Yield paths in this `.pth` file that appear to represent editable installations,
+    /// and should therefore be added as module-resolution search paths.
+    fn editable_installations(&'db self) -> impl Iterator<Item = ModuleResolutionPathBuf> + 'db {
+        let PthFile {
+            system,
+            path: _,
+            contents,
+            site_packages,
+        } = self;
+
+        // Empty lines or lines starting with '#' are ignored by the Python interpreter.
+        // Lines that start with "import " or "import\t" do not represent editable installs at all;
+        // instead, these are lines that are executed by Python at startup.
+        // https://docs.python.org/3/library/site.html#module-site
+        contents.lines().filter_map(move |line| {
+            let line = line.trim_end();
+            if line.is_empty()
+                || line.starts_with('#')
+                || line.starts_with("import ")
+                || line.starts_with("import\t")
+            {
+                return None;
+            }
+            let possible_editable_install = SystemPath::absolute(line, site_packages);
+            ModuleResolutionPathBuf::editable_installation_root(*system, possible_editable_install)
+        })
+    }
+}
+
+/// Iterator that yields a [`PthFile`] instance for every `.pth` file
+/// found in a given `site-packages` directory.
+struct PthFileIterator<'db> {
+    db: &'db dyn Db,
+    directory_iterator: Box<dyn Iterator<Item = std::io::Result<DirectoryEntry>> + 'db>,
+    site_packages: &'db SystemPath,
+}
+
+impl<'db> PthFileIterator<'db> {
+    fn new(db: &'db dyn Db, site_packages: &'db SystemPath) -> std::io::Result<Self> {
+        Ok(Self {
+            db,
+            directory_iterator: db.system().read_directory(site_packages)?,
+            site_packages,
+        })
+    }
+}
+
+impl<'db> Iterator for PthFileIterator<'db> {
+    type Item = PthFile<'db>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let PthFileIterator {
+            db,
+            directory_iterator,
+            site_packages,
+        } = self;
+
+        let system = db.system();
+
+        loop {
+            let entry_result = directory_iterator.next()?;
+            let Ok(entry) = entry_result else {
+                continue;
+            };
+            let file_type = entry.file_type();
+            if file_type.is_directory() {
+                continue;
+            }
+            let path = entry.into_path();
+            if path.extension() != Some("pth") {
+                continue;
+            }
+
+            let Ok(contents) = db.system().read_to_string(&path) else {
+                continue;
+            };
+
+            return Some(PthFile {
+                system,
+                path,
+                contents,
+                site_packages,
+            });
+        }
+    }
+}
+
+/// Validated and normalized module-resolution settings.
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub(crate) struct ModuleResolutionSettings {
-    search_paths: OrderedSearchPaths,
+    search_path_settings: ValidatedSearchPathSettings,
     target_version: TargetVersion,
 }
 
 impl ModuleResolutionSettings {
-    pub(crate) fn search_paths(&self) -> &[Arc<ModuleResolutionPathBuf>] {
-        &self.search_paths
+    fn target_version(&self) -> TargetVersion {
+        self.target_version
     }
 
-    pub(crate) fn target_version(&self) -> TargetVersion {
-        self.target_version
+    fn search_paths<'db>(&'db self, db: &'db dyn Db) -> SearchPathIterator<'db> {
+        SearchPathIterator {
+            db,
+            static_paths: self.search_path_settings.static_search_paths.iter(),
+            dynamic_paths: None,
+        }
     }
 }
 
@@ -245,7 +557,7 @@ fn resolve_name(
     let resolver_settings = module_resolver_settings(db);
     let resolver_state = ResolverState::new(db, resolver_settings.target_version());
 
-    for search_path in resolver_settings.search_paths() {
+    for search_path in resolver_settings.search_paths(db) {
         let mut components = name.components();
         let module_name = components.next_back()?;
 
@@ -388,6 +700,7 @@ mod tests {
     use ruff_db::files::{system_path_to_file, File, FilePath};
     use ruff_db::system::{DbWithTestSystem, OsSystem, SystemPath};
     use ruff_db::testing::assert_function_query_was_not_run;
+    use ruff_db::Db;
 
     use crate::db::tests::TestDb;
     use crate::module::ModuleKind;
@@ -1140,4 +1453,259 @@ mod tests {
             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(&db, foo_module_name.clone()).unwrap();
+        let foo_bar_module = resolve_module(&db, foo_bar_module_name.clone()).unwrap();
+
+        assert_eq!(
+            foo_module.file().path(&db),
+            &FilePath::system("/x/src/foo/__init__.py")
+        );
+        assert_eq!(
+            foo_bar_module.file().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(&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(&db, bar_module_name.clone()).unwrap();
+        assert_eq!(
+            bar_module.file().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(&db, foo_module_name.clone()).unwrap();
+
+        assert_eq!(
+            foo_module.file().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(&db, foo_module_name.clone()).unwrap();
+        let a_module = resolve_module(&db, a_module_name.clone()).unwrap();
+        let b_module = resolve_module(&db, b_module_name.clone()).unwrap();
+        let spam_module = resolve_module(&db, spam_module_name.clone()).unwrap();
+
+        assert_eq!(
+            foo_module.file().path(&db),
+            &FilePath::system("/x/y/src/foo.pyi")
+        );
+        assert_eq!(a_module.file().path(&db), &FilePath::system("/a.py"));
+        assert_eq!(b_module.file().path(&db), &FilePath::system("/baz/b.py"));
+        assert_eq!(
+            spam_module.file().path(&db),
+            &FilePath::System(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(&db, foo_module_name).unwrap();
+        assert_eq!(
+            foo_module.file().path(&db),
+            &FilePath::system("/x/src/foo.py")
+        );
+
+        db.clear_salsa_events();
+        let bar_module = resolve_module(&db, bar_module_name).unwrap();
+        assert_eq!(
+            bar_module.file().path(&db),
+            &FilePath::system("/y/src/bar.py")
+        );
+        let events = db.take_salsa_events();
+        assert_function_query_was_not_run::<editable_install_resolution_paths, _, _>(
+            &db,
+            |res| &res.function,
+            &(),
+            &events,
+        );
+    }
+
+    #[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(&db, foo_module_name.clone()).unwrap();
+        assert_eq!(
+            foo_module.file().path(&db),
+            &FilePath::system("/x/src/foo.py")
+        );
+
+        db.memory_file_system()
+            .remove_file(site_packages.join("_foo.pth"))
+            .unwrap();
+
+        // Why are we touching a random file in the path that's been editably installed,
+        // rather than the `.pth` file, when the `.pth` file is the one that has been deleted?
+        // It's because the `.pth` file isn't directly tracked as a dependency by Salsa
+        // currently (we don't use `system_path_to_file()` to get the file, and we don't use
+        // `source_text()` to read the source of the file). Instead of using these APIs which
+        // would automatically add the existence and contents of the file as a Salsa-tracked
+        // dependency, we use `.report_untracked_read()` to force Salsa to re-parse all
+        // `.pth` files on each new "revision". Making a random modification to a tracked
+        // Salsa file forces a new revision.
+        //
+        // TODO: get rid of the `.report_untracked_read()` call...
+        File::touch_path(&mut db, SystemPath::new("/x/src/foo.py"));
+
+        assert_eq!(resolve_module(&db, foo_module_name.clone()), 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(&db, foo_module_name.clone()).unwrap();
+        let src_path = SystemPathBuf::from("/x/src");
+        assert_eq!(
+            foo_module.file().path(&db),
+            &FilePath::System(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::touch_path(&mut db, &src_path.join("foo.py"));
+        File::touch_path(&mut db, &src_path);
+        assert_eq!(resolve_module(&db, foo_module_name.clone()), 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<&SearchPathRoot> =
+            module_resolver_settings(&db).search_paths(&db).collect();
+
+        assert!(search_paths.contains(&&Arc::new(
+            ModuleResolutionPathBuf::first_party("/src").unwrap()
+        )));
+
+        assert!(!search_paths.contains(&&Arc::new(
+            ModuleResolutionPathBuf::editable_installation_root(db.system(), "/src").unwrap()
+        )));
+    }
 }

crates/ruff_db/src/system.rs

@@ -150,6 +150,10 @@ impl DirectoryEntry {
         Self { path, file_type }
     }
 
+    pub fn into_path(self) -> SystemPathBuf {
+        self.path
+    }
+
     pub fn path(&self) -> &SystemPath {
         &self.path
     }