Eagerly validate search paths (#12783)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/red_knot/src/main.rs

@@ -184,7 +184,7 @@ fn run() -> anyhow::Result<ExitStatus> {
 
     // TODO: Use the `program_settings` to compute the key for the database's persistent
     //   cache and load the cache if it exists.
-    let mut db = RootDatabase::new(workspace_metadata, program_settings, system);
+    let mut db = RootDatabase::new(workspace_metadata, program_settings, system)?;
 
     let (main_loop, main_loop_cancellation_token) = MainLoop::new();
 

crates/red_knot/tests/file_watching.rs

@@ -4,7 +4,6 @@ use std::io::Write;
 use std::time::Duration;
 
 use anyhow::{anyhow, Context};
-use salsa::Setter;
 
 use red_knot_python_semantic::{
     resolve_module, ModuleName, Program, ProgramSettings, PythonVersion, SearchPathSettings,
@@ -26,6 +25,7 @@ struct TestCase {
     /// We need to hold on to it in the test case or the temp files get deleted.
     _temp_dir: tempfile::TempDir,
     root_dir: SystemPathBuf,
+    search_path_settings: SearchPathSettings,
 }
 
 impl TestCase {
@@ -108,18 +108,20 @@ impl TestCase {
     fn update_search_path_settings(
         &mut self,
         f: impl FnOnce(&SearchPathSettings) -> SearchPathSettings,
-    ) {
+    ) -> anyhow::Result<()> {
         let program = Program::get(self.db());
-        let search_path_settings = program.search_paths(self.db());
 
-        let new_settings = f(search_path_settings);
+        let new_settings = f(&self.search_path_settings);
 
-        program.set_search_paths(&mut self.db).to(new_settings);
+        program.update_search_paths(&mut self.db, new_settings.clone())?;
+        self.search_path_settings = new_settings;
 
         if let Some(watcher) = &mut self.watcher {
             watcher.update(&self.db);
             assert!(!watcher.has_errored_paths());
         }
+
+        Ok(())
     }
 
     fn collect_package_files(&self, path: &SystemPath) -> Vec<File> {
@@ -221,13 +223,13 @@ where
     let system = OsSystem::new(&workspace_path);
 
     let workspace = WorkspaceMetadata::from_path(&workspace_path, &system)?;
-    let search_paths = create_search_paths(&root_path, workspace.root());
+    let search_path_settings = create_search_paths(&root_path, workspace.root());
 
-    for path in search_paths
+    for path in search_path_settings
         .extra_paths
         .iter()
-        .chain(search_paths.site_packages.iter())
+        .chain(search_path_settings.site_packages.iter())
-        .chain(search_paths.custom_typeshed.iter())
+        .chain(search_path_settings.custom_typeshed.iter())
     {
         std::fs::create_dir_all(path.as_std_path())
             .with_context(|| format!("Failed to create search path '{path}'"))?;
@@ -235,10 +237,10 @@ where
 
     let settings = ProgramSettings {
         target_version: PythonVersion::default(),
-        search_paths,
+        search_paths: search_path_settings.clone(),
     };
 
-    let db = RootDatabase::new(workspace, settings, system);
+    let db = RootDatabase::new(workspace, settings, system)?;
 
     let (sender, receiver) = crossbeam::channel::unbounded();
     let watcher = directory_watcher(move |events| sender.send(events).unwrap())
@@ -253,6 +255,7 @@ where
         watcher: Some(watcher),
         _temp_dir: temp_dir,
         root_dir: root_path,
+        search_path_settings,
     };
 
     // Sometimes the file watcher reports changes for events that happened before the watcher was started.
@@ -737,7 +740,8 @@ fn add_search_path() -> anyhow::Result<()> {
     case.update_search_path_settings(|settings| SearchPathSettings {
         site_packages: vec![site_packages.clone()],
         ..settings.clone()
-    });
+    })
+    .expect("Search path settings to be valid");
 
     std::fs::write(site_packages.join("a.py").as_std_path(), "class A: ...")?;
 
@@ -767,7 +771,8 @@ fn remove_search_path() -> anyhow::Result<()> {
     case.update_search_path_settings(|settings| SearchPathSettings {
         site_packages: vec![],
         ..settings.clone()
-    });
+    })
+    .expect("Search path settings to be valid");
 
     std::fs::write(site_packages.join("a.py").as_std_path(), "class A: ...")?;
 

crates/red_knot_python_semantic/Cargo.toml

@@ -17,6 +17,7 @@ ruff_python_ast = { workspace = true }
 ruff_python_stdlib = { workspace = true }
 ruff_text_size = { workspace = true }
 
+anyhow = { workspace = true }
 bitflags = { workspace = true }
 camino = { workspace = true }
 compact_str = { workspace = true }

crates/red_knot_python_semantic/src/module_resolver/mod.rs

@@ -2,11 +2,13 @@ use std::iter::FusedIterator;
 
 pub(crate) use module::Module;
 pub use resolver::resolve_module;
+pub(crate) use resolver::SearchPaths;
 use ruff_db::system::SystemPath;
 pub use typeshed::vendored_typeshed_stubs;
 
+use crate::module_resolver::resolver::search_paths;
 use crate::Db;
-use resolver::{module_resolution_settings, SearchPathIterator};
+use resolver::SearchPathIterator;
 
 mod module;
 mod path;
@@ -20,7 +22,7 @@ mod testing;
 /// Returns an iterator over all search paths pointing to a system path
 pub fn system_module_search_paths(db: &dyn Db) -> SystemModuleSearchPathsIter {
     SystemModuleSearchPathsIter {
-        inner: module_resolution_settings(db).search_paths(db),
+        inner: search_paths(db),
     }
 }
 

crates/red_knot_python_semantic/src/module_resolver/resolver.rs

@@ -7,12 +7,13 @@ use ruff_db::files::{File, FilePath, FileRootKind};
 use ruff_db::system::{DirectoryEntry, SystemPath, SystemPathBuf};
 use ruff_db::vendored::VendoredPath;
 
+use crate::db::Db;
+use crate::module_name::ModuleName;
+use crate::{Program, SearchPathSettings};
+
 use super::module::{Module, ModuleKind};
 use super::path::{ModulePath, SearchPath, SearchPathValidationError};
 use super::state::ResolverState;
-use crate::db::Db;
-use crate::module_name::ModuleName;
-use crate::{Program, PythonVersion, SearchPathSettings};
 
 /// Resolves a module name to a module.
 pub fn resolve_module(db: &dyn Db, module_name: ModuleName) -> Option<Module> {
@@ -84,9 +85,7 @@ pub(crate) fn file_to_module(db: &dyn Db, file: File) -> Option<Module> {
         FilePath::SystemVirtual(_) => return None,
     };
 
-    let settings = module_resolution_settings(db);
+    let mut search_paths = search_paths(db);
-
-    let mut search_paths = settings.search_paths(db);
 
     let module_name = loop {
         let candidate = search_paths.next()?;
@@ -119,51 +118,67 @@ pub(crate) fn file_to_module(db: &dyn Db, file: File) -> Option<Module> {
     }
 }
 
+pub(crate) fn search_paths(db: &dyn Db) -> SearchPathIterator {
+    Program::get(db).search_paths(db).iter(db)
+}
+
+#[derive(Debug, PartialEq, Eq, Default)]
+pub(crate) struct SearchPaths {
+    /// 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.
+    static_paths: Vec<SearchPath>,
+
+    /// site-packages paths are not included in the above field:
+    /// if there are multiple site-packages paths, editable installations can appear
+    /// *between* the site-packages paths on `sys.path` at runtime.
+    /// That means we can't know where a second or third `site-packages` path should sit
+    /// in terms of module-resolution priority until we've discovered the editable installs
+    /// for the first `site-packages` path
+    site_packages: Vec<SearchPath>,
+}
+
+impl SearchPaths {
     /// Validate and normalize the raw settings given by the user
     /// into settings we can use for module resolution
     ///
     /// This method also implements the typing spec's [module resolution order].
     ///
     /// [module resolution order]: https://typing.readthedocs.io/en/latest/spec/distributing.html#import-resolution-ordering
-fn try_resolve_module_resolution_settings(
+    pub(crate) fn from_settings(
         db: &dyn Db,
-) -> Result<ModuleResolutionSettings, SearchPathValidationError> {
+        settings: SearchPathSettings,
-    let program = Program::get(db.upcast());
+    ) -> Result<Self, SearchPathValidationError> {
-
         let SearchPathSettings {
             extra_paths,
             src_root,
             custom_typeshed,
-        site_packages,
+            site_packages: site_packages_paths,
-    } = program.search_paths(db.upcast());
+        } = settings;
-
-    if !extra_paths.is_empty() {
-        tracing::info!("Extra search paths: {extra_paths:?}");
-    }
-
-    if let Some(custom_typeshed) = custom_typeshed {
-        tracing::info!("Custom typeshed directory: {custom_typeshed}");
-    }
 
         let system = db.system();
         let files = db.files();
 
-    let mut static_search_paths = vec![];
+        let mut static_paths = vec![];
 
         for path in extra_paths {
-        let search_path = SearchPath::extra(system, path.clone())?;
+            tracing::debug!("Adding static extra search-path '{path}'");
+
+            let search_path = SearchPath::extra(system, path)?;
             files.try_add_root(
                 db.upcast(),
                 search_path.as_system_path().unwrap(),
                 FileRootKind::LibrarySearchPath,
             );
-        static_search_paths.push(search_path);
+            static_paths.push(search_path);
         }
 
-    static_search_paths.push(SearchPath::first_party(system, src_root.clone())?);
+        tracing::debug!("Adding static search path '{src_root}'");
+        static_paths.push(SearchPath::first_party(system, src_root)?);
 
-    static_search_paths.push(if let Some(custom_typeshed) = custom_typeshed.as_ref() {
+        static_paths.push(if let Some(custom_typeshed) = custom_typeshed {
-        let search_path = SearchPath::custom_stdlib(db, custom_typeshed.clone())?;
+            tracing::debug!("Adding static custom-sdtlib search-path '{custom_typeshed}'");
+
+            let search_path = SearchPath::custom_stdlib(db, custom_typeshed)?;
             files.try_add_root(
                 db.upcast(),
                 search_path.as_system_path().unwrap(),
@@ -174,33 +189,31 @@ fn try_resolve_module_resolution_settings(
             SearchPath::vendored_stdlib()
         });
 
-    let mut site_packages_paths: Vec<_> = Vec::with_capacity(site_packages.len());
+        let mut site_packages: Vec<_> = Vec::with_capacity(site_packages_paths.len());
 
-    for path in site_packages {
+        for path in site_packages_paths {
-        let search_path = SearchPath::site_packages(system, path.to_path_buf())?;
+            tracing::debug!("Adding site-package path '{path}'");
+            let search_path = SearchPath::site_packages(system, path)?;
             files.try_add_root(
                 db.upcast(),
                 search_path.as_system_path().unwrap(),
                 FileRootKind::LibrarySearchPath,
             );
-        site_packages_paths.push(search_path);
+            site_packages.push(search_path);
         }
 
         // TODO vendor typeshed's third-party stubs as well as the stdlib and fallback to them as a final step
 
-    let target_version = program.target_version(db.upcast());
-    tracing::info!("Target version: {target_version}");
-
         // Filter out module resolution paths that point to the same directory on disk (the same invariant maintained by [`sys.path` at runtime]).
         // (Paths may, however, *overlap* -- e.g. you could have both `src/` and `src/foo`
         // as module resolution paths simultaneously.)
         //
-    // [`sys.path` at runtime]: https://docs.python.org/3/library/site.html#module-site
         // This code doesn't use an `IndexSet` because the key is the system path and not the search root.
-    let mut seen_paths =
+        //
-        FxHashSet::with_capacity_and_hasher(static_search_paths.len(), FxBuildHasher);
+        // [`sys.path` at runtime]: https://docs.python.org/3/library/site.html#module-site
+        let mut seen_paths = FxHashSet::with_capacity_and_hasher(static_paths.len(), FxBuildHasher);
 
-    static_search_paths.retain(|path| {
+        static_paths.retain(|path| {
             if let Some(path) = path.as_system_path() {
                 seen_paths.insert(path.to_path_buf())
             } else {
@@ -208,17 +221,19 @@ fn try_resolve_module_resolution_settings(
             }
         });
 
-    Ok(ModuleResolutionSettings {
+        Ok(SearchPaths {
-        target_version,
+            static_paths,
-        static_search_paths,
+            site_packages,
-        site_packages_paths,
         })
     }
 
-#[salsa::tracked(return_ref)]
+    pub(crate) fn iter<'a>(&'a self, db: &'a dyn Db) -> SearchPathIterator<'a> {
-pub(crate) fn module_resolution_settings(db: &dyn Db) -> ModuleResolutionSettings {
+        SearchPathIterator {
-    // TODO proper error handling if this returns an error:
+            db,
-    try_resolve_module_resolution_settings(db).unwrap()
+            static_paths: self.static_paths.iter(),
+            dynamic_paths: None,
+        }
+    }
 }
 
 /// Collect all dynamic search paths. For each `site-packages` path:
@@ -231,19 +246,20 @@ pub(crate) fn module_resolution_settings(db: &dyn Db) -> ModuleResolutionSetting
 /// module-resolution priority.
 #[salsa::tracked(return_ref)]
 pub(crate) fn dynamic_resolution_paths(db: &dyn Db) -> Vec<SearchPath> {
-    let ModuleResolutionSettings {
+    tracing::debug!("Resolving dynamic module resolution paths");
-        target_version: _,
+
-        static_search_paths,
+    let SearchPaths {
-        site_packages_paths,
+        static_paths,
-    } = module_resolution_settings(db);
+        site_packages,
+    } = Program::get(db).search_paths(db);
 
     let mut dynamic_paths = Vec::new();
 
-    if site_packages_paths.is_empty() {
+    if site_packages.is_empty() {
         return dynamic_paths;
     }
 
-    let mut existing_paths: FxHashSet<_> = static_search_paths
+    let mut existing_paths: FxHashSet<_> = static_paths
         .iter()
         .filter_map(|path| path.as_system_path())
         .map(Cow::Borrowed)
@@ -252,7 +268,7 @@ pub(crate) fn dynamic_resolution_paths(db: &dyn Db) -> Vec<SearchPath> {
     let files = db.files();
     let system = db.system();
 
-    for site_packages_search_path in site_packages_paths {
+    for site_packages_search_path in site_packages {
         let site_packages_dir = site_packages_search_path
             .as_system_path()
             .expect("Expected site package path to be a system path");
@@ -302,6 +318,10 @@ pub(crate) fn dynamic_resolution_paths(db: &dyn Db) -> Vec<SearchPath> {
             if existing_paths.insert(Cow::Owned(installation.clone())) {
                 match SearchPath::editable(system, installation) {
                     Ok(search_path) => {
+                        tracing::debug!(
+                            "Adding editable installation to module resolution path {path}",
+                            path = search_path.as_system_path().unwrap()
+                        );
                         dynamic_paths.push(search_path);
                     }
 
@@ -448,38 +468,6 @@ impl<'db> Iterator for PthFileIterator<'db> {
     }
 }
 
-/// Validated and normalized module-resolution settings.
-#[derive(Clone, Debug, PartialEq, Eq)]
-pub(crate) struct ModuleResolutionSettings {
-    target_version: PythonVersion,
-
-    /// 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.
-    static_search_paths: Vec<SearchPath>,
-
-    /// site-packages paths are not included in the above field:
-    /// if there are multiple site-packages paths, editable installations can appear
-    /// *between* the site-packages paths on `sys.path` at runtime.
-    /// That means we can't know where a second or third `site-packages` path should sit
-    /// in terms of module-resolution priority until we've discovered the editable installs
-    /// for the first `site-packages` path
-    site_packages_paths: Vec<SearchPath>,
-}
-
-impl ModuleResolutionSettings {
-    fn target_version(&self) -> PythonVersion {
-        self.target_version
-    }
-
-    pub(crate) fn search_paths<'db>(&'db self, db: &'db dyn Db) -> SearchPathIterator<'db> {
-        SearchPathIterator {
-            db,
-            static_paths: self.static_search_paths.iter(),
-            dynamic_paths: None,
-        }
-    }
-}
-
 /// A thin wrapper around `ModuleName` to make it a Salsa ingredient.
 ///
 /// This is needed because Salsa requires that all query arguments are salsa ingredients.
@@ -492,13 +480,13 @@ struct ModuleNameIngredient<'db> {
 /// Given a module name and a list of search paths in which to lookup modules,
 /// attempt to resolve the module name
 fn resolve_name(db: &dyn Db, name: &ModuleName) -> Option<(SearchPath, File, ModuleKind)> {
-    let resolver_settings = module_resolution_settings(db);
+    let program = Program::get(db);
-    let target_version = resolver_settings.target_version();
+    let target_version = program.target_version(db);
     let resolver_state = ResolverState::new(db, target_version);
     let is_builtin_module =
         ruff_python_stdlib::sys::is_builtin_module(target_version.minor, name.as_str());
 
-    for search_path in resolver_settings.search_paths(db) {
+    for search_path in search_paths(db) {
         // When a builtin module is imported, standard module resolution is bypassed:
         // the module name always resolves to the stdlib module,
         // even if there's a module of the same name in the first-party root
@@ -652,6 +640,8 @@ mod tests {
     use crate::module_name::ModuleName;
     use crate::module_resolver::module::ModuleKind;
     use crate::module_resolver::testing::{FileSpec, MockedTypeshed, TestCase, TestCaseBuilder};
+    use crate::ProgramSettings;
+    use crate::PythonVersion;
 
     use super::*;
 
@@ -1202,14 +1192,19 @@ mod tests {
         std::fs::write(foo.as_std_path(), "")?;
         std::os::unix::fs::symlink(foo.as_std_path(), bar.as_std_path())?;
 
-        let search_paths = SearchPathSettings {
+        Program::from_settings(
+            &db,
+            ProgramSettings {
+                target_version: PythonVersion::PY38,
+                search_paths: SearchPathSettings {
                     extra_paths: vec![],
                     src_root: src.clone(),
                     custom_typeshed: Some(custom_typeshed.clone()),
                     site_packages: vec![site_packages],
-        };
+                },
-
+            },
-        Program::new(&db, PythonVersion::PY38, search_paths);
+        )
+        .context("Invalid program settings")?;
 
         let foo_module = resolve_module(&db, ModuleName::new_static("foo").unwrap()).unwrap();
         let bar_module = resolve_module(&db, ModuleName::new_static("bar").unwrap()).unwrap();
@@ -1673,8 +1668,7 @@ not_a_directory
             .with_site_packages_files(&[("_foo.pth", "/src")])
             .build();
 
-        let search_paths: Vec<&SearchPath> =
+        let search_paths: Vec<&SearchPath> = search_paths(&db).collect();
-            module_resolution_settings(&db).search_paths(&db).collect();
 
         assert!(search_paths.contains(
             &&SearchPath::first_party(db.system(), SystemPathBuf::from("/src")).unwrap()
@@ -1703,16 +1697,19 @@ not_a_directory
         ])
         .unwrap();
 
-        Program::new(
+        Program::from_settings(
             &db,
-            PythonVersion::default(),
+            ProgramSettings {
-            SearchPathSettings {
+                target_version: PythonVersion::default(),
+                search_paths: SearchPathSettings {
                     extra_paths: vec![],
                     src_root: SystemPathBuf::from("/src"),
                     custom_typeshed: None,
                     site_packages: vec![venv_site_packages, system_site_packages],
                 },
-        );
+            },
+        )
+        .expect("Valid program settings");
 
         // The editable installs discovered from the `.pth` file in the first `site-packages` directory
         // take precedence over the second `site-packages` directory...

crates/red_knot_python_semantic/src/module_resolver/testing.rs

@@ -4,6 +4,7 @@ use ruff_db::vendored::VendoredPathBuf;
 use crate::db::tests::TestDb;
 use crate::program::{Program, SearchPathSettings};
 use crate::python_version::PythonVersion;
+use crate::ProgramSettings;
 
 /// A test case for the module resolver.
 ///
@@ -220,16 +221,19 @@ impl TestCaseBuilder<MockedTypeshed> {
         let src = Self::write_mock_directory(&mut db, "/src", first_party_files);
         let typeshed = Self::build_typeshed_mock(&mut db, &typeshed_option);
 
-        Program::new(
+        Program::from_settings(
             &db,
+            ProgramSettings {
                 target_version,
-            SearchPathSettings {
+                search_paths: SearchPathSettings {
                     extra_paths: vec![],
                     src_root: src.clone(),
                     custom_typeshed: Some(typeshed.clone()),
                     site_packages: vec![site_packages.clone()],
                 },
-        );
+            },
+        )
+        .expect("Valid program settings");
 
         TestCase {
             db,
@@ -273,16 +277,19 @@ impl TestCaseBuilder<VendoredTypeshed> {
             Self::write_mock_directory(&mut db, "/site-packages", site_packages_files);
         let src = Self::write_mock_directory(&mut db, "/src", first_party_files);
 
-        Program::new(
+        Program::from_settings(
             &db,
+            ProgramSettings {
                 target_version,
-            SearchPathSettings {
+                search_paths: SearchPathSettings {
                     extra_paths: vec![],
                     src_root: src.clone(),
                     custom_typeshed: None,
                     site_packages: vec![site_packages.clone()],
                 },
-        );
+            },
+        )
+        .expect("Valid search path settings");
 
         TestCase {
             db,

crates/red_knot_python_semantic/src/program.rs

@@ -1,21 +1,53 @@
 use crate::python_version::PythonVersion;
-use crate::Db;
+use anyhow::Context;
-use ruff_db::system::SystemPathBuf;
 use salsa::Durability;
+use salsa::Setter;
+
+use ruff_db::system::SystemPathBuf;
+
+use crate::module_resolver::SearchPaths;
+use crate::Db;
 
 #[salsa::input(singleton)]
 pub struct Program {
     pub target_version: PythonVersion,
 
+    #[default]
     #[return_ref]
-    pub search_paths: SearchPathSettings,
+    pub(crate) search_paths: SearchPaths,
 }
 
 impl Program {
-    pub fn from_settings(db: &dyn Db, settings: ProgramSettings) -> Self {
+    pub fn from_settings(db: &dyn Db, settings: ProgramSettings) -> anyhow::Result<Self> {
-        Program::builder(settings.target_version, settings.search_paths)
+        let ProgramSettings {
+            target_version,
+            search_paths,
+        } = settings;
+
+        tracing::info!("Target version: {target_version}");
+
+        let search_paths = SearchPaths::from_settings(db, search_paths)
+            .with_context(|| "Invalid search path settings")?;
+
+        Ok(Program::builder(settings.target_version)
             .durability(Durability::HIGH)
-            .new(db)
+            .search_paths(search_paths)
+            .new(db))
+    }
+
+    pub fn update_search_paths(
+        &self,
+        db: &mut dyn Db,
+        search_path_settings: SearchPathSettings,
+    ) -> anyhow::Result<()> {
+        let search_paths = SearchPaths::from_settings(db, search_path_settings)?;
+
+        if self.search_paths(db) != &search_paths {
+            tracing::debug!("Update search paths");
+            self.set_search_paths(db).to(search_paths);
+        }
+
+        Ok(())
     }
 }
 

crates/red_knot_python_semantic/src/semantic_model.rs

@@ -171,29 +171,32 @@ mod tests {
     use crate::program::{Program, SearchPathSettings};
     use crate::python_version::PythonVersion;
     use crate::types::Type;
-    use crate::{HasTy, SemanticModel};
+    use crate::{HasTy, ProgramSettings, SemanticModel};
 
-    fn setup_db() -> TestDb {
+    fn setup_db<'a>(files: impl IntoIterator<Item = (&'a str, &'a str)>) -> anyhow::Result<TestDb> {
-        let db = TestDb::new();
+        let mut db = TestDb::new();
-        Program::new(
+        db.write_files(files)?;
+
+        Program::from_settings(
             &db,
-            PythonVersion::default(),
+            ProgramSettings {
-            SearchPathSettings {
+                target_version: PythonVersion::default(),
+                search_paths: SearchPathSettings {
                     extra_paths: vec![],
                     src_root: SystemPathBuf::from("/src"),
                     site_packages: vec![],
                     custom_typeshed: None,
                 },
-        );
+            },
+        )?;
 
-        db
+        Ok(db)
     }
 
     #[test]
     fn function_ty() -> anyhow::Result<()> {
-        let mut db = setup_db();
+        let db = setup_db([("/src/foo.py", "def test(): pass")])?;
 
-        db.write_file("/src/foo.py", "def test(): pass")?;
         let foo = system_path_to_file(&db, "/src/foo.py").unwrap();
 
         let ast = parsed_module(&db, foo);
@@ -209,9 +212,8 @@ mod tests {
 
     #[test]
     fn class_ty() -> anyhow::Result<()> {
-        let mut db = setup_db();
+        let db = setup_db([("/src/foo.py", "class Test: pass")])?;
 
-        db.write_file("/src/foo.py", "class Test: pass")?;
         let foo = system_path_to_file(&db, "/src/foo.py").unwrap();
 
         let ast = parsed_module(&db, foo);
@@ -227,12 +229,11 @@ mod tests {
 
     #[test]
     fn alias_ty() -> anyhow::Result<()> {
-        let mut db = setup_db();
+        let db = setup_db([
-
-        db.write_files([
             ("/src/foo.py", "class Test: pass"),
             ("/src/bar.py", "from foo import Test"),
         ])?;
+
         let bar = system_path_to_file(&db, "/src/bar.py").unwrap();
 
         let ast = parsed_module(&db, bar);

crates/red_knot_python_semantic/src/types/infer.rs

@@ -1494,6 +1494,7 @@ impl<'db> TypeInferenceBuilder<'db> {
 
 #[cfg(test)]
 mod tests {
+    use anyhow::Context;
     use ruff_db::files::{system_path_to_file, File};
     use ruff_db::parsed::parsed_module;
     use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
@@ -1508,40 +1509,58 @@ mod tests {
     use crate::semantic_index::symbol::FileScopeId;
     use crate::semantic_index::{global_scope, semantic_index, symbol_table, use_def_map};
     use crate::types::{global_symbol_ty_by_name, infer_definition_types, symbol_ty_by_name, Type};
-    use crate::{HasTy, SemanticModel};
+    use crate::{HasTy, ProgramSettings, SemanticModel};
 
     fn setup_db() -> TestDb {
         let db = TestDb::new();
 
-        Program::new(
+        let src_root = SystemPathBuf::from("/src");
+        db.memory_file_system()
+            .create_directory_all(&src_root)
+            .unwrap();
+
+        Program::from_settings(
             &db,
-            PythonVersion::default(),
+            ProgramSettings {
-            SearchPathSettings {
+                target_version: PythonVersion::default(),
+                search_paths: SearchPathSettings {
                     extra_paths: Vec::new(),
-                src_root: SystemPathBuf::from("/src"),
+                    src_root,
                     site_packages: vec![],
                     custom_typeshed: None,
                 },
-        );
+            },
+        )
+        .expect("Valid search path settings");
 
         db
     }
 
-    fn setup_db_with_custom_typeshed(typeshed: &str) -> TestDb {
+    fn setup_db_with_custom_typeshed<'a>(
-        let db = TestDb::new();
+        typeshed: &str,
+        files: impl IntoIterator<Item = (&'a str, &'a str)>,
+    ) -> anyhow::Result<TestDb> {
+        let mut db = TestDb::new();
+        let src_root = SystemPathBuf::from("/src");
 
-        Program::new(
+        db.write_files(files)
+            .context("Failed to write test files")?;
+
+        Program::from_settings(
             &db,
-            PythonVersion::default(),
+            ProgramSettings {
-            SearchPathSettings {
+                target_version: PythonVersion::default(),
+                search_paths: SearchPathSettings {
                     extra_paths: Vec::new(),
-                src_root: SystemPathBuf::from("/src"),
+                    src_root,
                     site_packages: vec![],
                     custom_typeshed: Some(SystemPathBuf::from(typeshed)),
                 },
-        );
+            },
+        )
+        .context("Failed to create Program")?;
 
-        db
+        Ok(db)
     }
 
     fn assert_public_ty(db: &TestDb, file_name: &str, symbol_name: &str, expected: &str) {
@@ -2131,16 +2150,17 @@ mod tests {
 
     #[test]
     fn builtin_symbol_custom_stdlib() -> anyhow::Result<()> {
-        let mut db = setup_db_with_custom_typeshed("/typeshed");
+        let db = setup_db_with_custom_typeshed(
-
+            "/typeshed",
-        db.write_files([
+            [
                 ("/src/a.py", "c = copyright"),
                 (
                     "/typeshed/stdlib/builtins.pyi",
                     "def copyright() -> None: ...",
                 ),
                 ("/typeshed/stdlib/VERSIONS", "builtins: 3.8-"),
-        ])?;
+            ],
+        )?;
 
         assert_public_ty(&db, "/src/a.py", "c", "Literal[copyright]");
 
@@ -2160,13 +2180,14 @@ mod tests {
 
     #[test]
     fn unknown_builtin_later_defined() -> anyhow::Result<()> {
-        let mut db = setup_db_with_custom_typeshed("/typeshed");
+        let db = setup_db_with_custom_typeshed(
-
+            "/typeshed",
-        db.write_files([
+            [
                 ("/src/a.py", "x = foo"),
                 ("/typeshed/stdlib/builtins.pyi", "foo = bar; bar = 1"),
                 ("/typeshed/stdlib/VERSIONS", "builtins: 3.8-"),
-        ])?;
+            ],
+        )?;
 
         assert_public_ty(&db, "/src/a.py", "x", "Unbound");
 

crates/red_knot_server/src/session.rs

@@ -78,7 +78,8 @@ impl Session {
                     custom_typeshed: None,
                 },
             };
-            workspaces.insert(path, RootDatabase::new(metadata, program_settings, system));
+            // TODO(micha): Handle the case where the program settings are incorrect more gracefully.
+            workspaces.insert(path, RootDatabase::new(metadata, program_settings, system)?);
         }
 
         Ok(Self {

crates/red_knot_wasm/src/lib.rs

@@ -49,7 +49,8 @@ impl Workspace {
             search_paths: SearchPathSettings::default(),
         };
 
-        let db = RootDatabase::new(workspace, program_settings, system.clone());
+        let db =
+            RootDatabase::new(workspace, program_settings, system.clone()).map_err(into_error)?;
 
         Ok(Self { db, system })
     }

crates/red_knot_workspace/src/db.rs

@@ -28,7 +28,11 @@ pub struct RootDatabase {
 }
 
 impl RootDatabase {
-    pub fn new<S>(workspace: WorkspaceMetadata, settings: ProgramSettings, system: S) -> Self
+    pub fn new<S>(
+        workspace: WorkspaceMetadata,
+        settings: ProgramSettings,
+        system: S,
+    ) -> anyhow::Result<Self>
     where
         S: System + 'static + Send + Sync + RefUnwindSafe,
     {
@@ -41,10 +45,10 @@ impl RootDatabase {
 
         let workspace = Workspace::from_metadata(&db, workspace);
         // Initialize the `Program` singleton
-        Program::from_settings(&db, settings);
+        Program::from_settings(&db, settings)?;
 
         db.workspace = Some(workspace);
-        db
+        Ok(db)
     }
 
     pub fn workspace(&self) -> Workspace {

crates/red_knot_workspace/src/lint.rs

@@ -305,7 +305,7 @@ enum AnyImportRef<'a> {
 
 #[cfg(test)]
 mod tests {
-    use red_knot_python_semantic::{Program, PythonVersion, SearchPathSettings};
+    use red_knot_python_semantic::{Program, ProgramSettings, PythonVersion, SearchPathSettings};
     use ruff_db::files::system_path_to_file;
     use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
 
@@ -320,16 +320,23 @@ mod tests {
     fn setup_db_with_root(src_root: SystemPathBuf) -> TestDb {
         let db = TestDb::new();
 
-        Program::new(
+        db.memory_file_system()
+            .create_directory_all(&src_root)
+            .unwrap();
+
+        Program::from_settings(
             &db,
-            PythonVersion::default(),
+            ProgramSettings {
-            SearchPathSettings {
+                target_version: PythonVersion::default(),
+                search_paths: SearchPathSettings {
                     extra_paths: Vec::new(),
                     src_root,
                     site_packages: vec![],
                     custom_typeshed: None,
                 },
-        );
+            },
+        )
+        .expect("Valid program settings");
 
         db
     }

crates/red_knot_workspace/tests/check.rs

@@ -20,8 +20,7 @@ fn setup_db(workspace_root: SystemPathBuf) -> anyhow::Result<RootDatabase> {
         target_version: PythonVersion::default(),
         search_paths,
     };
-    let db = RootDatabase::new(workspace, settings, system);
+    RootDatabase::new(workspace, settings, system)
-    Ok(db)
 }
 
 /// Test that all snippets in testcorpus can be checked without panic

crates/ruff_benchmark/benches/red_knot.rs

@@ -52,7 +52,7 @@ fn setup_case() -> Case {
         },
     };
 
-    let mut db = RootDatabase::new(metadata, settings, system);
+    let mut db = RootDatabase::new(metadata, settings, system).unwrap();
     let parser = system_path_to_file(&db, parser_path).unwrap();
 
     db.workspace().open_file(&mut db, parser);