[ty] Test "list modules" versus "resolve module" in every mdtest

This ensures there is some level of consistency between the APIs.

This did require exposing a couple more things on `Module` for good
error messages. This also motivated a switch to an interned struct
instead of a tracked struct. This ensures that `list_modules` and
`resolve_modules` reuse the same `Module` values when the inputs are the
same.

Ref https://github.com/astral-sh/ruff/pull/19883#discussion_r2272520194

crates/ty_python_semantic/src/module_resolver/mod.rs

@@ -3,7 +3,7 @@ use std::iter::FusedIterator;
 pub use list::list_modules;
 pub(crate) use module::KnownModule;
 pub use module::Module;
-pub use path::SearchPathValidationError;
+pub use path::{SearchPath, SearchPathValidationError};
 pub use resolver::SearchPaths;
 pub(crate) use resolver::file_to_module;
 pub use resolver::{resolve_module, resolve_real_module};

crates/ty_python_semantic/src/module_resolver/module.rs

@@ -59,7 +59,7 @@ impl<'db> Module<'db> {
     }
 
     /// Is this a module that we special-case somehow? If so, which one?
-    pub(crate) fn known(self, db: &'db dyn Database) -> Option<KnownModule> {
+    pub fn known(self, db: &'db dyn Database) -> Option<KnownModule> {
         match self {
             Module::File(module) => module.known(db),
             Module::Namespace(_) => None,
@@ -75,7 +75,7 @@ impl<'db> Module<'db> {
     ///
     /// It is guaranteed that if `None` is returned, then this is a namespace
     /// package. Otherwise, this is a regular package or file module.
-    pub(crate) fn search_path(self, db: &'db dyn Database) -> Option<&'db SearchPath> {
+    pub fn search_path(self, db: &'db dyn Database) -> Option<&'db SearchPath> {
         match self {
             Module::File(module) => Some(module.search_path(db)),
             Module::Namespace(_) => None,
@@ -214,7 +214,7 @@ fn all_submodule_names_for_package(db: &dyn Db, file: File) -> Option<Vec<Name>>
 }
 
 /// A module that resolves to a file (`lib.py` or `package/__init__.py`)
-#[salsa::tracked(debug, heap_size=ruff_memory_usage::heap_size)]
+#[salsa::interned(debug, heap_size=ruff_memory_usage::heap_size)]
 pub struct FileModule<'db> {
     #[returns(ref)]
     pub(super) name: ModuleName,
@@ -229,7 +229,7 @@ pub struct FileModule<'db> {
 ///
 /// Namespace packages are special because there are
 /// multiple possible paths and they have no corresponding code file.
-#[salsa::tracked(debug, heap_size=ruff_memory_usage::heap_size)]
+#[salsa::interned(debug, heap_size=ruff_memory_usage::heap_size)]
 pub struct NamespacePackage<'db> {
     #[returns(ref)]
     pub(super) name: ModuleName,

crates/ty_test/src/lib.rs

@@ -18,8 +18,9 @@ use std::fmt::Write;
 use ty_python_semantic::pull_types::pull_types;
 use ty_python_semantic::types::check_types;
 use ty_python_semantic::{
-    Program, ProgramSettings, PythonEnvironment, PythonPlatform, PythonVersionSource,
-    PythonVersionWithSource, SearchPathSettings, SysPrefixPathOrigin,
+    Module, Program, ProgramSettings, PythonEnvironment, PythonPlatform, PythonVersionSource,
+    PythonVersionWithSource, SearchPath, SearchPathSettings, SysPrefixPathOrigin, list_modules,
+    resolve_module,
 };
 
 mod assertion;
@@ -68,13 +69,16 @@ pub fn run(
             Log::Filter(filter) => setup_logging_with_filter(filter),
         });
 
-        if let Err(failures) = run_test(&mut db, relative_fixture_path, snapshot_path, &test) {
-            any_failures = true;
+        let failures = run_test(&mut db, relative_fixture_path, snapshot_path, &test);
+        let inconsistencies = run_module_resolution_consistency_test(&db);
+        let this_test_failed = failures.is_err() || inconsistencies.is_err();
+        any_failures = any_failures || this_test_failed;
 
-            if output_format.is_cli() {
+        if this_test_failed && output_format.is_cli() {
             println!("\n{}\n", test.name().bold().underline());
         }
 
+        if let Err(failures) = run_test(&mut db, relative_fixture_path, snapshot_path, &test) {
             let md_index = LineIndex::from_source_text(&source);
 
             for test_failures in failures {
@@ -100,10 +104,24 @@ pub fn run(
                     }
                 }
             }
+        }
+        if let Err(inconsistencies) = run_module_resolution_consistency_test(&db) {
+            any_failures = true;
+            for inconsistency in inconsistencies {
+                match output_format {
+                    OutputFormat::Cli => {
+                        let info = relative_fixture_path.to_string().cyan();
+                        println!("  {info} {inconsistency}");
+                    }
+                    OutputFormat::GitHub => {
+                        println!("::error file={absolute_fixture_path}::{inconsistency}");
+                    }
+                }
+            }
+        }
 
+        if this_test_failed && output_format.is_cli() {
             let escaped_test_name = test.name().replace('\'', "\\'");
-
-            if output_format.is_cli() {
             println!(
                 "\nTo rerun this specific test, set the environment variable: {}='{escaped_test_name}'",
                 EnvVars::MDTEST_TEST_FILTER,
@@ -114,7 +132,6 @@ pub fn run(
             );
         }
     }
-    }
 
     println!("\n{}\n", "-".repeat(50));
 
@@ -406,6 +423,92 @@ fn run_test(
     }
 }
 
+/// Reports an inconsistency between "list modules" and "resolve module."
+///
+/// Values of this type are only constructed when `from_list` and
+/// `from_resolve` are not equivalent.
+struct ModuleInconsistency<'db> {
+    db: &'db db::Db,
+    /// The module returned from `list_module`.
+    from_list: Module<'db>,
+    /// The module returned, if any, from `resolve_module`.
+    from_resolve: Option<Module<'db>>,
+}
+
+/// Tests that "list modules" is consistent with "resolve module."
+///
+/// This only checks that everything returned by `list_module` is the
+/// identical module we get back from `resolve_module`. It does not
+/// check that all possible outputs of `resolve_module` are captured by
+/// `list_module`.
+fn run_module_resolution_consistency_test(db: &db::Db) -> Result<(), Vec<ModuleInconsistency<'_>>> {
+    let mut errs = vec![];
+    for from_list in list_modules(db) {
+        errs.push(match resolve_module(db, from_list.name(db)) {
+            None => ModuleInconsistency {
+                db,
+                from_list,
+                from_resolve: None,
+            },
+            Some(from_resolve) if from_list != from_resolve => ModuleInconsistency {
+                db,
+                from_list,
+                from_resolve: Some(from_resolve),
+            },
+            _ => continue,
+        });
+    }
+    if errs.is_empty() { Ok(()) } else { Err(errs) }
+}
+
+impl std::fmt::Display for ModuleInconsistency<'_> {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        fn fmt_module(
+            db: &db::Db,
+            f: &mut std::fmt::Formatter,
+            module: &Module<'_>,
+        ) -> std::fmt::Result {
+            let name = module.name(db);
+            let path = module
+                .file(db)
+                .map(|file| file.path(db).to_string())
+                .unwrap_or_else(|| "N/A".to_string());
+            let search_path = module
+                .search_path(db)
+                .map(SearchPath::to_string)
+                .unwrap_or_else(|| "N/A".to_string());
+            let known = module
+                .known(db)
+                .map(|known| known.to_string())
+                .unwrap_or_else(|| "N/A".to_string());
+            write!(
+                f,
+                "Module(\
+                   name={name}, \
+                   file={path}, \
+                   kind={kind:?}, \
+                   search_path={search_path}, \
+                   known={known}\
+                 )",
+                kind = module.kind(db),
+            )
+        }
+        write!(f, "Found ")?;
+        fmt_module(self.db, f, &self.from_list)?;
+        match self.from_resolve {
+            None => write!(
+                f,
+                " when listing modules, but `resolve_module` returned `None`",
+            )?,
+            Some(ref got) => {
+                write!(f, " when listing modules, but `resolve_module` returned ",)?;
+                fmt_module(self.db, f, got)?;
+            }
+        }
+        Ok(())
+    }
+}
+
 type Failures = Vec<FileFailures>;
 
 /// The failures for a single file in a test by line number.