[ty] Add tests to src.root if it exists and is not a package (#18286)

crates/ty/docs/configuration.md

@@ -172,6 +172,9 @@ If left unspecified, ty will try to detect common project layouts and initialize
 * if a `./<project-name>/<project-name>` directory exists, include `.` and `./<project-name>` in the first party search path
 * otherwise, default to `.` (flat layout)
 
+Besides, if a `./tests` directory exists and is not a package (i.e. it does not contain an `__init__.py` file),
+it will also be included in the first party search path.
+
 **Default value**: `null`
 
 **Type**: `str`

crates/ty_project/src/metadata.rs

@@ -1029,6 +1029,52 @@ expected `.`, `]`
         Ok(())
     }
 
+    #[test]
+    fn src_root_with_tests() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        // pytest will find `tests/test_foo.py` and realize it is NOT part of a package
+        // given that there's no `__init__.py` file in the same folder.
+        // It will then add `tests` to `sys.path`
+        // in order to import `test_foo.py` as the module `test_foo`.
+        system
+            .memory_file_system()
+            .write_files_all([
+                (root.join("src/main.py"), ""),
+                (root.join("tests/conftest.py"), ""),
+                (root.join("tests/test_foo.py"), ""),
+            ])
+            .context("Failed to write files")?;
+
+        let metadata = ProjectMetadata::discover(&root, &system)?;
+        let settings = metadata
+            .options
+            .to_program_settings(&root, "my_package", &system);
+
+        assert_eq!(
+            settings.search_paths.src_roots,
+            vec![root.clone(), root.join("src"), root.join("tests")]
+        );
+
+        // If `tests/__init__.py` is present, it is considered a package and `tests` is not added to `sys.path`.
+        system
+            .memory_file_system()
+            .write_file(root.join("tests/__init__.py"), "")
+            .context("Failed to write tests/__init__.py")?;
+        let metadata = ProjectMetadata::discover(&root, &system)?;
+        let settings = metadata
+            .options
+            .to_program_settings(&root, "my_package", &system);
+
+        assert_eq!(
+            settings.search_paths.src_roots,
+            vec![root.clone(), root.join("src")]
+        );
+
+        Ok(())
+    }
+
     #[track_caller]
     fn assert_error_eq(error: &ProjectDiscoveryError, message: &str) {
         assert_eq!(error.to_string().replace('\\', "/"), message);

crates/ty_project/src/metadata/options.rs

@@ -135,7 +135,7 @@ impl Options {
         } else {
             let src = project_root.join("src");
 
-            if system.is_directory(&src) {
+            let mut roots = if system.is_directory(&src) {
                 // Default to `src` and the project root if `src` exists and the root hasn't been specified.
                 // This corresponds to the `src-layout`
                 tracing::debug!(
@@ -154,7 +154,24 @@ impl Options {
                 // Default to a [flat project structure](https://packaging.python.org/en/latest/discussions/src-layout-vs-flat-layout/).
                 tracing::debug!("Defaulting `src.root` to `.`");
                 vec![project_root.to_path_buf()]
+            };
+
+            // Considering pytest test discovery conventions,
+            // we also include the `tests` directory if it exists and is not a package.
+            let tests_dir = project_root.join("tests");
+            if system.is_directory(&tests_dir)
+                && !system.is_file(&tests_dir.join("__init__.py"))
+                && !roots.contains(&tests_dir)
+            {
+                // If the `tests` directory exists and is not a package, include it as a source root.
+                tracing::debug!(
+                    "Including `./tests` in `src.root` because a `./tests` directory exists"
+                );
+
+                roots.push(tests_dir);
             }
+
+            roots
         };
 
         let (extra_paths, python, typeshed) = self
@@ -392,6 +409,9 @@ pub struct SrcOptions {
     /// * if a `./src` directory exists, include `.` and `./src` in the first party search path (src layout or flat)
     /// * if a `./<project-name>/<project-name>` directory exists, include `.` and `./<project-name>` in the first party search path
     /// * otherwise, default to `.` (flat layout)
+    ///
+    /// Besides, if a `./tests` directory exists and is not a package (i.e. it does not contain an `__init__.py` file),
+    /// it will also be included in the first party search path.
     #[serde(skip_serializing_if = "Option::is_none")]
     #[option(
         default = r#"null"#,