[ty] Support ephemeral uv virtual environments (#18335)

Author: AlexWaygood

crates/ruff_db/src/system/path.rs

@@ -596,6 +596,13 @@ impl AsRef<SystemPath> for Utf8PathBuf {
     }
 }
 
+impl AsRef<SystemPath> for camino::Utf8Component<'_> {
+    #[inline]
+    fn as_ref(&self) -> &SystemPath {
+        SystemPath::new(self.as_str())
+    }
+}
+
 impl AsRef<SystemPath> for str {
     #[inline]
     fn as_ref(&self) -> &SystemPath {
@@ -626,6 +633,22 @@ impl Deref for SystemPathBuf {
     }
 }
 
+impl<P: AsRef<SystemPath>> FromIterator<P> for SystemPathBuf {
+    fn from_iter<I: IntoIterator<Item = P>>(iter: I) -> Self {
+        let mut buf = SystemPathBuf::new();
+        buf.extend(iter);
+        buf
+    }
+}
+
+impl<P: AsRef<SystemPath>> Extend<P> for SystemPathBuf {
+    fn extend<I: IntoIterator<Item = P>>(&mut self, iter: I) {
+        for path in iter {
+            self.push(path);
+        }
+    }
+}
+
 impl std::fmt::Debug for SystemPath {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         self.0.fmt(f)

crates/ty_python_semantic/resources/mdtest/import/site_packages_discovery.md

@@ -0,0 +1,59 @@
+# Tests for `site-packages` discovery
+
+## Ephemeral uv environments
+
+If you use the `--with` flag when invoking `uv run`, uv will create an "ephemeral" virtual
+environment that is layered on top of the pre-existing environment. `site-packages` directories from
+the pre-existing environment will be added as an import search path at runtime as well as the
+`site-packages` directory from the ephemeral environment. The `VIRTUAL_ENV` environment variable
+will only point to the ephemeral virtual environment, but, following uv commit
+`7bba3d00d4ad1fb3daba86b98eb25d8d9e9836ae`, uv writes the `sys.prefix` path of the parent
+environment to an `extends-environment` key in the ephemeral environment's `pyvenv.cfg` file.
+
+This test ensures that we are able to resolve imports that point to packages in either
+`site-packages` directory (the one of the ephemeral environment or the one of the parent
+environment) if we detect that an ephemeral uv environment has been activated.
+
+```toml
+[environment]
+python = "/.venv"
+```
+
+`/.venv/pyvenv.cfg`:
+
+```cfg
+home = /doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin
+implementation = CPython
+uv = 0.7.6
+version_info = 3.13.2
+include-system-site-packages = false
+prompt = ruff
+extends-environment = /.other-environment
+```
+
+`/doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin/python`:
+
+```text
+```
+
+`/.venv/<path-to-site-packages>/foo.py`:
+
+```py
+X: int = 42
+```
+
+`/.other-environment/<path-to-site-packages>/bar.py`:
+
+```py
+Y: "str" = "Y"
+```
+
+`/src/main.py`:
+
+```py
+from foo import X
+from bar import Y
+
+reveal_type(X)  # revealed: int
+reveal_type(Y)  # revealed: str
+```

crates/ty_python_semantic/src/module_resolver/resolver.rs

@@ -14,7 +14,9 @@ use ruff_python_ast::PythonVersion;
 use crate::db::Db;
 use crate::module_name::ModuleName;
 use crate::module_resolver::typeshed::{TypeshedVersions, vendored_typeshed_versions};
-use crate::site_packages::{PythonEnvironment, SitePackagesDiscoveryError, SysPrefixPathOrigin};
+use crate::site_packages::{
+    PythonEnvironment, SitePackagesDiscoveryError, SitePackagesPaths, SysPrefixPathOrigin,
+};
 use crate::{Program, PythonPath, SearchPathSettings};
 
 use super::module::{Module, ModuleKind};
@@ -289,7 +291,7 @@ impl SearchPaths {
                             virtual_env_path,
                             error
                         );
-                        vec![]
+                        SitePackagesPaths::default()
                     };
 
                     match PythonEnvironment::new(
@@ -304,7 +306,7 @@ impl SearchPaths {
                     }
                 } else {
                     tracing::debug!("No virtual environment found");
-                    vec![]
+                    SitePackagesPaths::default()
                 }
             }