Change EnvironmentOptions::venv-path to Option<SystemPathBuf> (#15631)

## Summary

The `Options` struct is intended to capture the user's configuration
options but
`EnvironmentOptions::venv_path` supports both a `SitePackages::Known`
and `SitePackages::Derived`.

Users should only be able to provide `SitePackages::Derived`—they
specify a path to a venv, and Red Knot derives the path to the
site-packages directory. We'll only use the `Known` variant once we
automatically discover the Python installation.

That's why this PR changes `EnvironmentOptions::venv_path` from
`Option<SitePackages>` to `Option<SystemPathBuf>`.

This requires making some changes to the file watcher test, and I
decided to use `extra_paths` over venv path
because our venv validation is annoyingly correct -- making mocking a
venv rather involved.

## Test Plan

`cargo test`

crates/red_knot/src/main.rs

@@ -10,7 +10,6 @@ use red_knot_project::metadata::options::{EnvironmentOptions, Options};
 use red_knot_project::watch;
 use red_knot_project::watch::ProjectWatcher;
 use red_knot_project::{ProjectDatabase, ProjectMetadata};
-use red_knot_python_semantic::SitePackages;
 use red_knot_server::run_server;
 use ruff_db::diagnostic::Diagnostic;
 use ruff_db::system::{OsSystem, System, SystemPath, SystemPathBuf};
@@ -77,9 +76,7 @@ impl Args {
                 venv_path: self
                     .venv_path
                     .as_ref()
-                    .map(|venv_path| SitePackages::Derived {
-                        venv_path: SystemPath::absolute(venv_path, cli_cwd),
-                    }),
+                    .map(|venv_path| SystemPath::absolute(venv_path, cli_cwd)),
                 typeshed: self
                     .typeshed
                     .as_ref()

crates/red_knot/tests/file_watching.rs

@@ -8,9 +8,7 @@ use red_knot_project::metadata::options::{EnvironmentOptions, Options};
 use red_knot_project::metadata::pyproject::{PyProject, Tool};
 use red_knot_project::watch::{directory_watcher, ChangeEvent, ProjectWatcher};
 use red_knot_project::{Db, ProjectDatabase, ProjectMetadata};
-use red_knot_python_semantic::{
-    resolve_module, ModuleName, PythonPlatform, PythonVersion, SitePackages,
-};
+use red_knot_python_semantic::{resolve_module, ModuleName, PythonPlatform, PythonVersion};
 use ruff_db::files::{system_path_to_file, File, FileError};
 use ruff_db::source::source_text;
 use ruff_db::system::{OsSystem, SystemPath, SystemPathBuf};
@@ -324,7 +322,6 @@ where
         .extra_paths
         .iter()
         .chain(program_settings.search_paths.typeshed.as_ref())
-        .chain(program_settings.search_paths.site_packages.paths())
     {
         std::fs::create_dir_all(path.as_std_path())
             .with_context(|| format!("Failed to create search path `{path}`"))?;
@@ -794,7 +791,7 @@ fn search_path() -> anyhow::Result<()> {
     let mut case = setup_with_options([("bar.py", "import sub.a")], |root_path, _project_path| {
         Some(Options {
             environment: Some(EnvironmentOptions {
-                venv_path: Some(SitePackages::Known(vec![root_path.join("site_packages")])),
+                extra_paths: Some(vec![root_path.join("site_packages")]),
                 ..EnvironmentOptions::default()
             }),
             ..Options::default()
@@ -835,7 +832,7 @@ fn add_search_path() -> anyhow::Result<()> {
     // Register site-packages as a search path.
     case.update_options(Options {
         environment: Some(EnvironmentOptions {
-            venv_path: Some(SitePackages::Known(vec![site_packages.clone()])),
+            extra_paths: Some(vec![site_packages.clone()]),
             ..EnvironmentOptions::default()
         }),
         ..Options::default()
@@ -858,7 +855,7 @@ fn remove_search_path() -> anyhow::Result<()> {
     let mut case = setup_with_options([("bar.py", "import sub.a")], |root_path, _project_path| {
         Some(Options {
             environment: Some(EnvironmentOptions {
-                venv_path: Some(SitePackages::Known(vec![root_path.join("site_packages")])),
+                extra_paths: Some(vec![root_path.join("site_packages")]),
                 ..EnvironmentOptions::default()
             }),
             ..Options::default()
@@ -1381,9 +1378,8 @@ mod unix {
             |_root, project| {
                 Some(Options {
                     environment: Some(EnvironmentOptions {
-                        venv_path: Some(SitePackages::Known(vec![
-                            project.join(".venv/lib/python3.12/site-packages")
-                        ])),
+                        extra_paths: Some(vec![project.join(".venv/lib/python3.12/site-packages")]),
+                        python_version: Some(PythonVersion::PY312),
                         ..EnvironmentOptions::default()
                     }),
                     ..Options::default()

crates/red_knot_project/src/metadata/options.rs

@@ -74,7 +74,9 @@ impl Options {
             extra_paths: extra_paths.unwrap_or_default(),
             src_roots,
             typeshed,
-            site_packages: python.unwrap_or(SitePackages::Known(vec![])),
+            site_packages: python
+                .map(|venv_path| SitePackages::Derived { venv_path })
+                .unwrap_or(SitePackages::Known(vec![])),
         }
     }
 }
@@ -98,7 +100,7 @@ pub struct EnvironmentOptions {
 
     // TODO: Rename to python, see https://github.com/astral-sh/ruff/issues/15530
     /// The path to the user's `site-packages` directory, where third-party packages from ``PyPI`` are installed.
-    pub venv_path: Option<SitePackages>,
+    pub venv_path: Option<SystemPathBuf>,
 }
 
 #[derive(Debug, Default, Clone, Eq, PartialEq, Combine, Serialize, Deserialize)]

crates/red_knot_python_semantic/src/module_resolver/resolver.rs

@@ -222,8 +222,14 @@ impl SearchPaths {
         static_paths.push(stdlib_path);
 
         let site_packages_paths = match site_packages_paths {
-            SitePackages::Derived { venv_path } => VirtualEnvironment::new(venv_path, system)
-                .and_then(|venv| venv.site_packages_directories(system))?,
+            SitePackages::Derived { venv_path } => {
+                // TODO: We may want to warn here if the venv's python version is older
+                //  than the one resolved in the program settings because it indicates
+                //  that the `target-version` is incorrectly configured or that the
+                //  venv is out of date.
+                VirtualEnvironment::new(venv_path, system)
+                    .and_then(|venv| venv.site_packages_directories(system))?
+            }
             SitePackages::Known(paths) => paths
                 .iter()
                 .map(|path| canonicalize(path, system))

crates/red_knot_python_semantic/src/program.rs

@@ -134,12 +134,3 @@ pub enum SitePackages {
     /// Resolved site packages paths
     Known(Vec<SystemPathBuf>),
 }
-
-impl SitePackages {
-    pub fn paths(&self) -> &[SystemPathBuf] {
-        match self {
-            SitePackages::Derived { venv_path } => std::slice::from_ref(venv_path),
-            SitePackages::Known(paths) => paths,
-        }
-    }
-}