Always use base interpreter for cached environments (#4805)

Closes #4801.

crates/uv-python/src/interpreter.rs

@@ -140,6 +140,25 @@ impl Interpreter {
         }
     }
 
+    /// Return the [`Interpreter`] for the base executable, if it's available.
+    ///
+    /// If no such base executable is available, or if the base executable is the same as the
+    /// current executable, this method returns `None`.
+    pub fn to_base_interpreter(&self, cache: &Cache) -> Result<Option<Self>, Error> {
+        if let Some(base_executable) = self
+            .sys_base_executable()
+            .filter(|base_executable| *base_executable != self.sys_executable())
+        {
+            match Self::query(base_executable, cache) {
+                Ok(base_interpreter) => Ok(Some(base_interpreter)),
+                Err(Error::NotFound(_)) => Ok(None),
+                Err(err) => Err(err),
+            }
+        } else {
+            Ok(None)
+        }
+    }
+
     /// Returns the path to the Python virtual environment.
     #[inline]
     pub fn platform(&self) -> &Platform {

crates/uv/src/commands/project/environment.rs

@@ -42,6 +42,22 @@ impl CachedEnvironment {
     ) -> anyhow::Result<Self> {
         let spec = RequirementsSpecification::from_requirements(requirements);
 
+        // When caching, always use the base interpreter, rather than that of the virtual
+        // environment.
+        let interpreter = if let Some(interpreter) = interpreter.to_base_interpreter(cache)? {
+            debug!(
+                "Caching via base interpreter: `{}`",
+                interpreter.sys_executable().display()
+            );
+            interpreter
+        } else {
+            debug!(
+                "Caching via interpreter: `{}`",
+                interpreter.sys_executable().display()
+            );
+            interpreter
+        };
+
         // Resolve the requirements with the interpreter.
         let resolution = resolve_environment(
             &interpreter,
@@ -60,11 +76,13 @@ impl CachedEnvironment {
         // Hash the resolution by hashing the generated lockfile.
         // TODO(charlie): If the resolution contains any mutable metadata (like a path or URL
         // dependency), skip this step.
-        let lock = Lock::from_resolution_graph(&resolution)?;
+        let resolution_hash = digest(
-        let toml = lock.to_toml()?;
+            &Lock::from_resolution_graph(&resolution)?
-        let resolution_hash = digest(&toml);
+                .to_toml()?
+                .as_bytes(),
+        );
 
-        // Hash the interpreter by hashing the sysconfig data.
+        // Hash the interpreter based on its path.
         // TODO(charlie): Come up with a robust hash for the interpreter.
         let interpreter_hash = digest(&interpreter.sys_executable());