Allow caching for --fix

crates/ruff_cli/src/diagnostics.rs

@@ -140,6 +140,10 @@ impl FixMap {
         self.0.is_empty()
     }
 
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
     /// Returns an iterator over the fixes in the map, along with the file path.
     pub(crate) fn iter(&self) -> impl Iterator<Item = (&String, &FixTable)> {
         self.0.iter()
@@ -188,13 +192,8 @@ pub(crate) fn lint_path(
     unsafe_fixes: UnsafeFixes,
 ) -> Result<Diagnostics> {
     // Check the cache.
-    // TODO(charlie): `fixer::Mode::Apply` and `fixer::Mode::Diff` both have
-    // side-effects that aren't captured in the cache. (In practice, it's fine
-    // to cache `fixer::Mode::Apply`, since a file either has no fixes, or we'll
-    // write the fixes to disk, thus invalidating the cache. But it's a bit hard
-    // to reason about. We need to come up with a better solution here.)
     let caching = match cache {
-        Some(cache) if noqa.into() && fix_mode.is_generate() => {
+        Some(cache) if noqa.into() => {
             let relative_path = cache
                 .relative_path(path)
                 .expect("wrong package cache for file");
@@ -204,7 +203,14 @@ pub(crate) fn lint_path(
                 .get(relative_path, &cache_key)
                 .and_then(|entry| entry.to_diagnostics(path));
             if let Some(diagnostics) = cached_diagnostics {
-                return Ok(diagnostics);
+                if match fix_mode {
+                    flags::FixMode::Generate => true,
+                    flags::FixMode::Apply | flags::FixMode::Diff => {
+                        diagnostics.messages.is_empty() && diagnostics.fixed.is_empty()
+                    }
+                } {
+                    return Ok(diagnostics);
+                }
             }
 
             // Stash the file metadata for later so when we update the cache it reflects the prerun
@@ -304,15 +310,22 @@ pub(crate) fn lint_path(
     if let Some((cache, relative_path, key)) = caching {
         // We don't cache parsing errors.
         if parse_error.is_none() {
-            cache.update_lint(
-                relative_path.to_owned(),
-                &key,
-                LintCacheData::from_messages(
-                    &messages,
-                    imports.clone(),
-                    source_kind.as_ipy_notebook().map(Notebook::index).cloned(),
-                ),
-            );
+            if match fix_mode {
+                flags::FixMode::Generate => true,
+                flags::FixMode::Apply | flags::FixMode::Diff => {
+                    messages.is_empty() && fixed.is_empty()
+                }
+            } {
+                cache.update_lint(
+                    relative_path.to_owned(),
+                    &key,
+                    LintCacheData::from_messages(
+                        &messages,
+                        imports.clone(),
+                        source_kind.as_ipy_notebook().map(Notebook::index).cloned(),
+                    ),
+                );
+            }
         }
     }