Test builtin derives expansions

Via a hack to disable their fast path.

crates/hir-def/src/macro_expansion_tests/mod.rs

@@ -53,6 +53,8 @@ use crate::{
 
 #[track_caller]
 fn check_errors(#[rust_analyzer::rust_fixture] ra_fixture: &str, expect: Expect) {
+    crate::nameres::ENABLE_BUILTIN_DERIVE_FAST_PATH.set(false);
+
     let db = TestDB::with_files(ra_fixture);
     let krate = db.fetch_test_crate();
     let def_map = crate_def_map(&db, krate);
@@ -80,10 +82,15 @@ fn check_errors(#[rust_analyzer::rust_fixture] ra_fixture: &str, expect: Expect)
         .sorted_unstable_by_key(|(range, _)| range.start())
         .format_with("\n", |(range, err), format| format(&format_args!("{range:?}: {err}")))
         .to_string();
+
+    crate::nameres::ENABLE_BUILTIN_DERIVE_FAST_PATH.set(true);
+
     expect.assert_eq(&errors);
 }
 
 fn check(#[rust_analyzer::rust_fixture] ra_fixture: &str, mut expect: Expect) {
+    crate::nameres::ENABLE_BUILTIN_DERIVE_FAST_PATH.set(false);
+
     let extra_proc_macros = vec![(
         r#"
 #[proc_macro_attribute]
@@ -246,6 +253,8 @@ pub fn identity_when_valid(_attr: TokenStream, item: TokenStream) -> TokenStream
         }
     }
 
+    crate::nameres::ENABLE_BUILTIN_DERIVE_FAST_PATH.set(true);
+
     expect.indent(false);
     expect.assert_eq(&expanded_text);
 }

crates/hir-def/src/nameres.rs

@@ -87,6 +87,25 @@ use crate::{
 
 pub use self::path_resolution::ResolvePathResultPrefixInfo;
 
+#[cfg(test)]
+thread_local! {
+    /// HACK: In order to test builtin derive expansion, we gate their fast path with this atomic when cfg(test).
+    pub(crate) static ENABLE_BUILTIN_DERIVE_FAST_PATH: std::cell::Cell<bool> =
+        const { std::cell::Cell::new(true) };
+}
+
+#[inline]
+#[cfg(test)]
+fn enable_builtin_derive_fast_path() -> bool {
+    ENABLE_BUILTIN_DERIVE_FAST_PATH.get()
+}
+
+#[inline(always)]
+#[cfg(not(test))]
+fn enable_builtin_derive_fast_path() -> bool {
+    true
+}
+
 const PREDEFINED_TOOLS: &[SmolStr] = &[
     SmolStr::new_static("clippy"),
     SmolStr::new_static("rustfmt"),

crates/hir-def/src/nameres/collector.rs

@@ -1526,8 +1526,9 @@ impl<'db> DefCollector<'db> {
                                             }
                                         }
 
-                                        if let MacroDefKind::BuiltInDerive(_, builtin_derive) =
-                                            def_id.kind
+                                        if super::enable_builtin_derive_fast_path()
+                                            && let MacroDefKind::BuiltInDerive(_, builtin_derive) =
+                                                def_id.kind
                                         {
                                             self.deferred_builtin_derives
                                                 .entry(ast_id.ast_id.upcast())