perf: drop the full-crate AST walk in check_unused

compiler/rustc_resolve/src/check_unused.rs

@@ -10,9 +10,9 @@
 //
 // Checking for unused imports is split into three steps:
 //
-//  - `UnusedImportCheckVisitor` walks the AST to find all the unused imports
-//    inside of `UseTree`s, recording their `NodeId`s and grouping them by
-//    the parent `use` item
+//  - `UnusedImportCheckVisitor` visits the `use` items collected during late
+//    resolution to find all the unused imports inside of `UseTree`s, recording
+//    their `NodeId`s and grouping them by the parent `use` item
 //
 //  - `calc_unused_spans` then walks over all the `use` items marked in the
 //    previous step to collect the spans associated with the `NodeId`s and to
@@ -410,7 +410,7 @@ fn calc_unused_spans(
 }
 
 impl Resolver<'_, '_> {
-    pub(crate) fn check_unused(&mut self, krate: &ast::Crate) {
+    pub(crate) fn check_unused(&mut self, use_items: Vec<&ast::Item>) {
         let tcx = self.tcx;
         let mut maybe_unused_extern_crates = FxHashMap::default();
 
@@ -465,7 +465,10 @@ impl Resolver<'_, '_> {
             base_id: ast::DUMMY_NODE_ID,
             item_span: DUMMY_SP,
         };
-        visit::walk_crate(&mut visitor, krate);
+        // `use_items` is in crate DFS order, so diagnostics and side effects are unchanged.
+        for item in use_items {
+            visitor.visit_item(item);
+        }
 
         visitor.report_unused_extern_crate_items(maybe_unused_extern_crates);
 

compiler/rustc_resolve/src/late.rs

@@ -5587,12 +5587,15 @@ impl<'a, 'ast, 'ra, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
 }
 
 /// Walks the whole crate in DFS order, visiting each item, counting the declared number of
-/// lifetime generic parameters and function parameters.
-struct ItemInfoCollector<'a, 'ra, 'tcx> {
+/// lifetime generic parameters and function parameters. Also collects all `use` and
+/// `extern crate` items so that `check_unused` doesn't need to walk the crate again.
+struct ItemInfoCollector<'a, 'ast, 'ra, 'tcx> {
     r: &'a mut Resolver<'ra, 'tcx>,
+    /// All `use` and `extern crate` items, in the order in which they are visited.
+    use_items: Vec<&'ast Item>,
 }
 
-impl ItemInfoCollector<'_, '_, '_> {
+impl ItemInfoCollector<'_, '_, '_, '_> {
     fn collect_fn_info(&mut self, decl: &FnDecl, id: NodeId) {
         self.r
             .delegation_fn_sigs
@@ -5626,7 +5629,7 @@ fn required_generic_args_suggestion(generics: &ast::Generics) -> Option<String>
     if required.is_empty() { None } else { Some(format!("<{}>", required.join(", "))) }
 }
 
-impl<'ast> Visitor<'ast> for ItemInfoCollector<'_, '_, '_> {
+impl<'ast> Visitor<'ast> for ItemInfoCollector<'_, 'ast, '_, '_> {
     fn visit_item(&mut self, item: &'ast Item) {
         match &item.kind {
             ItemKind::TyAlias(TyAlias { generics, .. })
@@ -5659,11 +5662,13 @@ impl<'ast> Visitor<'ast> for ItemInfoCollector<'_, '_, '_> {
                 }
             }
 
+            ItemKind::Use(..) | ItemKind::ExternCrate(..) => {
+                self.use_items.push(item);
+            }
+
             ItemKind::Mod(..)
             | ItemKind::Static(..)
             | ItemKind::ConstBlock(..)
-            | ItemKind::Use(..)
-            | ItemKind::ExternCrate(..)
             | ItemKind::MacroDef(..)
             | ItemKind::GlobalAsm(..)
             | ItemKind::MacCall(..)
@@ -5694,9 +5699,12 @@ impl<'ast> Visitor<'ast> for ItemInfoCollector<'_, '_, '_> {
 }
 
 impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
-    pub(crate) fn late_resolve_crate(&mut self, krate: &Crate) {
+    /// Returns the `use` and `extern crate` items of the crate, for use by `check_unused`.
+    pub(crate) fn late_resolve_crate<'ast>(&mut self, krate: &'ast Crate) -> Vec<&'ast Item> {
         with_owner(self, CRATE_NODE_ID, |this| {
-            visit::walk_crate(&mut ItemInfoCollector { r: this }, krate);
+            let mut info_collector = ItemInfoCollector { r: this, use_items: Vec::new() };
+            visit::walk_crate(&mut info_collector, krate);
+            let use_items = info_collector.use_items;
             let mut late_resolution_visitor = LateResolutionVisitor::new(this);
             late_resolution_visitor
                 .resolve_doc_links(&krate.attrs, MaybeExported::Ok(CRATE_NODE_ID));
@@ -5709,6 +5717,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                     crate::diagnostics::UnusedLabel,
                 );
             }
+            use_items
         })
     }
 }

compiler/rustc_resolve/src/lib.rs

@@ -2082,9 +2082,10 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             self.tcx
                 .sess
                 .time("finalize_macro_resolutions", || self.finalize_macro_resolutions(krate));
-            self.tcx.sess.time("late_resolve_crate", || self.late_resolve_crate(krate));
+            let use_items =
+                self.tcx.sess.time("late_resolve_crate", || self.late_resolve_crate(krate));
             self.tcx.sess.time("resolve_main", || self.resolve_main());
-            self.tcx.sess.time("resolve_check_unused", || self.check_unused(krate));
+            self.tcx.sess.time("resolve_check_unused", || self.check_unused(use_items));
             self.tcx.sess.time("resolve_report_errors", || self.report_errors(krate));
             self.tcx
                 .sess