Record weak lang item candidates during the main lang item walk [~-0.35% incr-unchanged]

compiler/rustc_passes/src/lang_items.rs

@@ -40,6 +40,10 @@ struct LanguageItemCollector<'ast, 'tcx> {
     // so we can avoid constructing this map for local def-ids.
     item_spans: FxHashMap<DefId, Span>,
     parent_item: Option<&'ast ast::Item>,
+    /// Weak lang items declared by foreign items, in visit order. Whether
+    /// they end up missing is decided only once collection has finished,
+    /// which used to require a second full crate walk.
+    weak_foreign_candidates: Vec<LangItem>,
 }
 
 impl<'ast, 'tcx> LanguageItemCollector<'ast, 'tcx> {
@@ -51,6 +55,7 @@ impl<'ast, 'tcx> LanguageItemCollector<'ast, 'tcx> {
             tcx,
             resolver,
             items: LanguageItems::new(),
+            weak_foreign_candidates: Vec::new(),
             item_spans: FxHashMap::default(),
             parent_item: None,
         }
@@ -269,7 +274,8 @@ fn get_lang_items(tcx: TyCtxt<'_>, (): ()) -> LanguageItems {
     visit::Visitor::visit_crate(&mut collector, krate);
 
     // Find all required but not-yet-defined lang items.
-    weak_lang_items::check_crate(tcx, &mut collector.items, krate);
+    let weak_foreign_candidates = std::mem::take(&mut collector.weak_foreign_candidates);
+    weak_lang_items::check_crate(tcx, &mut collector.items, &weak_foreign_candidates);
 
     // Return all the lang items that were found.
     collector.items
@@ -314,6 +320,12 @@ impl<'ast, 'tcx> visit::Visitor<'ast> for LanguageItemCollector<'ast, 'tcx> {
     }
 
     fn visit_foreign_item(&mut self, i: &'ast ast::ForeignItem) {
+        if let Some((lang_item, _)) = extract_ast(&i.attrs)
+            && let Some(item) = LangItem::from_name(lang_item)
+            && item.is_weak()
+        {
+            self.weak_foreign_candidates.push(item);
+        }
         self.check_for_lang(
             Target::Fn,
             self.resolver.owners[&i.id].def_id,

compiler/rustc_passes/src/weak_lang_items.rs

@@ -1,7 +1,5 @@
 //! Validity checking for weak lang items
 
-use rustc_ast as ast;
-use rustc_ast::visit;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::lang_items::{self, LangItem};
 use rustc_hir::weak_lang_items::WEAK_LANG_ITEMS;
@@ -10,14 +8,18 @@ use rustc_middle::ty::TyCtxt;
 use rustc_session::config::CrateType;
 
 use crate::diagnostics::{MissingLangItem, MissingPanicHandler, PanicUnwindWithoutStd};
-use crate::lang_items::extract_ast;
 
 /// Checks the crate for usage of weak lang items, returning a vector of all the
 /// lang items required by this crate, but not defined yet.
+///
+/// `weak_foreign_candidates` holds the weak lang items declared by the
+/// crate's foreign items, in visit order; the lang item collector records
+/// them during its walk so no second crate walk is needed here. Whether a
+/// candidate is missing can only be decided now, after collection finished.
 pub(crate) fn check_crate(
     tcx: TyCtxt<'_>,
     items: &mut lang_items::LanguageItems,
-    krate: &ast::Crate,
+    weak_foreign_candidates: &[LangItem],
 ) {
     // This is never called by user code, it's generated by the compiler. It
     // will never implicitly be added to the `missing` array unless we do so
@@ -26,26 +28,13 @@ pub(crate) fn check_crate(
         items.missing.push(LangItem::EhPersonality);
     }
 
-    visit::Visitor::visit_crate(&mut WeakLangItemVisitor { items }, krate);
-
-    verify(tcx, items);
-}
-
-struct WeakLangItemVisitor<'a> {
-    items: &'a mut lang_items::LanguageItems,
-}
-
-impl<'ast> visit::Visitor<'ast> for WeakLangItemVisitor<'_> {
-    fn visit_foreign_item(&mut self, i: &'ast ast::ForeignItem) {
-        if let Some((lang_item, _)) = extract_ast(&i.attrs)
-            && let Some(item) = LangItem::from_name(lang_item)
-            && item.is_weak()
-        {
-            if self.items.get(item).is_none() {
-                self.items.missing.push(item);
-            }
+    for &item in weak_foreign_candidates {
+        if items.get(item).is_none() {
+            items.missing.push(item);
         }
     }
+
+    verify(tcx, items);
 }
 
 fn verify(tcx: TyCtxt<'_>, items: &lang_items::LanguageItems) {