optimization: do not add non-local reachable-non-generic items to mentioned_items

Author: RalfJung

compiler/rustc_mir_transform/src/mentioned_items.rs

@@ -1,3 +1,4 @@
+use rustc_hir::def_id::DefId;
 use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::{self, ConstOperand, Location, MentionedItem, MirPass};
 use rustc_middle::ty::{self, adjustment::PointerCoercion, TyCtxt};
@@ -34,10 +35,12 @@ impl<'tcx> Visitor<'tcx> for MentionedItemsVisitor<'_, 'tcx> {
         let const_ = constant.const_;
         // This is how function items get referenced: via constants of `FnDef` type. This handles
         // both functions that are called and those that are just turned to function pointers.
-        if let ty::FnDef(def_id, args) = const_.ty().kind() {
+        if let ty::FnDef(def_id, args) = *const_.ty().kind()
+            && may_codegen_locally(self.tcx, def_id)
+        {
             debug!("adding to required_items: {def_id:?}");
             self.mentioned_items
-                .push(Spanned { node: MentionedItem::Fn(*def_id, args), span: constant.span });
+                .push(Spanned { node: MentionedItem::Fn(def_id, args), span: constant.span });
         }
     }
 
@@ -95,3 +98,29 @@ impl<'tcx> Visitor<'tcx> for MentionedItemsVisitor<'_, 'tcx> {
         }
     }
 }
+
+/// Returns `true` if we should codegen an item in the local crate, or returns `false` if we
+/// can just link to the upstream crate and therefore don't need a mono item.
+///
+/// This is an approximation of the collector's `should_codegen_locally`, in the sense that this
+/// here may return `true` even if `should_codegen_locally` says `false`. The point is to let us
+/// filter out items that definitely will not be considered by the collector.
+fn may_codegen_locally<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
+    if tcx.is_foreign_item(def_id) {
+        // Foreign items are always linked against, there's no way of instantiating them.
+        return false;
+    }
+
+    if def_id.is_local() {
+        // Local items cannot be referred to locally without monomorphizing them locally.
+        return true;
+    }
+
+    if tcx.is_reachable_non_generic(def_id) {
+        // We can link to the item in question, no instance needed in this crate.
+        return false;
+    }
+
+    // Conservative fall-back.
+    true
+}