lang-items: Collect trait functions that are lang items

gcc/rust/ChangeLog:

	* ast/rust-collect-lang-items.cc (CollectLangItems::visit): Add visitor for collecting
	functions that might be lang items.
	* ast/rust-collect-lang-items.h: Likewise.

gcc/rust/ast/rust-collect-lang-items.cc

@@ -82,5 +82,13 @@ CollectLangItems::visit (AST::TraitItemType &item)
   DefaultASTVisitor::visit (item);
 }
 
+void
+CollectLangItems::visit (AST::Function &item)
+{
+  maybe_add_lang_item (item);
+
+  DefaultASTVisitor::visit (item);
+}
+
 } // namespace AST
 } // namespace Rust

gcc/rust/ast/rust-collect-lang-items.h

@@ -47,6 +47,7 @@ class CollectLangItems : public DefaultASTVisitor
 
   void visit (AST::Trait &item) override;
   void visit (AST::TraitItemType &item) override;
+  void visit (AST::Function &item) override;
 
 private:
   template <typename T> void maybe_add_lang_item (const T &item);