Add a cache for rustc_legacy_const_generics

compiler/rustc_ast_lowering/src/lib.rs

@@ -175,7 +175,7 @@ pub trait ResolverAstLowering {
 
     fn item_generics_num_lifetimes(&self, def: DefId, sess: &Session) -> usize;
 
-    fn legacy_const_generic_args(&self, expr: &Expr) -> Option<Vec<usize>>;
+    fn legacy_const_generic_args(&mut self, expr: &Expr) -> Option<Vec<usize>>;
 
     /// Obtains resolution for a `NodeId` with a single resolution.
     fn get_partial_res(&mut self, id: NodeId) -> Option<PartialRes>;

compiler/rustc_resolve/src/lib.rs

@@ -996,6 +996,8 @@ pub struct Resolver<'a> {
     /// Some way to know that we are in a *trait* impl in `visit_assoc_item`.
     /// FIXME: Replace with a more general AST map (together with some other fields).
     trait_impl_items: FxHashSet<LocalDefId>,
+
+    legacy_const_generic_args: FxHashMap<DefId, Option<Vec<usize>>>,
 }
 
 /// Nothing really interesting here; it just provides memory for the rest of the crate.
@@ -1077,7 +1079,7 @@ fn item_generics_num_lifetimes(&self, def_id: DefId, sess: &Session) -> usize {
         self.cstore().item_generics_num_lifetimes(def_id, sess)
     }
 
-    fn legacy_const_generic_args(&self, expr: &Expr) -> Option<Vec<usize>> {
+    fn legacy_const_generic_args(&mut self, expr: &Expr) -> Option<Vec<usize>> {
         self.legacy_const_generic_args(expr)
     }
 
@@ -1321,6 +1323,7 @@ pub fn new(
             invocation_parents,
             next_disambiguator: Default::default(),
             trait_impl_items: Default::default(),
+            legacy_const_generic_args: Default::default(),
         };
 
         let root_parent_scope = ParentScope::module(graph_root, &resolver);
@@ -3317,7 +3320,7 @@ pub fn opt_span(&self, def_id: DefId) -> Option<Span> {
     /// Checks if an expression refers to a function marked with
     /// `#[rustc_legacy_const_generics]` and returns the argument index list
     /// from the attribute.
-    pub fn legacy_const_generic_args(&self, expr: &Expr) -> Option<Vec<usize>> {
+    pub fn legacy_const_generic_args(&mut self, expr: &Expr) -> Option<Vec<usize>> {
         if let ExprKind::Path(None, path) = &expr.kind {
             // Don't perform legacy const generics rewriting if the path already
             // has generic arguments.
@@ -3338,6 +3341,11 @@ pub fn legacy_const_generic_args(&self, expr: &Expr) -> Option<Vec<usize>> {
                     return None;
                 }
 
+                if let Some(v) = self.legacy_const_generic_args.get(&def_id) {
+                    return v.clone();
+                }
+
+                let parse_attrs = || {
                     let attrs = self.cstore().item_attrs(def_id, self.session);
                     let attr = attrs
                         .iter()
@@ -3351,7 +3359,14 @@ pub fn legacy_const_generic_args(&self, expr: &Expr) -> Option<Vec<usize>> {
                             _ => panic!("invalid arg index"),
                         }
                     }
-                return Some(ret);
+                    Some(ret)
+                };
+
+                // Cache the lookup to avoid parsing attributes for an iterm
+                // multiple times.
+                let ret = parse_attrs();
+                self.legacy_const_generic_args.insert(def_id, ret.clone());
+                return ret;
             }
         }
         None