rustc_metadata: Make attribute decoding slightly faster and stricter

Rename `CStore::item_attrs` -> `CStore::item_attrs_untracked` top follow conventions

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1309,24 +1309,26 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
 
     fn get_item_attrs(
         &'a self,
-        node_id: DefIndex,
+        id: DefIndex,
         sess: &'a Session,
     ) -> impl Iterator<Item = ast::Attribute> + 'a {
-        // The attributes for a tuple struct/variant are attached to the definition, not the ctor;
-        // we assume that someone passing in a tuple struct ctor is actually wanting to
-        // look at the definition
-        let def_key = self.def_key(node_id);
-        let item_id = if def_key.disambiguated_data.data == DefPathData::Ctor {
-            def_key.parent.unwrap()
-        } else {
-            node_id
-        };
-
         self.root
             .tables
             .attributes
-            .get(self, item_id)
+            .get(self, id)
-            .unwrap_or_else(Lazy::empty)
+            .unwrap_or_else(|| {
+                // Structure and variant constructors don't have any attributes encoded for them,
+                // but we assume that someone passing a constructor ID actually wants to look at
+                // the attributes on the corresponding struct or variant.
+                let def_key = self.def_key(id);
+                assert_eq!(def_key.disambiguated_data.data, DefPathData::Ctor);
+                let parent_id = def_key.parent.expect("no parent for a constructor");
+                self.root
+                    .tables
+                    .attributes
+                    .get(self, parent_id)
+                    .expect("no encoded attributes for a structure or variant")
+            })
             .decode((self, sess))
     }
 

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -145,9 +145,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
     lookup_deprecation_entry => {
         cdata.get_deprecation(def_id.index).map(DeprecationEntry::external)
     }
-    item_attrs => { tcx.arena.alloc_from_iter(
+    item_attrs => { tcx.arena.alloc_from_iter(cdata.get_item_attrs(def_id.index, tcx.sess)) }
-        cdata.get_item_attrs(def_id.index, tcx.sess)
-    ) }
     fn_arg_names => { cdata.get_fn_param_names(tcx, def_id.index) }
     rendered_const => { cdata.get_rendered_const(def_id.index) }
     impl_parent => { cdata.get_parent_impl(def_id.index) }
@@ -470,7 +468,7 @@ impl CStore {
         self.get_crate_data(cnum).num_def_ids()
     }
 
-    pub fn item_attrs(&self, def_id: DefId, sess: &Session) -> Vec<ast::Attribute> {
+    pub fn item_attrs_untracked(&self, def_id: DefId, sess: &Session) -> Vec<ast::Attribute> {
         self.get_crate_data(def_id.krate).get_item_attrs(def_id.index, sess).collect()
     }
 

compiler/rustc_resolve/src/diagnostics.rs

@@ -895,8 +895,11 @@ impl<'a> Resolver<'a> {
                             // a note about editions
                             let note = if let Some(did) = did {
                                 let requires_note = !did.is_local()
-                                    && this.cstore().item_attrs(did, this.session).iter().any(
+                                    && this
-                                        |attr| {
+                                        .cstore()
+                                        .item_attrs_untracked(did, this.session)
+                                        .iter()
+                                        .any(|attr| {
                                             if attr.has_name(sym::rustc_diagnostic_item) {
                                                 [sym::TryInto, sym::TryFrom, sym::FromIterator]
                                                     .map(|x| Some(x))
@@ -904,8 +907,7 @@ impl<'a> Resolver<'a> {
                                             } else {
                                                 false
                                             }
-                                        },
+                                        });
-                                    );
 
                                 requires_note.then(|| {
                                     format!(

compiler/rustc_resolve/src/lib.rs

@@ -3420,7 +3420,7 @@ impl<'a> Resolver<'a> {
 
                 let attr = self
                     .cstore()
-                    .item_attrs(def_id, self.session)
+                    .item_attrs_untracked(def_id, self.session)
                     .into_iter()
                     .find(|a| a.has_name(sym::rustc_legacy_const_generics))?;
                 let mut ret = Vec::new();