Remove rustc_hir::HashStableContext::hash_attr_id.

It has a single impl, which does nothing, as you'd expect when hashing a
type called `HashIgnoredAttrId`!

So this commit just removes it, and `HashIgnoredAttrId::hash_stable`
ends up doing nothing (with a comment) instead of calling the trait
method to do nothing.

Author: nnethercote

compiler/rustc_hir/src/stable_hash_impls.rs

@@ -11,9 +11,7 @@ use crate::lints::DelayedLints;
 /// Requirements for a `StableHashingContext` to be used in this crate.
 /// This is a hack to allow using the `HashStable_Generic` derive macro
 /// instead of implementing everything in `rustc_middle`.
-pub trait HashStableContext: rustc_ast::HashStableContext + rustc_abi::HashStableContext {
-    fn hash_attr_id(&mut self, id: &HashIgnoredAttrId, hasher: &mut StableHasher);
-}
+pub trait HashStableContext: rustc_ast::HashStableContext + rustc_abi::HashStableContext {}
 
 impl<HirCtx: crate::HashStableContext> ToStableHashKey<HirCtx> for BodyId {
     type KeyType = (DefPathHash, ItemLocalId);
@@ -104,7 +102,7 @@ impl<HirCtx: crate::HashStableContext> HashStable<HirCtx> for Crate<'_> {
 }
 
 impl<HirCtx: crate::HashStableContext> HashStable<HirCtx> for HashIgnoredAttrId {
-    fn hash_stable(&self, hcx: &mut HirCtx, hasher: &mut StableHasher) {
-        hcx.hash_attr_id(self, hasher)
+    fn hash_stable(&self, _hcx: &mut HirCtx, _hasher: &mut StableHasher) {
+        /* we don't hash HashIgnoredAttrId, we ignore them */
     }
 }

compiler/rustc_query_system/src/ich/impls_syntax.rs

@@ -49,11 +49,7 @@ fn is_ignored_attr(name: Symbol) -> bool {
     IGNORED_ATTRIBUTES.contains(&name)
 }
 
-impl<'ctx> rustc_hir::HashStableContext for StableHashingContext<'ctx> {
-    fn hash_attr_id(&mut self, _id: &HashIgnoredAttrId, _hasher: &mut StableHasher) {
-        /* we don't hash HashIgnoredAttrId, we ignore them */
-    }
-}
+impl<'ctx> rustc_hir::HashStableContext for StableHashingContext<'ctx> {}
 
 impl<'a> HashStable<StableHashingContext<'a>> for SourceFile {
     fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {