Make doc comments cheaper with `AttrKind`.

`AttrKind` is a new type with two variants, `Normal` and `DocComment`. It's a
big performance win (over 10% in some cases) because `DocComment` lets doc
comments (which are common) be represented very cheaply.

`Attribute` gets some new helper methods to ease the transition:
- `has_name()`: check if the attribute name matches a single `Symbol`; for
  `DocComment` variants it succeeds if the symbol is `sym::doc`.
- `is_doc_comment()`: check if it has a `DocComment` kind.
- `{get,unwrap}_normal_item()`: extract the item from a `Normal` variant;
  panic otherwise.

Fixes #60935.

1 files changed, 11 insertions, 14 deletions

diff --git a/src/librustc/ich/impls_syntax.rs b/src/librustc/ich/impls_syntax.rs
index 304735fb1c7..aa147462e3d 100644
--- a/src/librustc/ich/impls_syntax.rs
+++ b/src/librustc/ich/impls_syntax.rs
@@ -177,7 +177,7 @@ impl<'a> HashStable<StableHashingContext<'a>> for [ast::Attribute] {
         let filtered: SmallVec<[&ast::Attribute; 8]> = self
             .iter()
             .filter(|attr| {
-                !attr.is_sugared_doc &&
+                !attr.is_doc_comment() &&
                 !attr.ident().map_or(false, |ident| hcx.is_ignored_attr(ident.name))
             })
             .collect();
@@ -207,19 +207,16 @@ impl<'a> HashStable<StableHashingContext<'a>> for ast::Attribute {
     fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
         // Make sure that these have been filtered out.
         debug_assert!(!self.ident().map_or(false, |ident| hcx.is_ignored_attr(ident.name)));
-        debug_assert!(!self.is_sugared_doc);
-
-        let ast::Attribute {
-            ref item,
-            id: _,
-            style,
-            is_sugared_doc: _,
-            span,
-        } = *self;
-
-        item.hash_stable(hcx, hasher);
-        style.hash_stable(hcx, hasher);
-        span.hash_stable(hcx, hasher);
+        debug_assert!(!self.is_doc_comment());
+
+        let ast::Attribute { kind, id: _, style, span } = self;
+        if let ast::AttrKind::Normal(item) = kind {
+            item.hash_stable(hcx, hasher);
+            style.hash_stable(hcx, hasher);
+            span.hash_stable(hcx, hasher);
+        } else {
+            unreachable!();
+        }
     }
 }