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.

src/libsyntax_ext/proc_macro_harness.rs

@@ -249,9 +249,11 @@ impl<'a> Visitor<'a> for CollectProcMacros<'a> {
         for attr in &item.attrs {
             if is_proc_macro_attr(&attr) {
                 if let Some(prev_attr) = found_attr {
-                    let path_str = pprust::path_to_string(&attr.item.path);
-                    let msg = if attr.item.path.segments[0].ident.name ==
-                                 prev_attr.item.path.segments[0].ident.name {
+                    let prev_item = prev_attr.get_normal_item();
+                    let item = attr.get_normal_item();
+                    let path_str = pprust::path_to_string(&item.path);
+                    let msg = if item.path.segments[0].ident.name ==
+                                 prev_item.path.segments[0].ident.name {
                         format!(
                             "only one `#[{}]` attribute is allowed on any given function",
                             path_str,
@@ -261,7 +263,7 @@ impl<'a> Visitor<'a> for CollectProcMacros<'a> {
                             "`#[{}]` and `#[{}]` attributes cannot both be applied
                             to the same function",
                             path_str,
-                            pprust::path_to_string(&prev_attr.item.path),
+                            pprust::path_to_string(&prev_item.path),
                         )
                     };
 
@@ -290,7 +292,7 @@ impl<'a> Visitor<'a> for CollectProcMacros<'a> {
         if !is_fn {
             let msg = format!(
                 "the `#[{}]` attribute may only be used on bare functions",
-                pprust::path_to_string(&attr.item.path),
+                pprust::path_to_string(&attr.get_normal_item().path),
             );
 
             self.handler.span_err(attr.span, &msg);
@@ -304,7 +306,7 @@ impl<'a> Visitor<'a> for CollectProcMacros<'a> {
         if !self.is_proc_macro_crate {
             let msg = format!(
                 "the `#[{}]` attribute is only usable with crates of the `proc-macro` crate type",
-                pprust::path_to_string(&attr.item.path),
+                pprust::path_to_string(&attr.get_normal_item().path),
             );
 
             self.handler.span_err(attr.span, &msg);