Improve is_doc_keyword.

This is part of the implementation of `#[doc(keyword = "match")]`
attributes used by `std` to provide documentation for keywords.

`is_doc_keyword` currently does a crude keyword range test that's
intended to catch all keywords but misses `kw::Yeet`. This commit
changes it to use `Symbol` methods, including the new `is_weak` method
(required for `union`). `Symbol` methods are much less prone to falling
out of date if new keywords are added.

Author: nnethercote

compiler/rustc_passes/src/check_attr.rs

@@ -35,7 +35,7 @@ use rustc_session::lint::builtin::{
     UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES, UNUSED_ATTRIBUTES,
 };
 use rustc_session::parse::feature_err;
-use rustc_span::{BytePos, DUMMY_SP, Span, Symbol, kw, sym};
+use rustc_span::{BytePos, DUMMY_SP, Span, Symbol, edition, kw, sym};
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
 use rustc_trait_selection::infer::{TyCtxtInferExt, ValuePairs};
 use rustc_trait_selection::traits::ObligationCtxt;
@@ -1038,7 +1038,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             // FIXME: Once rustdoc can handle URL conflicts on case insensitive file systems, we
             // can remove the `SelfTy` case here, remove `sym::SelfTy`, and update the
             // `#[doc(keyword = "SelfTy")` attribute in `library/std/src/keyword_docs.rs`.
-            s <= kw::Union || s == sym::SelfTy
+            s.is_reserved(|| edition::LATEST_STABLE_EDITION) || s.is_weak() || s == sym::SelfTy
         }
 
         let doc_keyword = match meta.value_str() {

compiler/rustc_span/src/symbol.rs

@@ -131,7 +131,7 @@ symbols! {
         // tidy-alphabetical-end
 
         // Weak keywords, have special meaning only in specific contexts.
-        // Matching predicates: none
+        // Matching predicates: `is_weak`
         // tidy-alphabetical-start
         Auto:               "auto",
         Builtin:            "builtin",
@@ -2725,6 +2725,10 @@ impl Symbol {
             || self.is_unused_keyword_conditional(edition)
     }
 
+    pub fn is_weak(self) -> bool {
+        self >= kw::Auto && self <= kw::Yeet
+    }
+
     /// A keyword or reserved identifier that can be used as a path segment.
     pub fn is_path_segment_keyword(self) -> bool {
         self == kw::Super