Remove rustc::existing_doc_keyword lint.

`CheckAttrVisitor::check_doc_keyword` checks `#[doc(keyword = "..")]`
attributes to ensure they are on an empty module, and that the value is
a non-empty identifier.

The `rustc::existing_doc_keyword` lint checks these attributes to ensure
that the value is the name of a keyword.

It's silly to have two different checking mechanisms for these
attributes. This commit does the following.
- Changes `check_doc_keyword` to check that the value is the name of a
  keyword (avoiding the need for the identifier check, which removes a
  dependency on `rustc_lexer`).
- Removes the lint.
- Updates tests accordingly.

There is one hack: the `SelfTy` FIXME case used to used to be handled by
disabling the lint, but now is handled with a special case in
`is_doc_keyword`. That hack will go away if/when the FIXME is fixed.

Co-Authored-By: Guillaume Gomez <guillaume1.gomez@gmail.com>

compiler/rustc_passes/src/check_attr.rs

@@ -914,6 +914,13 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
     }
 
     fn check_doc_keyword(&self, meta: &MetaItemInner, hir_id: HirId) {
+        fn is_doc_keyword(s: Symbol) -> bool {
+            // 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
+        }
+
         let doc_keyword = meta.value_str().unwrap_or(kw::Empty);
         if doc_keyword == kw::Empty {
             self.doc_attr_str_error(meta, "keyword");
@@ -935,10 +942,10 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                 return;
             }
         }
-        if !rustc_lexer::is_ident(doc_keyword.as_str()) {
-            self.dcx().emit_err(errors::DocKeywordInvalidIdent {
+        if !is_doc_keyword(doc_keyword) {
+            self.dcx().emit_err(errors::DocKeywordNotKeyword {
                 span: meta.name_value_literal_span().unwrap_or_else(|| meta.span()),
-                doc_keyword,
+                keyword: doc_keyword,
             });
         }
     }

compiler/rustc_passes/src/errors.rs

@@ -215,6 +215,15 @@ pub(crate) struct DocKeywordEmptyMod {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(passes_doc_keyword_not_keyword)]
+#[help]
+pub(crate) struct DocKeywordNotKeyword {
+    #[primary_span]
+    pub span: Span,
+    pub keyword: Symbol,
+}
+
 #[derive(Diagnostic)]
 #[diag(passes_doc_keyword_not_mod)]
 pub(crate) struct DocKeywordNotMod {
@@ -222,14 +231,6 @@ pub(crate) struct DocKeywordNotMod {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(passes_doc_keyword_invalid_ident)]
-pub(crate) struct DocKeywordInvalidIdent {
-    #[primary_span]
-    pub span: Span,
-    pub doc_keyword: Symbol,
-}
-
 #[derive(Diagnostic)]
 #[diag(passes_doc_fake_variadic_not_valid)]
 pub(crate) struct DocFakeVariadicNotValid {