Rollup merge of #124778 - fmease:fix-diag-msg-parse-meta-item, r=nnethercote

Fix parse error message for meta items

Addresses https://github.com/rust-lang/rust/issues/122796#issuecomment-2010803906, cc [``@]Thomasdezeeuw.``

For attrs inside of a macro like `#[doc(alias = $ident)]` or `#[cfg(feature = $ident)]` where `$ident` is a macro metavariable of fragment kind `ident`, we used to say the following when expanded (with `$ident` ⟼ `ident`):

```
error: expected unsuffixed literal or identifier, found `ident`
  --> weird.rs:6:19
   |
6  |      #[cfg(feature = $ident)]
   |                      ^^^^^^
...
11 | m!(id);
   | ------ in this macro invocation
   |
   = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
```

This was incorrect and caused confusion, justifiably so (see #122796).

In this position, we only accept/expect *unsuffixed literals* which consist of numeric & string literals as well as the boolean literals / the keywords / the reserved identifiers `false` & `true` **but not** arbitrary identifiers.

Furthermore, we used to suggest garbage when encountering unexpected non-identifier tokens:

```
error: expected unsuffixed literal, found `-`
  --> weird.rs:16:17
   |
16 | #[cfg(feature = -1)]
   |                 ^
   |
help: surround the identifier with quotation marks to parse it as a string
   |
16 | #[cfg(feature =" "-1)]
   |                + +
```

Now we no longer do.

1 files changed, 3 insertions, 11 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index d06f03a7c17..2f68a299f26 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -978,21 +978,13 @@ pub(crate) struct InvalidMetaItem {
     #[primary_span]
     pub span: Span,
     pub token: Token,
-}
-
-#[derive(Diagnostic)]
-#[diag(parse_invalid_meta_item_unquoted_ident)]
-pub(crate) struct InvalidMetaItemUnquotedIdent {
-    #[primary_span]
-    pub span: Span,
-    pub token: Token,
     #[subdiagnostic]
-    pub sugg: InvalidMetaItemSuggQuoteIdent,
+    pub quote_ident_sugg: Option<InvalidMetaItemQuoteIdentSugg>,
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
-pub(crate) struct InvalidMetaItemSuggQuoteIdent {
+#[multipart_suggestion(parse_quote_ident_sugg, applicability = "machine-applicable")]
+pub(crate) struct InvalidMetaItemQuoteIdentSugg {
     #[suggestion_part(code = "\"")]
     pub before: Span,
     #[suggestion_part(code = "\"")]