compiler: improve captured metavariables diagnostic

Adds a link to the relevant part of The Rust Reference in the eror
message, and suggests a possible fix (replacing the fragment specifier
with :tt in the macro definition).

Fixes typos in the original message.

Signed-off-by: Lena Milizé <me@lvmn.org>

Author: lovelymono

compiler/rustc_expand/src/mbe/diagnostics.rs

@@ -66,7 +66,9 @@ pub(super) fn failed_to_match_macro<'cx>(
         && (matches!(expected_token.kind, TokenKind::Interpolated(_))
             || matches!(token.kind, TokenKind::Interpolated(_)))
     {
-        err.note("captured metavariables except for `$tt`, `$ident` and `$lifetime` cannot be compared to other tokens");
+        err.note("captured metavariables except for `:tt`, `:ident` and `:lifetime` cannot be compared to other tokens");
+        err.note("see https://doc.rust-lang.org/nightly/reference/macros-by-example.html#forwarding-a-matched-fragment for more information");
+        err.help("try using `:tt` instead in the macro definition");
     }
 
     // Check whether there's a missing comma in this macro call, like `println!("{}" a);`

tests/ui/macros/nonterminal-matching.stderr

@@ -18,7 +18,9 @@ LL |     macro n(a $nt_item b) {
 ...
 LL | complex_nonterminal!(enum E {});
    | ------------------------------- in this macro invocation
-   = note: captured metavariables except for `$tt`, `$ident` and `$lifetime` cannot be compared to other tokens
+   = note: captured metavariables except for `:tt`, `:ident` and `:lifetime` cannot be compared to other tokens
+   = note: see https://doc.rust-lang.org/nightly/reference/macros-by-example.html#forwarding-a-matched-fragment for more information
+   = help: try using `:tt` instead in the macro definition
    = note: this error originates in the macro `complex_nonterminal` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error