rustc lexer/macro_rules diagnostics feature request: Color highlight acceptable characters

This applies probably to macro_rules/macro v.2 only (correct this otherwise). When the lexer/tokenizer reports errors like

note: allowed there are: `{`, `[`, `=>`, `,`, `>`, `=`, `:`, `;`, `|`, `as` or `where`

then highlight each allowed characters/tokens (in terminals that use colors). This should not be a rocket science, since the error reporting already makes "error" red and "warning" yellow (though that is surely higher up the control chain).

macro_rules! m {
    ($e:expr ~ $id:ident) => {};
}

generates error:

    Checking macro_ru v0.1.0 (/home/pkehl/macro_ru)
error: `$e:expr` is followed by `~`, which is not allowed for `expr` fragments
 --> src/lib.rs:2:14
  |
2 |     ($e:expr ~ $id:ident) => {};
  |              ^ not allowed after `expr` fragments
  |
  = note: allowed there are: `=>`, `,` or `;`

and

macro_rules! m {
    ($t:ty ~ $id:ident) => {};
}

generates even less readable:

...
2 |     ($t:ty ~ $id:ident) => {};
  |            ^ not allowed after `ty` fragments
  |
  = note: allowed there are: `{`, `[`, `=>`, `,`, `>`, `=`, `:`, `;`, `|`, `as` or `where`

As per https://internals.rust-lang.org/t/rustc-lexer-diagnostics-feature-request-color-highlight-acceptable-characters/24227.
@rustbot label +A-diagnostics +D-diagnostic-infra

Start at the rustc lexer/tokenizer diagnostics for macro_rules fragment-following errors, using the expr and ty examples in the issue to reproduce the current output. Trace where allowed tokens are rendered and determine how terminal color highlighting is handled; done means each allowed character or token is visibly highlighted without changing the diagnostic content.