rustc_expand: improve diagnostics for non-repeatable metavars in repetition

[Unique-Usman]

Enhance NoSyntaxVarsExprRepeat by suggesting similarly named metavariables, distinguishing repeatable vs non-repeatable bindings, and listing available repeatable variables when no match exists.

[rustbot]

r? @nnethercote

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[Unique-Usman]

r? @estebank

[Unique-Usman]

address typo with repetition part of this issue -> #47452

[Unique-Usman]

macro_rules! m {
    (begin $($ard:ident),* end) => {
        [$($arg),*]
    };
}

macro_rules! mn {
    (begin $($arg:ident),* end) => {
        [$($typo),*]
    };
}

macro_rules! mnr {
    (begin $arg:ident end) => {
        [$($ard),*]
    };
}

macro_rules! err {
    (begin $arg:ident end) => {
        [$($typo),*]
    };
}

fn main() {
    let x = 1;
    let _ = m![begin x end];
    let _ = mn![begin x end];
    let _ = mnr![begin x end];
    let _ = err![begin x  end];
}

will give this

error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
 --> foo.rs:3:11
  |
3 |         [$($arg),*] 
  |           ^^^^^^

error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
 --> foo.rs:9:11
  |
9 |         [$($typo),*] 
  |           ^^^^^^^

error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
  --> foo.rs:15:11
   |
15 |         [$($ard),*]
   |           ^^^^^^

error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
  --> foo.rs:21:11
   |
21 |         [$($typo),*]
   |           ^^^^^^^

error: aborting due to 4 previous errors

with this pr it will give this

[uniqueusman@archlinux rust]$ rustc +stage1 foo.rs 
error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
 --> foo.rs:3:11
  |
3 |         [$($arg),*] 
  |           ^^^^^^
  |
help: there's a macro argument with a similar name
  |
3 -         [$($arg),*] 
3 +         [$($ard),*] 
  |

error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
 --> foo.rs:9:11
  |
9 |         [$($typo),*] 
  |           ^^----^
  |             |
  |             expected one of repeatable variables arguments: $arg

error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
  --> foo.rs:15:11
   |
14 |     (begin $arg:ident end) => { 
   |             --- this macro argument is unrepeatable
15 |         [$($ard),*]
   |           ^^^^^^
   |
help: there's a macro argument with a similar name that is unrepeatable
   |
15 -         [$($ard),*]
15 +         [$($arg),*]
   |

error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
  --> foo.rs:21:11
   |
21 |         [$($typo),*]
   |           ^^----^
   |             |
   |             variable is not repeatable and there is no repeatable variable arguments

error: aborting due to 4 previous errors

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.