Duplicate metavariables allowed in unused macro definitions

Summary

Duplicate metavariables are allowed in macro definitions, as long as those macro definitions are unused. Also, we don't warn when a macro definition is unused. See #4405

Reproducer

I tried this code:

macro_rules! foo {
    ($a:ident, $a:ident) => {()}
}

pub fn foo() {
    //foo!(a, b)
}

Does the code make use of any (1.49) nightly feature ?

• Nightly

Godbolt link

https://godbolt.org/z/5G3ad4bTo

Actual behavior

No error

Expected behavior

error: duplicate matcher binding
 --> <source>:2:16
  |
2 |     ($a:ident, $a:ident) => {()}
  |      --------  ^^^^^^^^ duplicate binding
  |      |
  |      previous binding

warning: unused macro definition
 --> <source>:1:1
  |
1 | / macro_rules! foo {
2 | |     ($a:ident, $a:ident) => {()}
3 | | }
  | |_^
  |
  = note: `#[warn(unused_macros)]` on by default

GCC Version

d47331f13c3f16262ffa4fbe6f919b83f1056d7f