Tracking issue for duplicate_matcher_binding_names compatibility lint

[mark-i-m]

What is this lint about

In #57593, it was observed that duplicate macro matcher bindings are allowed in declarations, but unusable in invocations:

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

This is macro compiles fine on its own, but any attempt to use it will result in an error because $a is bound twice. This is likely an oversight in the original implementation, but it really feels like a bug, so we want to make this a hard error.

This compatibility lint warns on such a declaration. The end goal is to make this a hard error in a few releases.

How to fix this warning/error

Change the binding names to be different.

Current status

• PR Error on duplicate matcher bindings #57617 introduces the duplicate_matcher_binding_names lint as warn-by-default
• PR warn -> deny duplicate match bindings #59394 makes the duplicate_matcher_binding_names lint as deny-by-default
• PR Make duplicate matcher bindings a hard error #59858 makes the duplicate_matcher_binding_names a hard error

[mark-i-m]

cc @Centril

[mark-i-m]

Note: The future compatibility lint has hit stable by now...

[Centril]

@mark-i-m I think we have given it enough time; would you like to prepare a PR that turns it into a hard error directly and let's crater + merge that one?

[mark-i-m]

@Centril Sure, but should we not make it deny-by-default first? I don't there is a particular hurry on this, right?

[mark-i-m]

Note to self: The next step is to merge this branch: master...mark-i-m:dup-matcher-bindings-3