Remove interner_name argument from define_label! and make the interner private.

[kpreid]

Objective

Simplify the use of define_label!().

Solution

• Move the static interner inside the intern() method, so that it is not visible anywhere else and does not need to have a non-conflicting name. The only operation the interner has is interning, so the intern() trait method is all the access that is ever needed.
• Also added support for trailing commas.

This PR will conflict with #24444. I will rebase whichever one doesn’t get merged first.

Testing

• Ran tests with cargo run -p ci -- --keep-going test. There were some failures that I believe are existing/spurious.

[ChristopherBiscardi]

This moves the static from the top level to the interior of the intern function, which I believe is fine since statics are only initialized at compile time.

The old static is technically visible inside the module, so someone could be accessing it and it might warrant a migration guide?

[kpreid]

This moves the static from the top level to the interior of the intern function, which I believe is fine since statics are only initialized at compile time.

That’s right. This is a common Rust pattern, though more often for statics that need lazy initialization.

The old static is technically visible inside the module, so someone could be accessing it and it might warrant a migration guide?

More importantly, users of the macro need to change the way they call it! I have added a migration guide.

[alice-i-cecile]

Merging #24444; ping me once the conflicts from that are resolved.

[kpreid]

@alice-i-cecile Rebased.