Tracking issue for RFC 1624: `break` with values for `loop`

[aturon]

RFC.

Current status:

• Implemented.
• Decided to stabilize.
• Documentation PR: Feature/loop break value reference#56 (See final section of the Rust Forge stabilization instructions for advice.)
• Stabilization PR: Stabilize the loop_break_value feature #42016 (See the Rust Forge for instructions.)

[aturon]

Currently awaiting implementation.

cc @dhardy

[eddyb]

@aturon How would we go about advertising mentorship? If someone wants to implement this and they're not already familiar with the compiler internals, they can come to #rustc on IRC to discuss it.
This is the case for many such features with a relatively small footprint, we're glad to help.

(Although I may be spreading myself thin a bit too much instead of finishing my own refactors.)

[aturon]

@eddyb An important question we don't have a systematic answer for right now (cc @brson). We should probably move this to an internals thread or something, though.

[dhardy]

I hear you @aturon but don't have the time just now (and maybe for a few weeks).