Revert let_chains stabilization

[Noratrieb]

This reverts commit 3266460.

It was discovered in #100513 that they are not implemented correctly, which does not make them ready for stabilization.

The merge in the let parsing had a few conflicts, cc @compiler-errors and @c410-f3r to make sure I did it correctly (alternatively I could also revert @compiler-errors' let diagnostic improvement PR as well if a simpler revert is desired).

r? @Mark-Simulacrum

[rust-highfive]

⚠️ Warning ⚠️

• Pull requests are usually filed against the master branch for this repo, but this one is against beta. Please double check that you specified the right target!

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

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Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[Mark-Simulacrum]

r? @compiler-errors

[compiler-errors]

Discussed with nils on zulip, this looks fine.

@bors r+ rollup=never p=1

[bors]

📌 Commit ef569fef7e3ce8094c56ad98d56bc2149d0630ff has been approved by compiler-errors

It is now in the queue for this repository.

[Noratrieb]

Looking at CI, this doesn't look fine to me :)
Or is this normal that is failing for beta?

[compiler-errors]

@bors r-

[compiler-errors]

Yeah, that's broken I think 🤔

[Noratrieb]

The feature(let_chains) attribute in rustc_errors got an incorrect cfg_attr(bootstrap). I added it unconditionally and didn't see any other wrong features, so it should hopefully pass CI now.

[compiler-errors]

@bors delegate=nilstrieb

r=me when CI is green then

[bors]

✌️ @Nilstrieb can now approve this pull request

[Noratrieb]

i think i messed up my rebase, at least git printed a warning while trying to rebase something else on another branch :/

[Noratrieb]

I did the entire revert again (copying out the conflict merge results from the PR here), could you take a quick look anyways to make sure I didn't break it? @compiler-errors

[compiler-errors]

The force-push diff looks right: https://github.com/rust-lang/rust/compare/b1f53f864deb49b078cabe6f6ae372bea7c99ad4..4b77950bac03717796b98df65b45bd9fee44a3d5

[Noratrieb]

Oh no, it does not, the cfg_attr got in again 😢

[compiler-errors]

🤣

[est31]

I'm not sure it's worth it, but just an idea. You could change #![cfg_attr(bootstrap, feature(let_chains))] to #![cfg_attr(let_chains_unstable, feature(let_chains))] together with some bootstrap magic that sets the cfg. Because right now, after this PR merges, one has to touch all places twice: once during stabilization, and once during update of the bootstrap (to remove the feature attr altogether). With my proposal, the first step is reduced to just changing the bootstrap magic to emit the cfg depending on if bootstrap is active or not. It's a feature that's heavily used in the compiler at this point.

[wesleywiser]

Revert looks good to me but I think we should wait for the compiler team triage meeting on Thursday before merging.

[joshtriplett]

LGTM

[pnkfelix]

discussed at T-compiler meeting.

@rustbot label: beta-accepted

[compiler-errors]

@bors r+ p=1 rollup=never

[bors]

📌 Commit 11a367a has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit 11a367a with merge 82bf341...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing 82bf341 to beta...