[rustc_parse] Forbid lets in certain places

[c410-f3r]

Currently only forbids in locals to resolve #94927 (comment) but feel free to point any other places.

[rust-highfive]

r? @compiler-errors

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[compiler-errors]

Sorry for taking so long to review.

I don't think this should be implemented like this. I think we should be explicitly adding code to allow let-chains in if and while (i.e. make them deny by default), and not exhaustively forbid them in certain places like the RHS of a let statement.

Even if this is the right approach, I think this current implementation is missing a lot of positions where we'd accidentally stabilize parsing let as an expression as discussed in #94927 (comment), for example:

#[cfg(ignore)]
fn foo() {
    let _ = f(let y = 1);
    |x| let 1 = 2;
    array[let x = y];
}

[compiler-errors]

(whoops, should've left comment here)

[c410-f3r]

Sorry for taking so long to review.

I don't think this should be implemented like this. I think we should be explicitly adding code to allow let-chains in if and while (i.e. make them deny by default), and not exhaustively forbid them in certain places like the RHS of a let statement.

Even if this is the right approach, I think this current implementation is missing a lot of positions where we'd accidentally stabilize parsing let as an expression as discussed in #94927 (comment), for example:

#[cfg(ignore)]
fn foo() {
    let _ = f(let y = 1);
    |x| let 1 = 2;
    array[let x = y];
}

Done. Thanks for the review

[bors]

☔ The latest upstream changes (presumably #97474) made this pull request unmergeable. Please resolve the merge conflicts.

[c410-f3r]

@rustbot label +S-waiting-on-review -S-waiting-on-author

If possible, I would like to tackle https://github.com/rust-lang/rust/pull/97295/files#diff-52ef6fde627bdaac8e79bcf67beac39ef4cd9a2a0bd0ff7e3ec69dd0ce74af94 and duplicated diagnostics in a following PR.

[compiler-errors]

Alright, this is a good first step then.

@bors r+

[bors]

📌 Commit 1c460819c4dbd77dd22df0292a5b6052347245ef has been approved by compiler-errors

[bors]

☔ The latest upstream changes (presumably #98447) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

@bors r- please fix the issues then i can re-approve