Fix "consider borrowing" for else-if

[JonathanBrouwer]

Fixes #141810

When trying to suggest a borrow on a if or block expression, instead we now recurse into the if or block.
The comments in the code should explain the goal of the new code.

r? @jdonszelmann

[jdonszelmann]

nicely done! I cannot quite reproduce the case in which this is needed:
https://github.com/rust-lang/rust/pull/141812/files#diff-a188fa3dba59dd225d4a65e08961d2937f6418bc6126716e001303bf946d6ee3R2734

But I'm a little worried it's not always correct. At the very least, there's a difference here (https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=ac067fdd4897e207ee42b9d9f9a433b8) in putting the reference inside or outside the block. Am I right that that's the case it's trying to catch?

[JonathanBrouwer]

Indeed, you're right.
There are some cases in which the improvement would also break Expr::If would also have similar problems, such as

call(&if true {
    String::new()
} else {
    String::new()
})

Would you suggest just limiting this improvement to if then? I can quickly code that up, is probably better

[jdonszelmann]

I think the if suggestion is clearly wrong, any improvement is worth it. But I'd say the old block suggestion is more often correct. Let's limit it to that and I'll re-review @rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[JonathanBrouwer]

@rustbot ready

[jdonszelmann]

looking good! @bors r+ rollup

[bors]

📌 Commit b1a1df2 has been approved by jdonszelmann

It is now in the queue for this repository.