rustc_builtin_macros: rename bench parameter to avoid collisions with user-defined function names

[IntegralPilot]

Resolves #148275 by preventing name collisions in the #[bench] macro.

Previously, a user-defined function named "b" could not be benchmarked because
the macro-generated lambda identity collided with the same name. We now generate
the lambda ident as __bench_<function_name>, ensuring it is always distinct
from the user’s function.

Because the prefix is applied recursively (e.g. benchmarking __bench_b
produces a lambda ident __bench___bench_b), there is no possible function
name that can equal its corresponding lambda ident. This guarantees that
the user can safely bench a function of any valid name without risk of
identifier collision.

[rustbot]

r? @jackh726

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[hkBst]

LGTM except for a small style nit as mentioned above.

View changes since this review

[oli-obk]

r? hkBst

[rustbot]

Failed to set assignee to hkBst: invalid assignee

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[oli-obk]

oh heh

@bors delegate=hkBst

[bors]

✌️ @hkBst, you can now approve this pull request!

If @oli-obk told you to "r=me" after making some further change, please make that change, then do @bors r=@oli-obk

[hkBst]

Looks fine now, but please squash your commits.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[IntegralPilot]

@hkBst, just squashed the commits! Thanks again for your review & help :)

[hkBst]

@bors r=@hkBst

[bors]

📌 Commit 67802e0 has been approved by hkBst

It is now in the queue for this repository.