backport: move new c abi abort behavior behind feature gate #84722
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### Background
In rust-lang#76570, new ABI strings including `C-unwind` were introduced.
Their behavior is specified in RFC 2945 [1].
However, it was reported in the #ffi-unwind stream of the Rust
community Zulip that this had altered the way that `extern "C"`
functions behaved even when the `c_unwind` feature gate was not
active. [2]
### Overview
This makes a small patch to
`rustc_mir_build::build::should_abort_on_panic`, so that the same
behavior from before is in place when the `c_unwind` gate is not
active.
`rustc_middle::ty::layout::fn_can_unwind` is not touched, as the
visible behavior should not differ before/after rust-lang#76570. [3]
### Footnotes
[1]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md
[2]: https://rust-lang.zulipchat.com/#narrow/stream/210922-project-ffi-unwind/topic/Is.20unwinding.20through.20extern.20C.20UB.3F/near/230112325
[3]: https://github.com/rust-lang/rust/pull/76570/files#diff-b0320c2b8868f325d83c027fc5d71732636e9763551e35895488f30fe057c6e9L2599-R2617
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This is a backport of PR #84158 to the beta branch.
The original T-compiler plan was to revert PR #76570 in its entirety, as was attempted in PR #84672. But the revert did not go smoothly (details below).
Therefore, we are backporting PR #84158 instead, which was our established backup plan if a revert did not go smoothly.
I have manually confirmed that this backport fixes the luajit issue described on issue #83541
Click for details as to why revert of PR #76570 did not go smoothly.
It turns out that Miri had been subsequently updated to reflect changes to
rustc_targetthat landed in PR #76570. This meant that the attempt to land PR #84672 broke Miri builds.Normally we allow tools to break when landing PR's (and just expect follow-up PR's to fix the tools), but we don't allow it for tools in the run-up to a release.
(We shouldn't be using that uniform policy for all tools. Miri should be allow to break during the week before a release; but currently we cannot express that, due to issue #74709.)
Therefore, its a lot of pain to try to revert PR #76570. And we're going with the backup plan.
Original commit message follows:
Background
In #76570, new ABI strings including
C-unwindwere introduced. Their behavior is specified in RFC 2945 1.However, it was reported in the #ffi-unwind stream of the Rust community Zulip that this had altered the way that
extern "C"functions behaved even when thec_unwindfeature gate was not active. 2Overview
This makes a small patch to
rustc_mir_build::build::should_abort_on_panic, so that the same behavior from before is in place when thec_unwindgate is not active.rustc_middle::ty::layout::fn_can_unwindis not touched, as the visible behavior should not differ before/after #76570. 3Footnotes
1.: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md
2.: https://rust-lang.zulipchat.com/#narrow/stream/210922-project-ffi-unwind/topic/Is.20unwinding.20through.20extern.20C.20UB.3F/near/230112325
3.: https://github.com/rust-lang/rust/pull/76570/files#diff-b0320c2b8868f325d83c027fc5d71732636e9763551e35895488f30fe057c6e9L2599-R2617