move new c abi abort behavior behind feature gate
### Background In #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 #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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@ -579,7 +579,7 @@ fn should_abort_on_panic(tcx: TyCtxt<'_>, fn_def_id: LocalDefId, abi: Abi) -> bo
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Some(UnwindAttr::Aborts) => true,
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// If no attribute was found and the panic strategy is `unwind`, then we should examine
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// the function's ABI string to determine whether it should abort upon panic.
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None => {
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None if tcx.features().c_unwind => {
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use Abi::*;
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match abi {
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// In the case of ABI's that have an `-unwind` equivalent, check whether the ABI
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@ -609,6 +609,10 @@ fn should_abort_on_panic(tcx: TyCtxt<'_>, fn_def_id: LocalDefId, abi: Abi) -> bo
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| Unadjusted => true,
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}
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}
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// If the `c_unwind` feature gate is not active, follow the behavior that was in place
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// prior to #76570. This is a special case: some functions have a C ABI but are meant to
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// unwind anyway. Don't stop them.
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None => false, // FIXME(#58794); should be `!(abi == Abi::Rust || abi == Abi::RustCall)`
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}
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}
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