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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
This commit is contained in:
katelyn a. martin 2021-04-13 09:55:21 -04:00 committed by Felix S. Klock II
parent f97769a2b7
commit 8341f4274d
1 changed files with 5 additions and 1 deletions
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6
compiler/rustc_mir_build/src/build/mod.rs
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@ -579,7 +579,7 @@ fn should_abort_on_panic(tcx: TyCtxt<'_>, fn_def_id: LocalDefId, abi: Abi) -> bo
Some(UnwindAttr::Aborts) => true,
// If no attribute was found and the panic strategy is `unwind`, then we should examine
// the function's ABI string to determine whether it should abort upon panic.
None => {
None if tcx.features().c_unwind => {
use Abi::*;
match abi {
// In the case of ABI's that have an `-unwind` equivalent, check whether the ABI
@ -609,6 +609,10 @@ fn should_abort_on_panic(tcx: TyCtxt<'_>, fn_def_id: LocalDefId, abi: Abi) -> bo
| Unadjusted => true,
}
}
// If the `c_unwind` feature gate is not active, follow the behavior that was in place
// prior to #76570. This is a special case: some functions have a C ABI but are meant to
// unwind anyway. Don't stop them.
None => false, // FIXME(#58794); should be `!(abi == Abi::Rust || abi == Abi::RustCall)`
}
}