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Auto merge of #76570 - cratelyn:implement-rfc-2945-c-unwind-abi, r=Amanieu

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Implement RFC 2945: "C-unwind" ABI

## Implement RFC 2945: "C-unwind" ABI

This branch implements [RFC 2945]. The tracking issue for this RFC is #74990.

The feature gate for the issue is `#![feature(c_unwind)]`.

This RFC was created as part of the ffi-unwind project group tracked at rust-lang/lang-team#19.

### Changes

Further details will be provided in commit messages, but a high-level overview
of the changes follows:

* A boolean `unwind` payload is added to the `C`, `System`, `Stdcall`,
and `Thiscall` variants, marking whether unwinding across FFI boundaries is
acceptable. The cases where each of these variants' `unwind` member is true
correspond with the `C-unwind`, `system-unwind`, `stdcall-unwind`, and
`thiscall-unwind` ABI strings introduced in RFC 2945 [3].

* This commit adds a `c_unwind` feature gate for the new ABI strings.
Tests for this feature gate are included in `src/test/ui/c-unwind/`, which
ensure that this feature gate works correctly for each of the new ABIs.
A new language features entry in the unstable book is added as well.

* We adjust the `rustc_middle::ty::layout::fn_can_unwind` function,
used to compute whether or not a `FnAbi` object represents a function that
should be able to unwind when `panic=unwind` is in use.

* Changes are also made to
`rustc_mir_build::build::should_abort_on_panic` so that the function ABI is
used to determind whether it should abort, assuming that the `panic=unwind`
strategy is being used, and no explicit unwind attribute was provided.

[RFC 2945]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md
This commit is contained in:
bors 2021-03-10 16:44:04 +00:00
commit 17a07d71bf
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# `c_unwind`
The tracking issue for this feature is: [#74990]
[#74990]: https://github.com/rust-lang/rust/issues/74990
------------------------
Introduces four new ABI strings: "C-unwind", "stdcall-unwind",
"thiscall-unwind", and "system-unwind". These enable unwinding from other
languages (such as C++) into Rust frames and from Rust into other languages.
See [RFC 2945] for more information.
[RFC 2945]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md