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rustc_target: add "unwind" payloads to Abi

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### Overview

    This commit begins the implementation work for RFC 2945. For more
    information, see the rendered RFC [1] and tracking issue [2].

    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].

 ### Feature Gate and Unstable Book

    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.

 ### Further Work To Be Done

    This commit does not proceed to implement the new unwinding ABIs,
    and is intentionally scoped specifically to *defining* the ABIs and
    their feature flag.

 ### One Note on Test Churn

    This will lead to some test churn, in re-blessing hash tests, as the
    deleted comment in `src/librustc_target/spec/abi.rs` mentioned,
    because we can no longer guarantee the ordering of the `Abi`
    variants.

    While this is a downside, this decision was made bearing in mind
    that RFC 2945 states the following, in the "Other `unwind` Strings"
    section [3]:

    >  More unwind variants of existing ABI strings may be introduced,
    >  with the same semantics, without an additional RFC.

    Adding a new variant for each of these cases, rather than specifying
    a payload for a given ABI, would quickly become untenable, and make
    working with the `Abi` enum prone to mistakes.

    This approach encodes the unwinding information *into* a given ABI,
    to account for the future possibility of other `-unwind` ABI
    strings.

 ### Ignore Directives

    `ignore-*` directives are used in two of our `*-unwind` ABI test
    cases.

    Specifically, the `stdcall-unwind` and `thiscall-unwind` test cases
    ignore architectures that do not support `stdcall` and
    `thiscall`, respectively.

    These directives are cribbed from
    `src/test/ui/c-variadic/variadic-ffi-1.rs` for `stdcall`, and
    `src/test/ui/extern/extern-thiscall.rs` for `thiscall`.

    This would otherwise fail on some targets, see:
    fcf697f902

 ### Footnotes

[1]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md
[2]: https://github.com/rust-lang/rust/issues/74990
[3]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md#other-unwind-abi-strings
This commit is contained in:
katelyn a. martin 2020-08-27 11:49:18 -04:00
parent 3a5d45f68c
commit df45c579de
29 changed files with 277 additions and 52 deletions
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8
compiler/rustc_middle/src/ty/layout.rs
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@ -2654,14 +2654,14 @@ where
RustIntrinsic | PlatformIntrinsic | Rust | RustCall => Conv::Rust,
// It's the ABI's job to select this, not ours.
System => bug!("system abi should be selected elsewhere"),
System { .. } => bug!("system abi should be selected elsewhere"),
EfiApi => bug!("eficall abi should be selected elsewhere"),
Stdcall => Conv::X86Stdcall,
Stdcall { .. } => Conv::X86Stdcall,
Fastcall => Conv::X86Fastcall,
Vectorcall => Conv::X86VectorCall,
Thiscall => Conv::X86ThisCall,
C => Conv::C,
Thiscall { .. } => Conv::X86ThisCall,
C { .. } => Conv::C,
Unadjusted => Conv::C,
Win64 => Conv::X86_64Win64,
SysV64 => Conv::X86_64SysV,