Repository: rust
Issue: Tracking Issue for `"vectorcall"` ABI
Labels: O-windows,T-lang,C-tracking-issue,A-ABI

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This is a tracking issue for the `"vectorcall"` and `"vectorcall-unwind"` ABIs.
The feature gate for the issue is `#![feature(abi_vectorcall)]`.

(Note that this feature was added in 2015, but didn't have a tracking issue until now.)

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### Steps
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- [x] Implement (#30567)
- [ ] Adjust documentation ([see instructions on rustc-dev-guide][doc-guide])
- [ ] Formatting for new syntax has been added to the [Style Guide] ([nightly-style-procedure])
- [ ] Stabilization PR ([see instructions on rustc-dev-guide][stabilization-guide])

[stabilization-guide]: https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#stabilization-pr
[doc-guide]: https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#documentation-prs
[nightly-style-procedure]: https://github.com/rust-lang/style-team/blob/master/nightly-style-procedure.md 
[Style Guide]: https://github.com/rust-lang/rust/tree/master/src/doc/style-guide

### Unresolved Questions
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Currently an LLVM error is emitted if there is an attempt to use the ABI on a target without the required target features (e.g. i586), but only if the specific usage of the ABI would require use of the disabled registers (e.g. on i586 targets `extern "vectorcall" fn f() -> i64 { ... }` will compile fine but `extern "vectorcall" fn f() -> f64 { ... }` will cause an LLVM error). Should the compiler always error if the ABI is used and SSE2 is disabled? According to https://learn.microsoft.com/en-us/cpp/cpp/vectorcall the ABI is only supported when SSE2 is enabled. Closely related to #116558.

### Implementation history

`"vectorcall"` added in #30567.
`"vectorcall-unwind"` added in #93561.