Tracking issue for unsupported_calling_conventions (cdecl, stdcall, fastcall)

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This is a tracking issue for the unsupported_calling_conventions lint. This lint warns against

• Using extern "cdecl" on non-x86-32 targets. Use extern "C" instead, there is no difference in behavior.
• Using extern "stdcall" or extern "fastcall" on non-x86-32 Windows targets. These ABIs are only defined for x86-32.
  • If you want to use "stdcall on x86-32-windows, C everywhere else", then use extern "system".
  • Otherwise, if you really need the same function both on x86-32 and on other targets but with a different ABI, consider using the fn_abi crate. Also we'd be interested to hear about your use-case, so please leave a comment below.

Implementation history:

• https://github.com/rust-lang/rust/pull/141435

Timeline:

• This was introduced with Rust 1.89.
• There's no timeline yet for when this will show up in cargo's future-compat reports, or be turned into a hard error. Since windows-sys needs an update to v0.60 to avoid the lint, we'll have to move really slowly here.

Original issue description

We generally accept stdcall on non-x86-32 Windows targets (while rejecting it on other non-x86-32 targets). See the discussion about this at https://github.com/rust-lang/rust/pull/86231#issuecomment-866325136 and https://github.com/rust-lang/rust/pull/86231#discussion_r663851615. However, @ChrisDenton points out that the raw-dylib feature is more strict here. That's an inconsistency that we should probably fix?

@ChrisDenton can you give an example of what exactly is being rejected by raw-dylib here?

2025-05-05: See here for the current latest proposal.

Cc @workingjubilee @rust-lang/lang @nagisa @petrochenkov

Start by reviewing the unsupported_calling_conventions lint description, the implementation history in PR #141435, and the linked raw-dylib discussion. Compare the stated cdecl, stdcall, and fastcall behavior with the current proposal; done means resolving the documented inconsistency and agreeing on the future lint or error timeline.