Introduce perma-unstable wasm-c-abi flag

[daxpedda]

Proposal

Currently Rust uses a non-spec-compliant ABI for extern "C" fn when compiling to wasm32-unknown-unknown. Unfortunately, as a large part of the Rust ecosystem relies on wasm-bindgen, which didn't support the spec-compliant ABI, switching to the spec-compliant ABI was problematic as it would break a lot of code out there.

With wasm-bindgen v0.2.88, it now supports the spec-compliant ABI, so hopefully some day in the future Rust can switch to this ABI. In the meantime it would be important for users to test this new ABI, which is the purpose of this MCP: to introduce a new flag which switches to this new ABI when targetting wasm32-unknown-unknown.

This flag would be perma-unstable, as there is no point to stabilize it and then just deprecate it when the new ABI becomes the default.

Related issue: rust-lang/rust#71871
Implementation PR: rust-lang/rust#117919

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

@rustbot concern reason-for-concern 
<description of the concern> 

Concerns can be lifted with:

@rustbot resolve reason-for-concern 

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler @rust-lang/compiler-contributors

[oli-obk]

Please keep the conversation in the zulip thread linked in #703 (comment)

[jackh726]

@rustbot second

This will need a tracking issue. This second is also predicated on the overall plan of moving towards a spec-compliant ABI continues as expected. If that changes, the flag should be removed (or at least reevaluated!)

[apiraino]

@rustbot label -final-comment-period +major-change-accepted