Tracking Issue for `extern "tail"` calling convention

[WaffleLapkin]

This is a tracking issue for extern "tail" calling convention (part of the tail calls experiment, see #112788).
The feature gate for the issue is #![feature(rust_tail_cc)].

This feature allows using "tail" as the calling convention string on functions:

extern "tail" fn nyaa() {}

This calling convention is supposed to be efficient for tail calls.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implement the feature (add extern "tail" calling convention #157016)
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Can we expand the supported architectures (currently llvm only supports tail cc on x86_64 and aarch64)
• Can we make the calling convention inside rustc, instead of relying on llvm?
• Can we list the signature restriction of tail calls for extern "tail" functions?
• Should this feature have an RFC?

Implementation history

• add extern "tail" calling convention #157016