Lift the same-signature restriction for extern "tail"

[folkertdev]

tracking issue: #157427

The extern "tail" calling convention uses callee cleanup (i.e. the callee restores the stack, not the caller). Hence, the same-signature restriction that is normally required to codegen tail calls does not apply.

We need the ABI to deviate from extern "Rust" to make sure indirect arguments are passed by stack offset, not via a pointer into the caller's stack frame (the value would potentially be overwritten). For standard tail calls we work around this problem by storing the value in the caller's caller, but for extern "tail" that doesn't work reliably because the signatures can be different.

I'm not sure about unsized arguments. That feature seems really broken, so I'm not sure how much work we should put into trying to do something reasonable there. Fundamentally I don't think we can support unsized arguments in extern "tail" calls.

Also we can't really promote using this yet due to tailcc being a bit broken on LLVM 22 on x86_64.

r? WaffleLapkin
cc @bjorn3

[rustbot]

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[bjorn3]

I don't think it is possible supporting unsized arguments with tail calls except for the narrow case where every tail call passes the unsized argument it received from its own caller, in which case Indirect { on_stack: false } would work even with extern "tail". I don't think it is worth supporting this narrow of a rule, so I think disallowing unsized arguments on extern "tail" entirely is the right call.