Target-specific ABIs are allowed in compatible targets

[gnzlbg]

For example, one can use the ptx-kernel ABI on a x86_64 target. What the following does, I have absolutely no idea:

#![feature(abi_ptx)]

pub extern "ptx-kernel" fn foo(x: i32) -> i32 { x } 

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
    Finished dev [unoptimized + debuginfo] target(s) in 0.53s

cc @eddyb @rkruppe

[gnzlbg]

@jonas-schievink I'm not sure what the best label would be, but this isn't just an FFI issue, since one can call extern "ptx-kernel" functions defined in Rust from Rust. Basically, extern "ABI" just lets you tell the Rust compiler which calling convention should it use for a function (well, kind of, because of implicit no-mangle in some contexts, but that's another story).

I think that we do not check for any of the ABIs whether they make sense for the target, so it is very likely that there is a stable ABI that can be used in a stable target for which it doesn't make much sense.

For example, the win32 ABI "stdcall" can be used on stable Rust on x86_64-unknown-linux-gnu. . That does something, but I'm not sure what that something would be. If we provide no guarantees about what that does, then the behavior is undefined, and this would be a soundness bug since no unsafe Rust code is required to exhibit undefined behavior.

[jonas-schievink]

@gnzlbg Yeah, specifying an ABI is most commonly used for FFI though, and that was the closest label we have, so I picked that. Feel free to change it to something else.

[Centril]

@jonas-schievink btw, if the current label set is insufficient, feel free to add new ones. :)

[nagisa]

Similar to #57182.

[gnzlbg]

Closing as a duplicate. I will just post there.