`#[track_caller]` on `fn`s in `extern "Rust"`

[Centril]

Context: #69251 (comment)

Suppose we have:

extern "Rust" {
    #[track_caller]
    fn bar();
}

This currently produces an error, but @eddyb suggested that it could be allowed.
Then comes the question of what this would mean. One way to imagine this is that we'd alter the ABI of fn bar, as we usually do for #[track_caller] functions, to accept an additional parameter for the Location. When calling bar() we'd then propagate the location, again as usual.

However, the place which actually defines fn bar() { ... } would not be aware of the #[track_caller] annotation, so it seems to me that we'd have to insert a shim where the extern "Rust" block resides.

I've opened this issue to seek some clarity on what @eddyb meant as neither me nor @anp fully understood.

[eddyb]

However, the place which actually defines fn bar() { ... } would not be aware of the #[track_caller] annotation

It would be UB for the definition to not have it, just like unwinding is UB when the extern "Rust" { fn ... } import opted out of unwinding (or didn't opt in, not sure what's the default nowadays).