Provide fnptr impls for extern "system"

[CAD97]

extern "system" is a cross-platform ABI. Adding these impls means that we provide the fnptr impls for all cross-platform ABIs.

A perfect solution would be to have the compiler provide these implementations, so that they're available for all ABIs and all airities, not just arities up to 12. Until we have that perfect shiny future, there's imho three defensible positions to take:

• Provide impls for just extern "Rust" and extern "C" — status quo
• Provide impls for all cross-platform ABIs — this PR
• Provide impls for all ABIs, including platform-specific — assumed intractable

The downside of providing more impls is that the fn primitive documentation gets more crowded with more implementations. I think the benefit outweighs the downside here, especially since the trait implementations on that page are already effectively unusable.

(Side note: stable 1.60 only provides one Debug link in the sidebar, whereas beta 1.61 has one for each impl. This is the case for any impl which isn't parameterized.)

[rust-highfive]

Hey! It looks like you've submitted a new PR for the library teams!

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[rust-highfive]

r? @Mark-Simulacrum

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[CAD97]

r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs

[bors]

☔ The latest upstream changes (presumably #97135) made this pull request unmergeable. Please resolve the merge conflicts.

[RalfJung]

The downside of providing more impls is that the fn primitive documentation gets more crowded with more implementations. I think the benefit outweighs the downside here, especially since the trait implementations on that page are already effectively unusable.

doc(hidden) on all instances except extern "Rust" fn could potentially be used to prevent that page from blowing up?

[RalfJung]

This overlaps with #92964.

[CAD97]

I wasn't aware of #92964 when I made this PR; I'll close in favor of that one (or the referenced trait solution).