Disable const-stability checking for intrinsics with fallback MIR?

[scottmcm]

As suggested in #122582 (comment), I moved my new intrinsic to core::intrinsics. But that meant I started getting a new error:

error: `typed_swap` is not yet stable as a const fn
   --> library\core\src\mem\mod.rs:731:14
    |
731 |     unsafe { intrinsics::typed_swap(x, y) }
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Is that error useful for things with fallback MIR that's already const-checked? Could we skip it?

(Maybe doing that in the same PR that would make CTFE capable of using fallback MIR?)

cc @oli-obk

[oli-obk]

This is just stability, you can mark intrinsics as const stable without the intrinsic itself becoming stable

[RalfJung]

Your fallback body uses a const-unstable function, so this is still not an intrinsic we'd want to allow calling from stable const fn. So IMO it makes sense that you would have to write #[rustc_const_unstable] here, you'd have to write the same if this was a regular function.

We could consider doing something special for intrinsics where the fallback body is entirely stable. But OTOH, adding a quick #[rustc_const_stable_intrinsic] (with #132492) isn't too much of a big deal either, I think.

[RalfJung]

Turns out with the new const stability checks this actually isn't that hard -- we just have to ensure that fallback bodies are recursively const-stability checked, and then we can take this into account when const-checking an intrinsic call.