Inherent associated functions and constants defined on function pointer types won't be found

[fmease]

Since only core & std are allowed to define functions and constants on function pointer types (like fn() or for<'a> fn(&'a ()) -> bool) and since they maybe never will, this issue is of low priority. For other primitive types like !, (), bool etc., inherent associated functions and constants are correctly resolved.

#![feature(rustc_attrs)]
#![rustc_coherence_is_core]

impl fn() {
    const X: () = ();
}

fn user() {
    <fn()>::X; //~ ERROR no associated item named `X` found for fn pointer `fn()` in the current scope
}

Note that the perma-unstable attribute #![rustc_coherence_is_core] is used in the reproducer which doesn't come with any language or stability guarantees. However, this issue is not about it but about the fact that core which uses this attribute internally and std (which can use #[rustc_allow_incoherent_impl]) would not be able to properly define associated items on those types. For the same reason, I'm not labeling this as requires-nightly.

@rustbot label T-compiler A-resolve A-associated-items

[compiler-errors]

@fmease: my understanding is that this just requires to assemble candidates for incoherent impls as well, just like method candidate assembly does.

[fmease]

@compiler-errors Note that this not about inherent associated types but functions & constants. So we need to modify method candidate assembly itself.

I think we just need to add | ty::FnPtr(..) here in assemble_probe haha:

rust/compiler/rustc_hir_typeck/src/method/probe.rs

Line 696 in f9216b7

| ty::Tuple(..) => self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty),

I'm gonna do that later once I have access to my machine.
@rustbot claim

[compiler-errors]

@fmease oh lol, i just saw your name and thought inherent associated types. cool, good luck

[fmease]

The IAT counterpart is #106719 fyi ^^

[fmease]

See https://rust-lang.zulipchat.com/#narrow/stream/326132-t-types.2Fmeetings/topic/2023-08-07.20planning.20meeting/near/382646902