Tracking issue for Fn traits (`unboxed_closures` & `fn_traits` feature)

[aturon]

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Tracks stabilization for the Fn* traits.

Random bugs:

• FnOnce doesn't seem to support type-based dispatch (as Add, etc. do) #45510 – type-based dispatch not working
• Cannot call implementations of the Fn* traits on references #42736 – foo() sugar doesn't work where you have &Foo: FnOnce

[nagisa]

Inability to delegate calls to other FnOnce implementors like this:

struct A<T>(T);

impl<T, Args> FnOnce<Args> for A<T>
where T: FnOnce<Args> {
    type Output = <T as FnOnce<Args>>::Output;
    fn call_once(self, args: Args) -> Self::Output { FnOnce::call_once(self.0, args) }
}

is the reason I have a safety issue in libloading.

[petrochenkov]

FnOnce::Output is stabilized in #34365

[nrc]

Could someone summarise what is blocking stabilisation here please?

[nagisa]

@nrc uncertainty about Args being the right thing given the possibility of variadic generics coming along around 2030 is the most major reason this is unstable.

[aturon]

@nrc also possibly some questions around the inheritance relationship, see #19032