Functions pointers with associated type parameters can't cast to pointers

[michaelwu]

trait Zoo {
    type X;
}

impl Zoo for u16 {
    type X = usize;
}

fn foo(abc: <u16 as Zoo>::X) {}

fn main() {
    let x: *const u8 = foo as _;
}

(Playground)

Errors:

   Compiling playground v0.0.1 (file:///playground)
error[E0605]: non-primitive cast: `fn(<u16 as Zoo>::X) {foo}` as `*const u8`
  --> src/main.rs:13:24
   |
13 |     let x: *const u8 = foo as _;
   |                        ^^^^^^^^
   |
   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait

error: aborting due to previous error

For more information about this error, try `rustc --explain E0605`.
error: Could not compile `playground`.

To learn more, run the command again with --verbose.

Seems like this should work.

[Havvy]

The following works:

• Switching abc to be of type usize itself.
• Adding type X = usize, and then switching abc to be of type X

[eddyb]

Huh, what if you cast it to fn(_)? That works, right?

[michaelwu]

Adding an intermediate cast to fn(_) does make it work. Playground

[eddyb]

So having a projection in the type, blocks the cast from happening? That's a bit weird.
cc @nikomatsakis @arielb1

[arielb1]

The fn_sig function on fn item types is returning unnormalized types directly from the tcx.

Defined in

rust/src/librustc/ty/sty.rs

Lines 1739 to 1747 in f1aefb4

	pub fn fn_sig(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>) -> PolyFnSig<'tcx> {
	match self.sty {
	FnDef(def_id, substs) => {
	tcx.fn_sig(def_id).subst(tcx, substs)
	}
	FnPtr(f) => f,
	_ => bug!("Ty::fn_sig() called on non-fn type: {:?}", self)
	}
	}

The relevant call-site is at

rust/src/librustc_typeck/check/cast.rs

Lines 432 to 443 in f1aefb4

	if let ty::FnDef(..) = self.expr_ty.sty {
	// Attempt a coercion to a fn pointer type.
	let f = self.expr_ty.fn_sig(fcx.tcx);
	let res = fcx.try_coerce(self.expr,
	self.expr_ty,
	fcx.tcx.mk_fn_ptr(f),
	AllowTwoPhase::No);
	if !res.is_ok() {
	return Err(CastError::NonScalar);
	}
	(FnPtr, t_cast)
	} else {