dyn-star: "needs to be a pointer-sized type" but the type is pointer-sized

[RalfJung]

Consider this code:

#![feature(dyn_star)]
#![allow(incomplete_features)]

use std::fmt::Debug;

fn main() {
    let _dyn_i: dyn* Debug = [0u8; std::mem::size_of::<usize>()];
}

This raises an error:

error[E0277]: `[u8; 8]` needs to be a pointer-sized type

However, it is a pointer-sized type. Looks like somehow that's still not good enough?

On a related note, I am surprised that this PointerSize marker trait (according to its documentation) cares only about size and not alignment. I think things would go very wrong if we had a type that had ptr size but larger alignment than a ptr, and then turned that into a dyn*...

[RalfJung]

I think I found the code implementing this:

rust/compiler/rustc_trait_selection/src/solve/trait_goals.rs

Lines 134 to 155 in 3de7d7f

	fn consider_builtin_pointer_sized_candidate(
	ecx: &mut EvalCtxt<'_, 'tcx>,
	goal: Goal<'tcx, Self>,
	) -> QueryResult<'tcx> {
	if goal.predicate.self_ty().has_non_region_infer() {
	return ecx.make_canonical_response(Certainty::AMBIGUOUS);
	}
	let tcx = ecx.tcx();
	let self_ty = tcx.erase_regions(goal.predicate.self_ty());
	if let Ok(layout) = tcx.layout_of(goal.param_env.and(self_ty))
	&& let usize_layout = tcx.layout_of(ty::ParamEnv::empty().and(tcx.types.usize)).unwrap().layout
	&& layout.layout.size() == usize_layout.size()
	&& layout.layout.align().abi == usize_layout.align().abi
	{
	// FIXME: We could make this faster by making a no-constraints response
	ecx.make_canonical_response(Certainty::Yes)
	} else {
	Err(NoSolution)
	}
	}

So actually, this trait is about the type having both the size and alignment of a pointer type. That resolves my alignment-related concerns, but means that the doc comment and error message associated with PointerSize (and its name!) are missing part of the story.