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dyn-star: "needs to be a pointer-sized type" but the type is pointer-sized #107696
Description
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*...
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RalfJung commentedon Feb 5, 2023
I think I found the code implementing this:
rust/compiler/rustc_trait_selection/src/solve/trait_goals.rs
Lines 134 to 155 in 3de7d7f
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.[-]"needs to be a pointer-sized type" but the type is pointer-sized[/-][+]dyn-star: "needs to be a pointer-sized type" but the type is pointer-sized[/+]Rollup merge of rust-lang#107769 - compiler-errors:pointer-like, r=eholk
Rollup merge of rust-lang#107769 - compiler-errors:pointer-like, r=eholk