RalfJung
commented
1 year ago An example of code that runs into this comes from this issue:
// check-fail
// This test used to cause an ICE in rustc_mir::interpret::step::eval_rvalue_into_place
#![allow(incomplete_features)]
#![feature(generic_const_exprs)]
use std::fmt::Debug;
use std::marker::PhantomData;
use std::mem::size_of;
struct Inline<T>
where
[u8; size_of::<T>() + 1]: ,
{
_phantom: PhantomData<T>,
buf: [u8; size_of::<T>() + 1],
}
impl<T> Inline<T>
where
[u8; size_of::<T>() + 1]: ,
{
pub fn new(val: T) -> Inline<T> {
todo!()
}
}
fn main() {
let dst = Inline::<dyn Debug>::new(0); //~ ERROR
//~^ ERROR
}So, looks like a problem with const generics? Something is trying to evaluate size_of::<T>() + 1 despite T: ?Sized. That makes me wonder which other kind of invalid MIR const generics might execute?
Given that this code is clearly wrong, wouldn't it be better to ICE instead of having this SizeOfUnsizedType error variant that should not exist? It is marked as an incomplete feature after all, so ICEs are expected. And the current "fix" is papering over the ICE, not fixing the root cause.
Cc @rust-lang/project-const-generics
lcnr
matthiaskrgr
This is the rustc side of https://github.com/rust-lang/project-const-generics/issues/37.