offset_of! accepts unsized tuple field

[tmiasko]

I tried this code:

#![feature(offset_of)]
use std::fmt::Debug;
fn main() {
    core::mem::offset_of!((u8, dyn Debug), 1);
}

I expected the compilation to fail since the offset cannot be statically determined. Instead, the compilation succeeds.

[scottmcm]

cc @clubby789 #112069

[clubby789]

Technically this was possible before as there were no Sized checks on the tuple case, but it no longer ICEs when codegenning the value.
Should

struct S<T: ?Sized> {
    a: u64,
    b: T,
}
fn main() {
    let _a = core::mem::offset_of!(S<dyn std::fmt::Debug>, a);
}

from the linked issue be allowed - is this a problem specific to tuples?

[est31]

is this a problem specific to tuples?

I'd say so, yes. The example you gave fails to compile if you try to get the offset of the b field.

I'd say there is a missing call to require_type_is_sized in the match in compiler/rustc_hir_typeck/src/expr.rs. The arm for ADTs has it, the one for tuples doesn't.