Incorrect error when using `mem::size_of` with generics, and "const_size_of" feature enabled.

[peterjoel]

See playground.

I have a function like this:

fn make_empty_array_gen<T>() -> T {
    unsafe {
        let tmp = [0u8; mem::size_of::<T>()];
        mem::transmute(tmp)
    }
}
fn main() {
    let mut foo_gen: [u64; 5] = make_empty_array_gen();
    println!("foo_gen = {:?}", foo_gen);
}

Which, in this usage, I'd expect to be monomorphised like this:

fn make_empty_array() -> [u64; 5] {
    unsafe {
        let tmp = [0u8; mem::size_of::<[u64; 5]>()];
        mem::transmute(tmp)
    }
}

Which has no problems if I just write it like that instead of using generics. But the generic version gives this error:

error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
  --> src/main.rs:14:25
   |
14 |         let tmp = [0u8; mem::size_of::<T>()];
   |                         ^^^^^^^^^^^^^^^^^ `T` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `T`
   = help: consider adding a `where T: std::marker::Sized` bound
   = note: required by `std::mem::size_of`

I'm not sure if this use case is intended to be supported. But, even if it isn't, the error message is very bad, since T: Sized should always be an implied constraint here.

[jannschu]

Probably duplicate of issue #43408.

[peterjoel]

@jannschu Yes, you are right. I'll close this one.