Miri does not know about niches

[gnzlbg]

This passes on the playground:

#![feature(rustc_attrs)]
#[rustc_layout_scalar_valid_range_start(0)]
#[rustc_layout_scalar_valid_range_end(15)]
pub struct Imm4(pub u8);

fn main() {
    let _y = unsafe { Imm4(3_u8) };
    let _z = unsafe { Imm4(22_u8) };
}

[RalfJung]

This is a duplicate of #508.

If you look at the MIR, there is no moment here in which an Imm4 is actually assigned, meaning there is no reason to assume that we even have a valid Imm4. let _y = unsafe { Imm4(3_u8) }; becomes (_1.0: u8) = const 3u8;, which is an assignment at type u8.