Miri check does not support `slice::align_to` method

[earthengine]

The following

fn main() {
    let data:[u8;200] = [0;200];
    let (_,m,_) = unsafe { data[1..].align_to::<i32>() };
    println!("m.len()={}", m.len());
}

Playground

prints m.len()=49 in normal run, but m.len()=0 in Miri check.

[RalfJung]

This is not a bug, this is expected behavior. align_to is explicitly documented to give a best-effort answer:

The method does a best effort to make the middle slice the greatest length possible for a given type and input slice, but only your algorithm's performance should depend on that, not its correctness.

The reason is that miri cannot know "how aligned" the allocation is. The 49 in a normal run can change from execution to execution, it depends on which base address got picked. miri does not pick a base address.

[oli-obk]

Closing since it is working as intended as per the RFC rust-lang/rfcs#2043

If anything is unclear about the motivation or documentation, don't hesitate to open an issue about that or ask here