Miri engine is too permissive with type-changing MIR assignments

[oli-obk]

rust/src/librustc_mir/interpret/place.rs

Line 856 in 02046a5

src.layout.details == dest.layout.details,

just checks whether the layout of the target and value of an assignment is the same. The only time the types can differ is when assigning a &mut T to a &T variable. We should instead employ a function like

fn mir_assign_valid_types(dst: Ty<'tcx>, src: Ty<'tcx>) {
    dst == src || match (&dst.kind, &src.kind) {
        (ty::Ref(_, _, dst_pointee), ty::Ref(_, _, src_pointee)) => dst_pointee == src_pointee,
        _ => false,
    }
}

(source #69700 (comment))

be sure to also add appropriate mutability checks to the patterns (mutable for the source, immutable for the dest)

[RalfJung]

@eddyb do we want to keep this open, as you have some future plans for checking subtyping or some such thing?

[eddyb]

I'm not sure, I think we should experiment with deeper lifetime erasure but I likely won't have the time to get involved myself any time soon. Maybe that should have its own issue though.