ICE on invalid size/alignment in CTFE validation

[RalfJung]

The following code ICEs:

#[repr(transparent)]
struct W<T>(T);

// The drop fn is checked before size/align are, so get ourselves a "sufficiently valid" drop fn
fn drop_me(_: *mut usize) {}

const INVALID_VTABLE_SIZE: W<&dyn Send> =
    unsafe { std::mem::transmute((&92u8, &(drop_me as fn(*mut usize), usize::MAX, 1usize))) };

To fix this, this code here needs to properly catch the "invalid size/align" error and turn it into a validation failure:

rust/compiler/rustc_mir/src/interpret/validity.rs

Line 356 in 563ab4a

self.ecx.read_size_and_align_from_vtable(vtable),

But for this we first need to make these errors catchable by introducing separate variants into this enum:

rust/compiler/rustc_middle/src/mir/interpret/error.rs

Line 211 in 2779fc1

pub enum UndefinedBehaviorInfo<'tcx> {

You can use the existing InvalidDropFn variant as a model; it does the same job for invalid drop function pointers in the vtable.

Cc @lqd

[lqd]

I'll take care of this

@rustbot claim