Safe Transmute: Require that source referent is smaller than destination
`BikeshedIntrinsicFrom` currently models transmute-via-union; i.e., it attempts to provide a `where` bound for this function:
```rust
pub unsafe fn transmute_via_union<Src, Dst>(src: Src) -> Dst {
use core::mem::*;
#[repr(C)]
union Transmute<T, U> {
src: ManuallyDrop<T>,
dst: ManuallyDrop<U>,
}
let transmute = Transmute { src: ManuallyDrop::new(src) };
// SAFETY: The caller must guarantee that the transmutation is safe.
let dst = transmute.dst;
ManuallyDrop::into_inner(dst)
}
```
A quirk of this model is that it admits padding extensions in value-to-value transmutation: The destination type can be bigger than the source type, so long as the excess consists of uninitialized bytes. However, this isn't permissible for reference-to-reference transmutations (introduced in #110662) — extra referent bytes cannot come from thin air.
This PR patches our analysis for reference-to-reference transmutations to require that the destination referent is no larger than the source referent.
r? `@compiler-errors`
89c3fa92d4