…ductive, r=compiler-errors

Enable coinduction support for Safe Transmute

This patch adds the `#[rustc_coinductive]` annotation to `BikeshedIntrinsicFrom`, so that it's possible to compute transmutability for recursive types.

## Motivation
Safe Transmute currently already supports references (rust-lang#110662). However, if a type is implemented recursively, it leads to an infinite loop when we try to check if transmutation is safe.

A couple simple examples that one might want to write, that are currently not possible to check transmutability for:
```rs
#[repr(C)] struct A(&'static B);
#[repr(C)] struct B(&'static A);
```

```rs
#[repr(C)]
enum IList<'a> { Nil, Cons(isize, &'a IList<'a>) }
#[repr(C)]
enum UList<'a> { Nil, Cons(usize, &'a UList<'a>) }
```

Previously, `@jswrenn` was considering writing a co-inductive solver from scratch, just for the `rustc_tranmsute` crate. Later on as I started working on Safe Transmute myself, I came across the `#[rustc_coinductive]` annotation, which is currently only being used for the `Sized` trait. Leveraging this trait actually solved the problem entirely, and it saves a lot of duplicate work that would have had to happen in `rustc_transmute`.

…piler-errors

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 rust-lang#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`

Rollup merge of rust-lang#122438 - jswrenn:check-referent-size, r=compiler-errors

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 rust-lang#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`