Rollup merge of #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 #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`

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -3091,6 +3091,13 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     rustc_transmute::Reason::DstIsTooBig => {
                         format!("The size of `{src}` is smaller than the size of `{dst}`")
                     }
+                    rustc_transmute::Reason::DstRefIsTooBig { src, dst } => {
+                        let src_size = src.size;
+                        let dst_size = dst.size;
+                        format!(
+                            "The referent size of `{src}` ({src_size} bytes) is smaller than that of `{dst}` ({dst_size} bytes)"
+                        )
+                    }
                     rustc_transmute::Reason::SrcSizeOverflow => {
                         format!(
                             "values of the type `{src}` are too big for the current architecture"