Rollup merge of #104338 - compiler-errors:pointer-sized, r=eholk

Enforce that `dyn*` coercions are actually pointer-sized

Implement a perma-unstable, rudimentary `PointerSized` trait to enforce `dyn*` casts are `usize`-sized for now, at least to prevent ICEs and weird codegen issues from cropping up after monomorphization since currently we enforce *nothing*.

This probably can/should be removed in favor of a more sophisticated trait for handling `dyn*` conversions when we decide on one, but I just want to get something up for discussion and experimentation for now.

r? ```@eholk``` cc ```@tmandry``` (though feel free to claim/reassign)

Fixes #102141
Fixes #102173

Author: Manishearth

core/src/marker.rs

@@ -809,6 +809,15 @@ pub trait Destruct {}
 #[cfg_attr(not(bootstrap), rustc_deny_explicit_impl)]
 pub trait Tuple {}
 
+/// A marker for things
+#[unstable(feature = "pointer_sized_trait", issue = "none")]
+#[cfg_attr(not(bootstrap), lang = "pointer_sized")]
+#[rustc_on_unimplemented(
+    message = "`{Self}` needs to be a pointer-sized type",
+    label = "`{Self}` needs to be a pointer-sized type"
+)]
+pub trait PointerSized {}
+
 /// Implementations of `Copy` for primitive types.
 ///
 /// Implementations that cannot be described in Rust