elaborate why dropping principal in *dyn casts is non-trivial

Author: Lukas Markeffsky

Diff for: compiler/rustc_hir_typeck/src/cast.rs

@@ -959,8 +959,15 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     // dyn Auto -> dyn Auto'? ok.
                     (None, None) => Ok(CastKind::PtrPtrCast),
 
-                    // dyn Trait -> dyn Auto? should be ok, but we used to not allow it.
-                    // FIXME: allow this
+                    // dyn Trait -> dyn Auto? not ok (for now).
+                    //
+                    // As a validity invariant of pointers to trait objects, we
+                    // currently require the layout of the vtable in the metadata
+                    // to exactly match the pointee's expected vtable layout.
+                    //
+                    // Since ptr-to-ptr casts, unlike unsizing coercions, cannot change
+                    // the pointer metadata, dropping a the principal in a (non-coercion)
+                    // cast is currently not allowed.
                     (Some(_), None) => Err(CastError::DifferingKinds { src_kind, dst_kind }),
 
                     // dyn Auto -> dyn Trait? not ok.

Diff for: tests/ui/cast/ptr-to-trait-obj-drop-principal.rs

@@ -0,0 +1,21 @@
+//! Test that non-coercion casts aren't allowed to drop the principal,
+//! because they cannot modify the pointer metadata.
+//!
+//! We test this in a const context to guard against UB if this is allowed
+//! in the future.
+
+trait Trait {}
+impl Trait for () {}
+
+struct Wrapper<T: ?Sized>(T);
+
+const OBJECT: *const (dyn Trait + Send) = &();
+
+// coercions are allowed
+const _: *const dyn Send = OBJECT as _;
+
+// casts are **not** allowed
+const _: *const Wrapper<dyn Send> = OBJECT as _;
+//~^ ERROR casting `*const (dyn Trait + Send + 'static)` as `*const Wrapper<dyn Send>` is invalid
+
+fn main() {}

Diff for: tests/ui/cast/ptr-to-trait-obj-drop-principal.stderr

@@ -0,0 +1,11 @@
+error[E0606]: casting `*const (dyn Trait + Send + 'static)` as `*const Wrapper<dyn Send>` is invalid
+  --> $DIR/ptr-to-trait-obj-drop-principal.rs:18:37
+   |
+LL | const _: *const Wrapper<dyn Send> = OBJECT as _;
+   |                                     ^^^^^^^^^^^
+   |
+   = note: the trait objects may have different vtables
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0606`.