interpret: use ConstPropNonsense for more const-prop induced issues

Author: RalfJung

Diff for: compiler/rustc_const_eval/src/interpret/place.rs

@@ -826,10 +826,10 @@ where
                 // predicate like `where Self: Sized` with `Self = dyn Trait`.
                 // See #102553 for an example of such a predicate.
                 if src.layout().is_unsized() {
-                    throw_inval!(SizeOfUnsizedType(src.layout().ty));
+                    throw_inval!(ConstPropNonsense);
                 }
                 if dest.layout().is_unsized() {
-                    throw_inval!(SizeOfUnsizedType(dest.layout().ty));
+                    throw_inval!(ConstPropNonsense);
                 }
                 assert_eq!(src.layout().size, dest.layout().size);
                 // Yay, we got a value that we can write directly.

Diff for: compiler/rustc_const_eval/src/interpret/projection.rs

@@ -101,7 +101,7 @@ where
         let (meta, offset) = if field_layout.is_unsized() {
             if base.layout().is_sized() {
                 // An unsized field of a sized type? Sure...
-                // But const-prop actually feeds us such nonsense MIR!
+                // But const-prop actually feeds us such nonsense MIR! (see test `const_prop/issue-86351.rs`)
                 throw_inval!(ConstPropNonsense);
             }
             let base_meta = base.meta(self)?;

Diff for: compiler/rustc_const_eval/src/interpret/step.rs

@@ -269,7 +269,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let ty = self.subst_from_current_frame_and_normalize_erasing_regions(ty)?;
                 let layout = self.layout_of(ty)?;
                 if let mir::NullOp::SizeOf | mir::NullOp::AlignOf = null_op && layout.is_unsized() {
-                    // FIXME: This should be a span_bug (#80742)
+                    // FIXME: This should be a span_bug, but const generics can run MIR
+                    // that is not properly type-checked yet (#97477).
                     self.tcx.sess.delay_span_bug(
                         self.frame().current_span(),
                         format!("{null_op:?} MIR operator called for unsized type {ty}"),