transmute: fix check for whether newtypes have equal size

Author: RalfJung

compiler/rustc_middle/src/ty/layout.rs

@@ -435,6 +435,21 @@ impl<'tcx> SizeSkeleton<'tcx> {
                 if def.is_union() || def.variants().is_empty() || def.variants().len() > 2 {
                     return Err(err);
                 }
+                // Only default repr types.
+                {
+                    // We can ignore the seed and `repr(transparent)` but we have to check
+                    // everything else.
+                    let ReprOptions { int, align, pack, flags, scalable, field_shuffle_seed: _ } =
+                        def.repr();
+                    if int.is_some()
+                        || align.is_some()
+                        || pack.is_some()
+                        || flags.difference(ReprFlags::IS_TRANSPARENT) != Default::default()
+                        || scalable.is_some()
+                    {
+                        return Err(err);
+                    }
+                }
 
                 // Get a zero-sized variant or a pointer newtype.
                 let zero_or_ptr_variant = |i| {