Adjust justification comment

compiler/rustc_middle/src/ty/layout.rs

@@ -3553,12 +3553,17 @@ where
                 // We panic if creating this type with all 0x01 bytes would
                 // cause LLVM UB.
                 //
-                // Therefore, in order for us to not panic,
-                // * the alignment of the pointer must be 1
-                //   (or we would have an unaligned pointer)
+                // Therefore, in order for us to not panic, it must either be a
+                // reference to [T] where T has align 1 (where we don't statically know
+                // the size, so we don't emit any dereferenceable), or a reference to str
+                // which acts much like a [u8].
                 //
-                // * the statically known size of the pointee must be 0.
-                //   (or we would emit dereferenceable)
+                // We *do* need to panic for &dyn Trait, even though the layout of dyn Trait is
+                // size 0 align 1, because &dyn Trait holds a reference to a non-zero sized type,
+                // which also must be aligned.
+                //
+                // This even applies to *const dyn Trait, which holds a reference and therefore
+                // must be valid, so 1-initialization is not okay there.
                 //
                 // If this bypass didn't exist, old versions of `hyper` with no semver compatible
                 // fix (0.11, 0.12, 0.13) would panic, as they make uninit &[u8] and &str.