Constant SLICE_ELEM_PLACEHOLDER 

const SLICE_ELEM_PLACEHOLDER: &str = "u8";

Placeholder type name substituted for the $elem token before parsing. The JSON $elem is a bare type argument (e.g. Align($self, $elem)), and the real element type — which can be a composite like [T; N] that has no resolvable name — is injected afterwards by replace_ty_args. Any primitive placeholder resolves cleanly through the name-based type parser.