Function type_path_key 

fn type_path_key<'tcx>(
    tcx: TyCtxt<'tcx>,
    ty: Ty<'tcx>,
) -> (String, Option<Ty<'tcx>>)

Generate a normalised type path key for lookups in the type-invariants DB, together with the element type for the built-in slice entry.

ADTs map to their def path (e.g. core::num::NonZero); &[T] / &mut [T] slice references map to the built-in [T] key and carry their element type (resolved via $elem during instantiation); anything else falls back to the type’s Debug representation.