Currently, the Ty<'tcx> type is a reference to a struct (&TyS) that packages up a big ol' enum TypeVariants. As part of chalkification, we would like to be able to write more efficient "decision tree" like procedures that branch on this type. This would be made nicer by factoring out the type into two parts, a "core" and a "substs". This core would be roughly equivalent to the existing TypeVariants, but without the recursive references to other types, which would be uniformly packaged up in the substs.
Thus I imagine that the TyS struct would change to something like this:
pub struct TyS<'tcx> {
pub sty: TypeVariants<'tcx>, // this is the "core" part
pub substs: &'tcx Substs<'tcx>, // this is the "substs" part, extracted from `TypeVariants`
pub flags: TypeFlags,
// the maximal depth of any bound regions appearing in this type.
region_depth: u32,
}(Incidentally, this new notion of TypeVariants could maybe subsume the existing SimplifiedType, though that's not super important.)
Steps
This transition is best made in steps:
Unknowns
It's not 100% clear that this is a good plan, though the individual refactorings above all seem like good steps regardless. Some complicating factors:
cc @eddyb, who first suggested this to me
cc @tschottdorf, who implemented #42297, a step along this path (and an important step for ATC regardless)
Currently, the
Ty<'tcx>type is a reference to a struct (&TyS) that packages up a big ol' enumTypeVariants. As part of chalkification, we would like to be able to write more efficient "decision tree" like procedures that branch on this type. This would be made nicer by factoring out the type into two parts, a "core" and a "substs". This core would be roughly equivalent to the existingTypeVariants, but without the recursive references to other types, which would be uniformly packaged up in thesubsts.Thus I imagine that the
TySstruct would change to something like this:(Incidentally, this new notion of
TypeVariantscould maybe subsume the existingSimplifiedType, though that's not super important.)Steps
This transition is best made in steps:
TyTupleto use a substsClosureSubststype used inTyClosureand make it into a normal substsGeneratorSubsts]Unknowns
It's not 100% clear that this is a good plan, though the individual refactorings above all seem like good steps regardless. Some complicating factors:
TyFnPtrembodies aPolyFnSig, which carries a binder.cc @eddyb, who first suggested this to me
cc @tschottdorf, who implemented #42297, a step along this path (and an important step for ATC regardless)