MGCA: Support struct expressions without intermediary anon consts by BoxyUwU · Pull Request #149114 · rust-lang/rust (original) (raw)
r? oli-obk
tracking issue: #132980
based on: #150025
High level goal
Under feature(min_generic_const_args) this PR adds another kind of const argument. A struct/variant construction const arg kind. We represent the values of the fields as themselves being const arguments which allows for uses of generic parameters subject to the existing restrictions present in min_generic_const_args:
fn foo<const N: Option>() {}
trait Trait { #[type_const] const ASSOC: usize; }
fn bar<T: Trait, const N: u32>() {
// the initializer of _0 is a N which is a legal const argument
// so this is ok.
foo::<{ Some:: { 0: N } }>();
// this is allowed as mgca supports uses of assoc consts in the
// type system. ie `<T as Trait>::ASSOC` is a legal const argument
foo::<{ Some::<u32> { 0: <T as Trait>::ASSOC } }>();
// this on the other hand is not allowed as `N + 1` is not a legal
// const argument
foo::<{ Some::<u32> { 0: N + 1 } }>();}
This PR does not support uses of const ctors, e.g. None. And also does not support tuple constructors, e.g. Some(N). I believe that it would not be difficult to add support for such functionality after this PR lands so have left it out deliberately.
We currently require that all generic parameters on the type being constructed be explicitly specified. I haven't really looked into why that is but it doesn't seem desirable to me as it should be legal to write Some { ... } in a const argument inside of a body and have that desugar to Some::<_> { ... }. Regardless this can definitely be a follow-up PR and I assume this is some underlying consistency with the way that elided args are handled with type paths elsewhere.
This PRs implementation of supporting struct expressions is somewhat incomplete. We don't handle Foo { ..expr } at all and aren't handling privacy/stability. The printing of ConstArgKind::Struct HIR nodes doesn't really exist either :')
I've tried to keep the implementation here somewhat deliberately incomplete as I think a number of these issues are actually quite small and self contained after this PR lands and I'm hoping it could be a good set of issues to mentor newer contributors on 🤔 I just wanted the "bare minimum" required to actually demonstrate that the previous changes are "necessary".
ValTree now recurse through ty::Const
In order to actually represent struct/variant construction in ty::Const without going through an anon const we would need to introduce some new ConstKind variant. Let's say some hypothetical ConstKind::ADT(Ty<'tcx>, List<Const<'tcx>>).
This variant would represent things the same way that ValTree does with the first element representing the VariantIdx of the enum (if its an enum), and then followed by a list of field values in definition order.
This could work but there are a few reasons why it's suboptimal.
First it would mean we have a second kind of Const that can be normalized. Right now we only have ConstKind::Unevaluated which possibly needs normalization. Similarly with TyKind we only have TyKind::Alias. If we introduced ConstKind::ADT it would need to be normalized to a ConstKind::Value eventually. This feels to me like it has the potential to cause bugs in the long run where only ConstKind::Unevaluated is handled by some code paths.
Secondly it would make type equality/inference be kind of... weird... It's desirable for Some { 0: ?x } eq Some { 0: 1_u32 } to result in ?x=1_u32. I can't see a way for this to work with this ConstKind::ADT design under the current architecture for how we represent types/consts and generally do equality operations.
We would need to wholly special case these two variants in type equality and have a custom recursive walker separate from the existing architecture for doing type equality. It would also be somewhat unique in that it's a non-rigid ty::Const (it can be normalized more later on in type inference) while also having somewhat "structural" equality behaviour.
Lastly, it's worth noting that its not actually ConstKind::ADT that we want. It's desirable to extend this setup to also support tuples and arrays, or even references if we wind up supporting those in const generics. Therefore this isn't really ConstKind::ADT but a more general ConstKind::ShallowValue or something to that effect. It represents at least one "layer" of a types value :')
Instead of doing this implementation choice we instead change ValTree::Branch:
enum ValTree<'tcx> { Leaf(ScalarInt), // Before this PR: Branch(Box<[ValTree<'tcx>]>), // After this PR Branch(Box<[Const<'tcx>]>), }
The representation for so called "shallow values" is now the same as the representation for the entire full value. The desired inference/type equality behaviour just falls right out of this. We also don't wind up with these shallow values actually being non-rigid. And ValTree already supports references/tuples/arrays so we can handle those just fine.
I think in the future it might be worth considering inlining ValTree into ty::ConstKind. E.g:
enum ConstKind { Scalar(Ty<'tcx>, ScalarInt), ShallowValue(Ty<'tcx>, List<Const<'tcx>>), Unevaluated(UnevaluatedConst<'tcx>), ... }
This would imply that the usage of ValTrees in patterns would now be using ty::Const but they already kind of are anyway and I think that's probably okay in the long run. It also would mean that the set of things we could represent in const patterns is greater which may be desirable in the long run for supporting things such as const patterns of const generic parameters.
Regardless, this PR doesn't actually inline ValTree into ty::ConstKind, it only changes Branch to recurse through Const. This change could be split out of this PR if desired.
I'm not sure if there'll be a perf impact from this change. It's somewhat plausible as now all const pattern values that have nesting will be interning a lot more Tys. We shall see :>
Forbidding generic parameters under mgca
Under mgca we now allow all const arguments to resolve paths to generic parameters. We then later actually validate that the const arg should be allowed to access generic parameters if it did wind up resolving to any.
This winds up just being a lot simpler to implement than trying to make name resolution "keep track" of whether we're inside of a non-anon-const const arg and then encounter a const { ... } indicating we should now stop allowing resolving to generic parameters.
It's also somewhat in line with what we'll need for a feature(generic_const_args) where we'll want to decide whether an anon const should have any generic parameters based off syntactically whether any generic parameters were used. Though that design is entirely hypothetical at this point :)