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4 files changed, 217 insertions, 0 deletions
diff --git a/compiler/rustc_trait_selection/src/solve/assembly.rs b/compiler/rustc_trait_selection/src/solve/assembly.rs index e44fd82ba22..e6c36d37ffb 100644 --- a/compiler/rustc_trait_selection/src/solve/assembly.rs +++ b/compiler/rustc_trait_selection/src/solve/assembly.rs @@ -173,6 +173,14 @@ pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy + Eq { ecx: &mut EvalCtxt<'_, 'tcx>, goal: Goal<'tcx, Self>, ) -> QueryResult<'tcx>; + + // Implement unsizing. The most common forms of unsizing are array to slice, + // and concrete (Sized) type into a `dyn Trait`. ADTs and Tuples can also + // have their final field unsized if it's generic. + fn consider_builtin_unsize_candidate( + ecx: &mut EvalCtxt<'_, 'tcx>, + goal: Goal<'tcx, Self>, + ) -> QueryResult<'tcx>; } impl<'tcx> EvalCtxt<'_, 'tcx> { @@ -303,6 +311,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> { G::consider_builtin_future_candidate(self, goal) } else if lang_items.gen_trait() == Some(trait_def_id) { G::consider_builtin_generator_candidate(self, goal) + } else if lang_items.unsize_trait() == Some(trait_def_id) { + G::consider_builtin_unsize_candidate(self, goal) } else { Err(NoSolution) }; diff --git a/compiler/rustc_trait_selection/src/solve/project_goals.rs b/compiler/rustc_trait_selection/src/solve/project_goals.rs index b175a6dde17..00b4fa67d68 100644 --- a/compiler/rustc_trait_selection/src/solve/project_goals.rs +++ b/compiler/rustc_trait_selection/src/solve/project_goals.rs @@ -554,6 +554,13 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> { .to_predicate(tcx), ) } + + fn consider_builtin_unsize_candidate( + _ecx: &mut EvalCtxt<'_, 'tcx>, + goal: Goal<'tcx, Self>, + ) -> QueryResult<'tcx> { + bug!("`Unsize` does not have an associated type: {:?}", goal); + } } /// This behavior is also implemented in `rustc_ty_utils` and in the old `project` code. diff --git a/compiler/rustc_trait_selection/src/solve/trait_goals.rs b/compiler/rustc_trait_selection/src/solve/trait_goals.rs index 1ea8fb8fd3d..28144da1b1e 100644 --- a/compiler/rustc_trait_selection/src/solve/trait_goals.rs +++ b/compiler/rustc_trait_selection/src/solve/trait_goals.rs @@ -8,6 +8,7 @@ use super::{Certainty, EvalCtxt, Goal, QueryResult}; use rustc_hir::def_id::DefId; use rustc_infer::infer::InferCtxt; use rustc_infer::traits::query::NoSolution; +use rustc_infer::traits::util::supertraits; use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams}; use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt}; use rustc_middle::ty::{TraitPredicate, TypeVisitable}; @@ -238,6 +239,180 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> { .to_predicate(tcx), ) } + + fn consider_builtin_unsize_candidate( + ecx: &mut EvalCtxt<'_, 'tcx>, + goal: Goal<'tcx, Self>, + ) -> QueryResult<'tcx> { + let tcx = ecx.tcx(); + let a_ty = goal.predicate.self_ty(); + let b_ty = goal.predicate.trait_ref.substs.type_at(1); + if b_ty.is_ty_var() { + return ecx.make_canonical_response(Certainty::AMBIGUOUS); + } + ecx.infcx.probe(|_| { + match (a_ty.kind(), b_ty.kind()) { + // Trait upcasting, or `dyn Trait + Auto + 'a` -> `dyn Trait + 'b` + ( + &ty::Dynamic(a_data, a_region, ty::Dyn), + &ty::Dynamic(b_data, b_region, ty::Dyn), + ) => { + // All of a's auto traits need to be in b's auto traits. + let auto_traits_compatible = b_data + .auto_traits() + .all(|b| a_data.auto_traits().any(|a| a == b)); + if !auto_traits_compatible { + return Err(NoSolution); + } + + // If the principal def ids match (or are both none), then we're not doing + // trait upcasting. We're just removing auto traits (or shortening the lifetime). + if a_data.principal_def_id() == b_data.principal_def_id() { + // Require that all of the trait predicates from A match B, except for + // the auto traits. We do this by constructing a new A type with B's + // auto traits, and equating these types. + let new_a_data = a_data + .iter() + .filter(|a| { + matches!( + a.skip_binder(), + ty::ExistentialPredicate::Trait(_) | ty::ExistentialPredicate::Projection(_) + ) + }) + .chain( + b_data + .auto_traits() + .map(ty::ExistentialPredicate::AutoTrait) + .map(ty::Binder::dummy), + ); + let new_a_data = tcx.mk_poly_existential_predicates(new_a_data); + let new_a_ty = tcx.mk_dynamic(new_a_data, b_region, ty::Dyn); + + // We also require that A's lifetime outlives B's lifetime. + let mut nested_obligations = ecx.infcx.eq(goal.param_env, new_a_ty, b_ty)?; + nested_obligations.push(goal.with(tcx, ty::Binder::dummy(ty::OutlivesPredicate(a_region, b_region)))); + + ecx.evaluate_all_and_make_canonical_response(nested_obligations) + } else if let Some(a_principal) = a_data.principal() + && let Some(b_principal) = b_data.principal() + && supertraits(tcx, a_principal.with_self_ty(tcx, a_ty)) + .any(|trait_ref| trait_ref.def_id() == b_principal.def_id()) + { + // FIXME: Intentionally ignoring `need_migrate_deref_output_trait_object` here for now. + // Confirm upcasting candidate + todo!() + } else { + Err(NoSolution) + } + } + // `T` -> `dyn Trait` unsizing + (_, &ty::Dynamic(data, region, ty::Dyn)) => { + // Can only unsize to an object-safe type + // FIXME: Can auto traits be *not* object safe? + if data + .auto_traits() + .chain(data.principal_def_id()) + .any(|def_id| !tcx.is_object_safe(def_id)) + { + return Err(NoSolution); + } + + let Some(sized_def_id) = tcx.lang_items().sized_trait() else { + return Err(NoSolution); + }; + let nested_goals: Vec<_> = data + .iter() + // Check that the type implements all of the predicates of the def-id. + // (i.e. the principal, all of the associated types match, and any auto traits) + .map(|pred| goal.with(tcx, pred.with_self_ty(tcx, a_ty))) + .chain([ + // The type must be Sized to be unsized. + goal.with( + tcx, + ty::Binder::dummy(tcx.mk_trait_ref(sized_def_id, [a_ty])), + ), + // The type must outlive the lifetime of the `dyn` we're unsizing into. + goal.with( + tcx, + ty::Binder::dummy(ty::OutlivesPredicate(a_ty, region)), + ), + ]) + .collect(); + + ecx.evaluate_all_and_make_canonical_response(nested_goals) + } + // `[T; n]` -> `[T]` unsizing + (&ty::Array(a_elem_ty, ..), &ty::Slice(b_elem_ty)) => { + // We just require that the element type stays the same + let nested_goals = ecx.infcx.eq(goal.param_env, a_elem_ty, b_elem_ty)?; + ecx.evaluate_all_and_make_canonical_response(nested_goals) + } + // Struct unsizing `Struct<T>` -> `Struct<U>` where `T: Unsize<U>` + (&ty::Adt(a_def, a_substs), &ty::Adt(b_def, b_substs)) + if a_def.is_struct() && a_def.did() == b_def.did() => + { + let unsizing_params = tcx.unsizing_params_for_adt(a_def.did()); + // We must be unsizing some type parameters. This also implies + // that the struct has a tail field. + if unsizing_params.is_empty() { + return Err(NoSolution); + } + + let tail_field = a_def + .non_enum_variant() + .fields + .last() + .expect("expected unsized ADT to have a tail field"); + let tail_field_ty = tcx.bound_type_of(tail_field.did); + + let a_tail_ty = tail_field_ty.subst(tcx, a_substs); + let b_tail_ty = tail_field_ty.subst(tcx, b_substs); + + // Substitute just the unsizing params from B into A. The type after + // this substitution must be equal to B. This is so we don't unsize + // unrelated type parameters. + let new_a_substs = tcx.mk_substs(a_substs.iter().enumerate().map(|(i, a)| { + if unsizing_params.contains(i as u32) { b_substs[i] } else { a } + })); + let unsized_a_ty = tcx.mk_adt(a_def, new_a_substs); + + // Finally, we require that `TailA: Unsize<TailB>` for the tail field + // types. + let mut nested_goals = ecx.infcx.eq(goal.param_env, unsized_a_ty, b_ty)?; + nested_goals.push(goal.with( + tcx, + ty::Binder::dummy( + tcx.mk_trait_ref(goal.predicate.def_id(), [a_tail_ty, b_tail_ty]), + ), + )); + + ecx.evaluate_all_and_make_canonical_response(nested_goals) + } + // Tuple unsizing `(.., T)` -> `(.., U)` where `T: Unsize<U>` + (&ty::Tuple(a_tys), &ty::Tuple(b_tys)) + if a_tys.len() == b_tys.len() && !a_tys.is_empty() => + { + let (a_last_ty, a_rest_tys) = a_tys.split_last().unwrap(); + let b_last_ty = b_tys.last().unwrap(); + + // Substitute just the tail field of B., and require that they're equal. + let unsized_a_ty = tcx.mk_tup(a_rest_tys.iter().chain([b_last_ty])); + let mut nested_goals = ecx.infcx.eq(goal.param_env, unsized_a_ty, b_ty)?; + + // Similar to ADTs, require that the rest of the fields are equal. + nested_goals.push(goal.with( + tcx, + ty::Binder::dummy( + tcx.mk_trait_ref(goal.predicate.def_id(), [*a_last_ty, *b_last_ty]), + ), + )); + + ecx.evaluate_all_and_make_canonical_response(nested_goals) + } + _ => Err(NoSolution), + } + }) + } } impl<'tcx> EvalCtxt<'_, 'tcx> { diff --git a/tests/ui/traits/new-solver/unsize-good.rs b/tests/ui/traits/new-solver/unsize-good.rs new file mode 100644 index 00000000000..87ed9cfd10a --- /dev/null +++ b/tests/ui/traits/new-solver/unsize-good.rs @@ -0,0 +1,25 @@ +// compile-flags: -Ztrait-solver=next +// check-pass + +#![feature(unsized_tuple_coercion)] + +trait Foo {} + +impl Foo for i32 {} + +fn main() { + // Unsizing via struct + let _: Box<dyn Foo> = Box::new(1i32); + + // Slice unsizing + let y = [1, 2, 3]; + let _: &[i32] = &y; + + // Tuple unsizing + let hi = (1i32,); + let _: &(dyn Foo,) = &hi; + + // Dropping auto traits + let a: &(dyn Foo + Send) = &1; + let _: &dyn Foo = a; +} |