Implement selection for unsize for better coercion behavior
This commit is contained in:
Michael Goulet
parent
eee6b31c0c
commit
77c3cf1bfd
@ -1,5 +1,6 @@
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use std::ops::ControlFlow;
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use rustc_hir as hir;
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use rustc_hir::def_id::DefId;
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use rustc_infer::infer::{DefineOpaqueTypes, InferCtxt, InferOk};
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use rustc_infer::traits::util::supertraits;
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@ -11,7 +12,7 @@
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ImplSource, ImplSourceObjectData, ImplSourceTraitUpcastingData, ImplSourceUserDefinedData,
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ObligationCause, SelectionError,
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};
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use rustc_middle::ty::{self, TyCtxt};
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use rustc_middle::ty::{self, Ty, TyCtxt};
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use rustc_span::DUMMY_SP;
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use crate::solve::assembly::{BuiltinImplSource, Candidate, CandidateSource};
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@ -113,6 +114,12 @@ fn select_in_new_trait_solver(
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),
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) => rematch_object(self, goal, nested_obligations),
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(Certainty::Maybe(_), CandidateSource::BuiltinImpl(BuiltinImplSource::Misc))
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if self.tcx.lang_items().unsize_trait() == Some(goal.predicate.def_id()) =>
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{
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rematch_unsize(self, goal, nested_obligations)
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}
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// Technically some builtin impls have nested obligations, but if
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// `Certainty::Yes`, then they should've all been verified and don't
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// need re-checking.
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@ -232,6 +239,9 @@ fn rematch_object<'tcx>(
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{
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assert_eq!(source_kind, ty::Dyn, "cannot upcast dyn*");
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if let ty::Dynamic(data, _, ty::Dyn) = goal.predicate.trait_ref.substs.type_at(1).kind() {
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// FIXME: We also need to ensure that the source lifetime outlives the
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// target lifetime. This doesn't matter for codegen, though, and only
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// *really* matters if the goal's certainty is ambiguous.
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(true, data.principal().unwrap().with_self_ty(infcx.tcx, self_ty))
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} else {
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bug!()
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@ -305,3 +315,136 @@ fn rematch_object<'tcx>(
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ImplSource::Object(ImplSourceObjectData { vtable_base, nested })
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}))
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}
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/// The `Unsize` trait is particularly important to coercion, so we try rematch it.
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/// NOTE: This must stay in sync with `consider_builtin_unsize_candidate` in trait
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/// goal assembly in the solver, both for soundness and in order to avoid ICEs.
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fn rematch_unsize<'tcx>(
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infcx: &InferCtxt<'tcx>,
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goal: Goal<'tcx, ty::TraitPredicate<'tcx>>,
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mut nested: Vec<PredicateObligation<'tcx>>,
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) -> SelectionResult<'tcx, Selection<'tcx>> {
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let tcx = infcx.tcx;
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let a_ty = goal.predicate.self_ty();
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let b_ty = goal.predicate.trait_ref.substs.type_at(1);
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match (a_ty.kind(), b_ty.kind()) {
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(_, &ty::Dynamic(data, region, ty::Dyn)) => {
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// Check that the type implements all of the predicates of the def-id.
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// (i.e. the principal, all of the associated types match, and any auto traits)
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nested.extend(data.iter().map(|pred| {
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Obligation::new(
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infcx.tcx,
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ObligationCause::dummy(),
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goal.param_env,
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pred.with_self_ty(tcx, a_ty),
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)
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}));
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// The type must be Sized to be unsized.
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let sized_def_id = tcx.require_lang_item(hir::LangItem::Sized, None);
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nested.push(Obligation::new(
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infcx.tcx,
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ObligationCause::dummy(),
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goal.param_env,
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ty::TraitRef::new(tcx, sized_def_id, [a_ty]),
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));
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// The type must outlive the lifetime of the `dyn` we're unsizing into.
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nested.push(Obligation::new(
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infcx.tcx,
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ObligationCause::dummy(),
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goal.param_env,
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ty::Binder::dummy(ty::OutlivesPredicate(a_ty, region)),
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));
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}
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// `[T; n]` -> `[T]` unsizing
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(&ty::Array(a_elem_ty, ..), &ty::Slice(b_elem_ty)) => {
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nested.extend(
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infcx
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.at(&ObligationCause::dummy(), goal.param_env)
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.eq(DefineOpaqueTypes::No, a_elem_ty, b_elem_ty)
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.expect("expected rematch to succeed")
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.into_obligations(),
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);
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}
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// Struct unsizing `Struct<T>` -> `Struct<U>` where `T: Unsize<U>`
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(&ty::Adt(a_def, a_substs), &ty::Adt(b_def, b_substs))
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if a_def.is_struct() && a_def.did() == b_def.did() =>
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{
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let unsizing_params = tcx.unsizing_params_for_adt(a_def.did());
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// We must be unsizing some type parameters. This also implies
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// that the struct has a tail field.
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if unsizing_params.is_empty() {
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bug!("expected rematch to succeed")
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}
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let tail_field = a_def
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.non_enum_variant()
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.fields
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.raw
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.last()
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.expect("expected unsized ADT to have a tail field");
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let tail_field_ty = tcx.type_of(tail_field.did);
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let a_tail_ty = tail_field_ty.subst(tcx, a_substs);
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let b_tail_ty = tail_field_ty.subst(tcx, b_substs);
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// Substitute just the unsizing params from B into A. The type after
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// this substitution must be equal to B. This is so we don't unsize
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// unrelated type parameters.
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let new_a_substs =
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tcx.mk_substs_from_iter(a_substs.iter().enumerate().map(|(i, a)| {
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if unsizing_params.contains(i as u32) { b_substs[i] } else { a }
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}));
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let unsized_a_ty = Ty::new_adt(tcx, a_def, new_a_substs);
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nested.extend(
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infcx
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.at(&ObligationCause::dummy(), goal.param_env)
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.eq(DefineOpaqueTypes::No, unsized_a_ty, b_ty)
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.expect("expected rematch to succeed")
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.into_obligations(),
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);
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// Finally, we require that `TailA: Unsize<TailB>` for the tail field
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// types.
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nested.push(Obligation::new(
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tcx,
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ObligationCause::dummy(),
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goal.param_env,
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ty::TraitRef::new(tcx, goal.predicate.def_id(), [a_tail_ty, b_tail_ty]),
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));
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}
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// Tuple unsizing `(.., T)` -> `(.., U)` where `T: Unsize<U>`
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(&ty::Tuple(a_tys), &ty::Tuple(b_tys))
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if a_tys.len() == b_tys.len() && !a_tys.is_empty() =>
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{
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let (a_last_ty, a_rest_tys) = a_tys.split_last().unwrap();
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let b_last_ty = b_tys.last().unwrap();
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// Substitute just the tail field of B., and require that they're equal.
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let unsized_a_ty =
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Ty::new_tup_from_iter(tcx, a_rest_tys.iter().chain([b_last_ty]).copied());
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nested.extend(
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infcx
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.at(&ObligationCause::dummy(), goal.param_env)
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.eq(DefineOpaqueTypes::No, unsized_a_ty, b_ty)
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.expect("expected rematch to succeed")
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.into_obligations(),
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);
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// Similar to ADTs, require that the rest of the fields are equal.
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nested.push(Obligation::new(
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tcx,
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ObligationCause::dummy(),
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goal.param_env,
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ty::TraitRef::new(tcx, goal.predicate.def_id(), [*a_last_ty, *b_last_ty]),
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));
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}
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// FIXME: We *could* ICE here if either:
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// 1. the certainty is `Certainty::Yes`,
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// 2. we're in codegen (which should mean `Certainty::Yes`).
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_ => return Ok(None),
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}
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Ok(Some(ImplSource::Builtin(nested)))
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}
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@ -1,4 +1,6 @@
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// run-pass
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// revisions: classic next
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//[next] compile-flags: -Ztrait-solver=next
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trait Foo: Fn(i32) -> i32 + Send {}
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13
tests/ui/traits/new-solver/unsize-although-ambiguous.rs
Normal file
13
tests/ui/traits/new-solver/unsize-although-ambiguous.rs
Normal file
@ -0,0 +1,13 @@
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// check-pass
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// compile-flags: -Ztrait-solver=next
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use std::fmt::Display;
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fn box_dyn_display(_: Box<dyn Display>) {}
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fn main() {
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// During coercion, we don't necessarily know whether `{integer}` implements
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// `Display`. Before, that would cause us to bail out in the coercion loop when
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// checking `{integer}: Unsize<dyn Display>`.
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box_dyn_display(Box::new(1));
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}
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