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move confirmation into submodule

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Bastian Kauschke 2020-05-25 22:33:21 +02:00
parent b8172ec405
commit bf5c62da9e
2 changed files with 827 additions and 804 deletions
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src/librustc_trait_selection/traits/select/confirmation.rs Normal file
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//! Confirmation.
//!
//! Confirmation unifies the output type parameters of the trait
//! with the values found in the obligation, possibly yielding a
//! type error. See the [rustc dev guide] for more details.
//!
//! [rustc dev guide]:
//! https://rustc-dev-guide.rust-lang.org/traits/resolution.html#confirmation
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_hir::lang_items;
use rustc_index::bit_set::GrowableBitSet;
use rustc_infer::infer::InferOk;
use rustc_middle::ty::subst::{GenericArg, GenericArgKind, Subst, SubstsRef};
use rustc_middle::ty::{self, Ty};
use rustc_middle::ty::{ToPolyTraitRef, ToPredicate, WithConstness};
use rustc_span::def_id::DefId;
use crate::traits::project::{self, normalize_with_depth};
use crate::traits::select::TraitObligationExt;
use crate::traits::util;
use crate::traits::util::{closure_trait_ref_and_return_type, predicate_for_trait_def};
use crate::traits::Normalized;
use crate::traits::OutputTypeParameterMismatch;
use crate::traits::Selection;
use crate::traits::TraitNotObjectSafe;
use crate::traits::{BuiltinDerivedObligation, ImplDerivedObligation};
use crate::traits::{ObjectCastObligation, PredicateObligation, TraitObligation};
use crate::traits::{Obligation, ObligationCause};
use crate::traits::{SelectionError, Unimplemented};
use crate::traits::{
VtableAutoImpl, VtableBuiltin, VtableClosure, VtableDiscriminantKind, VtableFnPointer,
VtableGenerator, VtableImpl, VtableObject, VtableParam, VtableTraitAlias,
};
use crate::traits::{
VtableAutoImplData, VtableBuiltinData, VtableClosureData, VtableDiscriminantKindData,
VtableFnPointerData, VtableGeneratorData, VtableImplData, VtableObjectData,
VtableTraitAliasData,
};
use super::BuiltinImplConditions;
use super::SelectionCandidate::{self, *};
use super::SelectionContext;
use std::iter;
impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
pub(super) fn confirm_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
candidate: SelectionCandidate<'tcx>,
) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
debug!("confirm_candidate({:?}, {:?})", obligation, candidate);
match candidate {
BuiltinCandidate { has_nested } => {
let data = self.confirm_builtin_candidate(obligation, has_nested);
Ok(VtableBuiltin(data))
}
ParamCandidate(param) => {
let obligations = self.confirm_param_candidate(obligation, param);
Ok(VtableParam(obligations))
}
ImplCandidate(impl_def_id) => {
Ok(VtableImpl(self.confirm_impl_candidate(obligation, impl_def_id)))
}
AutoImplCandidate(trait_def_id) => {
let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
Ok(VtableAutoImpl(data))
}
ProjectionCandidate => {
self.confirm_projection_candidate(obligation);
Ok(VtableParam(Vec::new()))
}
ClosureCandidate => {
let vtable_closure = self.confirm_closure_candidate(obligation)?;
Ok(VtableClosure(vtable_closure))
}
GeneratorCandidate => {
let vtable_generator = self.confirm_generator_candidate(obligation)?;
Ok(VtableGenerator(vtable_generator))
}
FnPointerCandidate => {
let data = self.confirm_fn_pointer_candidate(obligation)?;
Ok(VtableFnPointer(data))
}
DiscriminantKindCandidate => Ok(VtableDiscriminantKind(VtableDiscriminantKindData)),
TraitAliasCandidate(alias_def_id) => {
let data = self.confirm_trait_alias_candidate(obligation, alias_def_id);
Ok(VtableTraitAlias(data))
}
ObjectCandidate => {
let data = self.confirm_object_candidate(obligation);
Ok(VtableObject(data))
}
BuiltinObjectCandidate => {
// This indicates something like `Trait + Send: Send`. In this case, we know that
// this holds because that's what the object type is telling us, and there's really
// no additional obligations to prove and no types in particular to unify, etc.
Ok(VtableParam(Vec::new()))
}
BuiltinUnsizeCandidate => {
let data = self.confirm_builtin_unsize_candidate(obligation)?;
Ok(VtableBuiltin(data))
}
}
}
fn confirm_projection_candidate(&mut self, obligation: &TraitObligation<'tcx>) {
self.infcx.commit_unconditionally(|snapshot| {
let result =
self.match_projection_obligation_against_definition_bounds(obligation, snapshot);
assert!(result);
})
}
fn confirm_param_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
param: ty::PolyTraitRef<'tcx>,
) -> Vec<PredicateObligation<'tcx>> {
debug!("confirm_param_candidate({:?},{:?})", obligation, param);
// During evaluation, we already checked that this
// where-clause trait-ref could be unified with the obligation
// trait-ref. Repeat that unification now without any
// transactional boundary; it should not fail.
match self.match_where_clause_trait_ref(obligation, param) {
Ok(obligations) => obligations,
Err(()) => {
bug!(
"Where clause `{:?}` was applicable to `{:?}` but now is not",
param,
obligation
);
}
}
}
fn confirm_builtin_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
has_nested: bool,
) -> VtableBuiltinData<PredicateObligation<'tcx>> {
debug!("confirm_builtin_candidate({:?}, {:?})", obligation, has_nested);
let lang_items = self.tcx().lang_items();
let obligations = if has_nested {
let trait_def = obligation.predicate.def_id();
let conditions = if Some(trait_def) == lang_items.sized_trait() {
self.sized_conditions(obligation)
} else if Some(trait_def) == lang_items.copy_trait() {
self.copy_clone_conditions(obligation)
} else if Some(trait_def) == lang_items.clone_trait() {
self.copy_clone_conditions(obligation)
} else {
bug!("unexpected builtin trait {:?}", trait_def)
};
let nested = match conditions {
BuiltinImplConditions::Where(nested) => nested,
_ => bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation),
};
let cause = obligation.derived_cause(BuiltinDerivedObligation);
ensure_sufficient_stack(|| {
self.collect_predicates_for_types(
obligation.param_env,
cause,
obligation.recursion_depth + 1,
trait_def,
nested,
)
})
} else {
vec![]
};
debug!("confirm_builtin_candidate: obligations={:?}", obligations);
VtableBuiltinData { nested: obligations }
}
/// This handles the case where a `auto trait Foo` impl is being used.
/// The idea is that the impl applies to `X : Foo` if the following conditions are met:
///
/// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
/// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
fn confirm_auto_impl_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
trait_def_id: DefId,
) -> VtableAutoImplData<PredicateObligation<'tcx>> {
debug!("confirm_auto_impl_candidate({:?}, {:?})", obligation, trait_def_id);
let types = obligation.predicate.map_bound(|inner| {
let self_ty = self.infcx.shallow_resolve(inner.self_ty());
self.constituent_types_for_ty(self_ty)
});
self.vtable_auto_impl(obligation, trait_def_id, types)
}
/// See `confirm_auto_impl_candidate`.
fn vtable_auto_impl(
&mut self,
obligation: &TraitObligation<'tcx>,
trait_def_id: DefId,
nested: ty::Binder<Vec<Ty<'tcx>>>,
) -> VtableAutoImplData<PredicateObligation<'tcx>> {
debug!("vtable_auto_impl: nested={:?}", nested);
ensure_sufficient_stack(|| {
let cause = obligation.derived_cause(BuiltinDerivedObligation);
let mut obligations = self.collect_predicates_for_types(
obligation.param_env,
cause,
obligation.recursion_depth + 1,
trait_def_id,
nested,
);
let trait_obligations: Vec<PredicateObligation<'_>> =
self.infcx.commit_unconditionally(|_| {
let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
let (trait_ref, _) =
self.infcx.replace_bound_vars_with_placeholders(&poly_trait_ref);
let cause = obligation.derived_cause(ImplDerivedObligation);
self.impl_or_trait_obligations(
cause,
obligation.recursion_depth + 1,
obligation.param_env,
trait_def_id,
&trait_ref.substs,
)
});
// Adds the predicates from the trait. Note that this contains a `Self: Trait`
// predicate as usual. It won't have any effect since auto traits are coinductive.
obligations.extend(trait_obligations);
debug!("vtable_auto_impl: obligations={:?}", obligations);
VtableAutoImplData { trait_def_id, nested: obligations }
})
}
fn confirm_impl_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
impl_def_id: DefId,
) -> VtableImplData<'tcx, PredicateObligation<'tcx>> {
debug!("confirm_impl_candidate({:?},{:?})", obligation, impl_def_id);
// First, create the substitutions by matching the impl again,
// this time not in a probe.
self.infcx.commit_unconditionally(|snapshot| {
let substs = self.rematch_impl(impl_def_id, obligation, snapshot);
debug!("confirm_impl_candidate: substs={:?}", substs);
let cause = obligation.derived_cause(ImplDerivedObligation);
ensure_sufficient_stack(|| {
self.vtable_impl(
impl_def_id,
substs,
cause,
obligation.recursion_depth + 1,
obligation.param_env,
)
})
})
}
fn vtable_impl(
&mut self,
impl_def_id: DefId,
mut substs: Normalized<'tcx, SubstsRef<'tcx>>,
cause: ObligationCause<'tcx>,
recursion_depth: usize,
param_env: ty::ParamEnv<'tcx>,
) -> VtableImplData<'tcx, PredicateObligation<'tcx>> {
debug!(
"vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={})",
impl_def_id, substs, recursion_depth,
);
let mut impl_obligations = self.impl_or_trait_obligations(
cause,
recursion_depth,
param_env,
impl_def_id,
&substs.value,
);
debug!(
"vtable_impl: impl_def_id={:?} impl_obligations={:?}",
impl_def_id, impl_obligations
);
// Because of RFC447, the impl-trait-ref and obligations
// are sufficient to determine the impl substs, without
// relying on projections in the impl-trait-ref.
//
// e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
impl_obligations.append(&mut substs.obligations);
VtableImplData { impl_def_id, substs: substs.value, nested: impl_obligations }
}
fn confirm_object_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> VtableObjectData<'tcx, PredicateObligation<'tcx>> {
debug!("confirm_object_candidate({:?})", obligation);
// FIXME(nmatsakis) skipping binder here seems wrong -- we should
// probably flatten the binder from the obligation and the binder
// from the object. Have to try to make a broken test case that
// results.
let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
let poly_trait_ref = match self_ty.kind {
ty::Dynamic(ref data, ..) => data
.principal()
.unwrap_or_else(|| {
span_bug!(obligation.cause.span, "object candidate with no principal")
})
.with_self_ty(self.tcx(), self_ty),
_ => span_bug!(obligation.cause.span, "object candidate with non-object"),
};
let mut upcast_trait_ref = None;
let mut nested = vec![];
let vtable_base;
{
let tcx = self.tcx();
// We want to find the first supertrait in the list of
// supertraits that we can unify with, and do that
// unification. We know that there is exactly one in the list
// where we can unify, because otherwise select would have
// reported an ambiguity. (When we do find a match, also
// record it for later.)
let nonmatching = util::supertraits(tcx, poly_trait_ref).take_while(|&t| {
match self.infcx.commit_if_ok(|_| self.match_poly_trait_ref(obligation, t)) {
Ok(obligations) => {
upcast_trait_ref = Some(t);
nested.extend(obligations);
false
}
Err(_) => true,
}
});
// Additionally, for each of the non-matching predicates that
// we pass over, we sum up the set of number of vtable
// entries, so that we can compute the offset for the selected
// trait.
vtable_base = nonmatching.map(|t| super::util::count_own_vtable_entries(tcx, t)).sum();
}
VtableObjectData { upcast_trait_ref: upcast_trait_ref.unwrap(), vtable_base, nested }
}
fn confirm_fn_pointer_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> Result<VtableFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
debug!("confirm_fn_pointer_candidate({:?})", obligation);
// Okay to skip binder; it is reintroduced below.
let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
let sig = self_ty.fn_sig(self.tcx());
let trait_ref = closure_trait_ref_and_return_type(
self.tcx(),
obligation.predicate.def_id(),
self_ty,
sig,
util::TupleArgumentsFlag::Yes,
)
.map_bound(|(trait_ref, _)| trait_ref);
let Normalized { value: trait_ref, obligations } = ensure_sufficient_stack(|| {
project::normalize_with_depth(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
&trait_ref,
)
});
self.confirm_poly_trait_refs(
obligation.cause.clone(),
obligation.param_env,
obligation.predicate.to_poly_trait_ref(),
trait_ref,
)?;
Ok(VtableFnPointerData { fn_ty: self_ty, nested: obligations })
}
fn confirm_trait_alias_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
alias_def_id: DefId,
) -> VtableTraitAliasData<'tcx, PredicateObligation<'tcx>> {
debug!("confirm_trait_alias_candidate({:?}, {:?})", obligation, alias_def_id);
self.infcx.commit_unconditionally(|_| {
let (predicate, _) =
self.infcx().replace_bound_vars_with_placeholders(&obligation.predicate);
let trait_ref = predicate.trait_ref;
let trait_def_id = trait_ref.def_id;
let substs = trait_ref.substs;
let trait_obligations = self.impl_or_trait_obligations(
obligation.cause.clone(),
obligation.recursion_depth,
obligation.param_env,
trait_def_id,
&substs,
);
debug!(
"confirm_trait_alias_candidate: trait_def_id={:?} trait_obligations={:?}",
trait_def_id, trait_obligations
);
VtableTraitAliasData { alias_def_id, substs, nested: trait_obligations }
})
}
fn confirm_generator_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> Result<VtableGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
// Okay to skip binder because the substs on generator types never
// touch bound regions, they just capture the in-scope
// type/region parameters.
let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
let (generator_def_id, substs) = match self_ty.kind {
ty::Generator(id, substs, _) => (id, substs),
_ => bug!("closure candidate for non-closure {:?}", obligation),
};
debug!("confirm_generator_candidate({:?},{:?},{:?})", obligation, generator_def_id, substs);
let trait_ref = self.generator_trait_ref_unnormalized(obligation, substs);
let Normalized { value: trait_ref, mut obligations } = ensure_sufficient_stack(|| {
normalize_with_depth(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
&trait_ref,
)
});
debug!(
"confirm_generator_candidate(generator_def_id={:?}, \
trait_ref={:?}, obligations={:?})",
generator_def_id, trait_ref, obligations
);
obligations.extend(self.confirm_poly_trait_refs(
obligation.cause.clone(),
obligation.param_env,
obligation.predicate.to_poly_trait_ref(),
trait_ref,
)?);
Ok(VtableGeneratorData { generator_def_id, substs, nested: obligations })
}
fn confirm_closure_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> Result<VtableClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
debug!("confirm_closure_candidate({:?})", obligation);
let kind = self
.tcx()
.fn_trait_kind_from_lang_item(obligation.predicate.def_id())
.unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
// Okay to skip binder because the substs on closure types never
// touch bound regions, they just capture the in-scope
// type/region parameters.
let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
let (closure_def_id, substs) = match self_ty.kind {
ty::Closure(id, substs) => (id, substs),
_ => bug!("closure candidate for non-closure {:?}", obligation),
};
let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
let Normalized { value: trait_ref, mut obligations } = ensure_sufficient_stack(|| {
normalize_with_depth(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
&trait_ref,
)
});
debug!(
"confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
closure_def_id, trait_ref, obligations
);
obligations.extend(self.confirm_poly_trait_refs(
obligation.cause.clone(),
obligation.param_env,
obligation.predicate.to_poly_trait_ref(),
trait_ref,
)?);
// FIXME: Chalk
if !self.tcx().sess.opts.debugging_opts.chalk {
obligations.push(Obligation::new(
obligation.cause.clone(),
obligation.param_env,
ty::PredicateKind::ClosureKind(closure_def_id, substs, kind)
.to_predicate(self.tcx()),
));
}
Ok(VtableClosureData { closure_def_id, substs, nested: obligations })
}
/// In the case of closure types and fn pointers,
/// we currently treat the input type parameters on the trait as
/// outputs. This means that when we have a match we have only
/// considered the self type, so we have to go back and make sure
/// to relate the argument types too. This is kind of wrong, but
/// since we control the full set of impls, also not that wrong,
/// and it DOES yield better error messages (since we don't report
/// errors as if there is no applicable impl, but rather report
/// errors are about mismatched argument types.
///
/// Here is an example. Imagine we have a closure expression
/// and we desugared it so that the type of the expression is
/// `Closure`, and `Closure` expects an int as argument. Then it
/// is "as if" the compiler generated this impl:
///
/// impl Fn(int) for Closure { ... }
///
/// Now imagine our obligation is `Fn(usize) for Closure`. So far
/// we have matched the self type `Closure`. At this point we'll
/// compare the `int` to `usize` and generate an error.
///
/// Note that this checking occurs *after* the impl has selected,
/// because these output type parameters should not affect the
/// selection of the impl. Therefore, if there is a mismatch, we
/// report an error to the user.
fn confirm_poly_trait_refs(
&mut self,
obligation_cause: ObligationCause<'tcx>,
obligation_param_env: ty::ParamEnv<'tcx>,
obligation_trait_ref: ty::PolyTraitRef<'tcx>,
expected_trait_ref: ty::PolyTraitRef<'tcx>,
) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
self.infcx
.at(&obligation_cause, obligation_param_env)
.sup(obligation_trait_ref, expected_trait_ref)
.map(|InferOk { obligations, .. }| obligations)
.map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
}
fn confirm_builtin_unsize_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> Result<VtableBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
let tcx = self.tcx();
// `assemble_candidates_for_unsizing` should ensure there are no late-bound
// regions here. See the comment there for more details.
let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
let target = self.infcx.shallow_resolve(target);
debug!("confirm_builtin_unsize_candidate(source={:?}, target={:?})", source, target);
let mut nested = vec![];
match (&source.kind, &target.kind) {
// Trait+Kx+'a -> Trait+Ky+'b (upcasts).
(&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
// See `assemble_candidates_for_unsizing` for more info.
let existential_predicates = data_a.map_bound(|data_a| {
let iter = data_a
.principal()
.map(ty::ExistentialPredicate::Trait)
.into_iter()
.chain(data_a.projection_bounds().map(ty::ExistentialPredicate::Projection))
.chain(data_b.auto_traits().map(ty::ExistentialPredicate::AutoTrait));
tcx.mk_existential_predicates(iter)
});
let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
// Require that the traits involved in this upcast are **equal**;
// only the **lifetime bound** is changed.
//
// FIXME: This condition is arguably too strong -- it would
// suffice for the source trait to be a *subtype* of the target
// trait. In particular, changing from something like
// `for<'a, 'b> Foo<'a, 'b>` to `for<'a> Foo<'a, 'a>` should be
// permitted. And, indeed, in the in commit
// 904a0bde93f0348f69914ee90b1f8b6e4e0d7cbc, this
// condition was loosened. However, when the leak check was
// added back, using subtype here actually guides the coercion
// code in such a way that it accepts `old-lub-glb-object.rs`.
// This is probably a good thing, but I've modified this to `.eq`
// because I want to continue rejecting that test (as we have
// done for quite some time) before we are firmly comfortable
// with what our behavior should be there. -nikomatsakis
let InferOk { obligations, .. } = self
.infcx
.at(&obligation.cause, obligation.param_env)
.eq(target, source_trait) // FIXME -- see below
.map_err(|_| Unimplemented)?;
nested.extend(obligations);
// Register one obligation for 'a: 'b.
let cause = ObligationCause::new(
obligation.cause.span,
obligation.cause.body_id,
ObjectCastObligation(target),
);
let outlives = ty::OutlivesPredicate(r_a, r_b);
nested.push(Obligation::with_depth(
cause,
obligation.recursion_depth + 1,
obligation.param_env,
ty::Binder::bind(outlives).to_predicate(tcx),
));
}
// `T` -> `Trait`
(_, &ty::Dynamic(ref data, r)) => {
let mut object_dids = data.auto_traits().chain(data.principal_def_id());
if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
return Err(TraitNotObjectSafe(did));
}
let cause = ObligationCause::new(
obligation.cause.span,
obligation.cause.body_id,
ObjectCastObligation(target),
);
let predicate_to_obligation = |predicate| {
Obligation::with_depth(
cause.clone(),
obligation.recursion_depth + 1,
obligation.param_env,
predicate,
)
};
// Create obligations:
// - Casting `T` to `Trait`
// - For all the various builtin bounds attached to the object cast. (In other
// words, if the object type is `Foo + Send`, this would create an obligation for
// the `Send` check.)
// - Projection predicates
nested.extend(
data.iter().map(|predicate| {
predicate_to_obligation(predicate.with_self_ty(tcx, source))
}),
);
// We can only make objects from sized types.
let tr = ty::TraitRef::new(
tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
tcx.mk_substs_trait(source, &[]),
);
nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
// If the type is `Foo + 'a`, ensure that the type
// being cast to `Foo + 'a` outlives `'a`:
let outlives = ty::OutlivesPredicate(source, r);
nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
}
// `[T; n]` -> `[T]`
(&ty::Array(a, _), &ty::Slice(b)) => {
let InferOk { obligations, .. } = self
.infcx
.at(&obligation.cause, obligation.param_env)
.eq(b, a)
.map_err(|_| Unimplemented)?;
nested.extend(obligations);
}
// `Struct<T>` -> `Struct<U>`
(&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
GenericArgKind::Type(ty) => match ty.kind {
ty::Param(p) => Some(p.index),
_ => None,
},
// Lifetimes aren't allowed to change during unsizing.
GenericArgKind::Lifetime(_) => None,
GenericArgKind::Const(ct) => match ct.val {
ty::ConstKind::Param(p) => Some(p.index),
_ => None,
},
};
// The last field of the structure has to exist and contain type/const parameters.
let (tail_field, prefix_fields) =
def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
let tail_field_ty = tcx.type_of(tail_field.did);
let mut unsizing_params = GrowableBitSet::new_empty();
let mut found = false;
for arg in tail_field_ty.walk() {
if let Some(i) = maybe_unsizing_param_idx(arg) {
unsizing_params.insert(i);
found = true;
}
}
if !found {
return Err(Unimplemented);
}
// Ensure none of the other fields mention the parameters used
// in unsizing.
// FIXME(eddyb) cache this (including computing `unsizing_params`)
// by putting it in a query; it would only need the `DefId` as it
// looks at declared field types, not anything substituted.
for field in prefix_fields {
for arg in tcx.type_of(field.did).walk() {
if let Some(i) = maybe_unsizing_param_idx(arg) {
if unsizing_params.contains(i) {
return Err(Unimplemented);
}
}
}
}
// Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`.
let source_tail = tail_field_ty.subst(tcx, substs_a);
let target_tail = tail_field_ty.subst(tcx, substs_b);
// Check that the source struct with the target's
// unsizing parameters is equal to the target.
let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
}));
let new_struct = tcx.mk_adt(def, substs);
let InferOk { obligations, .. } = self
.infcx
.at(&obligation.cause, obligation.param_env)
.eq(target, new_struct)
.map_err(|_| Unimplemented)?;
nested.extend(obligations);
// Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
nested.push(predicate_for_trait_def(
tcx,
obligation.param_env,
obligation.cause.clone(),
obligation.predicate.def_id(),
obligation.recursion_depth + 1,
source_tail,
&[target_tail.into()],
));
}
// `(.., T)` -> `(.., U)`
(&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
assert_eq!(tys_a.len(), tys_b.len());
// The last field of the tuple has to exist.
let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
let &b_last = tys_b.last().unwrap();
// Check that the source tuple with the target's
// last element is equal to the target.
let new_tuple = tcx.mk_tup(
a_mid.iter().map(|k| k.expect_ty()).chain(iter::once(b_last.expect_ty())),
);
let InferOk { obligations, .. } = self
.infcx
.at(&obligation.cause, obligation.param_env)
.eq(target, new_tuple)
.map_err(|_| Unimplemented)?;
nested.extend(obligations);
// Construct the nested `T: Unsize<U>` predicate.
nested.push(ensure_sufficient_stack(|| {
predicate_for_trait_def(
tcx,
obligation.param_env,
obligation.cause.clone(),
obligation.predicate.def_id(),
obligation.recursion_depth + 1,
a_last.expect_ty(),
&[b_last],
)
}));
}
_ => bug!(),
};
Ok(VtableBuiltinData { nested })
}
}

811
src/librustc_trait_selection/traits/select/mod.rs
View file

@ -9,29 +9,19 @@ use self::SelectionCandidate::*;
use super::coherence::{self, Conflict};
use super::project;
use super::project::{normalize_with_depth, normalize_with_depth_to};
use super::project::normalize_with_depth_to;
use super::util;
use super::util::{closure_trait_ref_and_return_type, predicate_for_trait_def};
use super::wf;
use super::DerivedObligationCause;
use super::Obligation;
use super::ObligationCauseCode;
use super::Selection;
use super::SelectionResult;
use super::TraitNotObjectSafe;
use super::TraitQueryMode;
use super::{BuiltinDerivedObligation, ImplDerivedObligation, ObligationCauseCode};
use super::{Normalized, ProjectionCacheKey};
use super::{ObjectCastObligation, Obligation};
use super::{ObligationCause, PredicateObligation, TraitObligation};
use super::{OutputTypeParameterMismatch, Overflow, SelectionError, Unimplemented};
use super::{
VtableAutoImpl, VtableBuiltin, VtableClosure, VtableDiscriminantKind, VtableFnPointer,
VtableGenerator, VtableImpl, VtableObject, VtableParam, VtableTraitAlias,
};
use super::{
VtableAutoImplData, VtableBuiltinData, VtableClosureData, VtableDiscriminantKindData,
VtableFnPointerData, VtableGeneratorData, VtableImplData, VtableObjectData,
VtableTraitAliasData,
};
use super::{Overflow, SelectionError, Unimplemented};
use crate::infer::{CombinedSnapshot, InferCtxt, InferOk, PlaceholderMap, TypeFreshener};
use crate::traits::error_reporting::InferCtxtExt;
@ -42,16 +32,12 @@ use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_errors::ErrorReported;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_hir::lang_items;
use rustc_index::bit_set::GrowableBitSet;
use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
use rustc_middle::mir::interpret::ErrorHandled;
use rustc_middle::ty::fast_reject;
use rustc_middle::ty::relate::TypeRelation;
use rustc_middle::ty::subst::{GenericArg, GenericArgKind, Subst, SubstsRef};
use rustc_middle::ty::{
self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
};
use rustc_middle::ty::subst::{GenericArgKind, Subst, SubstsRef};
use rustc_middle::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable};
use rustc_span::symbol::sym;
use std::cell::{Cell, RefCell};
@ -63,6 +49,7 @@ use std::rc::Rc;
pub use rustc_middle::traits::select::*;
mod candidate_assembly;
mod confirmation;
pub struct SelectionContext<'cx, 'tcx> {
infcx: &'cx InferCtxt<'cx, 'tcx>,
@ -1808,790 +1795,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
.collect()
}
///////////////////////////////////////////////////////////////////////////
// CONFIRMATION
//
// Confirmation unifies the output type parameters of the trait
// with the values found in the obligation, possibly yielding a
// type error. See the [rustc dev guide] for more details.
//
// [rustc dev guide]:
// https://rustc-dev-guide.rust-lang.org/traits/resolution.html#confirmation
fn confirm_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
candidate: SelectionCandidate<'tcx>,
) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
debug!("confirm_candidate({:?}, {:?})", obligation, candidate);
match candidate {
BuiltinCandidate { has_nested } => {
let data = self.confirm_builtin_candidate(obligation, has_nested);
Ok(VtableBuiltin(data))
}
ParamCandidate(param) => {
let obligations = self.confirm_param_candidate(obligation, param);
Ok(VtableParam(obligations))
}
ImplCandidate(impl_def_id) => {
Ok(VtableImpl(self.confirm_impl_candidate(obligation, impl_def_id)))
}
AutoImplCandidate(trait_def_id) => {
let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
Ok(VtableAutoImpl(data))
}
ProjectionCandidate => {
self.confirm_projection_candidate(obligation);
Ok(VtableParam(Vec::new()))
}
ClosureCandidate => {
let vtable_closure = self.confirm_closure_candidate(obligation)?;
Ok(VtableClosure(vtable_closure))
}
GeneratorCandidate => {
let vtable_generator = self.confirm_generator_candidate(obligation)?;
Ok(VtableGenerator(vtable_generator))
}
FnPointerCandidate => {
let data = self.confirm_fn_pointer_candidate(obligation)?;
Ok(VtableFnPointer(data))
}
DiscriminantKindCandidate => Ok(VtableDiscriminantKind(VtableDiscriminantKindData)),
TraitAliasCandidate(alias_def_id) => {
let data = self.confirm_trait_alias_candidate(obligation, alias_def_id);
Ok(VtableTraitAlias(data))
}
ObjectCandidate => {
let data = self.confirm_object_candidate(obligation);
Ok(VtableObject(data))
}
BuiltinObjectCandidate => {
// This indicates something like `Trait + Send: Send`. In this case, we know that
// this holds because that's what the object type is telling us, and there's really
// no additional obligations to prove and no types in particular to unify, etc.
Ok(VtableParam(Vec::new()))
}
BuiltinUnsizeCandidate => {
let data = self.confirm_builtin_unsize_candidate(obligation)?;
Ok(VtableBuiltin(data))
}
}
}
fn confirm_projection_candidate(&mut self, obligation: &TraitObligation<'tcx>) {
self.infcx.commit_unconditionally(|snapshot| {
let result =
self.match_projection_obligation_against_definition_bounds(obligation, snapshot);
assert!(result);
})
}
fn confirm_param_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
param: ty::PolyTraitRef<'tcx>,
) -> Vec<PredicateObligation<'tcx>> {
debug!("confirm_param_candidate({:?},{:?})", obligation, param);
// During evaluation, we already checked that this
// where-clause trait-ref could be unified with the obligation
// trait-ref. Repeat that unification now without any
// transactional boundary; it should not fail.
match self.match_where_clause_trait_ref(obligation, param) {
Ok(obligations) => obligations,
Err(()) => {
bug!(
"Where clause `{:?}` was applicable to `{:?}` but now is not",
param,
obligation
);
}
}
}
fn confirm_builtin_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
has_nested: bool,
) -> VtableBuiltinData<PredicateObligation<'tcx>> {
debug!("confirm_builtin_candidate({:?}, {:?})", obligation, has_nested);
let lang_items = self.tcx().lang_items();
let obligations = if has_nested {
let trait_def = obligation.predicate.def_id();
let conditions = if Some(trait_def) == lang_items.sized_trait() {
self.sized_conditions(obligation)
} else if Some(trait_def) == lang_items.copy_trait() {
self.copy_clone_conditions(obligation)
} else if Some(trait_def) == lang_items.clone_trait() {
self.copy_clone_conditions(obligation)
} else {
bug!("unexpected builtin trait {:?}", trait_def)
};
let nested = match conditions {
BuiltinImplConditions::Where(nested) => nested,
_ => bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation),
};
let cause = obligation.derived_cause(BuiltinDerivedObligation);
ensure_sufficient_stack(|| {
self.collect_predicates_for_types(
obligation.param_env,
cause,
obligation.recursion_depth + 1,
trait_def,
nested,
)
})
} else {
vec![]
};
debug!("confirm_builtin_candidate: obligations={:?}", obligations);
VtableBuiltinData { nested: obligations }
}
/// This handles the case where a `auto trait Foo` impl is being used.
/// The idea is that the impl applies to `X : Foo` if the following conditions are met:
///
/// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
/// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
fn confirm_auto_impl_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
trait_def_id: DefId,
) -> VtableAutoImplData<PredicateObligation<'tcx>> {
debug!("confirm_auto_impl_candidate({:?}, {:?})", obligation, trait_def_id);
let types = obligation.predicate.map_bound(|inner| {
let self_ty = self.infcx.shallow_resolve(inner.self_ty());
self.constituent_types_for_ty(self_ty)
});
self.vtable_auto_impl(obligation, trait_def_id, types)
}
/// See `confirm_auto_impl_candidate`.
fn vtable_auto_impl(
&mut self,
obligation: &TraitObligation<'tcx>,
trait_def_id: DefId,
nested: ty::Binder<Vec<Ty<'tcx>>>,
) -> VtableAutoImplData<PredicateObligation<'tcx>> {
debug!("vtable_auto_impl: nested={:?}", nested);
ensure_sufficient_stack(|| {
let cause = obligation.derived_cause(BuiltinDerivedObligation);
let mut obligations = self.collect_predicates_for_types(
obligation.param_env,
cause,
obligation.recursion_depth + 1,
trait_def_id,
nested,
);
let trait_obligations: Vec<PredicateObligation<'_>> =
self.infcx.commit_unconditionally(|_| {
let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
let (trait_ref, _) =
self.infcx.replace_bound_vars_with_placeholders(&poly_trait_ref);
let cause = obligation.derived_cause(ImplDerivedObligation);
self.impl_or_trait_obligations(
cause,
obligation.recursion_depth + 1,
obligation.param_env,
trait_def_id,
&trait_ref.substs,
)
});
// Adds the predicates from the trait. Note that this contains a `Self: Trait`
// predicate as usual. It won't have any effect since auto traits are coinductive.
obligations.extend(trait_obligations);
debug!("vtable_auto_impl: obligations={:?}", obligations);
VtableAutoImplData { trait_def_id, nested: obligations }
})
}
fn confirm_impl_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
impl_def_id: DefId,
) -> VtableImplData<'tcx, PredicateObligation<'tcx>> {
debug!("confirm_impl_candidate({:?},{:?})", obligation, impl_def_id);
// First, create the substitutions by matching the impl again,
// this time not in a probe.
self.infcx.commit_unconditionally(|snapshot| {
let substs = self.rematch_impl(impl_def_id, obligation, snapshot);
debug!("confirm_impl_candidate: substs={:?}", substs);
let cause = obligation.derived_cause(ImplDerivedObligation);
ensure_sufficient_stack(|| {
self.vtable_impl(
impl_def_id,
substs,
cause,
obligation.recursion_depth + 1,
obligation.param_env,
)
})
})
}
fn vtable_impl(
&mut self,
impl_def_id: DefId,
mut substs: Normalized<'tcx, SubstsRef<'tcx>>,
cause: ObligationCause<'tcx>,
recursion_depth: usize,
param_env: ty::ParamEnv<'tcx>,
) -> VtableImplData<'tcx, PredicateObligation<'tcx>> {
debug!(
"vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={})",
impl_def_id, substs, recursion_depth,
);
let mut impl_obligations = self.impl_or_trait_obligations(
cause,
recursion_depth,
param_env,
impl_def_id,
&substs.value,
);
debug!(
"vtable_impl: impl_def_id={:?} impl_obligations={:?}",
impl_def_id, impl_obligations
);
// Because of RFC447, the impl-trait-ref and obligations
// are sufficient to determine the impl substs, without
// relying on projections in the impl-trait-ref.
//
// e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
impl_obligations.append(&mut substs.obligations);
VtableImplData { impl_def_id, substs: substs.value, nested: impl_obligations }
}
fn confirm_object_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> VtableObjectData<'tcx, PredicateObligation<'tcx>> {
debug!("confirm_object_candidate({:?})", obligation);
// FIXME(nmatsakis) skipping binder here seems wrong -- we should
// probably flatten the binder from the obligation and the binder
// from the object. Have to try to make a broken test case that
// results.
let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
let poly_trait_ref = match self_ty.kind {
ty::Dynamic(ref data, ..) => data
.principal()
.unwrap_or_else(|| {
span_bug!(obligation.cause.span, "object candidate with no principal")
})
.with_self_ty(self.tcx(), self_ty),
_ => span_bug!(obligation.cause.span, "object candidate with non-object"),
};
let mut upcast_trait_ref = None;
let mut nested = vec![];
let vtable_base;
{
let tcx = self.tcx();
// We want to find the first supertrait in the list of
// supertraits that we can unify with, and do that
// unification. We know that there is exactly one in the list
// where we can unify, because otherwise select would have
// reported an ambiguity. (When we do find a match, also
// record it for later.)
let nonmatching = util::supertraits(tcx, poly_trait_ref).take_while(|&t| {
match self.infcx.commit_if_ok(|_| self.match_poly_trait_ref(obligation, t)) {
Ok(obligations) => {
upcast_trait_ref = Some(t);
nested.extend(obligations);
false
}
Err(_) => true,
}
});
// Additionally, for each of the non-matching predicates that
// we pass over, we sum up the set of number of vtable
// entries, so that we can compute the offset for the selected
// trait.
vtable_base = nonmatching.map(|t| super::util::count_own_vtable_entries(tcx, t)).sum();
}
VtableObjectData { upcast_trait_ref: upcast_trait_ref.unwrap(), vtable_base, nested }
}
fn confirm_fn_pointer_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> Result<VtableFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
debug!("confirm_fn_pointer_candidate({:?})", obligation);
// Okay to skip binder; it is reintroduced below.
let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
let sig = self_ty.fn_sig(self.tcx());
let trait_ref = closure_trait_ref_and_return_type(
self.tcx(),
obligation.predicate.def_id(),
self_ty,
sig,
util::TupleArgumentsFlag::Yes,
)
.map_bound(|(trait_ref, _)| trait_ref);
let Normalized { value: trait_ref, obligations } = ensure_sufficient_stack(|| {
project::normalize_with_depth(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
&trait_ref,
)
});
self.confirm_poly_trait_refs(
obligation.cause.clone(),
obligation.param_env,
obligation.predicate.to_poly_trait_ref(),
trait_ref,
)?;
Ok(VtableFnPointerData { fn_ty: self_ty, nested: obligations })
}
fn confirm_trait_alias_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
alias_def_id: DefId,
) -> VtableTraitAliasData<'tcx, PredicateObligation<'tcx>> {
debug!("confirm_trait_alias_candidate({:?}, {:?})", obligation, alias_def_id);
self.infcx.commit_unconditionally(|_| {
let (predicate, _) =
self.infcx().replace_bound_vars_with_placeholders(&obligation.predicate);
let trait_ref = predicate.trait_ref;
let trait_def_id = trait_ref.def_id;
let substs = trait_ref.substs;
let trait_obligations = self.impl_or_trait_obligations(
obligation.cause.clone(),
obligation.recursion_depth,
obligation.param_env,
trait_def_id,
&substs,
);
debug!(
"confirm_trait_alias_candidate: trait_def_id={:?} trait_obligations={:?}",
trait_def_id, trait_obligations
);
VtableTraitAliasData { alias_def_id, substs, nested: trait_obligations }
})
}
fn confirm_generator_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> Result<VtableGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
// Okay to skip binder because the substs on generator types never
// touch bound regions, they just capture the in-scope
// type/region parameters.
let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
let (generator_def_id, substs) = match self_ty.kind {
ty::Generator(id, substs, _) => (id, substs),
_ => bug!("closure candidate for non-closure {:?}", obligation),
};
debug!("confirm_generator_candidate({:?},{:?},{:?})", obligation, generator_def_id, substs);
let trait_ref = self.generator_trait_ref_unnormalized(obligation, substs);
let Normalized { value: trait_ref, mut obligations } = ensure_sufficient_stack(|| {
normalize_with_depth(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
&trait_ref,
)
});
debug!(
"confirm_generator_candidate(generator_def_id={:?}, \
trait_ref={:?}, obligations={:?})",
generator_def_id, trait_ref, obligations
);
obligations.extend(self.confirm_poly_trait_refs(
obligation.cause.clone(),
obligation.param_env,
obligation.predicate.to_poly_trait_ref(),
trait_ref,
)?);
Ok(VtableGeneratorData { generator_def_id, substs, nested: obligations })
}
fn confirm_closure_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> Result<VtableClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
debug!("confirm_closure_candidate({:?})", obligation);
let kind = self
.tcx()
.fn_trait_kind_from_lang_item(obligation.predicate.def_id())
.unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
// Okay to skip binder because the substs on closure types never
// touch bound regions, they just capture the in-scope
// type/region parameters.
let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
let (closure_def_id, substs) = match self_ty.kind {
ty::Closure(id, substs) => (id, substs),
_ => bug!("closure candidate for non-closure {:?}", obligation),
};
let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
let Normalized { value: trait_ref, mut obligations } = ensure_sufficient_stack(|| {
normalize_with_depth(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
&trait_ref,
)
});
debug!(
"confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
closure_def_id, trait_ref, obligations
);
obligations.extend(self.confirm_poly_trait_refs(
obligation.cause.clone(),
obligation.param_env,
obligation.predicate.to_poly_trait_ref(),
trait_ref,
)?);
// FIXME: Chalk
if !self.tcx().sess.opts.debugging_opts.chalk {
obligations.push(Obligation::new(
obligation.cause.clone(),
obligation.param_env,
ty::PredicateKind::ClosureKind(closure_def_id, substs, kind)
.to_predicate(self.tcx()),
));
}
Ok(VtableClosureData { closure_def_id, substs, nested: obligations })
}
/// In the case of closure types and fn pointers,
/// we currently treat the input type parameters on the trait as
/// outputs. This means that when we have a match we have only
/// considered the self type, so we have to go back and make sure
/// to relate the argument types too. This is kind of wrong, but
/// since we control the full set of impls, also not that wrong,
/// and it DOES yield better error messages (since we don't report
/// errors as if there is no applicable impl, but rather report
/// errors are about mismatched argument types.
///
/// Here is an example. Imagine we have a closure expression
/// and we desugared it so that the type of the expression is
/// `Closure`, and `Closure` expects an int as argument. Then it
/// is "as if" the compiler generated this impl:
///
/// impl Fn(int) for Closure { ... }
///
/// Now imagine our obligation is `Fn(usize) for Closure`. So far
/// we have matched the self type `Closure`. At this point we'll
/// compare the `int` to `usize` and generate an error.
///
/// Note that this checking occurs *after* the impl has selected,
/// because these output type parameters should not affect the
/// selection of the impl. Therefore, if there is a mismatch, we
/// report an error to the user.
fn confirm_poly_trait_refs(
&mut self,
obligation_cause: ObligationCause<'tcx>,
obligation_param_env: ty::ParamEnv<'tcx>,
obligation_trait_ref: ty::PolyTraitRef<'tcx>,
expected_trait_ref: ty::PolyTraitRef<'tcx>,
) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
self.infcx
.at(&obligation_cause, obligation_param_env)
.sup(obligation_trait_ref, expected_trait_ref)
.map(|InferOk { obligations, .. }| obligations)
.map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
}
fn confirm_builtin_unsize_candidate(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> Result<VtableBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
let tcx = self.tcx();
// `assemble_candidates_for_unsizing` should ensure there are no late-bound
// regions here. See the comment there for more details.
let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
let target = self.infcx.shallow_resolve(target);
debug!("confirm_builtin_unsize_candidate(source={:?}, target={:?})", source, target);
let mut nested = vec![];
match (&source.kind, &target.kind) {
// Trait+Kx+'a -> Trait+Ky+'b (upcasts).
(&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
// See `assemble_candidates_for_unsizing` for more info.
let existential_predicates = data_a.map_bound(|data_a| {
let iter = data_a
.principal()
.map(ty::ExistentialPredicate::Trait)
.into_iter()
.chain(data_a.projection_bounds().map(ty::ExistentialPredicate::Projection))
.chain(data_b.auto_traits().map(ty::ExistentialPredicate::AutoTrait));
tcx.mk_existential_predicates(iter)
});
let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
// Require that the traits involved in this upcast are **equal**;
// only the **lifetime bound** is changed.
//
// FIXME: This condition is arguably too strong -- it would
// suffice for the source trait to be a *subtype* of the target
// trait. In particular, changing from something like
// `for<'a, 'b> Foo<'a, 'b>` to `for<'a> Foo<'a, 'a>` should be
// permitted. And, indeed, in the in commit
// 904a0bde93f0348f69914ee90b1f8b6e4e0d7cbc, this
// condition was loosened. However, when the leak check was
// added back, using subtype here actually guides the coercion
// code in such a way that it accepts `old-lub-glb-object.rs`.
// This is probably a good thing, but I've modified this to `.eq`
// because I want to continue rejecting that test (as we have
// done for quite some time) before we are firmly comfortable
// with what our behavior should be there. -nikomatsakis
let InferOk { obligations, .. } = self
.infcx
.at(&obligation.cause, obligation.param_env)
.eq(target, source_trait) // FIXME -- see below
.map_err(|_| Unimplemented)?;
nested.extend(obligations);
// Register one obligation for 'a: 'b.
let cause = ObligationCause::new(
obligation.cause.span,
obligation.cause.body_id,
ObjectCastObligation(target),
);
let outlives = ty::OutlivesPredicate(r_a, r_b);
nested.push(Obligation::with_depth(
cause,
obligation.recursion_depth + 1,
obligation.param_env,
ty::Binder::bind(outlives).to_predicate(tcx),
));
}
// `T` -> `Trait`
(_, &ty::Dynamic(ref data, r)) => {
let mut object_dids = data.auto_traits().chain(data.principal_def_id());
if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
return Err(TraitNotObjectSafe(did));
}
let cause = ObligationCause::new(
obligation.cause.span,
obligation.cause.body_id,
ObjectCastObligation(target),
);
let predicate_to_obligation = |predicate| {
Obligation::with_depth(
cause.clone(),
obligation.recursion_depth + 1,
obligation.param_env,
predicate,
)
};
// Create obligations:
// - Casting `T` to `Trait`
// - For all the various builtin bounds attached to the object cast. (In other
// words, if the object type is `Foo + Send`, this would create an obligation for
// the `Send` check.)
// - Projection predicates
nested.extend(
data.iter().map(|predicate| {
predicate_to_obligation(predicate.with_self_ty(tcx, source))
}),
);
// We can only make objects from sized types.
let tr = ty::TraitRef::new(
tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
tcx.mk_substs_trait(source, &[]),
);
nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx)));
// If the type is `Foo + 'a`, ensure that the type
// being cast to `Foo + 'a` outlives `'a`:
let outlives = ty::OutlivesPredicate(source, r);
nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx)));
}
// `[T; n]` -> `[T]`
(&ty::Array(a, _), &ty::Slice(b)) => {
let InferOk { obligations, .. } = self
.infcx
.at(&obligation.cause, obligation.param_env)
.eq(b, a)
.map_err(|_| Unimplemented)?;
nested.extend(obligations);
}
// `Struct<T>` -> `Struct<U>`
(&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() {
GenericArgKind::Type(ty) => match ty.kind {
ty::Param(p) => Some(p.index),
_ => None,
},
// Lifetimes aren't allowed to change during unsizing.
GenericArgKind::Lifetime(_) => None,
GenericArgKind::Const(ct) => match ct.val {
ty::ConstKind::Param(p) => Some(p.index),
_ => None,
},
};
// The last field of the structure has to exist and contain type/const parameters.
let (tail_field, prefix_fields) =
def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
let tail_field_ty = tcx.type_of(tail_field.did);
let mut unsizing_params = GrowableBitSet::new_empty();
let mut found = false;
for arg in tail_field_ty.walk() {
if let Some(i) = maybe_unsizing_param_idx(arg) {
unsizing_params.insert(i);
found = true;
}
}
if !found {
return Err(Unimplemented);
}
// Ensure none of the other fields mention the parameters used
// in unsizing.
// FIXME(eddyb) cache this (including computing `unsizing_params`)
// by putting it in a query; it would only need the `DefId` as it
// looks at declared field types, not anything substituted.
for field in prefix_fields {
for arg in tcx.type_of(field.did).walk() {
if let Some(i) = maybe_unsizing_param_idx(arg) {
if unsizing_params.contains(i) {
return Err(Unimplemented);
}
}
}
}
// Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`.
let source_tail = tail_field_ty.subst(tcx, substs_a);
let target_tail = tail_field_ty.subst(tcx, substs_b);
// Check that the source struct with the target's
// unsizing parameters is equal to the target.
let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| {
if unsizing_params.contains(i as u32) { substs_b[i] } else { k }
}));
let new_struct = tcx.mk_adt(def, substs);
let InferOk { obligations, .. } = self
.infcx
.at(&obligation.cause, obligation.param_env)
.eq(target, new_struct)
.map_err(|_| Unimplemented)?;
nested.extend(obligations);
// Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
nested.push(predicate_for_trait_def(
tcx,
obligation.param_env,
obligation.cause.clone(),
obligation.predicate.def_id(),
obligation.recursion_depth + 1,
source_tail,
&[target_tail.into()],
));
}
// `(.., T)` -> `(.., U)`
(&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
assert_eq!(tys_a.len(), tys_b.len());
// The last field of the tuple has to exist.
let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
let &b_last = tys_b.last().unwrap();
// Check that the source tuple with the target's
// last element is equal to the target.
let new_tuple = tcx.mk_tup(
a_mid.iter().map(|k| k.expect_ty()).chain(iter::once(b_last.expect_ty())),
);
let InferOk { obligations, .. } = self
.infcx
.at(&obligation.cause, obligation.param_env)
.eq(target, new_tuple)
.map_err(|_| Unimplemented)?;
nested.extend(obligations);
// Construct the nested `T: Unsize<U>` predicate.
nested.push(ensure_sufficient_stack(|| {
predicate_for_trait_def(
tcx,
obligation.param_env,
obligation.cause.clone(),
obligation.predicate.def_id(),
obligation.recursion_depth + 1,
a_last.expect_ty(),
&[b_last],
)
}));
}
_ => bug!(),
};
Ok(VtableBuiltinData { nested })
}
///////////////////////////////////////////////////////////////////////////
// Matching
//