rust/compiler/rustc_trait_selection/src/solve/project_goals.rs
2023-01-26 03:18:36 +00:00

522 lines
21 KiB
Rust

use crate::traits::{specialization_graph, translate_substs};
use super::assembly::{self, Candidate, CandidateSource};
use super::infcx_ext::InferCtxtExt;
use super::trait_goals::structural_traits;
use super::{Certainty, EvalCtxt, Goal, QueryResult};
use rustc_errors::ErrorGuaranteed;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::DefId;
use rustc_hir::LangItem;
use rustc_infer::infer::InferCtxt;
use rustc_infer::traits::query::NoSolution;
use rustc_infer::traits::specialization_graph::LeafDef;
use rustc_infer::traits::Reveal;
use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_middle::ty::{ProjectionPredicate, TypeSuperVisitable, TypeVisitor};
use rustc_middle::ty::{ToPredicate, TypeVisitable};
use rustc_span::DUMMY_SP;
use std::iter;
use std::ops::ControlFlow;
impl<'tcx> EvalCtxt<'_, 'tcx> {
pub(super) fn compute_projection_goal(
&mut self,
goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
) -> QueryResult<'tcx> {
// To only compute normalization once for each projection we only
// normalize if the expected term is an unconstrained inference variable.
//
// E.g. for `<T as Trait>::Assoc = u32` we recursively compute the goal
// `exists<U> <T as Trait>::Assoc = U` and then take the resulting type for
// `U` and equate it with `u32`. This means that we don't need a separate
// projection cache in the solver.
if self.term_is_fully_unconstrained(goal) {
let candidates = self.assemble_and_evaluate_candidates(goal);
self.merge_project_candidates(candidates)
} else {
let predicate = goal.predicate;
let unconstrained_rhs = match predicate.term.unpack() {
ty::TermKind::Ty(_) => self.infcx.next_ty_infer().into(),
ty::TermKind::Const(ct) => self.infcx.next_const_infer(ct.ty()).into(),
};
let unconstrained_predicate = ty::Clause::Projection(ProjectionPredicate {
projection_ty: goal.predicate.projection_ty,
term: unconstrained_rhs,
});
let (_has_changed, normalize_certainty) =
self.evaluate_goal(goal.with(self.tcx(), unconstrained_predicate))?;
let nested_eq_goals =
self.infcx.eq(goal.param_env, unconstrained_rhs, predicate.term)?;
let eval_certainty = self.evaluate_all(nested_eq_goals)?;
self.make_canonical_response(normalize_certainty.unify_and(eval_certainty))
}
}
/// Is the projection predicate is of the form `exists<T> <Ty as Trait>::Assoc = T`.
///
/// This is the case if the `term` is an inference variable in the innermost universe
/// and does not occur in any other part of the predicate.
fn term_is_fully_unconstrained(&self, goal: Goal<'tcx, ProjectionPredicate<'tcx>>) -> bool {
let infcx = self.infcx;
let term_is_infer = match goal.predicate.term.unpack() {
ty::TermKind::Ty(ty) => {
if let &ty::Infer(ty::TyVar(vid)) = ty.kind() {
match infcx.probe_ty_var(vid) {
Ok(value) => bug!("resolved var in query: {goal:?} {value:?}"),
Err(universe) => universe == infcx.universe(),
}
} else {
false
}
}
ty::TermKind::Const(ct) => {
if let ty::ConstKind::Infer(ty::InferConst::Var(vid)) = ct.kind() {
match self.infcx.probe_const_var(vid) {
Ok(value) => bug!("resolved var in query: {goal:?} {value:?}"),
Err(universe) => universe == infcx.universe(),
}
} else {
false
}
}
};
// Guard against `<T as Trait<?0>>::Assoc = ?0>`.
struct ContainsTerm<'tcx> {
term: ty::Term<'tcx>,
}
impl<'tcx> TypeVisitor<'tcx> for ContainsTerm<'tcx> {
type BreakTy = ();
fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
if t.needs_infer() {
if ty::Term::from(t) == self.term {
ControlFlow::BREAK
} else {
t.super_visit_with(self)
}
} else {
ControlFlow::CONTINUE
}
}
fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
if c.needs_infer() {
if ty::Term::from(c) == self.term {
ControlFlow::BREAK
} else {
c.super_visit_with(self)
}
} else {
ControlFlow::CONTINUE
}
}
}
let mut visitor = ContainsTerm { term: goal.predicate.term };
term_is_infer
&& goal.predicate.projection_ty.visit_with(&mut visitor).is_continue()
&& goal.param_env.visit_with(&mut visitor).is_continue()
}
fn merge_project_candidates(
&mut self,
mut candidates: Vec<Candidate<'tcx>>,
) -> QueryResult<'tcx> {
match candidates.len() {
0 => return Err(NoSolution),
1 => return Ok(candidates.pop().unwrap().result),
_ => {}
}
if candidates.len() > 1 {
let mut i = 0;
'outer: while i < candidates.len() {
for j in (0..candidates.len()).filter(|&j| i != j) {
if self.project_candidate_should_be_dropped_in_favor_of(
&candidates[i],
&candidates[j],
) {
debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
candidates.swap_remove(i);
continue 'outer;
}
}
debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
// If there are *STILL* multiple candidates, give up
// and report ambiguity.
i += 1;
if i > 1 {
debug!("multiple matches, ambig");
// FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
unimplemented!();
}
}
}
Ok(candidates.pop().unwrap().result)
}
fn project_candidate_should_be_dropped_in_favor_of(
&self,
candidate: &Candidate<'tcx>,
other: &Candidate<'tcx>,
) -> bool {
// FIXME: implement this
match (candidate.source, other.source) {
(CandidateSource::Impl(_), _)
| (CandidateSource::ParamEnv(_), _)
| (CandidateSource::BuiltinImpl, _)
| (CandidateSource::AliasBound(_), _) => unimplemented!(),
}
}
}
impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
fn self_ty(self) -> Ty<'tcx> {
self.self_ty()
}
fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
self.with_self_ty(tcx, self_ty)
}
fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId {
self.trait_def_id(tcx)
}
fn consider_impl_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
impl_def_id: DefId,
) -> QueryResult<'tcx> {
let tcx = ecx.tcx();
let goal_trait_ref = goal.predicate.projection_ty.trait_ref(tcx);
let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
if iter::zip(goal_trait_ref.substs, impl_trait_ref.skip_binder().substs)
.any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
{
return Err(NoSolution);
}
ecx.infcx.probe(|_| {
let impl_substs = ecx.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
let mut nested_goals = ecx.infcx.eq(goal.param_env, goal_trait_ref, impl_trait_ref)?;
let where_clause_bounds = tcx
.predicates_of(impl_def_id)
.instantiate(tcx, impl_substs)
.predicates
.into_iter()
.map(|pred| goal.with(tcx, pred));
nested_goals.extend(where_clause_bounds);
let trait_ref_certainty = ecx.evaluate_all(nested_goals)?;
// In case the associated item is hidden due to specialization, we have to
// return ambiguity this would otherwise be incomplete, resulting in
// unsoundness during coherence (#105782).
let Some(assoc_def) = fetch_eligible_assoc_item_def(
ecx.infcx,
goal.param_env,
goal_trait_ref,
goal.predicate.def_id(),
impl_def_id
)? else {
return ecx.make_canonical_response(trait_ref_certainty.unify_and(Certainty::AMBIGUOUS));
};
if !assoc_def.item.defaultness(tcx).has_value() {
tcx.sess.delay_span_bug(
tcx.def_span(assoc_def.item.def_id),
"missing value for assoc item in impl",
);
}
// Getting the right substitutions here is complex, e.g. given:
// - a goal `<Vec<u32> as Trait<i32>>::Assoc<u64>`
// - the applicable impl `impl<T> Trait<i32> for Vec<T>`
// - and the impl which defines `Assoc` being `impl<T, U> Trait<U> for Vec<T>`
//
// We first rebase the goal substs onto the impl, going from `[Vec<u32>, i32, u64]`
// to `[u32, u64]`.
//
// And then map these substs to the substs of the defining impl of `Assoc`, going
// from `[u32, u64]` to `[u32, i32, u64]`.
let impl_substs_with_gat = goal.predicate.projection_ty.substs.rebase_onto(
tcx,
goal_trait_ref.def_id,
impl_substs,
);
let substs = translate_substs(
ecx.infcx,
goal.param_env,
impl_def_id,
impl_substs_with_gat,
assoc_def.defining_node,
);
// Finally we construct the actual value of the associated type.
let is_const = matches!(tcx.def_kind(assoc_def.item.def_id), DefKind::AssocConst);
let ty = tcx.bound_type_of(assoc_def.item.def_id);
let term: ty::EarlyBinder<ty::Term<'tcx>> = if is_const {
let identity_substs =
ty::InternalSubsts::identity_for_item(tcx, assoc_def.item.def_id);
let did = ty::WithOptConstParam::unknown(assoc_def.item.def_id);
let kind =
ty::ConstKind::Unevaluated(ty::UnevaluatedConst::new(did, identity_substs));
ty.map_bound(|ty| tcx.mk_const(kind, ty).into())
} else {
ty.map_bound(|ty| ty.into())
};
// The term of our goal should be fully unconstrained, so this should never fail.
//
// It can however be ambiguous when the resolved type is a projection.
let nested_goals = ecx
.infcx
.eq(goal.param_env, goal.predicate.term, term.subst(tcx, substs))
.expect("failed to unify with unconstrained term");
let rhs_certainty =
ecx.evaluate_all(nested_goals).expect("failed to unify with unconstrained term");
ecx.make_canonical_response(trait_ref_certainty.unify_and(rhs_certainty))
})
}
fn consider_assumption(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
assumption: ty::Predicate<'tcx>,
) -> QueryResult<'tcx> {
if let Some(poly_projection_pred) = assumption.to_opt_poly_projection_pred() {
ecx.infcx.probe(|_| {
let assumption_projection_pred =
ecx.infcx.instantiate_bound_vars_with_infer(poly_projection_pred);
let nested_goals = ecx.infcx.eq(
goal.param_env,
goal.predicate.projection_ty,
assumption_projection_pred.projection_ty,
)?;
let subst_certainty = ecx.evaluate_all(nested_goals)?;
// The term of our goal should be fully unconstrained, so this should never fail.
//
// It can however be ambiguous when the resolved type is a projection.
let nested_goals = ecx
.infcx
.eq(goal.param_env, goal.predicate.term, assumption_projection_pred.term)
.expect("failed to unify with unconstrained term");
let rhs_certainty = ecx
.evaluate_all(nested_goals)
.expect("failed to unify with unconstrained term");
ecx.make_canonical_response(subst_certainty.unify_and(rhs_certainty))
})
} else {
Err(NoSolution)
}
}
fn consider_auto_trait_candidate(
_ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
bug!("auto traits do not have associated types: {:?}", goal);
}
fn consider_trait_alias_candidate(
_ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
bug!("trait aliases do not have associated types: {:?}", goal);
}
fn consider_builtin_sized_candidate(
_ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
bug!("`Sized` does not have an associated type: {:?}", goal);
}
fn consider_builtin_copy_clone_candidate(
_ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
bug!("`Copy`/`Clone` does not have an associated type: {:?}", goal);
}
fn consider_builtin_pointer_sized_candidate(
_ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
bug!("`PointerSized` does not have an associated type: {:?}", goal);
}
fn consider_builtin_fn_trait_candidates(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
goal_kind: ty::ClosureKind,
) -> QueryResult<'tcx> {
if let Some(tupled_inputs_and_output) =
structural_traits::extract_tupled_inputs_and_output_from_callable(
ecx.tcx(),
goal.predicate.self_ty(),
goal_kind,
)?
{
let pred = tupled_inputs_and_output
.map_bound(|(inputs, output)| ty::ProjectionPredicate {
projection_ty: ecx
.tcx()
.mk_alias_ty(goal.predicate.def_id(), [goal.predicate.self_ty(), inputs]),
term: output.into(),
})
.to_predicate(ecx.tcx());
Self::consider_assumption(ecx, goal, pred)
} else {
ecx.make_canonical_response(Certainty::AMBIGUOUS)
}
}
fn consider_builtin_tuple_candidate(
_ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
bug!("`Tuple` does not have an associated type: {:?}", goal);
}
fn consider_builtin_pointee_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx> {
let tcx = ecx.tcx();
ecx.infcx.probe(|_| {
let metadata_ty = match goal.predicate.self_ty().kind() {
ty::Bool
| ty::Char
| ty::Int(..)
| ty::Uint(..)
| ty::Float(..)
| ty::Array(..)
| ty::RawPtr(..)
| ty::Ref(..)
| ty::FnDef(..)
| ty::FnPtr(..)
| ty::Closure(..)
| ty::Infer(ty::IntVar(..) | ty::FloatVar(..))
| ty::Generator(..)
| ty::GeneratorWitness(..)
| ty::Never
| ty::Foreign(..) => tcx.types.unit,
ty::Error(e) => tcx.ty_error_with_guaranteed(*e),
ty::Str | ty::Slice(_) => tcx.types.usize,
ty::Dynamic(_, _, _) => {
let dyn_metadata = tcx.require_lang_item(LangItem::DynMetadata, None);
tcx.bound_type_of(dyn_metadata)
.subst(tcx, &[ty::GenericArg::from(goal.predicate.self_ty())])
}
ty::Alias(_, _) | ty::Param(_) | ty::Placeholder(..) => {
// FIXME(ptr_metadata): It would also be possible to return a `Ok(Ambig)` with no constraints.
let sized_predicate = ty::Binder::dummy(tcx.at(DUMMY_SP).mk_trait_ref(
LangItem::Sized,
[ty::GenericArg::from(goal.predicate.self_ty())],
));
let mut nested_goals = ecx.infcx.eq(
goal.param_env,
goal.predicate.term.ty().unwrap(),
tcx.types.unit,
)?;
nested_goals.push(goal.with(tcx, sized_predicate));
return ecx.evaluate_all_and_make_canonical_response(nested_goals);
}
ty::Adt(def, substs) if def.is_struct() => {
match def.non_enum_variant().fields.last() {
None => tcx.types.unit,
Some(field_def) => {
let self_ty = field_def.ty(tcx, substs);
let new_goal = goal.with(
tcx,
ty::Binder::dummy(goal.predicate.with_self_ty(tcx, self_ty)),
);
return ecx.evaluate_all_and_make_canonical_response(vec![new_goal]);
}
}
}
ty::Adt(_, _) => tcx.types.unit,
ty::Tuple(elements) => match elements.last() {
None => tcx.types.unit,
Some(&self_ty) => {
let new_goal = goal.with(
tcx,
ty::Binder::dummy(goal.predicate.with_self_ty(tcx, self_ty)),
);
return ecx.evaluate_all_and_make_canonical_response(vec![new_goal]);
}
},
ty::Infer(
ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_),
)
| ty::Bound(..) => bug!(
"unexpected self ty `{:?}` when normalizing `<T as Pointee>::Metadata`",
goal.predicate.self_ty()
),
};
let nested_goals =
ecx.infcx.eq(goal.param_env, goal.predicate.term.ty().unwrap(), metadata_ty)?;
ecx.evaluate_all_and_make_canonical_response(nested_goals)
})
}
}
/// This behavior is also implemented in `rustc_ty_utils` and in the old `project` code.
///
/// FIXME: We should merge these 3 implementations as it's likely that they otherwise
/// diverge.
#[instrument(level = "debug", skip(infcx, param_env), ret)]
fn fetch_eligible_assoc_item_def<'tcx>(
infcx: &InferCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
goal_trait_ref: ty::TraitRef<'tcx>,
trait_assoc_def_id: DefId,
impl_def_id: DefId,
) -> Result<Option<LeafDef>, NoSolution> {
let node_item = specialization_graph::assoc_def(infcx.tcx, impl_def_id, trait_assoc_def_id)
.map_err(|ErrorGuaranteed { .. }| NoSolution)?;
let eligible = if node_item.is_final() {
// Non-specializable items are always projectable.
true
} else {
// Only reveal a specializable default if we're past type-checking
// and the obligation is monomorphic, otherwise passes such as
// transmute checking and polymorphic MIR optimizations could
// get a result which isn't correct for all monomorphizations.
if param_env.reveal() == Reveal::All {
let poly_trait_ref = infcx.resolve_vars_if_possible(goal_trait_ref);
!poly_trait_ref.still_further_specializable()
} else {
debug!(?node_item.item.def_id, "not eligible due to default");
false
}
};
if eligible { Ok(Some(node_item)) } else { Ok(None) }
}