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use crate::infer::{InferCtxt, TyOrConstInferVar};
use crate::traits::error_reporting::TypeErrCtxtExt;
use crate::traits::normalize::normalize_with_depth_to;
use rustc_data_structures::captures::Captures;
use rustc_data_structures::obligation_forest::ProcessResult;
use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome};
use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
use rustc_infer::infer::DefineOpaqueTypes;
use rustc_infer::traits::ProjectionCacheKey;
use rustc_infer::traits::{PolyTraitObligation, SelectionError, TraitEngine};
use rustc_middle::mir::interpret::ErrorHandled;
use rustc_middle::ty::abstract_const::NotConstEvaluatable;
use rustc_middle::ty::error::{ExpectedFound, TypeError};
use rustc_middle::ty::GenericArgsRef;
use rustc_middle::ty::{self, Binder, Const, TypeVisitableExt};
use std::marker::PhantomData;
use super::const_evaluatable;
use super::project::{self, ProjectAndUnifyResult};
use super::select::SelectionContext;
use super::wf;
use super::EvaluationResult;
use super::PredicateObligation;
use super::Unimplemented;
use super::{FulfillmentError, FulfillmentErrorCode};
use crate::traits::project::PolyProjectionObligation;
use crate::traits::project::ProjectionCacheKeyExt as _;
use crate::traits::query::evaluate_obligation::InferCtxtExt;
impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
/// Note that we include both the `ParamEnv` and the `Predicate`,
/// as the `ParamEnv` can influence whether fulfillment succeeds
/// or fails.
type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
fn as_cache_key(&self) -> Self::CacheKey {
self.obligation.param_env.and(self.obligation.predicate)
}
}
/// The fulfillment context is used to drive trait resolution. It
/// consists of a list of obligations that must be (eventually)
/// satisfied. The job is to track which are satisfied, which yielded
/// errors, and which are still pending. At any point, users can call
/// `select_where_possible`, and the fulfillment context will try to do
/// selection, retaining only those obligations that remain
/// ambiguous. This may be helpful in pushing type inference
/// along. Once all type inference constraints have been generated, the
/// method `select_all_or_error` can be used to report any remaining
/// ambiguous cases as errors.
pub struct FulfillmentContext<'tcx> {
/// A list of all obligations that have been registered with this
/// fulfillment context.
predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
/// The snapshot in which this context was created. Using the context
/// outside of this snapshot leads to subtle bugs if the snapshot
/// gets rolled back. Because of this we explicitly check that we only
/// use the context in exactly this snapshot.
usable_in_snapshot: usize,
}
#[derive(Clone, Debug)]
pub struct PendingPredicateObligation<'tcx> {
pub obligation: PredicateObligation<'tcx>,
// This is far more often read than modified, meaning that we
// should mostly optimize for reading speed, while modifying is not as relevant.
//
// For whatever reason using a boxed slice is slower than using a `Vec` here.
pub stalled_on: Vec<TyOrConstInferVar>,
}
// `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
#[cfg(target_pointer_width = "64")]
rustc_data_structures::static_assert_size!(PendingPredicateObligation<'_>, 72);
impl<'tcx> FulfillmentContext<'tcx> {
/// Creates a new fulfillment context.
pub(super) fn new(infcx: &InferCtxt<'tcx>) -> FulfillmentContext<'tcx> {
assert!(
!infcx.next_trait_solver(),
"old trait solver fulfillment context created when \
infcx is set up for new trait solver"
);
FulfillmentContext {
predicates: ObligationForest::new(),
usable_in_snapshot: infcx.num_open_snapshots(),
}
}
/// Attempts to select obligations using `selcx`.
fn select(&mut self, selcx: SelectionContext<'_, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
let _enter = span.enter();
// Process pending obligations.
let outcome: Outcome<_, _> =
self.predicates.process_obligations(&mut FulfillProcessor { selcx });
// FIXME: if we kept the original cache key, we could mark projection
// obligations as complete for the projection cache here.
let errors: Vec<FulfillmentError<'tcx>> =
outcome.errors.into_iter().map(to_fulfillment_error).collect();
debug!(
"select({} predicates remaining, {} errors) done",
self.predicates.len(),
errors.len()
);
errors
}
}
impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
#[inline]
fn register_predicate_obligation(
&mut self,
infcx: &InferCtxt<'tcx>,
mut obligation: PredicateObligation<'tcx>,
) {
assert_eq!(self.usable_in_snapshot, infcx.num_open_snapshots());
// this helps to reduce duplicate errors, as well as making
// debug output much nicer to read and so on.
debug_assert!(!obligation.param_env.has_non_region_infer());
obligation.predicate = infcx.resolve_vars_if_possible(obligation.predicate);
debug!(?obligation, "register_predicate_obligation");
self.predicates
.register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
}
fn collect_remaining_errors(
&mut self,
_infcx: &InferCtxt<'tcx>,
) -> Vec<FulfillmentError<'tcx>> {
self.predicates
.to_errors(FulfillmentErrorCode::Ambiguity { overflow: None })
.into_iter()
.map(to_fulfillment_error)
.collect()
}
fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
let selcx = SelectionContext::new(infcx);
self.select(selcx)
}
fn drain_unstalled_obligations(
&mut self,
infcx: &InferCtxt<'tcx>,
) -> Vec<PredicateObligation<'tcx>> {
let mut processor = DrainProcessor { removed_predicates: Vec::new(), infcx };
let outcome: Outcome<_, _> = self.predicates.process_obligations(&mut processor);
assert!(outcome.errors.is_empty());
return processor.removed_predicates;
struct DrainProcessor<'a, 'tcx> {
infcx: &'a InferCtxt<'tcx>,
removed_predicates: Vec<PredicateObligation<'tcx>>,
}
impl<'tcx> ObligationProcessor for DrainProcessor<'_, 'tcx> {
type Obligation = PendingPredicateObligation<'tcx>;
type Error = !;
type OUT = Outcome<Self::Obligation, Self::Error>;
fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
pending_obligation
.stalled_on
.iter()
.any(|&var| self.infcx.ty_or_const_infer_var_changed(var))
}
fn process_obligation(
&mut self,
pending_obligation: &mut PendingPredicateObligation<'tcx>,
) -> ProcessResult<PendingPredicateObligation<'tcx>, !> {
assert!(self.needs_process_obligation(pending_obligation));
self.removed_predicates.push(pending_obligation.obligation.clone());
ProcessResult::Changed(vec![])
}
fn process_backedge<'c, I>(
&mut self,
cycle: I,
_marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
) -> Result<(), !>
where
I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
{
self.removed_predicates.extend(cycle.map(|c| c.obligation.clone()));
Ok(())
}
}
}
fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
self.predicates.map_pending_obligations(|o| o.obligation.clone())
}
}
struct FulfillProcessor<'a, 'tcx> {
selcx: SelectionContext<'a, 'tcx>,
}
fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
os.into_iter()
.map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
.collect()
}
impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
type Obligation = PendingPredicateObligation<'tcx>;
type Error = FulfillmentErrorCode<'tcx>;
type OUT = Outcome<Self::Obligation, Self::Error>;
/// Compared to `needs_process_obligation` this and its callees
/// contain some optimizations that come at the price of false negatives.
///
/// They
/// - reduce branching by covering only the most common case
/// - take a read-only view of the unification tables which allows skipping undo_log
/// construction.
/// - bail out on value-cache misses in ena to avoid pointer chasing
/// - hoist RefCell locking out of the loop
#[inline]
fn skippable_obligations<'b>(
&'b self,
it: impl Iterator<Item = &'b Self::Obligation>,
) -> usize {
let is_unchanged = self.selcx.infcx.is_ty_infer_var_definitely_unchanged();
it.take_while(|o| match o.stalled_on.as_slice() {
[o] => is_unchanged(*o),
_ => false,
})
.count()
}
/// Identifies whether a predicate obligation needs processing.
///
/// This is always inlined because it has a single callsite and it is
/// called *very* frequently. Be careful modifying this code! Several
/// compile-time benchmarks are very sensitive to even small changes.
#[inline(always)]
fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
// If we were stalled on some unresolved variables, first check whether
// any of them have been resolved; if not, don't bother doing more work
// yet.
let stalled_on = &pending_obligation.stalled_on;
match stalled_on.len() {
// This case is the hottest most of the time, being hit up to 99%
// of the time. `keccak` and `cranelift-codegen-0.82.1` are
// benchmarks that particularly stress this path.
1 => self.selcx.infcx.ty_or_const_infer_var_changed(stalled_on[0]),
// In this case we haven't changed, but wish to make a change. Note
// that this is a special case, and is not equivalent to the `_`
// case below, which would return `false` for an empty `stalled_on`
// vector.
//
// This case is usually hit only 1% of the time or less, though it
// reaches 20% in `wasmparser-0.101.0`.
0 => true,
// This case is usually hit only 1% of the time or less, though it
// reaches 95% in `mime-0.3.16`, 64% in `wast-54.0.0`, and 12% in
// `inflate-0.4.5`.
//
// The obvious way of writing this, with a call to `any()` and no
// closure, is currently slower than this version.
_ => (|| {
for &infer_var in stalled_on {
if self.selcx.infcx.ty_or_const_infer_var_changed(infer_var) {
return true;
}
}
false
})(),
}
}
/// Processes a predicate obligation and returns either:
/// - `Changed(v)` if the predicate is true, presuming that `v` are also true
/// - `Unchanged` if we don't have enough info to be sure
/// - `Error(e)` if the predicate does not hold
///
/// This is called much less often than `needs_process_obligation`, so we
/// never inline it.
#[inline(never)]
#[instrument(level = "debug", skip(self, pending_obligation))]
fn process_obligation(
&mut self,
pending_obligation: &mut PendingPredicateObligation<'tcx>,
) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
pending_obligation.stalled_on.truncate(0);
let obligation = &mut pending_obligation.obligation;
debug!(?obligation, "pre-resolve");
if obligation.predicate.has_non_region_infer() {
obligation.predicate = self.selcx.infcx.resolve_vars_if_possible(obligation.predicate);
}
let obligation = &pending_obligation.obligation;
let infcx = self.selcx.infcx;
if obligation.predicate.has_aliases() {
let mut obligations = Vec::new();
let predicate = normalize_with_depth_to(
&mut self.selcx,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
obligation.predicate,
&mut obligations,
);
if predicate != obligation.predicate {
obligations.push(obligation.with(infcx.tcx, predicate));
return ProcessResult::Changed(mk_pending(obligations));
}
}
let binder = obligation.predicate.kind();
match binder.no_bound_vars() {
None => match binder.skip_binder() {
// Evaluation will discard candidates using the leak check.
// This means we need to pass it the bound version of our
// predicate.
ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_ref)) => {
let trait_obligation = obligation.with(infcx.tcx, binder.rebind(trait_ref));
self.process_trait_obligation(
obligation,
trait_obligation,
&mut pending_obligation.stalled_on,
)
}
ty::PredicateKind::Clause(ty::ClauseKind::Projection(data)) => {
let project_obligation = obligation.with(infcx.tcx, binder.rebind(data));
self.process_projection_obligation(
obligation,
project_obligation,
&mut pending_obligation.stalled_on,
)
}
ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(_))
| ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(_))
| ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(..))
| ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(_))
| ty::PredicateKind::ObjectSafe(_)
| ty::PredicateKind::Subtype(_)
| ty::PredicateKind::Coerce(_)
| ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(..))
| ty::PredicateKind::ConstEquate(..) => {
let pred = ty::Binder::dummy(infcx.enter_forall_and_leak_universe(binder));
ProcessResult::Changed(mk_pending(vec![obligation.with(infcx.tcx, pred)]))
}
ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
ty::PredicateKind::NormalizesTo(..) => {
bug!("NormalizesTo is only used by the new solver")
}
ty::PredicateKind::AliasRelate(..) => {
bug!("AliasRelate is only used by the new solver")
}
},
Some(pred) => match pred {
ty::PredicateKind::Clause(ty::ClauseKind::Trait(data)) => {
let trait_obligation = obligation.with(infcx.tcx, Binder::dummy(data));
self.process_trait_obligation(
obligation,
trait_obligation,
&mut pending_obligation.stalled_on,
)
}
ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(data)) => {
if infcx.considering_regions {
infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
}
ProcessResult::Changed(vec![])
}
ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(
t_a,
r_b,
))) => {
if infcx.considering_regions {
infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
}
ProcessResult::Changed(vec![])
}
ty::PredicateKind::Clause(ty::ClauseKind::Projection(ref data)) => {
let project_obligation = obligation.with(infcx.tcx, Binder::dummy(*data));
self.process_projection_obligation(
obligation,
project_obligation,
&mut pending_obligation.stalled_on,
)
}
ty::PredicateKind::ObjectSafe(trait_def_id) => {
if !self.selcx.tcx().check_is_object_safe(trait_def_id) {
ProcessResult::Error(FulfillmentErrorCode::SelectionError(Unimplemented))
} else {
ProcessResult::Changed(vec![])
}
}
ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
ty::PredicateKind::NormalizesTo(..) => {
bug!("NormalizesTo is only used by the new solver")
}
ty::PredicateKind::AliasRelate(..) => {
bug!("AliasRelate is only used by the new solver")
}
// Compute `ConstArgHasType` above the overflow check below.
// This is because this is not ever a useful obligation to report
// as the cause of an overflow.
ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, ty)) => {
match self.selcx.infcx.at(&obligation.cause, obligation.param_env).eq(
// Only really excercised by generic_const_exprs
DefineOpaqueTypes::Yes,
ct.ty(),
ty,
) {
Ok(inf_ok) => ProcessResult::Changed(mk_pending(inf_ok.into_obligations())),
Err(_) => ProcessResult::Error(FulfillmentErrorCode::SelectionError(
SelectionError::Unimplemented,
)),
}
}
// General case overflow check. Allow `process_trait_obligation`
// and `process_projection_obligation` to handle checking for
// the recursion limit themselves. Also don't check some
// predicate kinds that don't give further obligations.
_ if !self
.selcx
.tcx()
.recursion_limit()
.value_within_limit(obligation.recursion_depth) =>
{
self.selcx.infcx.err_ctxt().report_overflow_obligation(&obligation, false);
}
ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(arg)) => {
match wf::obligations(
self.selcx.infcx,
obligation.param_env,
obligation.cause.body_id,
obligation.recursion_depth + 1,
arg,
obligation.cause.span,
) {
None => {
pending_obligation.stalled_on =
vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
ProcessResult::Unchanged
}
Some(os) => ProcessResult::Changed(mk_pending(os)),
}
}
ty::PredicateKind::Subtype(subtype) => {
match self.selcx.infcx.subtype_predicate(
&obligation.cause,
obligation.param_env,
Binder::dummy(subtype),
) {
Err((a, b)) => {
// None means that both are unresolved.
pending_obligation.stalled_on =
vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
ProcessResult::Unchanged
}
Ok(Ok(mut ok)) => {
for subobligation in &mut ok.obligations {
subobligation.set_depth_from_parent(obligation.recursion_depth);
}
ProcessResult::Changed(mk_pending(ok.obligations))
}
Ok(Err(err)) => {
let expected_found =
ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
ProcessResult::Error(FulfillmentErrorCode::SubtypeError(
expected_found,
err,
))
}
}
}
ty::PredicateKind::Coerce(coerce) => {
match self.selcx.infcx.coerce_predicate(
&obligation.cause,
obligation.param_env,
Binder::dummy(coerce),
) {
Err((a, b)) => {
// None means that both are unresolved.
pending_obligation.stalled_on =
vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
ProcessResult::Unchanged
}
Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
Ok(Err(err)) => {
let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
ProcessResult::Error(FulfillmentErrorCode::SubtypeError(
expected_found,
err,
))
}
}
}
ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(uv)) => {
match const_evaluatable::is_const_evaluatable(
self.selcx.infcx,
uv,
obligation.param_env,
obligation.cause.span,
) {
Ok(()) => ProcessResult::Changed(vec![]),
Err(NotConstEvaluatable::MentionsInfer) => {
pending_obligation.stalled_on.clear();
pending_obligation.stalled_on.extend(
uv.walk().filter_map(TyOrConstInferVar::maybe_from_generic_arg),
);
ProcessResult::Unchanged
}
Err(
e @ NotConstEvaluatable::MentionsParam
| e @ NotConstEvaluatable::Error(_),
) => ProcessResult::Error(FulfillmentErrorCode::SelectionError(
SelectionError::NotConstEvaluatable(e),
)),
}
}
ty::PredicateKind::ConstEquate(c1, c2) => {
let tcx = self.selcx.tcx();
assert!(
tcx.features().generic_const_exprs,
"`ConstEquate` without a feature gate: {c1:?} {c2:?}",
);
// FIXME: we probably should only try to unify abstract constants
// if the constants depend on generic parameters.
//
// Let's just see where this breaks :shrug:
{
let c1 = tcx.expand_abstract_consts(c1);
let c2 = tcx.expand_abstract_consts(c2);
debug!("equating consts:\nc1= {:?}\nc2= {:?}", c1, c2);
use rustc_hir::def::DefKind;
use ty::Unevaluated;
match (c1.kind(), c2.kind()) {
(Unevaluated(a), Unevaluated(b))
if a.def == b.def && tcx.def_kind(a.def) == DefKind::AssocConst =>
{
if let Ok(new_obligations) = infcx
.at(&obligation.cause, obligation.param_env)
.trace(c1, c2)
// Can define opaque types as this is only reachable with
// `generic_const_exprs`
.eq(DefineOpaqueTypes::Yes, a.args, b.args)
{
return ProcessResult::Changed(mk_pending(
new_obligations.into_obligations(),
));
}
}
(_, Unevaluated(_)) | (Unevaluated(_), _) => (),
(_, _) => {
if let Ok(new_obligations) = infcx
.at(&obligation.cause, obligation.param_env)
// Can define opaque types as this is only reachable with
// `generic_const_exprs`
.eq(DefineOpaqueTypes::Yes, c1, c2)
{
return ProcessResult::Changed(mk_pending(
new_obligations.into_obligations(),
));
}
}
}
}
let stalled_on = &mut pending_obligation.stalled_on;
let mut evaluate = |c: Const<'tcx>| {
if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
match self.selcx.infcx.try_const_eval_resolve(
obligation.param_env,
unevaluated,
c.ty(),
obligation.cause.span,
) {
Ok(val) => Ok(val),
Err(e) => {
match e {
ErrorHandled::TooGeneric(..) => {
stalled_on.extend(unevaluated.args.iter().filter_map(
TyOrConstInferVar::maybe_from_generic_arg,
));
}
_ => {}
}
Err(e)
}
}
} else {
Ok(c)
}
};
match (evaluate(c1), evaluate(c2)) {
(Ok(c1), Ok(c2)) => {
match self.selcx.infcx.at(&obligation.cause, obligation.param_env).eq(
// Can define opaque types as this is only reachable with
// `generic_const_exprs`
DefineOpaqueTypes::Yes,
c1,
c2,
) {
Ok(inf_ok) => {
ProcessResult::Changed(mk_pending(inf_ok.into_obligations()))
}
Err(err) => {
ProcessResult::Error(FulfillmentErrorCode::ConstEquateError(
ExpectedFound::new(true, c1, c2),
err,
))
}
}
}
(Err(ErrorHandled::Reported(reported, _)), _)
| (_, Err(ErrorHandled::Reported(reported, _))) => {
ProcessResult::Error(FulfillmentErrorCode::SelectionError(
SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(
reported.into(),
)),
))
}
(Err(ErrorHandled::TooGeneric(_)), _)
| (_, Err(ErrorHandled::TooGeneric(_))) => {
if c1.has_non_region_infer() || c2.has_non_region_infer() {
ProcessResult::Unchanged
} else {
// Two different constants using generic parameters ~> error.
let expected_found = ExpectedFound::new(true, c1, c2);
ProcessResult::Error(FulfillmentErrorCode::ConstEquateError(
expected_found,
TypeError::ConstMismatch(expected_found),
))
}
}
}
}
},
}
}
#[inline(never)]
fn process_backedge<'c, I>(
&mut self,
cycle: I,
_marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
) -> Result<(), FulfillmentErrorCode<'tcx>>
where
I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
{
if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
debug!("process_child_obligations: coinductive match");
Ok(())
} else {
let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
Err(FulfillmentErrorCode::Cycle(cycle))
}
}
}
impl<'a, 'tcx> FulfillProcessor<'a, 'tcx> {
#[instrument(level = "debug", skip(self, obligation, stalled_on))]
fn process_trait_obligation(
&mut self,
obligation: &PredicateObligation<'tcx>,
trait_obligation: PolyTraitObligation<'tcx>,
stalled_on: &mut Vec<TyOrConstInferVar>,
) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
let infcx = self.selcx.infcx;
if obligation.predicate.is_global() && !self.selcx.is_intercrate() {
// no type variables present, can use evaluation for better caching.
// FIXME: consider caching errors too.
if infcx.predicate_must_hold_considering_regions(obligation) {
debug!(
"selecting trait at depth {} evaluated to holds",
obligation.recursion_depth
);
return ProcessResult::Changed(vec![]);
}
}
match self.selcx.poly_select(&trait_obligation) {
Ok(Some(impl_source)) => {
debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
}
Ok(None) => {
debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
// This is a bit subtle: for the most part, the
// only reason we can fail to make progress on
// trait selection is because we don't have enough
// information about the types in the trait.
stalled_on.clear();
stalled_on.extend(args_infer_vars(
&self.selcx,
trait_obligation.predicate.map_bound(|pred| pred.trait_ref.args),
));
debug!(
"process_predicate: pending obligation {:?} now stalled on {:?}",
infcx.resolve_vars_if_possible(obligation.clone()),
stalled_on
);
ProcessResult::Unchanged
}
Err(selection_err) => {
debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
ProcessResult::Error(FulfillmentErrorCode::SelectionError(selection_err))
}
}
}
fn process_projection_obligation(
&mut self,
obligation: &PredicateObligation<'tcx>,
project_obligation: PolyProjectionObligation<'tcx>,
stalled_on: &mut Vec<TyOrConstInferVar>,
) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
let tcx = self.selcx.tcx();
if obligation.predicate.is_global() && !self.selcx.is_intercrate() {
// no type variables present, can use evaluation for better caching.
// FIXME: consider caching errors too.
if self.selcx.infcx.predicate_must_hold_considering_regions(obligation) {
if let Some(key) = ProjectionCacheKey::from_poly_projection_obligation(
&mut self.selcx,
&project_obligation,
) {
// If `predicate_must_hold_considering_regions` succeeds, then we've
// evaluated all sub-obligations. We can therefore mark the 'root'
// obligation as complete, and skip evaluating sub-obligations.
self.selcx
.infcx
.inner
.borrow_mut()
.projection_cache()
.complete(key, EvaluationResult::EvaluatedToOk);
}
return ProcessResult::Changed(vec![]);
} else {
debug!("Does NOT hold: {:?}", obligation);
}
}
match project::poly_project_and_unify_type(&mut self.selcx, &project_obligation) {
ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
ProjectAndUnifyResult::FailedNormalization => {
stalled_on.clear();
stalled_on.extend(args_infer_vars(
&self.selcx,
project_obligation.predicate.map_bound(|pred| pred.projection_ty.args),
));
ProcessResult::Unchanged
}
// Let the caller handle the recursion
ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
project_obligation.with(tcx, project_obligation.predicate),
])),
ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
ProcessResult::Error(FulfillmentErrorCode::ProjectionError(e))
}
}
}
}
/// Returns the set of inference variables contained in `args`.
fn args_infer_vars<'a, 'tcx>(
selcx: &SelectionContext<'a, 'tcx>,
args: ty::Binder<'tcx, GenericArgsRef<'tcx>>,
) -> impl Iterator<Item = TyOrConstInferVar> + Captures<'tcx> {
selcx
.infcx
.resolve_vars_if_possible(args)
.skip_binder() // ok because this check doesn't care about regions
.iter()
.filter(|arg| arg.has_non_region_infer())
.flat_map(|arg| {
let mut walker = arg.walk();
while let Some(c) = walker.next() {
if !c.has_non_region_infer() {
walker.visited.remove(&c);
walker.skip_current_subtree();
}
}
walker.visited.into_iter()
})
.filter_map(TyOrConstInferVar::maybe_from_generic_arg)
}
fn to_fulfillment_error<'tcx>(
error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
) -> FulfillmentError<'tcx> {
let mut iter = error.backtrace.into_iter();
let obligation = iter.next().unwrap().obligation;
// The root obligation is the last item in the backtrace - if there's only
// one item, then it's the same as the main obligation
let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
FulfillmentError::new(obligation, error.error, root_obligation)
}