1 use crate::infer::{InferCtxt, ShallowResolver};
2 use crate::ty::error::ExpectedFound;
3 use crate::ty::{self, ToPolyTraitRef, Ty, TypeFoldable};
4 use rustc_data_structures::obligation_forest::ProcessResult;
5 use rustc_data_structures::obligation_forest::{DoCompleted, Error, ForestObligation};
6 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
7 use std::marker::PhantomData;
9 use super::engine::{TraitEngine, TraitEngineExt};
11 use super::select::SelectionContext;
13 use super::CodeAmbiguity;
14 use super::CodeProjectionError;
15 use super::CodeSelectionError;
16 use super::{ConstEvalFailure, Unimplemented};
17 use super::{FulfillmentError, FulfillmentErrorCode};
18 use super::{ObligationCause, PredicateObligation};
20 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
21 type Predicate = ty::Predicate<'tcx>;
23 fn as_predicate(&self) -> &Self::Predicate {
24 &self.obligation.predicate
28 /// The fulfillment context is used to drive trait resolution. It
29 /// consists of a list of obligations that must be (eventually)
30 /// satisfied. The job is to track which are satisfied, which yielded
31 /// errors, and which are still pending. At any point, users can call
32 /// `select_where_possible`, and the fulfillment context will try to do
33 /// selection, retaining only those obligations that remain
34 /// ambiguous. This may be helpful in pushing type inference
35 /// along. Once all type inference constraints have been generated, the
36 /// method `select_all_or_error` can be used to report any remaining
37 /// ambiguous cases as errors.
38 pub struct FulfillmentContext<'tcx> {
39 // A list of all obligations that have been registered with this
40 // fulfillment context.
41 predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
42 // Should this fulfillment context register type-lives-for-region
43 // obligations on its parent infcx? In some cases, region
44 // obligations are either already known to hold (normalization) or
45 // hopefully verifed elsewhere (type-impls-bound), and therefore
46 // should not be checked.
48 // Note that if we are normalizing a type that we already
49 // know is well-formed, there should be no harm setting this
50 // to true - all the region variables should be determinable
51 // using the RFC 447 rules, which don't depend on
52 // type-lives-for-region constraints, and because the type
53 // is well-formed, the constraints should hold.
54 register_region_obligations: bool,
55 // Is it OK to register obligations into this infcx inside
58 // The "primary fulfillment" in many cases in typeck lives
59 // outside of any snapshot, so any use of it inside a snapshot
60 // will lead to trouble and therefore is checked against, but
61 // other fulfillment contexts sometimes do live inside of
62 // a snapshot (they don't *straddle* a snapshot, so there
63 // is no trouble there).
64 usable_in_snapshot: bool,
67 #[derive(Clone, Debug)]
68 pub struct PendingPredicateObligation<'tcx> {
69 pub obligation: PredicateObligation<'tcx>,
70 pub stalled_on: Vec<ty::InferTy>,
73 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
74 #[cfg(target_arch = "x86_64")]
75 static_assert_size!(PendingPredicateObligation<'_>, 136);
77 impl<'a, 'tcx> FulfillmentContext<'tcx> {
78 /// Creates a new fulfillment context.
79 pub fn new() -> FulfillmentContext<'tcx> {
81 predicates: ObligationForest::new(),
82 register_region_obligations: true,
83 usable_in_snapshot: false,
87 pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
89 predicates: ObligationForest::new(),
90 register_region_obligations: true,
91 usable_in_snapshot: true,
95 pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
97 predicates: ObligationForest::new(),
98 register_region_obligations: false,
99 usable_in_snapshot: false,
103 /// Attempts to select obligations using `selcx`.
106 selcx: &mut SelectionContext<'a, 'tcx>,
107 ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
108 debug!("select(obligation-forest-size={})", self.predicates.len());
110 let mut errors = Vec::new();
113 debug!("select: starting another iteration");
115 // Process pending obligations.
116 let outcome = self.predicates.process_obligations(
117 &mut FulfillProcessor {
119 register_region_obligations: self.register_region_obligations,
123 debug!("select: outcome={:#?}", outcome);
125 // FIXME: if we kept the original cache key, we could mark projection
126 // obligations as complete for the projection cache here.
128 errors.extend(outcome.errors.into_iter().map(|e| to_fulfillment_error(e)));
130 // If nothing new was added, no need to keep looping.
137 "select({} predicates remaining, {} errors) done",
138 self.predicates.len(),
142 if errors.is_empty() { Ok(()) } else { Err(errors) }
146 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
147 /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
148 /// creating a fresh type variable `$0` as well as a projection
149 /// predicate `<SomeType as SomeTrait>::X == $0`. When the
150 /// inference engine runs, it will attempt to find an impl of
151 /// `SomeTrait` or a where-clause that lets us unify `$0` with
152 /// something concrete. If this fails, we'll unify `$0` with
153 /// `projection_ty` again.
154 fn normalize_projection_type(
156 infcx: &InferCtxt<'_, 'tcx>,
157 param_env: ty::ParamEnv<'tcx>,
158 projection_ty: ty::ProjectionTy<'tcx>,
159 cause: ObligationCause<'tcx>,
161 debug!("normalize_projection_type(projection_ty={:?})", projection_ty);
163 debug_assert!(!projection_ty.has_escaping_bound_vars());
165 // FIXME(#20304) -- cache
167 let mut selcx = SelectionContext::new(infcx);
168 let mut obligations = vec![];
169 let normalized_ty = project::normalize_projection_type(
177 self.register_predicate_obligations(infcx, obligations);
179 debug!("normalize_projection_type: result={:?}", normalized_ty);
184 fn register_predicate_obligation(
186 infcx: &InferCtxt<'_, 'tcx>,
187 obligation: PredicateObligation<'tcx>,
189 // this helps to reduce duplicate errors, as well as making
190 // debug output much nicer to read and so on.
191 let obligation = infcx.resolve_vars_if_possible(&obligation);
193 debug!("register_predicate_obligation(obligation={:?})", obligation);
195 assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
198 .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
201 fn select_all_or_error(
203 infcx: &InferCtxt<'_, 'tcx>,
204 ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
205 self.select_where_possible(infcx)?;
207 let errors: Vec<_> = self
209 .to_errors(CodeAmbiguity)
211 .map(|e| to_fulfillment_error(e))
213 if errors.is_empty() { Ok(()) } else { Err(errors) }
216 fn select_where_possible(
218 infcx: &InferCtxt<'_, 'tcx>,
219 ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
220 let mut selcx = SelectionContext::new(infcx);
221 self.select(&mut selcx)
224 fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
225 self.predicates.map_pending_obligations(|o| o.obligation.clone())
229 struct FulfillProcessor<'a, 'b, 'tcx> {
230 selcx: &'a mut SelectionContext<'b, 'tcx>,
231 register_region_obligations: bool,
234 fn mk_pending(os: Vec<PredicateObligation<'tcx>>) -> Vec<PendingPredicateObligation<'tcx>> {
236 .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
240 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
241 type Obligation = PendingPredicateObligation<'tcx>;
242 type Error = FulfillmentErrorCode<'tcx>;
244 /// Processes a predicate obligation and returns either:
245 /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
246 /// - `Unchanged` if we don't have enough info to be sure
247 /// - `Error(e)` if the predicate does not hold
249 /// This is always inlined, despite its size, because it has a single
250 /// callsite and it is called *very* frequently.
252 fn process_obligation(
254 pending_obligation: &mut Self::Obligation,
255 ) -> ProcessResult<Self::Obligation, Self::Error> {
256 // If we were stalled on some unresolved variables, first check whether
257 // any of them have been resolved; if not, don't bother doing more work
259 let change = match pending_obligation.stalled_on.len() {
260 // Match arms are in order of frequency, which matters because this
261 // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
263 let infer = pending_obligation.stalled_on[0];
264 ShallowResolver::new(self.selcx.infcx()).shallow_resolve_changed(infer)
267 // In this case we haven't changed, but wish to make a change.
271 // This `for` loop was once a call to `all()`, but this lower-level
272 // form was a perf win. See #64545 for details.
274 for &infer in &pending_obligation.stalled_on {
275 if ShallowResolver::new(self.selcx.infcx()).shallow_resolve_changed(infer) {
286 "process_predicate: pending obligation {:?} still stalled on {:?}",
287 self.selcx.infcx().resolve_vars_if_possible(&pending_obligation.obligation),
288 pending_obligation.stalled_on
290 return ProcessResult::Unchanged;
293 // This part of the code is much colder.
295 pending_obligation.stalled_on.truncate(0);
297 let obligation = &mut pending_obligation.obligation;
299 if obligation.predicate.has_infer_types() {
300 obligation.predicate =
301 self.selcx.infcx().resolve_vars_if_possible(&obligation.predicate);
304 debug!("process_obligation: obligation = {:?} cause = {:?}", obligation, obligation.cause);
306 fn infer_ty(ty: Ty<'tcx>) -> ty::InferTy {
308 ty::Infer(infer) => infer,
313 match obligation.predicate {
314 ty::Predicate::Trait(ref data) => {
315 let trait_obligation = obligation.with(data.clone());
317 if data.is_global() {
318 // no type variables present, can use evaluation for better caching.
319 // FIXME: consider caching errors too.
320 if self.selcx.infcx().predicate_must_hold_considering_regions(&obligation) {
322 "selecting trait `{:?}` at depth {} evaluated to holds",
323 data, obligation.recursion_depth
325 return ProcessResult::Changed(vec![]);
329 match self.selcx.select(&trait_obligation) {
330 Ok(Some(vtable)) => {
332 "selecting trait `{:?}` at depth {} yielded Ok(Some)",
333 data, obligation.recursion_depth
335 ProcessResult::Changed(mk_pending(vtable.nested_obligations()))
339 "selecting trait `{:?}` at depth {} yielded Ok(None)",
340 data, obligation.recursion_depth
343 // This is a bit subtle: for the most part, the
344 // only reason we can fail to make progress on
345 // trait selection is because we don't have enough
346 // information about the types in the trait. One
347 // exception is that we sometimes haven't decided
348 // what kind of closure a closure is. *But*, in
349 // that case, it turns out, the type of the
350 // closure will also change, because the closure
351 // also includes references to its upvars as part
352 // of its type, and those types are resolved at
355 // FIXME(#32286) logic seems false if no upvars
356 pending_obligation.stalled_on =
357 trait_ref_type_vars(self.selcx, data.to_poly_trait_ref());
360 "process_predicate: pending obligation {:?} now stalled on {:?}",
361 self.selcx.infcx().resolve_vars_if_possible(obligation),
362 pending_obligation.stalled_on
365 ProcessResult::Unchanged
367 Err(selection_err) => {
369 "selecting trait `{:?}` at depth {} yielded Err",
370 data, obligation.recursion_depth
373 ProcessResult::Error(CodeSelectionError(selection_err))
378 ty::Predicate::RegionOutlives(ref binder) => {
379 match self.selcx.infcx().region_outlives_predicate(&obligation.cause, binder) {
380 Ok(()) => ProcessResult::Changed(vec![]),
381 Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
385 ty::Predicate::TypeOutlives(ref binder) => {
386 // Check if there are higher-ranked vars.
387 match binder.no_bound_vars() {
388 // If there are, inspect the underlying type further.
390 // Convert from `Binder<OutlivesPredicate<Ty, Region>>` to `Binder<Ty>`.
391 let binder = binder.map_bound_ref(|pred| pred.0);
393 // Check if the type has any bound vars.
394 match binder.no_bound_vars() {
395 // If so, this obligation is an error (for now). Eventually we should be
396 // able to support additional cases here, like `for<'a> &'a str: 'a`.
397 // NOTE: this is duplicate-implemented between here and fulfillment.
398 None => ProcessResult::Error(CodeSelectionError(Unimplemented)),
399 // Otherwise, we have something of the form
400 // `for<'a> T: 'a where 'a not in T`, which we can treat as
403 let r_static = self.selcx.tcx().lifetimes.re_static;
404 if self.register_region_obligations {
405 self.selcx.infcx().register_region_obligation_with_cause(
411 ProcessResult::Changed(vec![])
415 // If there aren't, register the obligation.
416 Some(ty::OutlivesPredicate(t_a, r_b)) => {
417 if self.register_region_obligations {
418 self.selcx.infcx().register_region_obligation_with_cause(
424 ProcessResult::Changed(vec![])
429 ty::Predicate::Projection(ref data) => {
430 let project_obligation = obligation.with(data.clone());
431 match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
433 let tcx = self.selcx.tcx();
434 pending_obligation.stalled_on =
435 trait_ref_type_vars(self.selcx, data.to_poly_trait_ref(tcx));
436 ProcessResult::Unchanged
438 Ok(Some(os)) => ProcessResult::Changed(mk_pending(os)),
439 Err(e) => ProcessResult::Error(CodeProjectionError(e)),
443 ty::Predicate::ObjectSafe(trait_def_id) => {
444 if !self.selcx.tcx().is_object_safe(trait_def_id) {
445 ProcessResult::Error(CodeSelectionError(Unimplemented))
447 ProcessResult::Changed(vec![])
451 ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
452 match self.selcx.infcx().closure_kind(closure_def_id, closure_substs) {
453 Some(closure_kind) => {
454 if closure_kind.extends(kind) {
455 ProcessResult::Changed(vec![])
457 ProcessResult::Error(CodeSelectionError(Unimplemented))
460 None => ProcessResult::Unchanged,
464 ty::Predicate::WellFormed(ty) => {
465 match wf::obligations(
467 obligation.param_env,
468 obligation.cause.body_id,
470 obligation.cause.span,
473 pending_obligation.stalled_on = vec![infer_ty(ty)];
474 ProcessResult::Unchanged
476 Some(os) => ProcessResult::Changed(mk_pending(os)),
480 ty::Predicate::Subtype(ref subtype) => {
481 match self.selcx.infcx().subtype_predicate(
483 obligation.param_env,
487 // None means that both are unresolved.
488 pending_obligation.stalled_on = vec![
489 infer_ty(subtype.skip_binder().a),
490 infer_ty(subtype.skip_binder().b),
492 ProcessResult::Unchanged
494 Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
496 let expected_found = ExpectedFound::new(
497 subtype.skip_binder().a_is_expected,
498 subtype.skip_binder().a,
499 subtype.skip_binder().b,
501 ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
509 ty::Predicate::ConstEvaluatable(def_id, substs) => {
510 if obligation.param_env.has_local_value() {
511 ProcessResult::Unchanged
513 if !substs.has_local_value() {
514 match self.selcx.tcx().const_eval_resolve(
515 obligation.param_env,
519 Some(obligation.cause.span),
521 Ok(_) => ProcessResult::Changed(vec![]),
523 ProcessResult::Error(CodeSelectionError(ConstEvalFailure(err)))
527 pending_obligation.stalled_on =
528 substs.types().map(|ty| infer_ty(ty)).collect();
529 ProcessResult::Unchanged
536 fn process_backedge<'c, I>(
539 _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
541 I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
543 if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
544 debug!("process_child_obligations: coinductive match");
546 let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
547 self.selcx.infcx().report_overflow_error_cycle(&cycle);
552 /// Returns the set of type variables contained in a trait ref
553 fn trait_ref_type_vars<'a, 'tcx>(
554 selcx: &mut SelectionContext<'a, 'tcx>,
555 t: ty::PolyTraitRef<'tcx>,
556 ) -> Vec<ty::InferTy> {
557 t.skip_binder() // ok b/c this check doesn't care about regions
559 .map(|t| selcx.infcx().resolve_vars_if_possible(&t))
560 .filter(|t| t.has_infer_types())
561 .flat_map(|t| t.walk())
562 .filter_map(|t| match t.kind {
563 ty::Infer(infer) => Some(infer),
569 fn to_fulfillment_error<'tcx>(
570 error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
571 ) -> FulfillmentError<'tcx> {
572 let obligation = error.backtrace.into_iter().next().unwrap().obligation;
573 FulfillmentError::new(obligation, error.error)