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Diffstat (limited to 'compiler/rustc_trait_selection/src/traits/auto_trait.rs')
| -rw-r--r-- | compiler/rustc_trait_selection/src/traits/auto_trait.rs | 903 |
1 files changed, 903 insertions, 0 deletions
diff --git a/compiler/rustc_trait_selection/src/traits/auto_trait.rs b/compiler/rustc_trait_selection/src/traits/auto_trait.rs new file mode 100644 index 000000000..294c81d0b --- /dev/null +++ b/compiler/rustc_trait_selection/src/traits/auto_trait.rs @@ -0,0 +1,903 @@ +//! Support code for rustdoc and external tools. +//! You really don't want to be using this unless you need to. + +use super::*; + +use crate::infer::region_constraints::{Constraint, RegionConstraintData}; +use crate::infer::InferCtxt; +use crate::traits::project::ProjectAndUnifyResult; +use rustc_middle::mir::interpret::ErrorHandled; +use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable}; +use rustc_middle::ty::visit::TypeVisitable; +use rustc_middle::ty::{Region, RegionVid, Term}; + +use rustc_data_structures::fx::{FxHashMap, FxHashSet}; + +use std::collections::hash_map::Entry; +use std::collections::VecDeque; +use std::iter; + +// FIXME(twk): this is obviously not nice to duplicate like that +#[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)] +pub enum RegionTarget<'tcx> { + Region(Region<'tcx>), + RegionVid(RegionVid), +} + +#[derive(Default, Debug, Clone)] +pub struct RegionDeps<'tcx> { + larger: FxHashSet<RegionTarget<'tcx>>, + smaller: FxHashSet<RegionTarget<'tcx>>, +} + +pub enum AutoTraitResult<A> { + ExplicitImpl, + PositiveImpl(A), + NegativeImpl, +} + +#[allow(dead_code)] +impl<A> AutoTraitResult<A> { + fn is_auto(&self) -> bool { + matches!(self, AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl) + } +} + +pub struct AutoTraitInfo<'cx> { + pub full_user_env: ty::ParamEnv<'cx>, + pub region_data: RegionConstraintData<'cx>, + pub vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'cx>>, +} + +pub struct AutoTraitFinder<'tcx> { + tcx: TyCtxt<'tcx>, +} + +impl<'tcx> AutoTraitFinder<'tcx> { + pub fn new(tcx: TyCtxt<'tcx>) -> Self { + AutoTraitFinder { tcx } + } + + /// Makes a best effort to determine whether and under which conditions an auto trait is + /// implemented for a type. For example, if you have + /// + /// ``` + /// struct Foo<T> { data: Box<T> } + /// ``` + /// + /// then this might return that Foo<T>: Send if T: Send (encoded in the AutoTraitResult type). + /// The analysis attempts to account for custom impls as well as other complex cases. This + /// result is intended for use by rustdoc and other such consumers. + /// + /// (Note that due to the coinductive nature of Send, the full and correct result is actually + /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field + /// types are all Send. So, in our example, we might have that Foo<T>: Send if Box<T>: Send. + /// But this is often not the best way to present to the user.) + /// + /// Warning: The API should be considered highly unstable, and it may be refactored or removed + /// in the future. + pub fn find_auto_trait_generics<A>( + &self, + ty: Ty<'tcx>, + orig_env: ty::ParamEnv<'tcx>, + trait_did: DefId, + mut auto_trait_callback: impl FnMut(AutoTraitInfo<'tcx>) -> A, + ) -> AutoTraitResult<A> { + let tcx = self.tcx; + + let trait_ref = ty::TraitRef { def_id: trait_did, substs: tcx.mk_substs_trait(ty, &[]) }; + + let trait_pred = ty::Binder::dummy(trait_ref); + + let bail_out = tcx.infer_ctxt().enter(|infcx| { + let mut selcx = SelectionContext::new(&infcx); + let result = selcx.select(&Obligation::new( + ObligationCause::dummy(), + orig_env, + trait_pred.to_poly_trait_predicate(), + )); + + match result { + Ok(Some(ImplSource::UserDefined(_))) => { + debug!( + "find_auto_trait_generics({:?}): \ + manual impl found, bailing out", + trait_ref + ); + return true; + } + _ => {} + } + + let result = selcx.select(&Obligation::new( + ObligationCause::dummy(), + orig_env, + trait_pred.to_poly_trait_predicate_negative_polarity(), + )); + + match result { + Ok(Some(ImplSource::UserDefined(_))) => { + debug!( + "find_auto_trait_generics({:?}): \ + manual impl found, bailing out", + trait_ref + ); + true + } + _ => false, + } + }); + + // If an explicit impl exists, it always takes priority over an auto impl + if bail_out { + return AutoTraitResult::ExplicitImpl; + } + + tcx.infer_ctxt().enter(|infcx| { + let mut fresh_preds = FxHashSet::default(); + + // Due to the way projections are handled by SelectionContext, we need to run + // evaluate_predicates twice: once on the original param env, and once on the result of + // the first evaluate_predicates call. + // + // The problem is this: most of rustc, including SelectionContext and traits::project, + // are designed to work with a concrete usage of a type (e.g., Vec<u8> + // fn<T>() { Vec<T> }. This information will generally never change - given + // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'. + // If we're unable to prove that 'T' implements a particular trait, we're done - + // there's nothing left to do but error out. + // + // However, synthesizing an auto trait impl works differently. Here, we start out with + // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing + // with - and progressively discover the conditions we need to fulfill for it to + // implement a certain auto trait. This ends up breaking two assumptions made by trait + // selection and projection: + // + // * We can always cache the result of a particular trait selection for the lifetime of + // an InfCtxt + // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T: + // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K' + // + // We fix the first assumption by manually clearing out all of the InferCtxt's caches + // in between calls to SelectionContext.select. This allows us to keep all of the + // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift + // them between calls. + // + // We fix the second assumption by reprocessing the result of our first call to + // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first + // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass, + // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing + // SelectionContext to return it back to us. + + let Some((new_env, user_env)) = self.evaluate_predicates( + &infcx, + trait_did, + ty, + orig_env, + orig_env, + &mut fresh_preds, + false, + ) else { + return AutoTraitResult::NegativeImpl; + }; + + let (full_env, full_user_env) = self + .evaluate_predicates( + &infcx, + trait_did, + ty, + new_env, + user_env, + &mut fresh_preds, + true, + ) + .unwrap_or_else(|| { + panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env) + }); + + debug!( + "find_auto_trait_generics({:?}): fulfilling \ + with {:?}", + trait_ref, full_env + ); + infcx.clear_caches(); + + // At this point, we already have all of the bounds we need. FulfillmentContext is used + // to store all of the necessary region/lifetime bounds in the InferContext, as well as + // an additional sanity check. + let mut fulfill = <dyn TraitEngine<'tcx>>::new(tcx); + fulfill.register_bound(&infcx, full_env, ty, trait_did, ObligationCause::dummy()); + let errors = fulfill.select_all_or_error(&infcx); + + if !errors.is_empty() { + panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors); + } + + infcx.process_registered_region_obligations(&Default::default(), full_env); + + let region_data = infcx + .inner + .borrow_mut() + .unwrap_region_constraints() + .region_constraint_data() + .clone(); + + let vid_to_region = self.map_vid_to_region(®ion_data); + + let info = AutoTraitInfo { full_user_env, region_data, vid_to_region }; + + AutoTraitResult::PositiveImpl(auto_trait_callback(info)) + }) + } +} + +impl<'tcx> AutoTraitFinder<'tcx> { + /// The core logic responsible for computing the bounds for our synthesized impl. + /// + /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like + /// `FulfillmentContext`, we recursively select the nested obligations of predicates we + /// encounter. However, whenever we encounter an `UnimplementedError` involving a type + /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular + /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met. + /// + /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key + /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete + /// user code. According, it considers all possible ways that a `Predicate` could be met, which + /// isn't always what we want for a synthesized impl. For example, given the predicate `T: + /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T: + /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`, + /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up. + /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl + /// like this: + /// ```ignore (illustrative) + /// impl<T> Send for Foo<T> where T: IntoIterator + /// ``` + /// While it might be technically true that Foo implements Send where `T: IntoIterator`, + /// the bound is overly restrictive - it's really only necessary that `T: Iterator`. + /// + /// For this reason, `evaluate_predicates` handles predicates with type variables specially. + /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately + /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later + /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator` + /// needs to hold. + /// + /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever + /// constructed once for a given type. As part of the construction process, the `ParamEnv` will + /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the + /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our + /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or + /// else `SelectionContext` will choke on the missing predicates. However, this should never + /// show up in the final synthesized generics: we don't want our generated docs page to contain + /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a + /// separate `user_env`, which only holds the predicates that will actually be displayed to the + /// user. + fn evaluate_predicates( + &self, + infcx: &InferCtxt<'_, 'tcx>, + trait_did: DefId, + ty: Ty<'tcx>, + param_env: ty::ParamEnv<'tcx>, + user_env: ty::ParamEnv<'tcx>, + fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>, + only_projections: bool, + ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> { + let tcx = infcx.tcx; + + // Don't try to process any nested obligations involving predicates + // that are already in the `ParamEnv` (modulo regions): we already + // know that they must hold. + for predicate in param_env.caller_bounds() { + fresh_preds.insert(self.clean_pred(infcx, predicate)); + } + + let mut select = SelectionContext::new(&infcx); + + let mut already_visited = FxHashSet::default(); + let mut predicates = VecDeque::new(); + predicates.push_back(ty::Binder::dummy(ty::TraitPredicate { + trait_ref: ty::TraitRef { + def_id: trait_did, + substs: infcx.tcx.mk_substs_trait(ty, &[]), + }, + constness: ty::BoundConstness::NotConst, + // Auto traits are positive + polarity: ty::ImplPolarity::Positive, + })); + + let computed_preds = param_env.caller_bounds().iter(); + let mut user_computed_preds: FxHashSet<_> = user_env.caller_bounds().iter().collect(); + + let mut new_env = param_env; + let dummy_cause = ObligationCause::dummy(); + + while let Some(pred) = predicates.pop_front() { + infcx.clear_caches(); + + if !already_visited.insert(pred) { + continue; + } + + // Call `infcx.resolve_vars_if_possible` to see if we can + // get rid of any inference variables. + let obligation = + infcx.resolve_vars_if_possible(Obligation::new(dummy_cause.clone(), new_env, pred)); + let result = select.select(&obligation); + + match result { + Ok(Some(ref impl_source)) => { + // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`), + // we immediately bail out, since it's impossible for us to continue. + + if let ImplSource::UserDefined(ImplSourceUserDefinedData { + impl_def_id, .. + }) = impl_source + { + // Blame 'tidy' for the weird bracket placement. + if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative { + debug!( + "evaluate_nested_obligations: found explicit negative impl\ + {:?}, bailing out", + impl_def_id + ); + return None; + } + } + + let obligations = impl_source.clone().nested_obligations().into_iter(); + + if !self.evaluate_nested_obligations( + ty, + obligations, + &mut user_computed_preds, + fresh_preds, + &mut predicates, + &mut select, + only_projections, + ) { + return None; + } + } + Ok(None) => {} + Err(SelectionError::Unimplemented) => { + if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) { + already_visited.remove(&pred); + self.add_user_pred(&mut user_computed_preds, pred.to_predicate(self.tcx)); + predicates.push_back(pred); + } else { + debug!( + "evaluate_nested_obligations: `Unimplemented` found, bailing: \ + {:?} {:?} {:?}", + ty, + pred, + pred.skip_binder().trait_ref.substs + ); + return None; + } + } + _ => panic!("Unexpected error for '{:?}': {:?}", ty, result), + }; + + let normalized_preds = elaborate_predicates( + tcx, + computed_preds.clone().chain(user_computed_preds.iter().cloned()), + ) + .map(|o| o.predicate); + new_env = ty::ParamEnv::new( + tcx.mk_predicates(normalized_preds), + param_env.reveal(), + param_env.constness(), + ); + } + + let final_user_env = ty::ParamEnv::new( + tcx.mk_predicates(user_computed_preds.into_iter()), + user_env.reveal(), + user_env.constness(), + ); + debug!( + "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \ + '{:?}'", + ty, trait_did, new_env, final_user_env + ); + + Some((new_env, final_user_env)) + } + + /// This method is designed to work around the following issue: + /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`, + /// progressively building a `ParamEnv` based on the results we get. + /// However, our usage of `SelectionContext` differs from its normal use within the compiler, + /// in that we capture and re-reprocess predicates from `Unimplemented` errors. + /// + /// This can lead to a corner case when dealing with region parameters. + /// During our selection loop in `evaluate_predicates`, we might end up with + /// two trait predicates that differ only in their region parameters: + /// one containing a HRTB lifetime parameter, and one containing a 'normal' + /// lifetime parameter. For example: + /// ```ignore (illustrative) + /// T as MyTrait<'a> + /// T as MyTrait<'static> + /// ``` + /// If we put both of these predicates in our computed `ParamEnv`, we'll + /// confuse `SelectionContext`, since it will (correctly) view both as being applicable. + /// + /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB + /// Our end goal is to generate a user-visible description of the conditions + /// under which a type implements an auto trait. A trait predicate involving + /// a HRTB means that the type needs to work with any choice of lifetime, + /// not just one specific lifetime (e.g., `'static`). + fn add_user_pred( + &self, + user_computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>, + new_pred: ty::Predicate<'tcx>, + ) { + let mut should_add_new = true; + user_computed_preds.retain(|&old_pred| { + if let (ty::PredicateKind::Trait(new_trait), ty::PredicateKind::Trait(old_trait)) = + (new_pred.kind().skip_binder(), old_pred.kind().skip_binder()) + { + if new_trait.def_id() == old_trait.def_id() { + let new_substs = new_trait.trait_ref.substs; + let old_substs = old_trait.trait_ref.substs; + + if !new_substs.types().eq(old_substs.types()) { + // We can't compare lifetimes if the types are different, + // so skip checking `old_pred`. + return true; + } + + for (new_region, old_region) in + iter::zip(new_substs.regions(), old_substs.regions()) + { + match (*new_region, *old_region) { + // If both predicates have an `ReLateBound` (a HRTB) in the + // same spot, we do nothing. + (ty::ReLateBound(_, _), ty::ReLateBound(_, _)) => {} + + (ty::ReLateBound(_, _), _) | (_, ty::ReVar(_)) => { + // One of these is true: + // The new predicate has a HRTB in a spot where the old + // predicate does not (if they both had a HRTB, the previous + // match arm would have executed). A HRBT is a 'stricter' + // bound than anything else, so we want to keep the newer + // predicate (with the HRBT) in place of the old predicate. + // + // OR + // + // The old predicate has a region variable where the new + // predicate has some other kind of region. An region + // variable isn't something we can actually display to a user, + // so we choose their new predicate (which doesn't have a region + // variable). + // + // In both cases, we want to remove the old predicate, + // from `user_computed_preds`, and replace it with the new + // one. Having both the old and the new + // predicate in a `ParamEnv` would confuse `SelectionContext`. + // + // We're currently in the predicate passed to 'retain', + // so we return `false` to remove the old predicate from + // `user_computed_preds`. + return false; + } + (_, ty::ReLateBound(_, _)) | (ty::ReVar(_), _) => { + // This is the opposite situation as the previous arm. + // One of these is true: + // + // The old predicate has a HRTB lifetime in a place where the + // new predicate does not. + // + // OR + // + // The new predicate has a region variable where the old + // predicate has some other type of region. + // + // We want to leave the old + // predicate in `user_computed_preds`, and skip adding + // new_pred to `user_computed_params`. + should_add_new = false + } + _ => {} + } + } + } + } + true + }); + + if should_add_new { + user_computed_preds.insert(new_pred); + } + } + + /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s + /// to each other, we match `ty::RegionVid`s to `ty::Region`s. + fn map_vid_to_region<'cx>( + &self, + regions: &RegionConstraintData<'cx>, + ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> { + let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default(); + let mut finished_map = FxHashMap::default(); + + for constraint in regions.constraints.keys() { + match constraint { + &Constraint::VarSubVar(r1, r2) => { + { + let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default(); + deps1.larger.insert(RegionTarget::RegionVid(r2)); + } + + let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default(); + deps2.smaller.insert(RegionTarget::RegionVid(r1)); + } + &Constraint::RegSubVar(region, vid) => { + { + let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default(); + deps1.larger.insert(RegionTarget::RegionVid(vid)); + } + + let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default(); + deps2.smaller.insert(RegionTarget::Region(region)); + } + &Constraint::VarSubReg(vid, region) => { + finished_map.insert(vid, region); + } + &Constraint::RegSubReg(r1, r2) => { + { + let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default(); + deps1.larger.insert(RegionTarget::Region(r2)); + } + + let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default(); + deps2.smaller.insert(RegionTarget::Region(r1)); + } + } + } + + while !vid_map.is_empty() { + let target = *vid_map.keys().next().expect("Keys somehow empty"); + let deps = vid_map.remove(&target).expect("Entry somehow missing"); + + for smaller in deps.smaller.iter() { + for larger in deps.larger.iter() { + match (smaller, larger) { + (&RegionTarget::Region(_), &RegionTarget::Region(_)) => { + if let Entry::Occupied(v) = vid_map.entry(*smaller) { + let smaller_deps = v.into_mut(); + smaller_deps.larger.insert(*larger); + smaller_deps.larger.remove(&target); + } + + if let Entry::Occupied(v) = vid_map.entry(*larger) { + let larger_deps = v.into_mut(); + larger_deps.smaller.insert(*smaller); + larger_deps.smaller.remove(&target); + } + } + (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => { + finished_map.insert(v1, r1); + } + (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => { + // Do nothing; we don't care about regions that are smaller than vids. + } + (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => { + if let Entry::Occupied(v) = vid_map.entry(*smaller) { + let smaller_deps = v.into_mut(); + smaller_deps.larger.insert(*larger); + smaller_deps.larger.remove(&target); + } + + if let Entry::Occupied(v) = vid_map.entry(*larger) { + let larger_deps = v.into_mut(); + larger_deps.smaller.insert(*smaller); + larger_deps.smaller.remove(&target); + } + } + } + } + } + } + finished_map + } + + fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool { + self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types()) + } + + pub fn is_of_param(&self, ty: Ty<'_>) -> bool { + match ty.kind() { + ty::Param(_) => true, + ty::Projection(p) => self.is_of_param(p.self_ty()), + _ => false, + } + } + + fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool { + if let Term::Ty(ty) = p.term().skip_binder() { + matches!(ty.kind(), ty::Projection(proj) if proj == &p.skip_binder().projection_ty) + } else { + false + } + } + + fn evaluate_nested_obligations( + &self, + ty: Ty<'_>, + nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>, + computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>, + fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>, + predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>, + select: &mut SelectionContext<'_, 'tcx>, + only_projections: bool, + ) -> bool { + let dummy_cause = ObligationCause::dummy(); + + for obligation in nested { + let is_new_pred = + fresh_preds.insert(self.clean_pred(select.infcx(), obligation.predicate)); + + // Resolve any inference variables that we can, to help selection succeed + let predicate = select.infcx().resolve_vars_if_possible(obligation.predicate); + + // We only add a predicate as a user-displayable bound if + // it involves a generic parameter, and doesn't contain + // any inference variables. + // + // Displaying a bound involving a concrete type (instead of a generic + // parameter) would be pointless, since it's always true + // (e.g. u8: Copy) + // Displaying an inference variable is impossible, since they're + // an internal compiler detail without a defined visual representation + // + // We check this by calling is_of_param on the relevant types + // from the various possible predicates + + let bound_predicate = predicate.kind(); + match bound_predicate.skip_binder() { + ty::PredicateKind::Trait(p) => { + // Add this to `predicates` so that we end up calling `select` + // with it. If this predicate ends up being unimplemented, + // then `evaluate_predicates` will handle adding it the `ParamEnv` + // if possible. + predicates.push_back(bound_predicate.rebind(p)); + } + ty::PredicateKind::Projection(p) => { + let p = bound_predicate.rebind(p); + debug!( + "evaluate_nested_obligations: examining projection predicate {:?}", + predicate + ); + + // As described above, we only want to display + // bounds which include a generic parameter but don't include + // an inference variable. + // Additionally, we check if we've seen this predicate before, + // to avoid rendering duplicate bounds to the user. + if self.is_param_no_infer(p.skip_binder().projection_ty.substs) + && !p.term().skip_binder().has_infer_types() + && is_new_pred + { + debug!( + "evaluate_nested_obligations: adding projection predicate \ + to computed_preds: {:?}", + predicate + ); + + // Under unusual circumstances, we can end up with a self-referential + // projection predicate. For example: + // <T as MyType>::Value == <T as MyType>::Value + // Not only is displaying this to the user pointless, + // having it in the ParamEnv will cause an issue if we try to call + // poly_project_and_unify_type on the predicate, since this kind of + // predicate will normally never end up in a ParamEnv. + // + // For these reasons, we ignore these weird predicates, + // ensuring that we're able to properly synthesize an auto trait impl + if self.is_self_referential_projection(p) { + debug!( + "evaluate_nested_obligations: encountered a projection + predicate equating a type with itself! Skipping" + ); + } else { + self.add_user_pred(computed_preds, predicate); + } + } + + // There are three possible cases when we project a predicate: + // + // 1. We encounter an error. This means that it's impossible for + // our current type to implement the auto trait - there's bound + // that we could add to our ParamEnv that would 'fix' this kind + // of error, as it's not caused by an unimplemented type. + // + // 2. We successfully project the predicate (Ok(Some(_))), generating + // some subobligations. We then process these subobligations + // like any other generated sub-obligations. + // + // 3. We receive an 'ambiguous' result (Ok(None)) + // If we were actually trying to compile a crate, + // we would need to re-process this obligation later. + // However, all we care about is finding out what bounds + // are needed for our type to implement a particular auto trait. + // We've already added this obligation to our computed ParamEnv + // above (if it was necessary). Therefore, we don't need + // to do any further processing of the obligation. + // + // Note that we *must* try to project *all* projection predicates + // we encounter, even ones without inference variable. + // This ensures that we detect any projection errors, + // which indicate that our type can *never* implement the given + // auto trait. In that case, we will generate an explicit negative + // impl (e.g. 'impl !Send for MyType'). However, we don't + // try to process any of the generated subobligations - + // they contain no new information, since we already know + // that our type implements the projected-through trait, + // and can lead to weird region issues. + // + // Normally, we'll generate a negative impl as a result of encountering + // a type with an explicit negative impl of an auto trait + // (for example, raw pointers have !Send and !Sync impls) + // However, through some **interesting** manipulations of the type + // system, it's actually possible to write a type that never + // implements an auto trait due to a projection error, not a normal + // negative impl error. To properly handle this case, we need + // to ensure that we catch any potential projection errors, + // and turn them into an explicit negative impl for our type. + debug!("Projecting and unifying projection predicate {:?}", predicate); + + match project::poly_project_and_unify_type(select, &obligation.with(p)) { + ProjectAndUnifyResult::MismatchedProjectionTypes(e) => { + debug!( + "evaluate_nested_obligations: Unable to unify predicate \ + '{:?}' '{:?}', bailing out", + ty, e + ); + return false; + } + ProjectAndUnifyResult::Recursive => { + debug!("evaluate_nested_obligations: recursive projection predicate"); + return false; + } + ProjectAndUnifyResult::Holds(v) => { + // We only care about sub-obligations + // when we started out trying to unify + // some inference variables. See the comment above + // for more information + if p.term().skip_binder().has_infer_types() { + if !self.evaluate_nested_obligations( + ty, + v.into_iter(), + computed_preds, + fresh_preds, + predicates, + select, + only_projections, + ) { + return false; + } + } + } + ProjectAndUnifyResult::FailedNormalization => { + // It's ok not to make progress when have no inference variables - + // in that case, we were only performing unification to check if an + // error occurred (which would indicate that it's impossible for our + // type to implement the auto trait). + // However, we should always make progress (either by generating + // subobligations or getting an error) when we started off with + // inference variables + if p.term().skip_binder().has_infer_types() { + panic!("Unexpected result when selecting {:?} {:?}", ty, obligation) + } + } + } + } + ty::PredicateKind::RegionOutlives(binder) => { + let binder = bound_predicate.rebind(binder); + select.infcx().region_outlives_predicate(&dummy_cause, binder) + } + ty::PredicateKind::TypeOutlives(binder) => { + let binder = bound_predicate.rebind(binder); + match ( + binder.no_bound_vars(), + binder.map_bound_ref(|pred| pred.0).no_bound_vars(), + ) { + (None, Some(t_a)) => { + select.infcx().register_region_obligation_with_cause( + t_a, + select.infcx().tcx.lifetimes.re_static, + &dummy_cause, + ); + } + (Some(ty::OutlivesPredicate(t_a, r_b)), _) => { + select.infcx().register_region_obligation_with_cause( + t_a, + r_b, + &dummy_cause, + ); + } + _ => {} + }; + } + ty::PredicateKind::ConstEquate(c1, c2) => { + let evaluate = |c: ty::Const<'tcx>| { + if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() { + match select.infcx().const_eval_resolve( + obligation.param_env, + unevaluated, + Some(obligation.cause.span), + ) { + Ok(Some(valtree)) => { + Ok(ty::Const::from_value(select.tcx(), valtree, c.ty())) + } + Ok(None) => { + let tcx = self.tcx; + let def_id = unevaluated.def.did; + let reported = tcx.sess.struct_span_err(tcx.def_span(def_id), &format!("unable to construct a constant value for the unevaluated constant {:?}", unevaluated)).emit(); + + Err(ErrorHandled::Reported(reported)) + } + Err(err) => Err(err), + } + } else { + Ok(c) + } + }; + + match (evaluate(c1), evaluate(c2)) { + (Ok(c1), Ok(c2)) => { + match select + .infcx() + .at(&obligation.cause, obligation.param_env) + .eq(c1, c2) + { + Ok(_) => (), + Err(_) => return false, + } + } + _ => return false, + } + } + // There's not really much we can do with these predicates - + // we start out with a `ParamEnv` with no inference variables, + // and these don't correspond to adding any new bounds to + // the `ParamEnv`. + ty::PredicateKind::WellFormed(..) + | ty::PredicateKind::ObjectSafe(..) + | ty::PredicateKind::ClosureKind(..) + | ty::PredicateKind::Subtype(..) + | ty::PredicateKind::ConstEvaluatable(..) + | ty::PredicateKind::Coerce(..) + | ty::PredicateKind::TypeWellFormedFromEnv(..) => {} + }; + } + true + } + + pub fn clean_pred( + &self, + infcx: &InferCtxt<'_, 'tcx>, + p: ty::Predicate<'tcx>, + ) -> ty::Predicate<'tcx> { + infcx.freshen(p) + } +} + +// Replaces all ReVars in a type with ty::Region's, using the provided map +pub struct RegionReplacer<'a, 'tcx> { + vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>, + tcx: TyCtxt<'tcx>, +} + +impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> { + fn tcx<'b>(&'b self) -> TyCtxt<'tcx> { + self.tcx + } + + fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> { + (match *r { + ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(), + _ => None, + }) + .unwrap_or_else(|| r.super_fold_with(self)) + } +} |