| //! 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::ty::fold::TypeFolder; |
| use crate::ty::{Region, RegionVid}; |
| |
| use rustc_data_structures::fx::{FxHashMap, FxHashSet}; |
| |
| use std::collections::hash_map::Entry; |
| use std::collections::VecDeque; |
| |
| // 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, |
| } |
| |
| impl<A> AutoTraitResult<A> { |
| fn is_auto(&self) -> bool { |
| match *self { |
| AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl => true, |
| _ => false, |
| } |
| } |
| } |
| |
| 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, |
| auto_trait_callback: impl Fn(&InferCtxt<'_, 'tcx>, 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::bind(trait_ref); |
| |
| let bail_out = tcx.infer_ctxt().enter(|infcx| { |
| let mut selcx = SelectionContext::with_negative(&infcx, true); |
| let result = selcx.select(&Obligation::new( |
| ObligationCause::dummy(), |
| orig_env, |
| trait_pred.to_poly_trait_predicate(), |
| )); |
| |
| match result { |
| Ok(Some(Vtable::VtableImpl(_))) => { |
| 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; |
| } |
| |
| return tcx.infer_ctxt().enter(|mut 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 (new_env, user_env) = match self.evaluate_predicates( |
| &mut infcx, |
| trait_did, |
| ty, |
| orig_env, |
| orig_env, |
| &mut fresh_preds, |
| false, |
| ) { |
| Some(e) => e, |
| None => return AutoTraitResult::NegativeImpl, |
| }; |
| |
| let (full_env, full_user_env) = self |
| .evaluate_predicates( |
| &mut 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 = FulfillmentContext::new(); |
| fulfill.register_bound( |
| &infcx, |
| full_env, |
| ty, |
| trait_did, |
| ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID), |
| ); |
| fulfill.select_all_or_error(&infcx).unwrap_or_else(|e| { |
| panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, e) |
| }); |
| |
| let body_id_map: FxHashMap<_, _> = |
| infcx.region_obligations.borrow().iter().map(|&(id, _)| (id, vec![])).collect(); |
| |
| infcx.process_registered_region_obligations(&body_id_map, None, full_env); |
| |
| let region_data = infcx.borrow_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 }; |
| |
| return AutoTraitResult::PositiveImpl(auto_trait_callback(&infcx, info)); |
| }); |
| } |
| } |
| |
| impl 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: |
| /// |
| /// 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; |
| |
| let mut select = SelectionContext::with_negative(&infcx, true); |
| |
| let mut already_visited = FxHashSet::default(); |
| let mut predicates = VecDeque::new(); |
| predicates.push_back(ty::Binder::bind(ty::TraitPredicate { |
| trait_ref: ty::TraitRef { |
| def_id: trait_did, |
| substs: infcx.tcx.mk_substs_trait(ty, &[]), |
| }, |
| })); |
| |
| let mut computed_preds: FxHashSet<_> = param_env.caller_bounds.iter().cloned().collect(); |
| let mut user_computed_preds: FxHashSet<_> = |
| user_env.caller_bounds.iter().cloned().collect(); |
| |
| let mut new_env = param_env; |
| let dummy_cause = ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID); |
| |
| 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 vtable)) => { |
| // 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. |
| match vtable { |
| Vtable::VtableImpl(VtableImplData { impl_def_id, .. }) => { |
| // 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 = vtable.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, |
| ty::Predicate::Trait(pred, ast::Constness::NotConst), |
| ); |
| 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), |
| }; |
| |
| computed_preds.extend(user_computed_preds.iter().cloned()); |
| let normalized_preds = |
| elaborate_predicates(tcx, computed_preds.iter().cloned().collect()); |
| new_env = |
| ty::ParamEnv::new(tcx.mk_predicates(normalized_preds), param_env.reveal, None); |
| } |
| |
| let final_user_env = ty::ParamEnv::new( |
| tcx.mk_predicates(user_computed_preds.into_iter()), |
| user_env.reveal, |
| None, |
| ); |
| debug!( |
| "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \ |
| '{:?}'", |
| ty, trait_did, new_env, final_user_env |
| ); |
| |
| return 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: |
| /// |
| /// 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<'c>( |
| &self, |
| user_computed_preds: &mut FxHashSet<ty::Predicate<'c>>, |
| new_pred: ty::Predicate<'c>, |
| ) { |
| let mut should_add_new = true; |
| user_computed_preds.retain(|&old_pred| { |
| match (&new_pred, old_pred) { |
| (&ty::Predicate::Trait(new_trait, _), ty::Predicate::Trait(old_trait, _)) => { |
| if new_trait.def_id() == old_trait.def_id() { |
| let new_substs = new_trait.skip_binder().trait_ref.substs; |
| let old_substs = old_trait.skip_binder().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 |
| new_substs.regions().zip(old_substs.regions()) |
| { |
| match (new_region, old_region) { |
| // If both predicates have an `ReLateBound` (a HRTB) in the |
| // same spot, we do nothing. |
| ( |
| ty::RegionKind::ReLateBound(_, _), |
| ty::RegionKind::ReLateBound(_, _), |
| ) => {} |
| |
| (ty::RegionKind::ReLateBound(_, _), _) |
| | (_, ty::RegionKind::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 |
| // varaible). |
| // |
| // 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::RegionKind::ReLateBound(_, _)) |
| | (ty::RegionKind::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 |
| } |
| _ => {} |
| } |
| } |
| } |
| } |
| _ => {} |
| } |
| return 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").clone(); |
| 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 { |
| return 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 { |
| return 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 { |
| match p.ty().skip_binder().kind { |
| ty::Projection(proj) if proj == p.skip_binder().projection_ty => true, |
| _ => 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::misc(DUMMY_SP, hir::DUMMY_HIR_ID); |
| |
| for (obligation, mut predicate) in nested.map(|o| (o.clone(), o.predicate)) { |
| let is_new_pred = fresh_preds.insert(self.clean_pred(select.infcx(), predicate)); |
| |
| // Resolve any inference variables that we can, to help selection succeed |
| predicate = select.infcx().resolve_vars_if_possible(&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 |
| match &predicate { |
| &ty::Predicate::Trait(p, _) => { |
| if self.is_param_no_infer(p.skip_binder().trait_ref.substs) |
| && !only_projections |
| && is_new_pred |
| { |
| self.add_user_pred(computed_preds, predicate); |
| } |
| predicates.push_back(p); |
| } |
| &ty::Predicate::Projection(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.ty().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-refeential |
| // 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 receieve 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 poly_project_and_unify_type(select, &obligation.with(p)) { |
| Err(e) => { |
| debug!( |
| "evaluate_nested_obligations: Unable to unify predicate \ |
| '{:?}' '{:?}', bailing out", |
| ty, e |
| ); |
| return false; |
| } |
| Ok(Some(v)) => { |
| // We only care about sub-obligations |
| // when we started out trying to unify |
| // some inference variables. See the comment above |
| // for more infomration |
| if p.ty().skip_binder().has_infer_types() { |
| if !self.evaluate_nested_obligations( |
| ty, |
| v.clone().iter().cloned(), |
| computed_preds, |
| fresh_preds, |
| predicates, |
| select, |
| only_projections, |
| ) { |
| return false; |
| } |
| } |
| } |
| Ok(None) => { |
| // It's ok not to make progress when hvave no inference variables - |
| // in that case, we were only performing unifcation 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.ty().skip_binder().has_infer_types() { |
| panic!("Unexpected result when selecting {:?} {:?}", ty, obligation) |
| } |
| } |
| } |
| } |
| &ty::Predicate::RegionOutlives(ref binder) => { |
| if select.infcx().region_outlives_predicate(&dummy_cause, binder).is_err() { |
| return false; |
| } |
| } |
| &ty::Predicate::TypeOutlives(ref 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, |
| ); |
| } |
| _ => {} |
| }; |
| } |
| _ => panic!("Unexpected predicate {:?} {:?}", ty, predicate), |
| }; |
| } |
| return 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)) |
| } |
| } |
