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// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. use hir::def_id::DefId; use hir; use hir::Node; use infer::{self, InferCtxt, InferOk, TypeVariableOrigin}; use infer::outlives::free_region_map::FreeRegionRelations; use rustc_data_structures::fx::FxHashMap; use syntax::ast; use traits::{self, PredicateObligation}; use ty::{self, Ty, TyCtxt, GenericParamDefKind}; use ty::fold::{BottomUpFolder, TypeFoldable, TypeFolder}; use ty::outlives::Component; use ty::subst::{Kind, Substs, UnpackedKind}; use util::nodemap::DefIdMap; pub type OpaqueTypeMap<'tcx> = DefIdMap<OpaqueTypeDecl<'tcx>>; /// Information about the opaque, abstract types whose values we /// are inferring in this function (these are the `impl Trait` that /// appear in the return type). #[derive(Copy, Clone, Debug)] pub struct OpaqueTypeDecl<'tcx> { /// The substitutions that we apply to the abstract that that this /// `impl Trait` desugars to. e.g., if: /// /// fn foo<'a, 'b, T>() -> impl Trait<'a> /// /// winds up desugared to: /// /// abstract type Foo<'x, T>: Trait<'x> /// fn foo<'a, 'b, T>() -> Foo<'a, T> /// /// then `substs` would be `['a, T]`. pub substs: &'tcx Substs<'tcx>, /// The type variable that represents the value of the abstract type /// that we require. In other words, after we compile this function, /// we will be created a constraint like: /// /// Foo<'a, T> = ?C /// /// where `?C` is the value of this type variable. =) It may /// naturally refer to the type and lifetime parameters in scope /// in this function, though ultimately it should only reference /// those that are arguments to `Foo` in the constraint above. (In /// other words, `?C` should not include `'b`, even though it's a /// lifetime parameter on `foo`.) pub concrete_ty: Ty<'tcx>, /// True if the `impl Trait` bounds include region bounds. /// For example, this would be true for: /// /// fn foo<'a, 'b, 'c>() -> impl Trait<'c> + 'a + 'b /// /// but false for: /// /// fn foo<'c>() -> impl Trait<'c> /// /// unless `Trait` was declared like: /// /// trait Trait<'c>: 'c /// /// in which case it would be true. /// /// This is used during regionck to decide whether we need to /// impose any additional constraints to ensure that region /// variables in `concrete_ty` wind up being constrained to /// something from `substs` (or, at minimum, things that outlive /// the fn body). (Ultimately, writeback is responsible for this /// check.) pub has_required_region_bounds: bool, } impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> { /// Replace all opaque types in `value` with fresh inference variables /// and creates appropriate obligations. For example, given the input: /// /// impl Iterator<Item = impl Debug> /// /// this method would create two type variables, `?0` and `?1`. It would /// return the type `?0` but also the obligations: /// /// ?0: Iterator<Item = ?1> /// ?1: Debug /// /// Moreover, it returns a `OpaqueTypeMap` that would map `?0` to /// info about the `impl Iterator<..>` type and `?1` to info about /// the `impl Debug` type. /// /// # Parameters /// /// - `parent_def_id` -- we will only instantiate opaque types /// with this parent. This is typically the def-id of the function /// in whose return type opaque types are being instantiated. /// - `body_id` -- the body-id with which the resulting obligations should /// be associated /// - `param_env` -- the in-scope parameter environment to be used for /// obligations /// - `value` -- the value within which we are instantiating opaque types pub fn instantiate_opaque_types<T: TypeFoldable<'tcx>>( &self, parent_def_id: DefId, body_id: ast::NodeId, param_env: ty::ParamEnv<'tcx>, value: &T, ) -> InferOk<'tcx, (T, OpaqueTypeMap<'tcx>)> { debug!( "instantiate_opaque_types(value={:?}, parent_def_id={:?}, body_id={:?}, param_env={:?})", value, parent_def_id, body_id, param_env, ); let mut instantiator = Instantiator { infcx: self, parent_def_id, body_id, param_env, opaque_types: DefIdMap(), obligations: vec![], }; let value = instantiator.instantiate_opaque_types_in_map(value); InferOk { value: (value, instantiator.opaque_types), obligations: instantiator.obligations, } } /// Given the map `opaque_types` containing the existential `impl /// Trait` types whose underlying, hidden types are being /// inferred, this method adds constraints to the regions /// appearing in those underlying hidden types to ensure that they /// at least do not refer to random scopes within the current /// function. These constraints are not (quite) sufficient to /// guarantee that the regions are actually legal values; that /// final condition is imposed after region inference is done. /// /// # The Problem /// /// Let's work through an example to explain how it works. Assume /// the current function is as follows: /// /// ```text /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>) /// ``` /// /// Here, we have two `impl Trait` types whose values are being /// inferred (the `impl Bar<'a>` and the `impl /// Bar<'b>`). Conceptually, this is sugar for a setup where we /// define underlying abstract types (`Foo1`, `Foo2`) and then, in /// the return type of `foo`, we *reference* those definitions: /// /// ```text /// abstract type Foo1<'x>: Bar<'x>; /// abstract type Foo2<'x>: Bar<'x>; /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. } /// // ^^^^ ^^ /// // | | /// // | substs /// // def_id /// ``` /// /// As indicating in the comments above, each of those references /// is (in the compiler) basically a substitution (`substs`) /// applied to the type of a suitable `def_id` (which identifies /// `Foo1` or `Foo2`). /// /// Now, at this point in compilation, what we have done is to /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with /// fresh inference variables C1 and C2. We wish to use the values /// of these variables to infer the underlying types of `Foo1` and /// `Foo2`. That is, this gives rise to higher-order (pattern) unification /// constraints like: /// /// ```text /// for<'a> (Foo1<'a> = C1) /// for<'b> (Foo1<'b> = C2) /// ``` /// /// For these equation to be satisfiable, the types `C1` and `C2` /// can only refer to a limited set of regions. For example, `C1` /// can only refer to `'static` and `'a`, and `C2` can only refer /// to `'static` and `'b`. The job of this function is to impose that /// constraint. /// /// Up to this point, C1 and C2 are basically just random type /// inference variables, and hence they may contain arbitrary /// regions. In fact, it is fairly likely that they do! Consider /// this possible definition of `foo`: /// /// ```text /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) { /// (&*x, &*y) /// } /// ``` /// /// Here, the values for the concrete types of the two impl /// traits will include inference variables: /// /// ```text /// &'0 i32 /// &'1 i32 /// ``` /// /// Ordinarily, the subtyping rules would ensure that these are /// sufficiently large. But since `impl Bar<'a>` isn't a specific /// type per se, we don't get such constraints by default. This /// is where this function comes into play. It adds extra /// constraints to ensure that all the regions which appear in the /// inferred type are regions that could validly appear. /// /// This is actually a bit of a tricky constraint in general. We /// want to say that each variable (e.g., `'0`) can only take on /// values that were supplied as arguments to the abstract type /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in /// scope. We don't have a constraint quite of this kind in the current /// region checker. /// /// # The Solution /// /// We make use of the constraint that we *do* have in the `<=` /// relation. To do that, we find the "minimum" of all the /// arguments that appear in the substs: that is, some region /// which is less than all the others. In the case of `Foo1<'a>`, /// that would be `'a` (it's the only choice, after all). Then we /// apply that as a least bound to the variables (e.g., `'a <= /// '0`). /// /// In some cases, there is no minimum. Consider this example: /// /// ```text /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... } /// ``` /// /// Here we would report an error, because `'a` and `'b` have no /// relation to one another. /// /// # The `free_region_relations` parameter /// /// The `free_region_relations` argument is used to find the /// "minimum" of the regions supplied to a given abstract type. /// It must be a relation that can answer whether `'a <= 'b`, /// where `'a` and `'b` are regions that appear in the "substs" /// for the abstract type references (the `<'a>` in `Foo1<'a>`). /// /// Note that we do not impose the constraints based on the /// generic regions from the `Foo1` definition (e.g., `'x`). This /// is because the constraints we are imposing here is basically /// the concern of the one generating the constraining type C1, /// which is the current function. It also means that we can /// take "implied bounds" into account in some cases: /// /// ```text /// trait SomeTrait<'a, 'b> { } /// fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. } /// ``` /// /// Here, the fact that `'b: 'a` is known only because of the /// implied bounds from the `&'a &'b u32` parameter, and is not /// "inherent" to the abstract type definition. /// /// # Parameters /// /// - `opaque_types` -- the map produced by `instantiate_opaque_types` /// - `free_region_relations` -- something that can be used to relate /// the free regions (`'a`) that appear in the impl trait. pub fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>( &self, opaque_types: &OpaqueTypeMap<'tcx>, free_region_relations: &FRR, ) { debug!("constrain_opaque_types()"); for (&def_id, opaque_defn) in opaque_types { self.constrain_opaque_type(def_id, opaque_defn, free_region_relations); } } fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>( &self, def_id: DefId, opaque_defn: &OpaqueTypeDecl<'tcx>, free_region_relations: &FRR, ) { debug!("constrain_opaque_type()"); debug!("constrain_opaque_type: def_id={:?}", def_id); debug!("constrain_opaque_type: opaque_defn={:#?}", opaque_defn); let concrete_ty = self.resolve_type_vars_if_possible(&opaque_defn.concrete_ty); debug!("constrain_opaque_type: concrete_ty={:?}", concrete_ty); let abstract_type_generics = self.tcx.generics_of(def_id); let span = self.tcx.def_span(def_id); // If there are required region bounds, we can just skip // ahead. There will already be a registered region // obligation related `concrete_ty` to those regions. if opaque_defn.has_required_region_bounds { return; } // There were no `required_region_bounds`, // so we have to search for a `least_region`. // Go through all the regions used as arguments to the // abstract type. These are the parameters to the abstract // type; so in our example above, `substs` would contain // `['a]` for the first impl trait and `'b` for the // second. let mut least_region = None; for param in &abstract_type_generics.params { match param.kind { GenericParamDefKind::Lifetime => {} _ => continue } // Get the value supplied for this region from the substs. let subst_arg = opaque_defn.substs.region_at(param.index as usize); // Compute the least upper bound of it with the other regions. debug!("constrain_opaque_types: least_region={:?}", least_region); debug!("constrain_opaque_types: subst_arg={:?}", subst_arg); match least_region { None => least_region = Some(subst_arg), Some(lr) => { if free_region_relations.sub_free_regions(lr, subst_arg) { // keep the current least region } else if free_region_relations.sub_free_regions(subst_arg, lr) { // switch to `subst_arg` least_region = Some(subst_arg); } else { // There are two regions (`lr` and // `subst_arg`) which are not relatable. We can't // find a best choice. self.tcx .sess .struct_span_err(span, "ambiguous lifetime bound in `impl Trait`") .span_label( span, format!("neither `{}` nor `{}` outlives the other", lr, subst_arg), ) .emit(); least_region = Some(self.tcx.mk_region(ty::ReEmpty)); break; } } } } let least_region = least_region.unwrap_or(self.tcx.types.re_static); debug!("constrain_opaque_types: least_region={:?}", least_region); // Require that the type `concrete_ty` outlives // `least_region`, modulo any type parameters that appear // in the type, which we ignore. This is because impl // trait values are assumed to capture all the in-scope // type parameters. This little loop here just invokes // `outlives` repeatedly, draining all the nested // obligations that result. let mut types = vec![concrete_ty]; let bound_region = |r| self.sub_regions(infer::CallReturn(span), least_region, r); while let Some(ty) = types.pop() { let mut components = self.tcx.outlives_components(ty); while let Some(component) = components.pop() { match component { Component::Region(r) => { bound_region(r); } Component::Param(_) => { // ignore type parameters like `T`, they are captured // implicitly by the `impl Trait` } Component::UnresolvedInferenceVariable(_) => { // we should get an error that more type // annotations are needed in this case self.tcx .sess .delay_span_bug(span, "unresolved inf var in opaque"); } Component::Projection(ty::ProjectionTy { substs, item_def_id: _, }) => { for r in substs.regions() { bound_region(r); } types.extend(substs.types()); } Component::EscapingProjection(more_components) => { components.extend(more_components); } } } } } /// Given the fully resolved, instantiated type for an opaque /// type, i.e., the value of an inference variable like C1 or C2 /// (*), computes the "definition type" for an abstract type /// definition -- that is, the inferred value of `Foo1<'x>` or /// `Foo2<'x>` that we would conceptually use in its definition: /// /// abstract type Foo1<'x>: Bar<'x> = AAA; <-- this type AAA /// abstract type Foo2<'x>: Bar<'x> = BBB; <-- or this type BBB /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. } /// /// Note that these values are defined in terms of a distinct set of /// generic parameters (`'x` instead of `'a`) from C1 or C2. The main /// purpose of this function is to do that translation. /// /// (*) C1 and C2 were introduced in the comments on /// `constrain_opaque_types`. Read that comment for more context. /// /// # Parameters /// /// - `def_id`, the `impl Trait` type /// - `opaque_defn`, the opaque definition created in `instantiate_opaque_types` /// - `instantiated_ty`, the inferred type C1 -- fully resolved, lifted version of /// `opaque_defn.concrete_ty` pub fn infer_opaque_definition_from_instantiation( &self, def_id: DefId, opaque_defn: &OpaqueTypeDecl<'tcx>, instantiated_ty: Ty<'gcx>, ) -> Ty<'gcx> { debug!( "infer_opaque_definition_from_instantiation(def_id={:?}, instantiated_ty={:?})", def_id, instantiated_ty ); let gcx = self.tcx.global_tcx(); // Use substs to build up a reverse map from regions to their // identity mappings. This is necessary because of `impl // Trait` lifetimes are computed by replacing existing // lifetimes with 'static and remapping only those used in the // `impl Trait` return type, resulting in the parameters // shifting. let id_substs = Substs::identity_for_item(gcx, def_id); let map: FxHashMap<Kind<'tcx>, Kind<'gcx>> = opaque_defn .substs .iter() .enumerate() .map(|(index, subst)| (*subst, id_substs[index])) .collect(); // Convert the type from the function into a type valid outside // the function, by replacing invalid regions with 'static, // after producing an error for each of them. let definition_ty = instantiated_ty.fold_with(&mut ReverseMapper::new( self.tcx, self.is_tainted_by_errors(), def_id, map, instantiated_ty, )); debug!( "infer_opaque_definition_from_instantiation: definition_ty={:?}", definition_ty ); // We can unwrap here because our reverse mapper always // produces things with 'gcx lifetime, though the type folder // obscures that. let definition_ty = gcx.lift(&definition_ty).unwrap(); definition_ty } } struct ReverseMapper<'cx, 'gcx: 'tcx, 'tcx: 'cx> { tcx: TyCtxt<'cx, 'gcx, 'tcx>, /// If errors have already been reported in this fn, we suppress /// our own errors because they are sometimes derivative. tainted_by_errors: bool, opaque_type_def_id: DefId, map: FxHashMap<Kind<'tcx>, Kind<'gcx>>, map_missing_regions_to_empty: bool, /// initially `Some`, set to `None` once error has been reported hidden_ty: Option<Ty<'tcx>>, } impl<'cx, 'gcx, 'tcx> ReverseMapper<'cx, 'gcx, 'tcx> { fn new( tcx: TyCtxt<'cx, 'gcx, 'tcx>, tainted_by_errors: bool, opaque_type_def_id: DefId, map: FxHashMap<Kind<'tcx>, Kind<'gcx>>, hidden_ty: Ty<'tcx>, ) -> Self { Self { tcx, tainted_by_errors, opaque_type_def_id, map, map_missing_regions_to_empty: false, hidden_ty: Some(hidden_ty), } } fn fold_kind_mapping_missing_regions_to_empty(&mut self, kind: Kind<'tcx>) -> Kind<'tcx> { assert!(!self.map_missing_regions_to_empty); self.map_missing_regions_to_empty = true; let kind = kind.fold_with(self); self.map_missing_regions_to_empty = false; kind } fn fold_kind_normally(&mut self, kind: Kind<'tcx>) -> Kind<'tcx> { assert!(!self.map_missing_regions_to_empty); kind.fold_with(self) } } impl<'cx, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ReverseMapper<'cx, 'gcx, 'tcx> { fn tcx(&self) -> TyCtxt<'_, 'gcx, 'tcx> { self.tcx } fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> { match r { // ignore bound regions that appear in the type (e.g., this // would ignore `'r` in a type like `for<'r> fn(&'r u32)`. ty::ReLateBound(..) | // ignore `'static`, as that can appear anywhere ty::ReStatic | // ignore `ReScope`, as that can appear anywhere // See `src/test/run-pass/issue-49556.rs` for example. ty::ReScope(..) => return r, _ => { } } match self.map.get(&r.into()).map(|k| k.unpack()) { Some(UnpackedKind::Lifetime(r1)) => r1, Some(u) => panic!("region mapped to unexpected kind: {:?}", u), None => { if !self.map_missing_regions_to_empty && !self.tainted_by_errors { if let Some(hidden_ty) = self.hidden_ty.take() { let span = self.tcx.def_span(self.opaque_type_def_id); let mut err = struct_span_err!( self.tcx.sess, span, E0700, "hidden type for `impl Trait` captures lifetime that \ does not appear in bounds", ); // Assuming regionck succeeded, then we must // be capturing *some* region from the fn // header, and hence it must be free, so it's // ok to invoke this fn (which doesn't accept // all regions, and would ICE if an // inappropriate region is given). We check // `is_tainted_by_errors` by errors above, so // we don't get in here unless regionck // succeeded. (Note also that if regionck // failed, then the regions we are attempting // to map here may well be giving errors // *because* the constraints were not // satisfiable.) self.tcx.note_and_explain_free_region( &mut err, &format!("hidden type `{}` captures ", hidden_ty), r, "" ); err.emit(); } } self.tcx.types.re_empty }, } } fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> { match ty.sty { ty::Closure(def_id, substs) => { // I am a horrible monster and I pray for death. When // we encounter a closure here, it is always a closure // from within the function that we are currently // type-checking -- one that is now being encapsulated // in an existential abstract type. Ideally, we would // go through the types/lifetimes that it references // and treat them just like we would any other type, // which means we would error out if we find any // reference to a type/region that is not in the // "reverse map". // // **However,** in the case of closures, there is a // somewhat subtle (read: hacky) consideration. The // problem is that our closure types currently include // all the lifetime parameters declared on the // enclosing function, even if they are unused by the // closure itself. We can't readily filter them out, // so here we replace those values with `'empty`. This // can't really make a difference to the rest of the // compiler; those regions are ignored for the // outlives relation, and hence don't affect trait // selection or auto traits, and they are erased // during codegen. let generics = self.tcx.generics_of(def_id); let substs = self.tcx.mk_substs(substs.substs.iter().enumerate().map( |(index, &kind)| { if index < generics.parent_count { // Accommodate missing regions in the parent kinds... self.fold_kind_mapping_missing_regions_to_empty(kind) } else { // ...but not elsewhere. self.fold_kind_normally(kind) } }, )); self.tcx.mk_closure(def_id, ty::ClosureSubsts { substs }) } _ => ty.super_fold_with(self), } } } struct Instantiator<'a, 'gcx: 'tcx, 'tcx: 'a> { infcx: &'a InferCtxt<'a, 'gcx, 'tcx>, parent_def_id: DefId, body_id: ast::NodeId, param_env: ty::ParamEnv<'tcx>, opaque_types: OpaqueTypeMap<'tcx>, obligations: Vec<PredicateObligation<'tcx>>, } impl<'a, 'gcx, 'tcx> Instantiator<'a, 'gcx, 'tcx> { fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: &T) -> T { debug!("instantiate_opaque_types_in_map(value={:?})", value); let tcx = self.infcx.tcx; value.fold_with(&mut BottomUpFolder { tcx, reg_op: |reg| reg, fldop: |ty| { if let ty::Opaque(def_id, substs) = ty.sty { // Check that this is `impl Trait` type is // declared by `parent_def_id` -- i.e., one whose // value we are inferring. At present, this is // always true during the first phase of // type-check, but not always true later on during // NLL. Once we support named abstract types more fully, // this same scenario will be able to arise during all phases. // // Here is an example using `abstract type` that indicates // the distinction we are checking for: // // ```rust // mod a { // pub abstract type Foo: Iterator; // pub fn make_foo() -> Foo { .. } // } // // mod b { // fn foo() -> a::Foo { a::make_foo() } // } // ``` // // Here, the return type of `foo` references a // `Opaque` indeed, but not one whose value is // presently being inferred. You can get into a // similar situation with closure return types // today: // // ```rust // fn foo() -> impl Iterator { .. } // fn bar() { // let x = || foo(); // returns the Opaque assoc with `foo` // } // ``` if let Some(opaque_node_id) = tcx.hir.as_local_node_id(def_id) { let parent_def_id = self.parent_def_id; let def_scope_default = || { let opaque_parent_node_id = tcx.hir.get_parent(opaque_node_id); parent_def_id == tcx.hir.local_def_id(opaque_parent_node_id) }; let in_definition_scope = match tcx.hir.find(opaque_node_id) { Some(Node::Item(item)) => match item.node { // impl trait hir::ItemKind::Existential(hir::ExistTy { impl_trait_fn: Some(parent), .. }) => parent == self.parent_def_id, // named existential types hir::ItemKind::Existential(hir::ExistTy { impl_trait_fn: None, .. }) => may_define_existential_type( tcx, self.parent_def_id, opaque_node_id, ), _ => def_scope_default(), }, Some(Node::ImplItem(item)) => match item.node { hir::ImplItemKind::Existential(_) => may_define_existential_type( tcx, self.parent_def_id, opaque_node_id, ), _ => def_scope_default(), }, _ => bug!( "expected (impl) item, found {}", tcx.hir.node_to_string(opaque_node_id), ), }; if in_definition_scope { return self.fold_opaque_ty(ty, def_id, substs); } debug!( "instantiate_opaque_types_in_map: \ encountered opaque outside it's definition scope \ def_id={:?}", def_id, ); } } ty }, }) } fn fold_opaque_ty( &mut self, ty: Ty<'tcx>, def_id: DefId, substs: &'tcx Substs<'tcx>, ) -> Ty<'tcx> { let infcx = self.infcx; let tcx = infcx.tcx; debug!( "instantiate_opaque_types: Opaque(def_id={:?}, substs={:?})", def_id, substs ); // Use the same type variable if the exact same Opaque appears more // than once in the return type (e.g. if it's passed to a type alias). if let Some(opaque_defn) = self.opaque_types.get(&def_id) { return opaque_defn.concrete_ty; } let span = tcx.def_span(def_id); let ty_var = infcx.next_ty_var(TypeVariableOrigin::TypeInference(span)); let predicates_of = tcx.predicates_of(def_id); debug!( "instantiate_opaque_types: predicates: {:#?}", predicates_of, ); let bounds = predicates_of.instantiate(tcx, substs); debug!("instantiate_opaque_types: bounds={:?}", bounds); let required_region_bounds = tcx.required_region_bounds(ty, bounds.predicates.clone()); debug!( "instantiate_opaque_types: required_region_bounds={:?}", required_region_bounds ); // make sure that we are in fact defining the *entire* type // e.g. `existential type Foo<T: Bound>: Bar;` needs to be // defined by a function like `fn foo<T: Bound>() -> Foo<T>`. debug!( "instantiate_opaque_types: param_env: {:#?}", self.param_env, ); debug!( "instantiate_opaque_types: generics: {:#?}", tcx.generics_of(def_id), ); self.opaque_types.insert( def_id, OpaqueTypeDecl { substs, concrete_ty: ty_var, has_required_region_bounds: !required_region_bounds.is_empty(), }, ); debug!("instantiate_opaque_types: ty_var={:?}", ty_var); for predicate in bounds.predicates { // Change the predicate to refer to the type variable, // which will be the concrete type, instead of the Opaque. // This also instantiates nested `impl Trait`. let predicate = self.instantiate_opaque_types_in_map(&predicate); let cause = traits::ObligationCause::new(span, self.body_id, traits::SizedReturnType); // Require that the predicate holds for the concrete type. debug!("instantiate_opaque_types: predicate={:?}", predicate); self.obligations .push(traits::Obligation::new(cause, self.param_env, predicate)); } ty_var } } /// Whether `opaque_node_id` is a sibling or a child of a sibling of `def_id` /// /// ```rust /// pub mod foo { /// pub mod bar { /// pub existential type Baz; /// /// fn f1() -> Baz { .. } /// } /// /// fn f2() -> bar::Baz { .. } /// } /// ``` /// /// Here, `def_id` will be the `DefId` of the existential type `Baz`. /// `opaque_node_id` is the `NodeId` of the reference to Baz -- /// so either the return type of f1 or f2. /// We will return true if the reference is within the same module as the existential type /// So true for f1, false for f2. pub fn may_define_existential_type( tcx: TyCtxt, def_id: DefId, opaque_node_id: ast::NodeId, ) -> bool { let mut node_id = tcx .hir .as_local_node_id(def_id) .unwrap(); // named existential types can be defined by any siblings or // children of siblings let mod_id = tcx.hir.get_parent(opaque_node_id); // so we walk up the node tree until we hit the root or the parent // of the opaque type while node_id != mod_id && node_id != ast::CRATE_NODE_ID { node_id = tcx.hir.get_parent(node_id); } // syntactically we are allowed to define the concrete type node_id == mod_id }