772 lines
31 KiB
Rust
772 lines
31 KiB
Rust
// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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use hir::def_id::DefId;
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use infer::{self, InferCtxt, InferOk, TypeVariableOrigin};
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use infer::outlives::free_region_map::FreeRegionRelations;
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use rustc_data_structures::fx::FxHashMap;
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use syntax::ast;
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use traits::{self, PredicateObligation};
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use ty::{self, Ty, TyCtxt, GenericParamDefKind};
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use ty::fold::{BottomUpFolder, TypeFoldable, TypeFolder};
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use ty::outlives::Component;
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use ty::subst::{Kind, Substs, UnpackedKind};
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use util::nodemap::DefIdMap;
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pub type AnonTypeMap<'tcx> = DefIdMap<AnonTypeDecl<'tcx>>;
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/// Information about the anonymous, abstract types whose values we
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/// are inferring in this function (these are the `impl Trait` that
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/// appear in the return type).
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#[derive(Copy, Clone, Debug)]
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pub struct AnonTypeDecl<'tcx> {
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/// The substitutions that we apply to the abstract that that this
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/// `impl Trait` desugars to. e.g., if:
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///
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/// fn foo<'a, 'b, T>() -> impl Trait<'a>
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///
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/// winds up desugared to:
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///
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/// abstract type Foo<'x, T>: Trait<'x>
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/// fn foo<'a, 'b, T>() -> Foo<'a, T>
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///
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/// then `substs` would be `['a, T]`.
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pub substs: &'tcx Substs<'tcx>,
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/// The type variable that represents the value of the abstract type
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/// that we require. In other words, after we compile this function,
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/// we will be created a constraint like:
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///
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/// Foo<'a, T> = ?C
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///
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/// where `?C` is the value of this type variable. =) It may
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/// naturally refer to the type and lifetime parameters in scope
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/// in this function, though ultimately it should only reference
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/// those that are arguments to `Foo` in the constraint above. (In
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/// other words, `?C` should not include `'b`, even though it's a
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/// lifetime parameter on `foo`.)
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pub concrete_ty: Ty<'tcx>,
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/// True if the `impl Trait` bounds include region bounds.
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/// For example, this would be true for:
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///
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/// fn foo<'a, 'b, 'c>() -> impl Trait<'c> + 'a + 'b
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///
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/// but false for:
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///
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/// fn foo<'c>() -> impl Trait<'c>
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///
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/// unless `Trait` was declared like:
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///
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/// trait Trait<'c>: 'c
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///
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/// in which case it would be true.
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///
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/// This is used during regionck to decide whether we need to
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/// impose any additional constraints to ensure that region
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/// variables in `concrete_ty` wind up being constrained to
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/// something from `substs` (or, at minimum, things that outlive
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/// the fn body). (Ultimately, writeback is responsible for this
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/// check.)
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pub has_required_region_bounds: bool,
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}
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impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
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/// Replace all anonymized types in `value` with fresh inference variables
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/// and creates appropriate obligations. For example, given the input:
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///
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/// impl Iterator<Item = impl Debug>
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///
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/// this method would create two type variables, `?0` and `?1`. It would
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/// return the type `?0` but also the obligations:
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///
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/// ?0: Iterator<Item = ?1>
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/// ?1: Debug
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///
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/// Moreover, it returns a `AnonTypeMap` that would map `?0` to
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/// info about the `impl Iterator<..>` type and `?1` to info about
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/// the `impl Debug` type.
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///
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/// # Parameters
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///
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/// - `parent_def_id` -- we will only instantiate anonymous types
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/// with this parent. This is typically the def-id of the function
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/// in whose return type anon types are being instantiated.
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/// - `body_id` -- the body-id with which the resulting obligations should
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/// be associated
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/// - `param_env` -- the in-scope parameter environment to be used for
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/// obligations
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/// - `value` -- the value within which we are instantiating anon types
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pub fn instantiate_anon_types<T: TypeFoldable<'tcx>>(
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&self,
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parent_def_id: DefId,
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body_id: ast::NodeId,
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param_env: ty::ParamEnv<'tcx>,
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value: &T,
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) -> InferOk<'tcx, (T, AnonTypeMap<'tcx>)> {
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debug!(
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"instantiate_anon_types(value={:?}, parent_def_id={:?}, body_id={:?}, param_env={:?})",
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value, parent_def_id, body_id, param_env,
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);
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let mut instantiator = Instantiator {
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infcx: self,
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parent_def_id,
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body_id,
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param_env,
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anon_types: DefIdMap(),
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obligations: vec![],
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};
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let value = instantiator.instantiate_anon_types_in_map(value);
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InferOk {
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value: (value, instantiator.anon_types),
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obligations: instantiator.obligations,
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}
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}
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/// Given the map `anon_types` containing the existential `impl
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/// Trait` types whose underlying, hidden types are being
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/// inferred, this method adds constraints to the regions
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/// appearing in those underlying hidden types to ensure that they
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/// at least do not refer to random scopes within the current
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/// function. These constraints are not (quite) sufficient to
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/// guarantee that the regions are actually legal values; that
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/// final condition is imposed after region inference is done.
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///
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/// # The Problem
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///
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/// Let's work through an example to explain how it works. Assume
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/// the current function is as follows:
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///
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/// ```text
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/// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
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/// ```
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///
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/// Here, we have two `impl Trait` types whose values are being
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/// inferred (the `impl Bar<'a>` and the `impl
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/// Bar<'b>`). Conceptually, this is sugar for a setup where we
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/// define underlying abstract types (`Foo1`, `Foo2`) and then, in
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/// the return type of `foo`, we *reference* those definitions:
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///
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/// ```text
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/// abstract type Foo1<'x>: Bar<'x>;
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/// abstract type Foo2<'x>: Bar<'x>;
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/// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
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/// // ^^^^ ^^
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/// // | |
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/// // | substs
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/// // def_id
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/// ```
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///
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/// As indicating in the comments above, each of those references
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/// is (in the compiler) basically a substitution (`substs`)
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/// applied to the type of a suitable `def_id` (which identifies
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/// `Foo1` or `Foo2`).
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///
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/// Now, at this point in compilation, what we have done is to
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/// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
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/// fresh inference variables C1 and C2. We wish to use the values
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/// of these variables to infer the underlying types of `Foo1` and
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/// `Foo2`. That is, this gives rise to higher-order (pattern) unification
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/// constraints like:
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///
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/// ```text
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/// for<'a> (Foo1<'a> = C1)
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/// for<'b> (Foo1<'b> = C2)
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/// ```
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///
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/// For these equation to be satisfiable, the types `C1` and `C2`
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/// can only refer to a limited set of regions. For example, `C1`
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/// can only refer to `'static` and `'a`, and `C2` can only refer
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/// to `'static` and `'b`. The job of this function is to impose that
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/// constraint.
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///
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/// Up to this point, C1 and C2 are basically just random type
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/// inference variables, and hence they may contain arbitrary
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/// regions. In fact, it is fairly likely that they do! Consider
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/// this possible definition of `foo`:
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///
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/// ```text
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/// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
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/// (&*x, &*y)
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/// }
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/// ```
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///
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/// Here, the values for the concrete types of the two impl
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/// traits will include inference variables:
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///
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/// ```text
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/// &'0 i32
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/// &'1 i32
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/// ```
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///
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/// Ordinarily, the subtyping rules would ensure that these are
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/// sufficiently large. But since `impl Bar<'a>` isn't a specific
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/// type per se, we don't get such constraints by default. This
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/// is where this function comes into play. It adds extra
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/// constraints to ensure that all the regions which appear in the
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/// inferred type are regions that could validly appear.
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///
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/// This is actually a bit of a tricky constraint in general. We
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/// want to say that each variable (e.g., `'0`) can only take on
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/// values that were supplied as arguments to the abstract type
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/// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
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/// scope. We don't have a constraint quite of this kind in the current
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/// region checker.
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///
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/// # The Solution
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///
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/// We make use of the constraint that we *do* have in the `<=`
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/// relation. To do that, we find the "minimum" of all the
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/// arguments that appear in the substs: that is, some region
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/// which is less than all the others. In the case of `Foo1<'a>`,
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/// that would be `'a` (it's the only choice, after all). Then we
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/// apply that as a least bound to the variables (e.g., `'a <=
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/// '0`).
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///
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/// In some cases, there is no minimum. Consider this example:
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///
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/// ```text
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/// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
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/// ```
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///
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/// Here we would report an error, because `'a` and `'b` have no
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/// relation to one another.
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///
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/// # The `free_region_relations` parameter
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///
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/// The `free_region_relations` argument is used to find the
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/// "minimum" of the regions supplied to a given abstract type.
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/// It must be a relation that can answer whether `'a <= 'b`,
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/// where `'a` and `'b` are regions that appear in the "substs"
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/// for the abstract type references (the `<'a>` in `Foo1<'a>`).
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///
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/// Note that we do not impose the constraints based on the
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/// generic regions from the `Foo1` definition (e.g., `'x`). This
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/// is because the constraints we are imposing here is basically
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/// the concern of the one generating the constraining type C1,
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/// which is the current function. It also means that we can
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/// take "implied bounds" into account in some cases:
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///
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/// ```text
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/// trait SomeTrait<'a, 'b> { }
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/// fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. }
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/// ```
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///
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/// Here, the fact that `'b: 'a` is known only because of the
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/// implied bounds from the `&'a &'b u32` parameter, and is not
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/// "inherent" to the abstract type definition.
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///
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/// # Parameters
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///
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/// - `anon_types` -- the map produced by `instantiate_anon_types`
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/// - `free_region_relations` -- something that can be used to relate
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/// the free regions (`'a`) that appear in the impl trait.
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pub fn constrain_anon_types<FRR: FreeRegionRelations<'tcx>>(
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&self,
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anon_types: &AnonTypeMap<'tcx>,
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free_region_relations: &FRR,
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) {
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debug!("constrain_anon_types()");
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for (&def_id, anon_defn) in anon_types {
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self.constrain_anon_type(def_id, anon_defn, free_region_relations);
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}
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}
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fn constrain_anon_type<FRR: FreeRegionRelations<'tcx>>(
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&self,
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def_id: DefId,
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anon_defn: &AnonTypeDecl<'tcx>,
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free_region_relations: &FRR,
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) {
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debug!("constrain_anon_type()");
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debug!("constrain_anon_type: def_id={:?}", def_id);
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debug!("constrain_anon_type: anon_defn={:#?}", anon_defn);
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let concrete_ty = self.resolve_type_vars_if_possible(&anon_defn.concrete_ty);
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debug!("constrain_anon_type: concrete_ty={:?}", concrete_ty);
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let abstract_type_generics = self.tcx.generics_of(def_id);
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let span = self.tcx.def_span(def_id);
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// If there are required region bounds, we can just skip
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// ahead. There will already be a registered region
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// obligation related `concrete_ty` to those regions.
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if anon_defn.has_required_region_bounds {
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return;
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}
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// There were no `required_region_bounds`,
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// so we have to search for a `least_region`.
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// Go through all the regions used as arguments to the
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// abstract type. These are the parameters to the abstract
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// type; so in our example above, `substs` would contain
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// `['a]` for the first impl trait and `'b` for the
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// second.
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let mut least_region = None;
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for param in &abstract_type_generics.params {
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match param.kind {
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GenericParamDefKind::Lifetime => {}
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_ => continue
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}
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// Get the value supplied for this region from the substs.
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let subst_arg = anon_defn.substs.region_at(param.index as usize);
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// Compute the least upper bound of it with the other regions.
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debug!("constrain_anon_types: least_region={:?}", least_region);
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debug!("constrain_anon_types: subst_arg={:?}", subst_arg);
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match least_region {
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None => least_region = Some(subst_arg),
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Some(lr) => {
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if free_region_relations.sub_free_regions(lr, subst_arg) {
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// keep the current least region
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} else if free_region_relations.sub_free_regions(subst_arg, lr) {
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// switch to `subst_arg`
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least_region = Some(subst_arg);
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} else {
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// There are two regions (`lr` and
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// `subst_arg`) which are not relatable. We can't
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// find a best choice.
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self.tcx
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.sess
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.struct_span_err(span, "ambiguous lifetime bound in `impl Trait`")
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.span_label(
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span,
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format!("neither `{}` nor `{}` outlives the other", lr, subst_arg),
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)
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.emit();
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least_region = Some(self.tcx.mk_region(ty::ReEmpty));
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break;
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}
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}
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}
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}
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let least_region = least_region.unwrap_or(self.tcx.types.re_static);
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debug!("constrain_anon_types: least_region={:?}", least_region);
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// Require that the type `concrete_ty` outlives
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// `least_region`, modulo any type parameters that appear
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// in the type, which we ignore. This is because impl
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// trait values are assumed to capture all the in-scope
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// type parameters. This little loop here just invokes
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// `outlives` repeatedly, draining all the nested
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// obligations that result.
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let mut types = vec![concrete_ty];
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let bound_region = |r| self.sub_regions(infer::CallReturn(span), least_region, r);
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while let Some(ty) = types.pop() {
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let mut components = self.tcx.outlives_components(ty);
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while let Some(component) = components.pop() {
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match component {
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Component::Region(r) => {
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bound_region(r);
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}
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Component::Param(_) => {
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// ignore type parameters like `T`, they are captured
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// implicitly by the `impl Trait`
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}
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Component::UnresolvedInferenceVariable(_) => {
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// we should get an error that more type
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// annotations are needed in this case
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self.tcx
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.sess
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.delay_span_bug(span, "unresolved inf var in anon");
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}
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Component::Projection(ty::ProjectionTy {
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substs,
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item_def_id: _,
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}) => {
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for r in substs.regions() {
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bound_region(r);
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}
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types.extend(substs.types());
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}
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Component::EscapingProjection(more_components) => {
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components.extend(more_components);
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}
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}
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}
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}
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}
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/// Given the fully resolved, instantiated type for an anonymous
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/// type, i.e., the value of an inference variable like C1 or C2
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/// (*), computes the "definition type" for an abstract type
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/// definition -- that is, the inferred value of `Foo1<'x>` or
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/// `Foo2<'x>` that we would conceptually use in its definition:
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///
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/// abstract type Foo1<'x>: Bar<'x> = AAA; <-- this type AAA
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/// abstract type Foo2<'x>: Bar<'x> = BBB; <-- or this type BBB
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/// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
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///
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/// Note that these values are defined in terms of a distinct set of
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/// generic parameters (`'x` instead of `'a`) from C1 or C2. The main
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/// purpose of this function is to do that translation.
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///
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/// (*) C1 and C2 were introduced in the comments on
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/// `constrain_anon_types`. Read that comment for more context.
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///
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/// # Parameters
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///
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/// - `def_id`, the `impl Trait` type
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/// - `anon_defn`, the anonymous definition created in `instantiate_anon_types`
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/// - `instantiated_ty`, the inferred type C1 -- fully resolved, lifted version of
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/// `anon_defn.concrete_ty`
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pub fn infer_anon_definition_from_instantiation(
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&self,
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def_id: DefId,
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anon_defn: &AnonTypeDecl<'tcx>,
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instantiated_ty: Ty<'gcx>,
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) -> Ty<'gcx> {
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debug!(
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"infer_anon_definition_from_instantiation(instantiated_ty={:?})",
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instantiated_ty
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);
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let gcx = self.tcx.global_tcx();
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|
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// Use substs to build up a reverse map from regions to their
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// identity mappings. This is necessary because of `impl
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// Trait` lifetimes are computed by replacing existing
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// lifetimes with 'static and remapping only those used in the
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// `impl Trait` return type, resulting in the parameters
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// shifting.
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let id_substs = Substs::identity_for_item(gcx, def_id);
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let map: FxHashMap<Kind<'tcx>, Kind<'gcx>> = anon_defn
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.substs
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.iter()
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.enumerate()
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.map(|(index, subst)| (*subst, id_substs[index]))
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.collect();
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// Convert the type from the function into a type valid outside
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// the function, by replacing invalid regions with 'static,
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// 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_anon_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,
|
|
|
|
anon_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,
|
|
anon_type_def_id: DefId,
|
|
map: FxHashMap<Kind<'tcx>, Kind<'gcx>>,
|
|
hidden_ty: Ty<'tcx>,
|
|
) -> Self {
|
|
Self {
|
|
tcx,
|
|
tainted_by_errors,
|
|
anon_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.anon_type_def_id);
|
|
let mut err = struct_span_err!(
|
|
self.tcx.sess,
|
|
span,
|
|
E0909,
|
|
"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::TyClosure(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>,
|
|
anon_types: AnonTypeMap<'tcx>,
|
|
obligations: Vec<PredicateObligation<'tcx>>,
|
|
}
|
|
|
|
impl<'a, 'gcx, 'tcx> Instantiator<'a, 'gcx, 'tcx> {
|
|
fn instantiate_anon_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
|
|
debug!("instantiate_anon_types_in_map(value={:?})", value);
|
|
let tcx = self.infcx.tcx;
|
|
value.fold_with(&mut BottomUpFolder {
|
|
tcx,
|
|
fldop: |ty| {
|
|
if let ty::TyAnon(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
|
|
// `TyAnon` 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 Anon assoc with `foo`
|
|
// }
|
|
// ```
|
|
if let Some(anon_node_id) = tcx.hir.as_local_node_id(def_id) {
|
|
let anon_parent_node_id = tcx.hir.get_parent(anon_node_id);
|
|
let anon_parent_def_id = tcx.hir.local_def_id(anon_parent_node_id);
|
|
if self.parent_def_id == anon_parent_def_id {
|
|
return self.fold_anon_ty(ty, def_id, substs);
|
|
}
|
|
|
|
debug!(
|
|
"instantiate_anon_types_in_map: \
|
|
encountered anon with wrong parent \
|
|
def_id={:?} \
|
|
anon_parent_def_id={:?}",
|
|
def_id, anon_parent_def_id
|
|
);
|
|
}
|
|
}
|
|
|
|
ty
|
|
},
|
|
})
|
|
}
|
|
|
|
fn fold_anon_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_anon_types: TyAnon(def_id={:?}, substs={:?})",
|
|
def_id, substs
|
|
);
|
|
|
|
// Use the same type variable if the exact same TyAnon appears more
|
|
// than once in the return type (e.g. if it's passed to a type alias).
|
|
if let Some(anon_defn) = self.anon_types.get(&def_id) {
|
|
return anon_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);
|
|
let bounds = predicates_of.instantiate(tcx, substs);
|
|
debug!("instantiate_anon_types: bounds={:?}", bounds);
|
|
|
|
let required_region_bounds = tcx.required_region_bounds(ty, bounds.predicates.clone());
|
|
debug!(
|
|
"instantiate_anon_types: required_region_bounds={:?}",
|
|
required_region_bounds
|
|
);
|
|
|
|
self.anon_types.insert(
|
|
def_id,
|
|
AnonTypeDecl {
|
|
substs,
|
|
concrete_ty: ty_var,
|
|
has_required_region_bounds: !required_region_bounds.is_empty(),
|
|
},
|
|
);
|
|
debug!("instantiate_anon_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 TyAnon.
|
|
// This also instantiates nested `impl Trait`.
|
|
let predicate = self.instantiate_anon_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_anon_types: predicate={:?}", predicate);
|
|
self.obligations
|
|
.push(traits::Obligation::new(cause, self.param_env, predicate));
|
|
}
|
|
|
|
ty_var
|
|
}
|
|
}
|