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mod.rs
641 lines (596 loc) · 25.9 KB
/
mod.rs
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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 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};
use ty::fold::{BottomUpFolder, TypeFoldable};
use ty::outlives::Component;
use ty::subst::{Kind, UnpackedKind, Substs};
use util::nodemap::DefIdMap;
pub type AnonTypeMap<'tcx> = DefIdMap<AnonTypeDecl<'tcx>>;
/// Information about the anonymous, 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 AnonTypeDecl<'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 anonymized 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 `AnonTypeMap` 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 anonymous types
/// with this parent. This is typically the def-id of the function
/// in whose return type anon 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 anon types
pub fn instantiate_anon_types<T: TypeFoldable<'tcx>>(
&self,
parent_def_id: DefId,
body_id: ast::NodeId,
param_env: ty::ParamEnv<'tcx>,
value: &T,
) -> InferOk<'tcx, (T, AnonTypeMap<'tcx>)> {
debug!(
"instantiate_anon_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,
anon_types: DefIdMap(),
obligations: vec![],
};
let value = instantiator.instantiate_anon_types_in_map(value);
InferOk {
value: (value, instantiator.anon_types),
obligations: instantiator.obligations,
}
}
/// Given the map `anon_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
///
/// - `anon_types` -- the map produced by `instantiate_anon_types`
/// - `free_region_relations` -- something that can be used to relate
/// the free regions (`'a`) that appear in the impl trait.
pub fn constrain_anon_types<FRR: FreeRegionRelations<'tcx>>(
&self,
anon_types: &AnonTypeMap<'tcx>,
free_region_relations: &FRR,
) {
debug!("constrain_anon_types()");
for (&def_id, anon_defn) in anon_types {
self.constrain_anon_type(def_id, anon_defn, free_region_relations);
}
}
fn constrain_anon_type<FRR: FreeRegionRelations<'tcx>>(
&self,
def_id: DefId,
anon_defn: &AnonTypeDecl<'tcx>,
free_region_relations: &FRR,
) {
debug!("constrain_anon_type()");
debug!("constrain_anon_type: def_id={:?}", def_id);
debug!("constrain_anon_type: anon_defn={:#?}", anon_defn);
let concrete_ty = self.resolve_type_vars_if_possible(&anon_defn.concrete_ty);
debug!("constrain_anon_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 anon_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 region_def in &abstract_type_generics.regions {
// Find the index of this region in the list of substitutions.
let index = region_def.index as usize;
// Get the value supplied for this region from the substs.
let subst_arg = anon_defn.substs.region_at(index);
// Compute the least upper bound of it with the other regions.
debug!("constrain_anon_types: least_region={:?}", least_region);
debug!("constrain_anon_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_anon_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 anon");
}
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 anonymous
/// 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_anon_types`. Read that comment for more context.
///
/// # Parameters
///
/// - `def_id`, the `impl Trait` type
/// - `anon_defn`, the anonymous definition created in `instantiate_anon_types`
/// - `instantiated_ty`, the inferred type C1 -- fully resolved, lifted version of
/// `anon_defn.concrete_ty`
pub fn infer_anon_definition_from_instantiation(
&self,
def_id: DefId,
anon_defn: &AnonTypeDecl<'tcx>,
instantiated_ty: Ty<'gcx>,
) -> Ty<'gcx> {
debug!(
"infer_anon_definition_from_instantiation(instantiated_ty={:?})",
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>> = anon_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 = gcx.fold_regions(&instantiated_ty, &mut false, |r, _| {
match *r {
// 'static and early-bound regions are valid.
ty::ReStatic | ty::ReEmpty => r,
// All other regions, we map them appropriately to their adjusted
// indices, erroring if we find any lifetimes that were not mapped
// into the new set.
_ => if let Some(UnpackedKind::Lifetime(r1)) = map.get(&r.into())
.map(|k| k.unpack()) {
r1
} else {
// No mapping was found. This means that
// it is either a disallowed lifetime,
// which will be caught by regionck, or it
// is a region in a non-upvar closure
// generic, which is explicitly
// allowed. If that surprises you, read
// on.
//
// The case of closure 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
// trans.
gcx.types.re_empty
},
}
});
debug!(
"infer_anon_definition_from_instantiation: definition_ty={:?}",
definition_ty
);
definition_ty
}
}
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
}
}