+use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
+use crate::infer::{InferCtxt, InferOk};
+use crate::traits;
+use rustc_data_structures::sync::Lrc;
use rustc_data_structures::vec_map::VecMap;
use rustc_hir as hir;
-use rustc_middle::ty::{OpaqueTypeKey, Ty};
+use rustc_hir::def_id::LocalDefId;
+use rustc_middle::ty::fold::BottomUpFolder;
+use rustc_middle::ty::subst::{GenericArgKind, Subst};
+use rustc_middle::ty::{self, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeVisitor};
use rustc_span::Span;
+use std::ops::ControlFlow;
+
pub type OpaqueTypeMap<'tcx> = VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>;
/// Information about the opaque types whose values we
/// The origin of the opaque type.
pub origin: hir::OpaqueTyOrigin,
}
+
+impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
+ /// Replaces 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 an `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` -- the `DefId` of the function in which the opaque type
+ /// is defined
+ /// - `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
+ /// - `value_span` -- the span where the value came from, used in error reporting
+ pub fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
+ &self,
+ body_id: hir::HirId,
+ param_env: ty::ParamEnv<'tcx>,
+ value: T,
+ value_span: Span,
+ ) -> InferOk<'tcx, T> {
+ debug!(
+ "instantiate_opaque_types(value={:?}, body_id={:?}, \
+ param_env={:?}, value_span={:?})",
+ value, body_id, param_env, value_span,
+ );
+ let mut instantiator =
+ Instantiator { infcx: self, body_id, param_env, value_span, obligations: vec![] };
+ let value = instantiator.instantiate_opaque_types_in_map(value);
+ InferOk { value, obligations: instantiator.obligations }
+ }
+
+ /// Given the map `opaque_types` containing the opaque
+ /// `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 opaque types (`Foo1`, `Foo2`) and then, in
+ /// the return type of `foo`, we *reference* those definitions:
+ ///
+ /// ```text
+ /// type Foo1<'x> = impl Bar<'x>;
+ /// type Foo2<'x> = impl 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 opaque 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 generally prefer to make `<=` constraints, since they
+ /// integrate best into the region solver. 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 a more complex "in constraint", like `'r
+ /// in ['a, 'b, 'static]` (where `'r` is some region appearing in
+ /// the hidden type).
+ ///
+ /// # Constrain regions, not the hidden concrete type
+ ///
+ /// Note that generating constraints on each region `Rc` is *not*
+ /// the same as generating an outlives constraint on `Tc` iself.
+ /// For example, if we had a function like this:
+ ///
+ /// ```rust
+ /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
+ /// (x, y)
+ /// }
+ ///
+ /// // Equivalent to:
+ /// type FooReturn<'a, T> = impl Foo<'a>;
+ /// fn foo<'a, T>(..) -> FooReturn<'a, T> { .. }
+ /// ```
+ ///
+ /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
+ /// is an inference variable). If we generated a constraint that
+ /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
+ /// but this is not necessary, because the opaque type we
+ /// create will be allowed to reference `T`. So we only generate a
+ /// constraint that `'0: 'a`.
+ ///
+ /// # The `free_region_relations` parameter
+ ///
+ /// The `free_region_relations` argument is used to find the
+ /// "minimum" of the regions supplied to a given opaque 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 opaque 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 opaque 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.
+ #[instrument(level = "debug", skip(self))]
+ pub fn constrain_opaque_type(
+ &self,
+ opaque_type_key: OpaqueTypeKey<'tcx>,
+ opaque_defn: &OpaqueTypeDecl<'tcx>,
+ ) {
+ let def_id = opaque_type_key.def_id;
+
+ let tcx = self.tcx;
+
+ let concrete_ty = self.resolve_vars_if_possible(opaque_defn.concrete_ty);
+
+ debug!(?concrete_ty);
+
+ let first_own_region = match opaque_defn.origin {
+ hir::OpaqueTyOrigin::FnReturn | hir::OpaqueTyOrigin::AsyncFn => {
+ // We lower
+ //
+ // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
+ //
+ // into
+ //
+ // type foo::<'p0..'pn>::Foo<'q0..'qm>
+ // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
+ //
+ // For these types we only iterate over `'l0..lm` below.
+ tcx.generics_of(def_id).parent_count
+ }
+ // These opaque type inherit all lifetime parameters from their
+ // parent, so we have to check them all.
+ hir::OpaqueTyOrigin::TyAlias => 0,
+ };
+
+ // For a case like `impl Foo<'a, 'b>`, we would generate a constraint
+ // `'r in ['a, 'b, 'static]` for each region `'r` that appears in the
+ // hidden type (i.e., it must be equal to `'a`, `'b`, or `'static`).
+ //
+ // `conflict1` and `conflict2` are the two region bounds that we
+ // detected which were unrelated. They are used for diagnostics.
+
+ // Create the set of choice regions: each region in the hidden
+ // type can be equal to any of the region parameters of the
+ // opaque type definition.
+ let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
+ opaque_type_key.substs[first_own_region..]
+ .iter()
+ .filter_map(|arg| match arg.unpack() {
+ GenericArgKind::Lifetime(r) => Some(r),
+ GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
+ })
+ .chain(std::iter::once(self.tcx.lifetimes.re_static))
+ .collect(),
+ );
+
+ concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
+ tcx: self.tcx,
+ op: |r| {
+ self.member_constraint(
+ opaque_type_key.def_id,
+ opaque_defn.definition_span,
+ concrete_ty,
+ r,
+ &choice_regions,
+ )
+ },
+ });
+ }
+}
+
+// Visitor that requires that (almost) all regions in the type visited outlive
+// `least_region`. We cannot use `push_outlives_components` because regions in
+// closure signatures are not included in their outlives components. We need to
+// ensure all regions outlive the given bound so that we don't end up with,
+// say, `ReVar` appearing in a return type and causing ICEs when other
+// functions end up with region constraints involving regions from other
+// functions.
+//
+// We also cannot use `for_each_free_region` because for closures it includes
+// the regions parameters from the enclosing item.
+//
+// We ignore any type parameters because impl trait values are assumed to
+// capture all the in-scope type parameters.
+struct ConstrainOpaqueTypeRegionVisitor<'tcx, OP> {
+ tcx: TyCtxt<'tcx>,
+ op: OP,
+}
+
+impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
+where
+ OP: FnMut(ty::Region<'tcx>),
+{
+ fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
+ Some(self.tcx)
+ }
+
+ fn visit_binder<T: TypeFoldable<'tcx>>(
+ &mut self,
+ t: &ty::Binder<'tcx, T>,
+ ) -> ControlFlow<Self::BreakTy> {
+ t.as_ref().skip_binder().visit_with(self);
+ ControlFlow::CONTINUE
+ }
+
+ fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+ match *r {
+ // ignore bound regions, keep visiting
+ ty::ReLateBound(_, _) => ControlFlow::CONTINUE,
+ _ => {
+ (self.op)(r);
+ ControlFlow::CONTINUE
+ }
+ }
+ }
+
+ fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+ // We're only interested in types involving regions
+ if !ty.flags().intersects(ty::TypeFlags::HAS_POTENTIAL_FREE_REGIONS) {
+ return ControlFlow::CONTINUE;
+ }
+
+ match ty.kind() {
+ ty::Closure(_, ref substs) => {
+ // Skip lifetime parameters of the enclosing item(s)
+
+ substs.as_closure().tupled_upvars_ty().visit_with(self);
+ substs.as_closure().sig_as_fn_ptr_ty().visit_with(self);
+ }
+
+ ty::Generator(_, ref substs, _) => {
+ // Skip lifetime parameters of the enclosing item(s)
+ // Also skip the witness type, because that has no free regions.
+
+ substs.as_generator().tupled_upvars_ty().visit_with(self);
+ substs.as_generator().return_ty().visit_with(self);
+ substs.as_generator().yield_ty().visit_with(self);
+ substs.as_generator().resume_ty().visit_with(self);
+ }
+ _ => {
+ ty.super_visit_with(self);
+ }
+ }
+
+ ControlFlow::CONTINUE
+ }
+}
+
+struct Instantiator<'a, 'tcx> {
+ infcx: &'a InferCtxt<'a, 'tcx>,
+ body_id: hir::HirId,
+ param_env: ty::ParamEnv<'tcx>,
+ value_span: Span,
+ obligations: Vec<traits::PredicateObligation<'tcx>>,
+}
+
+impl<'a, 'tcx> Instantiator<'a, 'tcx> {
+ fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
+ let tcx = self.infcx.tcx;
+ value.fold_with(&mut BottomUpFolder {
+ tcx,
+ ty_op: |ty| {
+ if ty.references_error() {
+ return tcx.ty_error();
+ } else if let ty::Opaque(def_id, substs) = ty.kind() {
+ // 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 opaque types more fully,
+ // this same scenario will be able to arise during all phases.
+ //
+ // Here is an example using type alias `impl Trait`
+ // that indicates the distinction we are checking for:
+ //
+ // ```rust
+ // mod a {
+ // pub type Foo = impl Iterator;
+ // pub fn make_foo() -> Foo { .. }
+ // }
+ //
+ // mod b {
+ // fn foo() -> a::Foo { a::make_foo() }
+ // }
+ // ```
+ //
+ // Here, the return type of `foo` references an
+ // `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(def_id) = def_id.as_local() {
+ let opaque_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+ let parent_def_id = self.infcx.defining_use_anchor;
+ let def_scope_default = || {
+ let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
+ parent_def_id == tcx.hir().local_def_id(opaque_parent_hir_id)
+ };
+ let (in_definition_scope, origin) =
+ match tcx.hir().expect_item(opaque_hir_id).kind {
+ // Anonymous `impl Trait`
+ hir::ItemKind::OpaqueTy(hir::OpaqueTy {
+ impl_trait_fn: Some(parent),
+ origin,
+ ..
+ }) => (parent == parent_def_id.to_def_id(), origin),
+ // Named `type Foo = impl Bar;`
+ hir::ItemKind::OpaqueTy(hir::OpaqueTy {
+ impl_trait_fn: None,
+ origin,
+ ..
+ }) => (
+ may_define_opaque_type(tcx, parent_def_id, opaque_hir_id),
+ origin,
+ ),
+ _ => (def_scope_default(), hir::OpaqueTyOrigin::TyAlias),
+ };
+ if in_definition_scope {
+ let opaque_type_key =
+ OpaqueTypeKey { def_id: def_id.to_def_id(), substs };
+ return self.fold_opaque_ty(ty, opaque_type_key, origin);
+ }
+
+ debug!(
+ "instantiate_opaque_types_in_map: \
+ encountered opaque outside its definition scope \
+ def_id={:?}",
+ def_id,
+ );
+ }
+ }
+
+ ty
+ },
+ lt_op: |lt| lt,
+ ct_op: |ct| ct,
+ })
+ }
+
+ #[instrument(skip(self), level = "debug")]
+ fn fold_opaque_ty(
+ &mut self,
+ ty: Ty<'tcx>,
+ opaque_type_key: OpaqueTypeKey<'tcx>,
+ origin: hir::OpaqueTyOrigin,
+ ) -> Ty<'tcx> {
+ let infcx = self.infcx;
+ let tcx = infcx.tcx;
+ let OpaqueTypeKey { def_id, substs } = opaque_type_key;
+
+ // Use the same type variable if the exact same opaque type appears more
+ // than once in the return type (e.g., if it's passed to a type alias).
+ if let Some(opaque_defn) = infcx.inner.borrow().opaque_types.get(&opaque_type_key) {
+ debug!("re-using cached concrete type {:?}", opaque_defn.concrete_ty.kind());
+ return opaque_defn.concrete_ty;
+ }
+
+ let ty_var = infcx.next_ty_var(TypeVariableOrigin {
+ kind: TypeVariableOriginKind::TypeInference,
+ span: self.value_span,
+ });
+
+ // Ideally, we'd get the span where *this specific `ty` came
+ // from*, but right now we just use the span from the overall
+ // value being folded. In simple cases like `-> impl Foo`,
+ // these are the same span, but not in cases like `-> (impl
+ // Foo, impl Bar)`.
+ let definition_span = self.value_span;
+
+ {
+ let mut infcx = self.infcx.inner.borrow_mut();
+ infcx.opaque_types.insert(
+ OpaqueTypeKey { def_id, substs },
+ OpaqueTypeDecl { opaque_type: ty, definition_span, concrete_ty: ty_var, origin },
+ );
+ infcx.opaque_types_vars.insert(ty_var, ty);
+ }
+
+ debug!("generated new type inference var {:?}", ty_var.kind());
+
+ let item_bounds = tcx.explicit_item_bounds(def_id);
+
+ self.obligations.reserve(item_bounds.len());
+ for (predicate, _) in item_bounds {
+ debug!(?predicate);
+ let predicate = predicate.subst(tcx, substs);
+ debug!(?predicate);
+
+ // We can't normalize associated types from `rustc_infer`, but we can eagerly register inference variables for them.
+ let predicate = predicate.fold_with(&mut BottomUpFolder {
+ tcx,
+ ty_op: |ty| match ty.kind() {
+ ty::Projection(projection_ty) => infcx.infer_projection(
+ self.param_env,
+ *projection_ty,
+ traits::ObligationCause::misc(self.value_span, self.body_id),
+ 0,
+ &mut self.obligations,
+ ),
+ _ => ty,
+ },
+ lt_op: |lt| lt,
+ ct_op: |ct| ct,
+ });
+ debug!(?predicate);
+
+ if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
+ if projection.ty.references_error() {
+ // No point on adding these obligations since there's a type error involved.
+ return tcx.ty_error();
+ }
+ }
+ // Change the predicate to refer to the type variable,
+ // which will be the concrete type instead of the opaque type.
+ // This also instantiates nested instances of `impl Trait`.
+ let predicate = self.instantiate_opaque_types_in_map(predicate);
+
+ let cause =
+ traits::ObligationCause::new(self.value_span, self.body_id, traits::OpaqueType);
+
+ // Require that the predicate holds for the concrete type.
+ debug!(?predicate);
+ self.obligations.push(traits::Obligation::new(cause, self.param_env, predicate));
+ }
+
+ ty_var
+ }
+}
+
+/// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
+///
+/// Example:
+/// ```rust
+/// pub mod foo {
+/// pub mod bar {
+/// pub trait Bar { .. }
+///
+/// pub type Baz = impl Bar;
+///
+/// fn f1() -> Baz { .. }
+/// }
+///
+/// fn f2() -> bar::Baz { .. }
+/// }
+/// ```
+///
+/// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
+/// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
+/// For the above example, this function returns `true` for `f1` and `false` for `f2`.
+fn may_define_opaque_type(tcx: TyCtxt<'_>, def_id: LocalDefId, opaque_hir_id: hir::HirId) -> bool {
+ let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+
+ // Named opaque types can be defined by any siblings or children of siblings.
+ let scope = tcx.hir().get_defining_scope(opaque_hir_id);
+ // We walk up the node tree until we hit the root or the scope of the opaque type.
+ while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
+ hir_id = tcx.hir().get_parent_item(hir_id);
+ }
+ // Syntactically, we are allowed to define the concrete type if:
+ let res = hir_id == scope;
+ trace!(
+ "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
+ tcx.hir().find(hir_id),
+ tcx.hir().get(opaque_hir_id),
+ res
+ );
+ res
+}
projects / rust.git / blobdiff