Move instantiate_opaque_types to rustc_infer.

It does not depend on anything from rustc_trait_selection anymore.

6 files changed, 283 insertions, 294 deletions

diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index 7e69e710d68..7e6a481ca69 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -36,7 +36,6 @@ use rustc_span::def_id::CRATE_DEF_ID;
 use rustc_span::{Span, DUMMY_SP};
 use rustc_target::abi::VariantIdx;
 use rustc_trait_selection::infer::InferCtxtExt as _;
-use rustc_trait_selection::opaque_types::InferCtxtExt;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
 use rustc_trait_selection::traits::query::type_op;
 use rustc_trait_selection::traits::query::type_op::custom::CustomTypeOp;
diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs
index 25489b73613..e2e07f2072e 100644
--- a/compiler/rustc_infer/src/infer/opaque_types.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types.rs
@@ -1,8 +1,12 @@
-use crate::infer::InferCtxt;
+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::subst::GenericArgKind;
+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;
 
@@ -52,6 +56,49 @@ pub struct OpaqueTypeDecl<'tcx> {
 }
 
 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
@@ -359,3 +406,232 @@ where
         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
+}
diff --git a/compiler/rustc_trait_selection/src/opaque_types.rs b/compiler/rustc_trait_selection/src/opaque_types.rs
index 3be5f2e59db..75d57d78e3b 100644
--- a/compiler/rustc_trait_selection/src/opaque_types.rs
+++ b/compiler/rustc_trait_selection/src/opaque_types.rs
@@ -1,25 +1,14 @@
-use crate::traits::{self, ObligationCause, PredicateObligation};
+use crate::traits;
 use rustc_data_structures::fx::FxHashMap;
-use rustc_hir as hir;
-use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::def_id::DefId;
 use rustc_infer::infer::error_reporting::unexpected_hidden_region_diagnostic;
-use rustc_infer::infer::opaque_types::OpaqueTypeDecl;
-use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
-use rustc_infer::infer::{InferCtxt, InferOk};
-use rustc_middle::ty::fold::{BottomUpFolder, TypeFoldable, TypeFolder};
-use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, Subst};
+use rustc_infer::infer::InferCtxt;
+use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
+use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts};
 use rustc_middle::ty::{self, OpaqueTypeKey, Ty, TyCtxt};
 use rustc_span::Span;
 
 pub trait InferCtxtExt<'tcx> {
-    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>;
-
     fn infer_opaque_definition_from_instantiation(
         &self,
         opaque_type_key: OpaqueTypeKey<'tcx>,
@@ -29,49 +18,6 @@ pub trait InferCtxtExt<'tcx> {
 }
 
 impl<'a, 'tcx> InferCtxtExt<'tcx> for 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
-    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 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 opaque type
@@ -375,235 +321,6 @@ impl TypeFolder<'tcx> for ReverseMapper<'tcx> {
     }
 }
 
-struct Instantiator<'a, 'tcx> {
-    infcx: &'a InferCtxt<'a, 'tcx>,
-    body_id: hir::HirId,
-    param_env: ty::ParamEnv<'tcx>,
-    value_span: Span,
-    obligations: Vec<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,
-                        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
-}
-
 /// Given a set of predicates that apply to an object type, returns
 /// the region bounds that the (erased) `Self` type must
 /// outlive. Precisely *because* the `Self` type is erased, the
diff --git a/compiler/rustc_typeck/src/check/_match.rs b/compiler/rustc_typeck/src/check/_match.rs
index c17c42c497f..a8160313228 100644
--- a/compiler/rustc_typeck/src/check/_match.rs
+++ b/compiler/rustc_typeck/src/check/_match.rs
@@ -6,7 +6,6 @@ use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKi
 use rustc_infer::traits::Obligation;
 use rustc_middle::ty::{self, ToPredicate, Ty, TyS};
 use rustc_span::{MultiSpan, Span};
-use rustc_trait_selection::opaque_types::InferCtxtExt as _;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
 use rustc_trait_selection::traits::{
     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
diff --git a/compiler/rustc_typeck/src/check/check.rs b/compiler/rustc_typeck/src/check/check.rs
index 66316214e5e..5040c4db951 100644
--- a/compiler/rustc_typeck/src/check/check.rs
+++ b/compiler/rustc_typeck/src/check/check.rs
@@ -21,7 +21,6 @@ use rustc_session::lint::builtin::{UNINHABITED_STATIC, UNSUPPORTED_CALLING_CONVE
 use rustc_span::symbol::sym;
 use rustc_span::{self, MultiSpan, Span};
 use rustc_target::spec::abi::Abi;
-use rustc_trait_selection::opaque_types::InferCtxtExt as _;
 use rustc_trait_selection::traits;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
 use rustc_ty_utils::representability::{self, Representability};
diff --git a/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs b/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
index 41e43321a8f..5179b425f88 100644
--- a/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
+++ b/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
@@ -35,7 +35,6 @@ use rustc_span::source_map::{original_sp, DUMMY_SP};
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{self, BytePos, MultiSpan, Span};
 use rustc_trait_selection::infer::InferCtxtExt as _;
-use rustc_trait_selection::opaque_types::InferCtxtExt as _;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
 use rustc_trait_selection::traits::{
     self, ObligationCause, ObligationCauseCode, StatementAsExpression, TraitEngine, TraitEngineExt,