Split out opaque from type_of

2 files changed, 303 insertions, 305 deletions

diff --git a/compiler/rustc_hir_analysis/src/collect/type_of.rs b/compiler/rustc_hir_analysis/src/collect/type_of.rs
index 97c6cb491d1..8e082d3c532 100644
--- a/compiler/rustc_hir_analysis/src/collect/type_of.rs
+++ b/compiler/rustc_hir_analysis/src/collect/type_of.rs
@@ -1,10 +1,7 @@
 use rustc_errors::{Applicability, StashKey};
 use rustc_hir as hir;
 use rustc_hir::def_id::LocalDefId;
-use rustc_hir::intravisit;
-use rustc_hir::intravisit::Visitor;
-use rustc_hir::{HirId, Node};
-use rustc_middle::hir::nested_filter;
+use rustc_hir::HirId;
 use rustc_middle::ty::print::with_forced_trimmed_paths;
 use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::util::IntTypeExt;
@@ -14,7 +11,8 @@ use rustc_span::{Span, DUMMY_SP};
 
 use super::ItemCtxt;
 use super::{bad_placeholder, is_suggestable_infer_ty};
-use crate::errors::UnconstrainedOpaqueType;
+
+mod opaque;
 
 fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
     use hir::*;
@@ -429,7 +427,7 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::EarlyBinder<Ty
                 ItemKind::OpaqueTy(OpaqueTy {
                     origin: hir::OpaqueTyOrigin::TyAlias { .. },
                     ..
-                }) => find_opaque_ty_constraints_for_tait(tcx, def_id),
+                }) => opaque::find_opaque_ty_constraints_for_tait(tcx, def_id),
                 // Opaque types desugared from `impl Trait`.
                 ItemKind::OpaqueTy(OpaqueTy {
                     origin:
@@ -443,7 +441,7 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::EarlyBinder<Ty
                             "tried to get type of this RPITIT with no definition"
                         );
                     }
-                    find_opaque_ty_constraints_for_rpit(tcx, def_id, owner)
+                    opaque::find_opaque_ty_constraints_for_rpit(tcx, def_id, owner)
                 }
                 ItemKind::Trait(..)
                 | ItemKind::TraitAlias(..)
@@ -502,304 +500,6 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::EarlyBinder<Ty
     ty::EarlyBinder(output)
 }
 
-#[instrument(skip(tcx), level = "debug")]
-/// Checks "defining uses" of opaque `impl Trait` types to ensure that they meet the restrictions
-/// laid for "higher-order pattern unification".
-/// This ensures that inference is tractable.
-/// In particular, definitions of opaque types can only use other generics as arguments,
-/// and they cannot repeat an argument. Example:
-///
-/// ```ignore (illustrative)
-/// type Foo<A, B> = impl Bar<A, B>;
-///
-/// // Okay -- `Foo` is applied to two distinct, generic types.
-/// fn a<T, U>() -> Foo<T, U> { .. }
-///
-/// // Not okay -- `Foo` is applied to `T` twice.
-/// fn b<T>() -> Foo<T, T> { .. }
-///
-/// // Not okay -- `Foo` is applied to a non-generic type.
-/// fn b<T>() -> Foo<T, u32> { .. }
-/// ```
-///
-fn find_opaque_ty_constraints_for_tait(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
-    use rustc_hir::{Expr, ImplItem, Item, TraitItem};
-
-    struct ConstraintLocator<'tcx> {
-        tcx: TyCtxt<'tcx>,
-
-        /// def_id of the opaque type whose defining uses are being checked
-        def_id: LocalDefId,
-
-        /// as we walk the defining uses, we are checking that all of them
-        /// define the same hidden type. This variable is set to `Some`
-        /// with the first type that we find, and then later types are
-        /// checked against it (we also carry the span of that first
-        /// type).
-        found: Option<ty::OpaqueHiddenType<'tcx>>,
-
-        /// In the presence of dead code, typeck may figure out a hidden type
-        /// while borrowck will not. We collect these cases here and check at
-        /// the end that we actually found a type that matches (modulo regions).
-        typeck_types: Vec<ty::OpaqueHiddenType<'tcx>>,
-    }
-
-    impl ConstraintLocator<'_> {
-        #[instrument(skip(self), level = "debug")]
-        fn check(&mut self, item_def_id: LocalDefId) {
-            // Don't try to check items that cannot possibly constrain the type.
-            if !self.tcx.has_typeck_results(item_def_id) {
-                debug!("no constraint: no typeck results");
-                return;
-            }
-            // Calling `mir_borrowck` can lead to cycle errors through
-            // const-checking, avoid calling it if we don't have to.
-            // ```rust
-            // type Foo = impl Fn() -> usize; // when computing type for this
-            // const fn bar() -> Foo {
-            //     || 0usize
-            // }
-            // const BAZR: Foo = bar(); // we would mir-borrowck this, causing cycles
-            // // because we again need to reveal `Foo` so we can check whether the
-            // // constant does not contain interior mutability.
-            // ```
-            let tables = self.tcx.typeck(item_def_id);
-            if let Some(guar) = tables.tainted_by_errors {
-                self.found =
-                    Some(ty::OpaqueHiddenType { span: DUMMY_SP, ty: self.tcx.ty_error(guar) });
-                return;
-            }
-            let Some(&typeck_hidden_ty) = tables.concrete_opaque_types.get(&self.def_id) else {
-                debug!("no constraints in typeck results");
-                return;
-            };
-            if self.typeck_types.iter().all(|prev| prev.ty != typeck_hidden_ty.ty) {
-                self.typeck_types.push(typeck_hidden_ty);
-            }
-
-            // Use borrowck to get the type with unerased regions.
-            let concrete_opaque_types = &self.tcx.mir_borrowck(item_def_id).concrete_opaque_types;
-            debug!(?concrete_opaque_types);
-            if let Some(&concrete_type) = concrete_opaque_types.get(&self.def_id) {
-                debug!(?concrete_type, "found constraint");
-                if let Some(prev) = &mut self.found {
-                    if concrete_type.ty != prev.ty && !(concrete_type, prev.ty).references_error() {
-                        let guar =
-                            prev.report_mismatch(&concrete_type, self.def_id, self.tcx).emit();
-                        prev.ty = self.tcx.ty_error(guar);
-                    }
-                } else {
-                    self.found = Some(concrete_type);
-                }
-            }
-        }
-    }
-
-    impl<'tcx> intravisit::Visitor<'tcx> for ConstraintLocator<'tcx> {
-        type NestedFilter = nested_filter::All;
-
-        fn nested_visit_map(&mut self) -> Self::Map {
-            self.tcx.hir()
-        }
-        fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
-            if let hir::ExprKind::Closure(closure) = ex.kind {
-                self.check(closure.def_id);
-            }
-            intravisit::walk_expr(self, ex);
-        }
-        fn visit_item(&mut self, it: &'tcx Item<'tcx>) {
-            trace!(?it.owner_id);
-            // The opaque type itself or its children are not within its reveal scope.
-            if it.owner_id.def_id != self.def_id {
-                self.check(it.owner_id.def_id);
-                intravisit::walk_item(self, it);
-            }
-        }
-        fn visit_impl_item(&mut self, it: &'tcx ImplItem<'tcx>) {
-            trace!(?it.owner_id);
-            // The opaque type itself or its children are not within its reveal scope.
-            if it.owner_id.def_id != self.def_id {
-                self.check(it.owner_id.def_id);
-                intravisit::walk_impl_item(self, it);
-            }
-        }
-        fn visit_trait_item(&mut self, it: &'tcx TraitItem<'tcx>) {
-            trace!(?it.owner_id);
-            self.check(it.owner_id.def_id);
-            intravisit::walk_trait_item(self, it);
-        }
-    }
-
-    let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
-    let scope = tcx.hir().get_defining_scope(hir_id);
-    let mut locator = ConstraintLocator { def_id, tcx, found: None, typeck_types: vec![] };
-
-    debug!(?scope);
-
-    if scope == hir::CRATE_HIR_ID {
-        tcx.hir().walk_toplevel_module(&mut locator);
-    } else {
-        trace!("scope={:#?}", tcx.hir().get(scope));
-        match tcx.hir().get(scope) {
-            // We explicitly call `visit_*` methods, instead of using `intravisit::walk_*` methods
-            // This allows our visitor to process the defining item itself, causing
-            // it to pick up any 'sibling' defining uses.
-            //
-            // For example, this code:
-            // ```
-            // fn foo() {
-            //     type Blah = impl Debug;
-            //     let my_closure = || -> Blah { true };
-            // }
-            // ```
-            //
-            // requires us to explicitly process `foo()` in order
-            // to notice the defining usage of `Blah`.
-            Node::Item(it) => locator.visit_item(it),
-            Node::ImplItem(it) => locator.visit_impl_item(it),
-            Node::TraitItem(it) => locator.visit_trait_item(it),
-            other => bug!("{:?} is not a valid scope for an opaque type item", other),
-        }
-    }
-
-    let Some(hidden) = locator.found else {
-        let reported = tcx.sess.emit_err(UnconstrainedOpaqueType {
-            span: tcx.def_span(def_id),
-            name: tcx.item_name(tcx.local_parent(def_id).to_def_id()),
-            what: match tcx.hir().get(scope) {
-                _ if scope == hir::CRATE_HIR_ID => "module",
-                Node::Item(hir::Item { kind: hir::ItemKind::Mod(_), .. }) => "module",
-                Node::Item(hir::Item { kind: hir::ItemKind::Impl(_), .. }) => "impl",
-                _ => "item",
-            },
-        });
-        return tcx.ty_error(reported);
-    };
-
-    // Only check against typeck if we didn't already error
-    if !hidden.ty.references_error() {
-        for concrete_type in locator.typeck_types {
-            if concrete_type.ty != tcx.erase_regions(hidden.ty)
-                && !(concrete_type, hidden).references_error()
-            {
-                hidden.report_mismatch(&concrete_type, def_id, tcx).emit();
-            }
-        }
-    }
-
-    hidden.ty
-}
-
-fn find_opaque_ty_constraints_for_rpit(
-    tcx: TyCtxt<'_>,
-    def_id: LocalDefId,
-    owner_def_id: LocalDefId,
-) -> Ty<'_> {
-    use rustc_hir::{Expr, ImplItem, Item, TraitItem};
-
-    struct ConstraintChecker<'tcx> {
-        tcx: TyCtxt<'tcx>,
-
-        /// def_id of the opaque type whose defining uses are being checked
-        def_id: LocalDefId,
-
-        found: ty::OpaqueHiddenType<'tcx>,
-    }
-
-    impl ConstraintChecker<'_> {
-        #[instrument(skip(self), level = "debug")]
-        fn check(&self, def_id: LocalDefId) {
-            // Use borrowck to get the type with unerased regions.
-            let concrete_opaque_types = &self.tcx.mir_borrowck(def_id).concrete_opaque_types;
-            debug!(?concrete_opaque_types);
-            for (&def_id, &concrete_type) in concrete_opaque_types {
-                if def_id != self.def_id {
-                    // Ignore constraints for other opaque types.
-                    continue;
-                }
-
-                debug!(?concrete_type, "found constraint");
-
-                if concrete_type.ty != self.found.ty
-                    && !(concrete_type, self.found).references_error()
-                {
-                    self.found.report_mismatch(&concrete_type, self.def_id, self.tcx).emit();
-                }
-            }
-        }
-    }
-
-    impl<'tcx> intravisit::Visitor<'tcx> for ConstraintChecker<'tcx> {
-        type NestedFilter = nested_filter::OnlyBodies;
-
-        fn nested_visit_map(&mut self) -> Self::Map {
-            self.tcx.hir()
-        }
-        fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
-            if let hir::ExprKind::Closure(closure) = ex.kind {
-                self.check(closure.def_id);
-            }
-            intravisit::walk_expr(self, ex);
-        }
-        fn visit_item(&mut self, it: &'tcx Item<'tcx>) {
-            trace!(?it.owner_id);
-            // The opaque type itself or its children are not within its reveal scope.
-            if it.owner_id.def_id != self.def_id {
-                self.check(it.owner_id.def_id);
-                intravisit::walk_item(self, it);
-            }
-        }
-        fn visit_impl_item(&mut self, it: &'tcx ImplItem<'tcx>) {
-            trace!(?it.owner_id);
-            // The opaque type itself or its children are not within its reveal scope.
-            if it.owner_id.def_id != self.def_id {
-                self.check(it.owner_id.def_id);
-                intravisit::walk_impl_item(self, it);
-            }
-        }
-        fn visit_trait_item(&mut self, it: &'tcx TraitItem<'tcx>) {
-            trace!(?it.owner_id);
-            self.check(it.owner_id.def_id);
-            intravisit::walk_trait_item(self, it);
-        }
-    }
-
-    let concrete = tcx.mir_borrowck(owner_def_id).concrete_opaque_types.get(&def_id).copied();
-
-    if let Some(concrete) = concrete {
-        let scope = tcx.hir().local_def_id_to_hir_id(owner_def_id);
-        debug!(?scope);
-        let mut locator = ConstraintChecker { def_id, tcx, found: concrete };
-
-        match tcx.hir().get(scope) {
-            Node::Item(it) => intravisit::walk_item(&mut locator, it),
-            Node::ImplItem(it) => intravisit::walk_impl_item(&mut locator, it),
-            Node::TraitItem(it) => intravisit::walk_trait_item(&mut locator, it),
-            other => bug!("{:?} is not a valid scope for an opaque type item", other),
-        }
-    }
-
-    concrete.map(|concrete| concrete.ty).unwrap_or_else(|| {
-        let table = tcx.typeck(owner_def_id);
-        if let Some(guar) = table.tainted_by_errors {
-            // Some error in the
-            // owner fn prevented us from populating
-            // the `concrete_opaque_types` table.
-            tcx.ty_error(guar)
-        } else {
-            table.concrete_opaque_types.get(&def_id).map(|ty| ty.ty).unwrap_or_else(|| {
-                // We failed to resolve the opaque type or it
-                // resolves to itself. We interpret this as the
-                // no values of the hidden type ever being constructed,
-                // so we can just make the hidden type be `!`.
-                // For backwards compatibility reasons, we fall back to
-                // `()` until we the diverging default is changed.
-                tcx.mk_diverging_default()
-            })
-        }
-    })
-}
-
 fn infer_placeholder_type<'a>(
     tcx: TyCtxt<'a>,
     def_id: LocalDefId,
diff --git a/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs b/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
new file mode 100644
index 00000000000..f7c5b44678f
--- /dev/null
+++ b/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
@@ -0,0 +1,298 @@
+use rustc_hir::def_id::LocalDefId;
+use rustc_hir::intravisit::{self, Visitor};
+use rustc_hir::{self as hir, Expr, ImplItem, Item, Node, TraitItem};
+use rustc_middle::hir::nested_filter;
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
+use rustc_span::DUMMY_SP;
+
+use crate::errors::UnconstrainedOpaqueType;
+
+/// Checks "defining uses" of opaque `impl Trait` types to ensure that they meet the restrictions
+/// laid for "higher-order pattern unification".
+/// This ensures that inference is tractable.
+/// In particular, definitions of opaque types can only use other generics as arguments,
+/// and they cannot repeat an argument. Example:
+///
+/// ```ignore (illustrative)
+/// type Foo<A, B> = impl Bar<A, B>;
+///
+/// // Okay -- `Foo` is applied to two distinct, generic types.
+/// fn a<T, U>() -> Foo<T, U> { .. }
+///
+/// // Not okay -- `Foo` is applied to `T` twice.
+/// fn b<T>() -> Foo<T, T> { .. }
+///
+/// // Not okay -- `Foo` is applied to a non-generic type.
+/// fn b<T>() -> Foo<T, u32> { .. }
+/// ```
+#[instrument(skip(tcx), level = "debug")]
+pub(super) fn find_opaque_ty_constraints_for_tait(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
+    let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+    let scope = tcx.hir().get_defining_scope(hir_id);
+    let mut locator = TaitConstraintLocator { def_id, tcx, found: None, typeck_types: vec![] };
+
+    debug!(?scope);
+
+    if scope == hir::CRATE_HIR_ID {
+        tcx.hir().walk_toplevel_module(&mut locator);
+    } else {
+        trace!("scope={:#?}", tcx.hir().get(scope));
+        match tcx.hir().get(scope) {
+            // We explicitly call `visit_*` methods, instead of using `intravisit::walk_*` methods
+            // This allows our visitor to process the defining item itself, causing
+            // it to pick up any 'sibling' defining uses.
+            //
+            // For example, this code:
+            // ```
+            // fn foo() {
+            //     type Blah = impl Debug;
+            //     let my_closure = || -> Blah { true };
+            // }
+            // ```
+            //
+            // requires us to explicitly process `foo()` in order
+            // to notice the defining usage of `Blah`.
+            Node::Item(it) => locator.visit_item(it),
+            Node::ImplItem(it) => locator.visit_impl_item(it),
+            Node::TraitItem(it) => locator.visit_trait_item(it),
+            other => bug!("{:?} is not a valid scope for an opaque type item", other),
+        }
+    }
+
+    let Some(hidden) = locator.found else {
+        let reported = tcx.sess.emit_err(UnconstrainedOpaqueType {
+            span: tcx.def_span(def_id),
+            name: tcx.item_name(tcx.local_parent(def_id).to_def_id()),
+            what: match tcx.hir().get(scope) {
+                _ if scope == hir::CRATE_HIR_ID => "module",
+                Node::Item(hir::Item { kind: hir::ItemKind::Mod(_), .. }) => "module",
+                Node::Item(hir::Item { kind: hir::ItemKind::Impl(_), .. }) => "impl",
+                _ => "item",
+            },
+        });
+        return tcx.ty_error(reported);
+    };
+
+    // Only check against typeck if we didn't already error
+    if !hidden.ty.references_error() {
+        for concrete_type in locator.typeck_types {
+            if concrete_type.ty != tcx.erase_regions(hidden.ty)
+                && !(concrete_type, hidden).references_error()
+            {
+                hidden.report_mismatch(&concrete_type, def_id, tcx).emit();
+            }
+        }
+    }
+
+    hidden.ty
+}
+
+struct TaitConstraintLocator<'tcx> {
+    tcx: TyCtxt<'tcx>,
+
+    /// def_id of the opaque type whose defining uses are being checked
+    def_id: LocalDefId,
+
+    /// as we walk the defining uses, we are checking that all of them
+    /// define the same hidden type. This variable is set to `Some`
+    /// with the first type that we find, and then later types are
+    /// checked against it (we also carry the span of that first
+    /// type).
+    found: Option<ty::OpaqueHiddenType<'tcx>>,
+
+    /// In the presence of dead code, typeck may figure out a hidden type
+    /// while borrowck will not. We collect these cases here and check at
+    /// the end that we actually found a type that matches (modulo regions).
+    typeck_types: Vec<ty::OpaqueHiddenType<'tcx>>,
+}
+
+impl TaitConstraintLocator<'_> {
+    #[instrument(skip(self), level = "debug")]
+    fn check(&mut self, item_def_id: LocalDefId) {
+        // Don't try to check items that cannot possibly constrain the type.
+        if !self.tcx.has_typeck_results(item_def_id) {
+            debug!("no constraint: no typeck results");
+            return;
+        }
+        // Calling `mir_borrowck` can lead to cycle errors through
+        // const-checking, avoid calling it if we don't have to.
+        // ```rust
+        // type Foo = impl Fn() -> usize; // when computing type for this
+        // const fn bar() -> Foo {
+        //     || 0usize
+        // }
+        // const BAZR: Foo = bar(); // we would mir-borrowck this, causing cycles
+        // // because we again need to reveal `Foo` so we can check whether the
+        // // constant does not contain interior mutability.
+        // ```
+        let tables = self.tcx.typeck(item_def_id);
+        if let Some(guar) = tables.tainted_by_errors {
+            self.found = Some(ty::OpaqueHiddenType { span: DUMMY_SP, ty: self.tcx.ty_error(guar) });
+            return;
+        }
+        let Some(&typeck_hidden_ty) = tables.concrete_opaque_types.get(&self.def_id) else {
+            debug!("no constraints in typeck results");
+            return;
+        };
+        if self.typeck_types.iter().all(|prev| prev.ty != typeck_hidden_ty.ty) {
+            self.typeck_types.push(typeck_hidden_ty);
+        }
+
+        // Use borrowck to get the type with unerased regions.
+        let concrete_opaque_types = &self.tcx.mir_borrowck(item_def_id).concrete_opaque_types;
+        debug!(?concrete_opaque_types);
+        if let Some(&concrete_type) = concrete_opaque_types.get(&self.def_id) {
+            debug!(?concrete_type, "found constraint");
+            if let Some(prev) = &mut self.found {
+                if concrete_type.ty != prev.ty && !(concrete_type, prev.ty).references_error() {
+                    let guar = prev.report_mismatch(&concrete_type, self.def_id, self.tcx).emit();
+                    prev.ty = self.tcx.ty_error(guar);
+                }
+            } else {
+                self.found = Some(concrete_type);
+            }
+        }
+    }
+}
+
+impl<'tcx> intravisit::Visitor<'tcx> for TaitConstraintLocator<'tcx> {
+    type NestedFilter = nested_filter::All;
+
+    fn nested_visit_map(&mut self) -> Self::Map {
+        self.tcx.hir()
+    }
+    fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
+        if let hir::ExprKind::Closure(closure) = ex.kind {
+            self.check(closure.def_id);
+        }
+        intravisit::walk_expr(self, ex);
+    }
+    fn visit_item(&mut self, it: &'tcx Item<'tcx>) {
+        trace!(?it.owner_id);
+        // The opaque type itself or its children are not within its reveal scope.
+        if it.owner_id.def_id != self.def_id {
+            self.check(it.owner_id.def_id);
+            intravisit::walk_item(self, it);
+        }
+    }
+    fn visit_impl_item(&mut self, it: &'tcx ImplItem<'tcx>) {
+        trace!(?it.owner_id);
+        // The opaque type itself or its children are not within its reveal scope.
+        if it.owner_id.def_id != self.def_id {
+            self.check(it.owner_id.def_id);
+            intravisit::walk_impl_item(self, it);
+        }
+    }
+    fn visit_trait_item(&mut self, it: &'tcx TraitItem<'tcx>) {
+        trace!(?it.owner_id);
+        self.check(it.owner_id.def_id);
+        intravisit::walk_trait_item(self, it);
+    }
+}
+
+pub(super) fn find_opaque_ty_constraints_for_rpit(
+    tcx: TyCtxt<'_>,
+    def_id: LocalDefId,
+    owner_def_id: LocalDefId,
+) -> Ty<'_> {
+    let concrete = tcx.mir_borrowck(owner_def_id).concrete_opaque_types.get(&def_id).copied();
+
+    if let Some(concrete) = concrete {
+        let scope = tcx.hir().local_def_id_to_hir_id(owner_def_id);
+        debug!(?scope);
+        let mut locator = RpitConstraintChecker { def_id, tcx, found: concrete };
+
+        match tcx.hir().get(scope) {
+            Node::Item(it) => intravisit::walk_item(&mut locator, it),
+            Node::ImplItem(it) => intravisit::walk_impl_item(&mut locator, it),
+            Node::TraitItem(it) => intravisit::walk_trait_item(&mut locator, it),
+            other => bug!("{:?} is not a valid scope for an opaque type item", other),
+        }
+    }
+
+    concrete.map(|concrete| concrete.ty).unwrap_or_else(|| {
+        let table = tcx.typeck(owner_def_id);
+        if let Some(guar) = table.tainted_by_errors {
+            // Some error in the
+            // owner fn prevented us from populating
+            // the `concrete_opaque_types` table.
+            tcx.ty_error(guar)
+        } else {
+            table.concrete_opaque_types.get(&def_id).map(|ty| ty.ty).unwrap_or_else(|| {
+                // We failed to resolve the opaque type or it
+                // resolves to itself. We interpret this as the
+                // no values of the hidden type ever being constructed,
+                // so we can just make the hidden type be `!`.
+                // For backwards compatibility reasons, we fall back to
+                // `()` until we the diverging default is changed.
+                tcx.mk_diverging_default()
+            })
+        }
+    })
+}
+
+struct RpitConstraintChecker<'tcx> {
+    tcx: TyCtxt<'tcx>,
+
+    /// def_id of the opaque type whose defining uses are being checked
+    def_id: LocalDefId,
+
+    found: ty::OpaqueHiddenType<'tcx>,
+}
+
+impl RpitConstraintChecker<'_> {
+    #[instrument(skip(self), level = "debug")]
+    fn check(&self, def_id: LocalDefId) {
+        // Use borrowck to get the type with unerased regions.
+        let concrete_opaque_types = &self.tcx.mir_borrowck(def_id).concrete_opaque_types;
+        debug!(?concrete_opaque_types);
+        for (&def_id, &concrete_type) in concrete_opaque_types {
+            if def_id != self.def_id {
+                // Ignore constraints for other opaque types.
+                continue;
+            }
+
+            debug!(?concrete_type, "found constraint");
+
+            if concrete_type.ty != self.found.ty && !(concrete_type, self.found).references_error()
+            {
+                self.found.report_mismatch(&concrete_type, self.def_id, self.tcx).emit();
+            }
+        }
+    }
+}
+
+impl<'tcx> intravisit::Visitor<'tcx> for RpitConstraintChecker<'tcx> {
+    type NestedFilter = nested_filter::OnlyBodies;
+
+    fn nested_visit_map(&mut self) -> Self::Map {
+        self.tcx.hir()
+    }
+    fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
+        if let hir::ExprKind::Closure(closure) = ex.kind {
+            self.check(closure.def_id);
+        }
+        intravisit::walk_expr(self, ex);
+    }
+    fn visit_item(&mut self, it: &'tcx Item<'tcx>) {
+        trace!(?it.owner_id);
+        // The opaque type itself or its children are not within its reveal scope.
+        if it.owner_id.def_id != self.def_id {
+            self.check(it.owner_id.def_id);
+            intravisit::walk_item(self, it);
+        }
+    }
+    fn visit_impl_item(&mut self, it: &'tcx ImplItem<'tcx>) {
+        trace!(?it.owner_id);
+        // The opaque type itself or its children are not within its reveal scope.
+        if it.owner_id.def_id != self.def_id {
+            self.check(it.owner_id.def_id);
+            intravisit::walk_impl_item(self, it);
+        }
+    }
+    fn visit_trait_item(&mut self, it: &'tcx TraitItem<'tcx>) {
+        trace!(?it.owner_id);
+        self.check(it.owner_id.def_id);
+        intravisit::walk_trait_item(self, it);
+    }
+}