rust-lang · compiler-errors · Apr 5, 2025
diff --git a/compiler/rustc_hir_typeck/src/writeback.rs b/compiler/rustc_hir_typeck/src/writeback.rs
@@ -3,17 +3,21 @@
 // generic parameters.
 
 use std::mem;
+use std::ops::ControlFlow;
 
+use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_data_structures::unord::ExtendUnord;
 use rustc_errors::ErrorGuaranteed;
+use rustc_hir::def_id::LocalDefId;
 use rustc_hir::intravisit::{self, InferKind, Visitor};
 use rustc_hir::{self as hir, AmbigArg, HirId};
 use rustc_middle::span_bug;
 use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::adjustment::{Adjust, Adjustment, PointerCoercion};
 use rustc_middle::ty::{
-    self, DefiningScopeKind, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable,
-    TypeVisitableExt, fold_regions,
+    self, DefiningScopeKind, OpaqueHiddenType, Ty, TyCtxt, TypeFoldable, TypeFolder,
+    TypeSuperFoldable, TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor,
+    fold_regions,
 };
 use rustc_span::{Span, sym};
 use rustc_trait_selection::error_reporting::infer::need_type_info::TypeAnnotationNeeded;
@@ -622,6 +626,30 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
                 entry.span = prev.span.substitute_dummy(hidden_type.span);
             }
         }
+
+        let recursive_opaques: Vec<_> = self
+            .typeck_results
+            .concrete_opaque_types
+            .iter()
+            .filter(|&(&def_id, hidden_ty)| {
+                hidden_ty
+                    .ty
+                    .visit_with(&mut HasRecursiveOpaque {
+                        def_id,
+                        seen: Default::default(),
+                        opaques: &self.typeck_results.concrete_opaque_types,
+                        tcx,
+                    })
+                    .is_break()
+            })
+            .map(|(def_id, hidden_ty)| (*def_id, hidden_ty.span))
+            .collect();
+        for (def_id, span) in recursive_opaques {
+            let guar = self.fcx.dcx().span_err(span, "cannot resolve opaque type");
+            self.typeck_results
+                .concrete_opaque_types
+                .insert(def_id, OpaqueHiddenType { span, ty: Ty::new_error(tcx, guar) });
+        }
     }
 
     fn visit_field_id(&mut self, hir_id: HirId) {
@@ -948,3 +976,34 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for EagerlyNormalizeConsts<'tcx> {
         self.tcx.try_normalize_erasing_regions(self.typing_env, ct).unwrap_or(ct)
     }
 }
+
+struct HasRecursiveOpaque<'a, 'tcx> {
+    def_id: LocalDefId,
+    seen: FxHashSet<LocalDefId>,
+    opaques: &'a FxIndexMap<LocalDefId, ty::OpaqueHiddenType<'tcx>>,
+    tcx: TyCtxt<'tcx>,
+}
+
+impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for HasRecursiveOpaque<'_, 'tcx> {
+    type Result = ControlFlow<()>;
+
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> Self::Result {
+        if let ty::Alias(ty::Opaque, alias_ty) = *t.kind()
+            && let Some(def_id) = alias_ty.def_id.as_local()
+        {
+            if self.def_id == def_id {
+                return ControlFlow::Break(());
+            }
+
+            if self.seen.insert(def_id)
+                && let Some(hidden_ty) = self.opaques.get(&def_id)
+            {
+                ty::EarlyBinder::bind(hidden_ty.ty)
+                    .instantiate(self.tcx, alias_ty.args)
+                    .visit_with(self)?;
+            }
+        }
+
+        t.super_visit_with(self)
+    }
+}
diff --git a/tests/ui/impl-trait/issue-100075-2.stderr b/tests/ui/impl-trait/issue-100075-2.stderr
@@ -1,3 +1,9 @@
+error: cannot resolve opaque type
+  --> $DIR/issue-100075-2.rs:1:23
+   |
+LL | fn opaque<T>(t: T) -> impl Sized {
+   |                       ^^^^^^^^^^
+
 warning: function cannot return without recursing
   --> $DIR/issue-100075-2.rs:1:1
    |
@@ -10,15 +16,5 @@ LL |     opaque(Some(t))
    = help: a `loop` may express intention better if this is on purpose
    = note: `#[warn(unconditional_recursion)]` on by default
 
-error[E0720]: cannot resolve opaque type
-  --> $DIR/issue-100075-2.rs:1:23
-   |
-LL | fn opaque<T>(t: T) -> impl Sized {
-   |                       ^^^^^^^^^^ recursive opaque type
-...
-LL |     opaque(Some(t))
-   |     --------------- returning here with type `impl Sized`
-
 error: aborting due to 1 previous error; 1 warning emitted
 
-For more information about this error, try `rustc --explain E0720`.
diff --git a/tests/ui/impl-trait/issue-100075.stderr b/tests/ui/impl-trait/issue-100075.stderr
@@ -1,12 +1,8 @@
-error[E0720]: cannot resolve opaque type
+error: cannot resolve opaque type
   --> $DIR/issue-100075.rs:13:37
    |
 LL | fn _g<T>(t: &'static T) -> &'static impl Marker {
-   |                                     ^^^^^^^^^^^ recursive opaque type
-...
-LL |         return _g(t);
-   |                ----- returning here with type `&impl Marker`
+   |                                     ^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0720`.
diff --git a/tests/ui/impl-trait/issue-87450.stderr b/tests/ui/impl-trait/issue-87450.stderr
@@ -1,3 +1,9 @@
+error: cannot resolve opaque type
+  --> $DIR/issue-87450.rs:5:13
+   |
+LL | fn foo() -> impl Fn() {
+   |             ^^^^^^^^^
+
 warning: function cannot return without recursing
   --> $DIR/issue-87450.rs:5:1
    |
@@ -10,18 +16,5 @@ LL |     wrap(wrap(wrap(wrap(wrap(wrap(wrap(foo())))))))
    = help: a `loop` may express intention better if this is on purpose
    = note: `#[warn(unconditional_recursion)]` on by default
 
-error[E0720]: cannot resolve opaque type
-  --> $DIR/issue-87450.rs:5:13
-   |
-LL | fn foo() -> impl Fn() {
-   |             ^^^^^^^^^ recursive opaque type
-...
-LL |     wrap(wrap(wrap(wrap(wrap(wrap(wrap(foo())))))))
-   |     ----------------------------------------------- returning here with type `impl Fn()`
-...
-LL | fn wrap(f: impl Fn()) -> impl Fn() {
-   |                          --------- returning this opaque type `impl Fn()`
-
 error: aborting due to 1 previous error; 1 warning emitted
 
-For more information about this error, try `rustc --explain E0720`.
diff --git a/tests/ui/impl-trait/recursive-impl-trait-type-indirect.stderr b/tests/ui/impl-trait/recursive-impl-trait-type-indirect.stderr
@@ -1,113 +1,68 @@
-error[E0720]: cannot resolve opaque type
+error: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:6:22
    |
 LL | fn option(i: i32) -> impl Sized {
-   |                      ^^^^^^^^^^ recursive opaque type
-LL |
-LL |     if i < 0 { None } else { Some((option(i - 1), i)) }
-   |                ----          ------------------------ returning here with type `Option<(impl Sized, i32)>`
-   |                |
-   |                returning here with type `Option<(impl Sized, i32)>`
+   |                      ^^^^^^^^^^
 
-error[E0720]: cannot resolve opaque type
+error: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:11:15
    |
 LL | fn tuple() -> impl Sized {
-   |               ^^^^^^^^^^ recursive opaque type
-LL |
-LL |     (tuple(),)
-   |     ---------- returning here with type `(impl Sized,)`
+   |               ^^^^^^^^^^
 
-error[E0720]: cannot resolve opaque type
+error: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:16:15
    |
 LL | fn array() -> impl Sized {
-   |               ^^^^^^^^^^ recursive opaque type
-LL |
-LL |     [array()]
-   |     --------- returning here with type `[impl Sized; 1]`
+   |               ^^^^^^^^^^
 
-error[E0720]: cannot resolve opaque type
+error: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:21:13
    |
 LL | fn ptr() -> impl Sized {
-   |             ^^^^^^^^^^ recursive opaque type
-LL |
-LL |     &ptr() as *const _
-   |     ------------------ returning here with type `*const impl Sized`
+   |             ^^^^^^^^^^
 
-error[E0720]: cannot resolve opaque type
+error: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:26:16
    |
 LL | fn fn_ptr() -> impl Sized {
-   |                ^^^^^^^^^^ recursive opaque type
-LL |
-LL |     fn_ptr as fn() -> _
-   |     ------------------- returning here with type `fn() -> impl Sized`
+   |                ^^^^^^^^^^
 
-error[E0720]: cannot resolve opaque type
+error: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:31:25
    |
-LL |   fn closure_capture() -> impl Sized {
-   |                           ^^^^^^^^^^ recursive opaque type
-...
-LL | /     move || {
-LL | |         x;
-   | |         - closure captures itself here
-LL | |     }
-   | |_____- returning here with type `{closure@$DIR/recursive-impl-trait-type-indirect.rs:34:5: 34:12}`
+LL | fn closure_capture() -> impl Sized {
+   |                         ^^^^^^^^^^
 
-error[E0720]: cannot resolve opaque type
+error: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:39:29
    |
-LL |   fn closure_ref_capture() -> impl Sized {
-   |                               ^^^^^^^^^^ recursive opaque type
-...
-LL | /     move || {
-LL | |         &x;
-   | |          - closure captures itself here
-LL | |     }
-   | |_____- returning here with type `{closure@$DIR/recursive-impl-trait-type-indirect.rs:42:5: 42:12}`
+LL | fn closure_ref_capture() -> impl Sized {
+   |                             ^^^^^^^^^^
 
-error[E0720]: cannot resolve opaque type
+error: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:47:21
    |
 LL | fn closure_sig() -> impl Sized {
-   |                     ^^^^^^^^^^ recursive opaque type
-LL |
-LL |     || closure_sig()
-   |     ---------------- returning here with type `{closure@$DIR/recursive-impl-trait-type-indirect.rs:49:5: 49:7}`
+   |                     ^^^^^^^^^^
 
-error[E0720]: cannot resolve opaque type
+error: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:52:23
    |
 LL | fn coroutine_sig() -> impl Sized {
-   |                       ^^^^^^^^^^ recursive opaque type
-LL |
-LL |     || coroutine_sig()
-   |     ------------------ returning here with type `{closure@$DIR/recursive-impl-trait-type-indirect.rs:54:5: 54:7}`
+   |                       ^^^^^^^^^^
 
-error[E0720]: cannot resolve opaque type
+error: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:57:27
    |
-LL |   fn coroutine_capture() -> impl Sized {
-   |                             ^^^^^^^^^^ recursive opaque type
-...
-LL | /     move || {
-LL | |         yield;
-LL | |         x;
-   | |         - coroutine captures itself here
-LL | |     }
-   | |_____- returning here with type `{coroutine@$DIR/recursive-impl-trait-type-indirect.rs:62:5: 62:12}`
+LL | fn coroutine_capture() -> impl Sized {
+   |                           ^^^^^^^^^^
 
-error[E0720]: cannot resolve opaque type
+error: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:68:35
    |
 LL | fn substs_change<T: 'static>() -> impl Sized {
-   |                                   ^^^^^^^^^^ recursive opaque type
-LL |
-LL |     (substs_change::<&T>(),)
-   |     ------------------------ returning here with type `(impl Sized,)`
+   |                                   ^^^^^^^^^^
 
 error[E0720]: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:78:26

diff --git a/tests/ui/type-alias-impl-trait/infinite-cycle-involving-weak.stderr b/tests/ui/type-alias-impl-trait/infinite-cycle-involving-weak.stderr
@@ -1,9 +1,8 @@
-error[E0720]: cannot resolve opaque type
-  --> $DIR/infinite-cycle-involving-weak.rs:3:10
+error: cannot resolve opaque type
+  --> $DIR/infinite-cycle-involving-weak.rs:7:13
    |
-LL | type T = impl Copy;
-   |          ^^^^^^^^^ cannot resolve opaque type
+LL | fn foo() -> T {
+   |             ^
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0720`.
diff --git a/tests/ui/type-alias-impl-trait/recursive-fn-tait.stderr b/tests/ui/type-alias-impl-trait/recursive-fn-tait.stderr
@@ -1,14 +1,8 @@
-error: concrete type differs from previous defining opaque type use
+error: cannot resolve opaque type
   --> $DIR/recursive-fn-tait.rs:15:6
    |
 LL | ) -> Diff {
-   |      ^^^^ expected `{closure@$DIR/recursive-fn-tait.rs:8:5: 8:16}`, got `{closure@$DIR/recursive-fn-tait.rs:17:5: 17:20}`
-   |
-note: previous use here
-  --> $DIR/recursive-fn-tait.rs:7:18
-   |
-LL | pub fn lift() -> Diff {
-   |                  ^^^^
+   |      ^^^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/type-alias-impl-trait/recursive-tait-conflicting-defn-2.stderr b/tests/ui/type-alias-impl-trait/recursive-tait-conflicting-defn-2.stderr
@@ -1,13 +1,7 @@
-error: concrete type differs from previous defining opaque type use
+error: cannot resolve opaque type
   --> $DIR/recursive-tait-conflicting-defn-2.rs:15:13
    |
 LL | fn bad() -> Op {
-   |             ^^ expected `&&str`, got `impl std::fmt::Display`
-   |
-note: previous use here
-  --> $DIR/recursive-tait-conflicting-defn-2.rs:7:13
-   |
-LL | fn foo() -> Op {
    |             ^^
 
 error: aborting due to 1 previous error

diff --git a/tests/ui/type-alias-impl-trait/recursive-tait-conflicting-defn.stderr b/tests/ui/type-alias-impl-trait/recursive-tait-conflicting-defn.stderr
@@ -1,14 +1,8 @@
-error: concrete type differs from previous defining opaque type use
+error: cannot resolve opaque type
   --> $DIR/recursive-tait-conflicting-defn.rs:25:22
    |
 LL | fn make_option2() -> Option<TestImpl> {
-   |                      ^^^^^^^^^^^^^^^^ expected `A`, got `B<TestImpl>`
-   |
-note: previous use here
-  --> $DIR/recursive-tait-conflicting-defn.rs:20:18
-   |
-LL | pub fn test() -> TestImpl {
-   |                  ^^^^^^^^
+   |                      ^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error