rust-lang · rust-bors · Aug 19, 2026 · Aug 14, 2026 · Aug 18, 2026
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
@@ -1593,6 +1593,45 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         }
     }
 
+    /// Whether `error`, a projection goal, only failed because the trait goal it rests on
+    /// did: `<T as Trait>::Assoc == U` cannot hold when `T: Trait` doesn't, so an error on
+    /// the latter says everything the former would.
+    pub(super) fn trait_error_implies_projection_error(
+        &self,
+        cond: Goal<'tcx, ty::Predicate<'tcx>>,
+        error: Goal<'tcx, ty::Predicate<'tcx>>,
+    ) -> bool {
+        if cond.param_env != error.param_env {
+            return false;
+        }
+        let Some(error) = error.predicate.as_projection_clause() else {
+            return false;
+        };
+
+        self.enter_forall(error, |error| {
+            if !error.projection_term.kind.is_trait_projection() {
+                return false;
+            }
+            let trait_pred = ty::TraitPredicate {
+                trait_ref: error.projection_term.trait_ref(self.tcx),
+                polarity: ty::PredicatePolarity::Positive,
+            };
+            // Elaborating is what pairs a failing `C: FnMut(..)` with the
+            // `<C as FnOnce<..>>::Output` projection resting on it. A supertrait can hold
+            // while `cond` fails though, so the projection is only covered if its own trait
+            // goal is unproven too, otherwise it failed for its own reasons.
+            elaborate(self.tcx, std::iter::once(cond.predicate))
+                .filter_map(|implied| implied.as_trait_clause())
+                .any(|implied| self.can_match_trait(cond.param_env, trait_pred, implied))
+                && !self.predicate_must_hold_modulo_regions(&Obligation::new(
+                    self.tcx,
+                    ObligationCause::dummy(),
+                    cond.param_env,
+                    trait_pred,
+                ))
+        })
+    }
+
     #[instrument(level = "debug", skip_all)]
     pub(super) fn report_projection_error(
         &self,

diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
@@ -224,6 +224,21 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         // We do this in 2 passes because we want to display errors in order, though
         // maybe it *is* better to sort errors by span or something.
         let mut is_suppressed = vec![false; errors.len()];
+        // A failing trait goal also fails every projection goal resting on it. Ambiguity
+        // errors are exempt: they get merged into a single diagnostic whose notes list all
+        // the constraints the annotation has to satisfy, so their projections still say
+        // something the trait error doesn't.
+        let covered_by_trait_error =
+            |cond: &ErrorDescriptor<'tcx>, error: &ErrorDescriptor<'tcx>| {
+                let is_definite = |error: &ErrorDescriptor<'tcx>| {
+                    error.index.is_some_and(|index| {
+                        !matches!(errors[index].code, FulfillmentErrorCode::Ambiguity { .. })
+                    })
+                };
+                is_definite(cond)
+                    && is_definite(error)
+                    && self.trait_error_implies_projection_error(cond.goal, error.goal)
+            };
         for (_, error_set) in error_map.iter() {
             // We want to suppress "duplicate" errors with the same span.
             for error in error_set {
@@ -239,9 +254,10 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             continue;
                         }
 
-                        if self.error_implies(error2.goal, error.goal)
+                        if (self.error_implies(error2.goal, error.goal)
                             && !(error2.index >= error.index
-                                && self.error_implies(error.goal, error2.goal))
+                                && self.error_implies(error.goal, error2.goal)))
+                            || covered_by_trait_error(error2, error)
                         {
                             info!("skipping {:?} (implied by {:?})", error, error2);
                             is_suppressed[index] = true;

diff --git a/tests/ui/mismatched_types/closure-arg-type-mismatch-issue-45727.current.fixed b/tests/ui/mismatched_types/closure-arg-type-mismatch-issue-45727.current.fixed
@@ -6,9 +6,7 @@ fn main() {
     let _ = (-10..=10).find(|x: &i32| x.signum() == 0);
     //[current]~^ ERROR type mismatch in closure arguments
     //[next]~^^ ERROR: expected an `FnMut(&{integer})` closure, found
-    //[next]~| ERROR: type mismatch resolving `<{closure@closure-arg-type-mismatch-issue-45727.rs:6:29} as FnOnce<(&{integer},)>>::Output == bool`
     let _ = (-10..=10).find(|x: &i32| x.signum() == 0);
     //[current]~^ ERROR type mismatch in closure arguments
     //[next]~^^ ERROR: expected an `FnMut(&{integer})` closure, found
-    //[next]~| ERROR: type mismatch resolving `<{closure@closure-arg-type-mismatch-issue-45727.rs:10:29} as FnOnce<(&{integer},)>>::Output == bool`
 }
diff --git a/tests/ui/mismatched_types/closure-arg-type-mismatch-issue-45727.current.stderr b/tests/ui/mismatched_types/closure-arg-type-mismatch-issue-45727.current.stderr
@@ -16,7 +16,7 @@ LL |     let _ = (-10..=10).find(|x: &i32| x.signum() == 0);
    |                                 +
 
 error[E0631]: type mismatch in closure arguments
-  --> $DIR/closure-arg-type-mismatch-issue-45727.rs:10:24
+  --> $DIR/closure-arg-type-mismatch-issue-45727.rs:9:24
    |
 LL |     let _ = (-10..=10).find(|x: &&&i32| x.signum() == 0);
    |                        ^^^^ ----------- found signature defined here

diff --git a/tests/ui/mismatched_types/closure-arg-type-mismatch-issue-45727.next.stderr b/tests/ui/mismatched_types/closure-arg-type-mismatch-issue-45727.next.stderr
@@ -12,45 +12,20 @@ LL |     let _ = (-10..=10).find(|x: i32| x.signum() == 0);
 note: required by a bound in `find`
   --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
 
-error[E0271]: type mismatch resolving `<{closure@closure-arg-type-mismatch-issue-45727.rs:6:29} as FnOnce<(&{integer},)>>::Output == bool`
-  --> $DIR/closure-arg-type-mismatch-issue-45727.rs:6:38
-   |
-LL |     let _ = (-10..=10).find(|x: i32| x.signum() == 0);
-   |                        ---- -------- ^^^^^^^^^^^^^^^ types differ
-   |                        |    |
-   |                        |    this closure
-   |                        required by a bound introduced by this call
-   |
-note: required by a bound in `find`
-  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
-
-error[E0277]: expected an `FnMut(&{integer})` closure, found `{closure@$DIR/closure-arg-type-mismatch-issue-45727.rs:10:29: 10:40}`
-  --> $DIR/closure-arg-type-mismatch-issue-45727.rs:10:29
+error[E0277]: expected an `FnMut(&{integer})` closure, found `{closure@$DIR/closure-arg-type-mismatch-issue-45727.rs:9:29: 9:40}`
+  --> $DIR/closure-arg-type-mismatch-issue-45727.rs:9:29
    |
 LL |     let _ = (-10..=10).find(|x: &&&i32| x.signum() == 0);
-   |                        ---- ^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an `FnMut(&{integer})` closure, found `{closure@$DIR/closure-arg-type-mismatch-issue-45727.rs:10:29: 10:40}`
+   |                        ---- ^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an `FnMut(&{integer})` closure, found `{closure@$DIR/closure-arg-type-mismatch-issue-45727.rs:9:29: 9:40}`
    |                        |
    |                        required by a bound introduced by this call
    |
-   = help: the trait `for<'a> FnMut(&'a {integer})` is not implemented for closure `{closure@$DIR/closure-arg-type-mismatch-issue-45727.rs:10:29: 10:40}`
+   = help: the trait `for<'a> FnMut(&'a {integer})` is not implemented for closure `{closure@$DIR/closure-arg-type-mismatch-issue-45727.rs:9:29: 9:40}`
    = note: expected a closure with signature `for<'a> fn(&'a {integer})`
               found a closure with signature `fn(&&&i32)`
 note: required by a bound in `find`
   --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
 
-error[E0271]: type mismatch resolving `<{closure@closure-arg-type-mismatch-issue-45727.rs:10:29} as FnOnce<(&{integer},)>>::Output == bool`
-  --> $DIR/closure-arg-type-mismatch-issue-45727.rs:10:41
-   |
-LL |     let _ = (-10..=10).find(|x: &&&i32| x.signum() == 0);
-   |                        ---- ----------- ^^^^^^^^^^^^^^^ types differ
-   |                        |    |
-   |                        |    this closure
-   |                        required by a bound introduced by this call
-   |
-note: required by a bound in `find`
-  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
-
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0271, E0277.
-For more information about an error, try `rustc --explain E0271`.
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/mismatched_types/closure-arg-type-mismatch-issue-45727.rs b/tests/ui/mismatched_types/closure-arg-type-mismatch-issue-45727.rs
@@ -6,9 +6,7 @@ fn main() {
     let _ = (-10..=10).find(|x: i32| x.signum() == 0);
     //[current]~^ ERROR type mismatch in closure arguments
     //[next]~^^ ERROR: expected an `FnMut(&{integer})` closure, found
-    //[next]~| ERROR: type mismatch resolving `<{closure@closure-arg-type-mismatch-issue-45727.rs:6:29} as FnOnce<(&{integer},)>>::Output == bool`
     let _ = (-10..=10).find(|x: &&&i32| x.signum() == 0);
     //[current]~^ ERROR type mismatch in closure arguments
     //[next]~^^ ERROR: expected an `FnMut(&{integer})` closure, found
-    //[next]~| ERROR: type mismatch resolving `<{closure@closure-arg-type-mismatch-issue-45727.rs:10:29} as FnOnce<(&{integer},)>>::Output == bool`
 }
diff --git a/tests/ui/traits/next-solver/diagnostics/const-host-effect-hrtb-no-ice.rs b/tests/ui/traits/next-solver/diagnostics/const-host-effect-hrtb-no-ice.rs
@@ -8,7 +8,6 @@ const fn with_positive<F: for<'a> [const] Fn(&'a ())>() {}
 const _: () = {
     with_positive::<()>();
     //~^ ERROR expected an `Fn(&'a ())` closure, found `()`
-    //~| ERROR type mismatch resolving `<() as FnOnce<(&(),)>>::Output == ()`
 };
 
 fn main() {}
diff --git a/tests/ui/traits/next-solver/diagnostics/const-host-effect-hrtb-no-ice.stderr b/tests/ui/traits/next-solver/diagnostics/const-host-effect-hrtb-no-ice.stderr
@@ -11,19 +11,6 @@ note: required by a bound in `with_positive`
 LL | const fn with_positive<F: for<'a> [const] Fn(&'a ())>() {}
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `with_positive`
 
-error[E0271]: type mismatch resolving `<() as FnOnce<(&(),)>>::Output == ()`
-  --> $DIR/const-host-effect-hrtb-no-ice.rs:9:21
-   |
-LL |     with_positive::<()>();
-   |                     ^^ types differ
-   |
-note: required by a bound in `with_positive`
-  --> $DIR/const-host-effect-hrtb-no-ice.rs:6:43
-   |
-LL | const fn with_positive<F: for<'a> [const] Fn(&'a ())>() {}
-   |                                           ^^^^^^^^^^ required by this bound in `with_positive`
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0271, E0277.
-For more information about an error, try `rustc --explain E0271`.
+For more information about this error, try `rustc --explain E0277`.
diff --git a/.../traits/next-solver/diagnostics/dont-suppress-independent-projection-error.current.stderr b/.../traits/next-solver/diagnostics/dont-suppress-independent-projection-error.current.stderr
@@ -0,0 +1,53 @@
+error[E0277]: the trait bound `S: Sub` is not satisfied
+  --> $DIR/dont-suppress-independent-projection-error.rs:28:7
+   |
+LL |     f(S);
+   |     - ^ unsatisfied trait bound
+   |     |
+   |     required by a bound introduced by this call
+   |
+help: the trait `Sub` is not implemented for `S`
+  --> $DIR/dont-suppress-independent-projection-error.rs:14:1
+   |
+LL | struct S;
+   | ^^^^^^^^
+help: this trait has no implementations, consider adding one
+  --> $DIR/dont-suppress-independent-projection-error.rs:12:1
+   |
+LL | trait Sub: Super {}
+   | ^^^^^^^^^^^^^^^^
+note: required by a bound in `f`
+  --> $DIR/dont-suppress-independent-projection-error.rs:22:8
+   |
+LL | fn f<T>(_: T)
+   |    - required by a bound in this function
+LL | where
+LL |     T: Sub,
+   |        ^^^ required by this bound in `f`
+
+error[E0271]: type mismatch resolving `<S as Super>::Assoc == u32`
+  --> $DIR/dont-suppress-independent-projection-error.rs:28:7
+   |
+LL |     f(S);
+   |     - ^ type mismatch resolving `<S as Super>::Assoc == u32`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: expected this to be `u32`
+  --> $DIR/dont-suppress-independent-projection-error.rs:17:18
+   |
+LL |     type Assoc = u8;
+   |                  ^^
+note: required by a bound in `f`
+  --> $DIR/dont-suppress-independent-projection-error.rs:23:14
+   |
+LL | fn f<T>(_: T)
+   |    - required by a bound in this function
+...
+LL |     T: Super<Assoc = u32>,
+   |              ^^^^^^^^^^^ required by this bound in `f`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0271, E0277.
+For more information about an error, try `rustc --explain E0271`.
diff --git a/.../ui/traits/next-solver/diagnostics/dont-suppress-independent-projection-error.next.stderr b/.../ui/traits/next-solver/diagnostics/dont-suppress-independent-projection-error.next.stderr
@@ -0,0 +1,53 @@
+error[E0277]: the trait bound `S: Sub` is not satisfied
+  --> $DIR/dont-suppress-independent-projection-error.rs:28:7
+   |
+LL |     f(S);
+   |     - ^ unsatisfied trait bound
+   |     |
+   |     required by a bound introduced by this call
+   |
+help: the trait `Sub` is not implemented for `S`
+  --> $DIR/dont-suppress-independent-projection-error.rs:14:1
+   |
+LL | struct S;
+   | ^^^^^^^^
+help: this trait has no implementations, consider adding one
+  --> $DIR/dont-suppress-independent-projection-error.rs:12:1
+   |
+LL | trait Sub: Super {}
+   | ^^^^^^^^^^^^^^^^
+note: required by a bound in `f`
+  --> $DIR/dont-suppress-independent-projection-error.rs:22:8
+   |
+LL | fn f<T>(_: T)
+   |    - required by a bound in this function
+LL | where
+LL |     T: Sub,
+   |        ^^^ required by this bound in `f`
+
+error[E0271]: type mismatch resolving `<S as Super>::Assoc == u32`
+  --> $DIR/dont-suppress-independent-projection-error.rs:28:7
+   |
+LL |     f(S);
+   |     - ^ type mismatch resolving `<S as Super>::Assoc == u32`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: expected this to be `u32`
+  --> $DIR/dont-suppress-independent-projection-error.rs:17:18
+   |
+LL |     type Assoc = u8;
+   |                  ^^
+note: required by a bound in `f`
+  --> $DIR/dont-suppress-independent-projection-error.rs:23:14
+   |
+LL | fn f<T>(_: T)
+   |    - required by a bound in this function
+...
+LL |     T: Super<Assoc = u32>,
+   |              ^^^^^^^^^^^ required by this bound in `f`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0271, E0277.
+For more information about an error, try `rustc --explain E0271`.
diff --git a/tests/ui/traits/next-solver/diagnostics/dont-suppress-independent-projection-error.rs b/tests/ui/traits/next-solver/diagnostics/dont-suppress-independent-projection-error.rs
@@ -0,0 +1,31 @@
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
+
+// A projection error is only redundant when the trait goal it rests on failed too.
+// Here `S: Super` holds, so `<S as Super>::Assoc == u32` failed on its own and has to
+// be reported even though the `S: Sub` error at the same span elaborates to `S: Super`.
+
+trait Super {
+    type Assoc;
+}
+trait Sub: Super {}
+
+struct S;
+
+impl Super for S {
+    type Assoc = u8;
+}
+
+fn f<T>(_: T)
+where
+    T: Sub,
+    T: Super<Assoc = u32>,
+{
+}
+
+fn main() {
+    f(S);
+    //~^ ERROR the trait bound `S: Sub` is not satisfied
+    //~| ERROR type mismatch resolving `<S as Super>::Assoc == u32`
+}
diff --git a/tests/ui/traits/next-solver/diagnostics/iterator-item-suggest-no-ice.rs b/tests/ui/traits/next-solver/diagnostics/iterator-item-suggest-no-ice.rs
@@ -15,7 +15,6 @@ trait FooMut {
         //~^ ERROR: cannot find value `_I` in this scope
         self.bar(collection);
         //~^ ERROR: `&'b _` is not an iterator
-        //~| ERROR: type mismatch resolving `<&_ as Iterator>::Item == &()`
     }
 }
 

diff --git a/tests/ui/traits/next-solver/diagnostics/iterator-item-suggest-no-ice.stderr b/tests/ui/traits/next-solver/diagnostics/iterator-item-suggest-no-ice.stderr
@@ -40,32 +40,7 @@ LL |     where
 LL |         for<'b> &'b I: Iterator<Item = &'b ()>;
    |                        ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `FooMut::bar`
 
-error[E0271]: type mismatch resolving `<&_ as Iterator>::Item == &()`
-  --> $DIR/iterator-item-suggest-no-ice.rs:16:18
-   |
-LL |         self.bar(collection);
-   |              --- ^^^^^^^^^^ types differ
-   |              |
-   |              required by a bound introduced by this call
-   |
-note: the method call chain might not have had the expected associated types
-  --> $DIR/iterator-item-suggest-no-ice.rs:14:35
-   |
-LL |         let collection = vec![_I].iter().map(|x| ());
-   |                          -------- ^^^^^^ ----------- `Iterator::Item` remains `<{type error} as Iterator>::Item` here
-   |                          |        |
-   |                          |        `Iterator::Item` is `<{type error} as Iterator>::Item` here
-   |                          this expression has type `Vec<{type error}>`
-note: required by a bound in `FooMut::bar`
-  --> $DIR/iterator-item-suggest-no-ice.rs:7:33
-   |
-LL |     fn bar<I>(&self, _: I)
-   |        --- required by a bound in this associated function
-LL |     where
-LL |         for<'b> &'b I: Iterator<Item = &'b ()>;
-   |                                 ^^^^^^^^^^^^^ required by this bound in `FooMut::bar`
-
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0271, E0277, E0425, E0428.
-For more information about an error, try `rustc --explain E0271`.
+Some errors have detailed explanations: E0277, E0425, E0428.
+For more information about an error, try `rustc --explain E0277`.