Deduplicate diagnostics for const trait supertraits

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -1442,6 +1442,31 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         self.can_eq(param_env, goal.trait_ref, trait_assumption.trait_ref)
     }
 
+    fn can_match_host_effect(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        goal: ty::HostEffectPredicate<'tcx>,
+        assumption: ty::Binder<'tcx, ty::HostEffectPredicate<'tcx>>,
+    ) -> bool {
+        let assumption = self.instantiate_binder_with_fresh_vars(
+            DUMMY_SP,
+            infer::BoundRegionConversionTime::HigherRankedType,
+            assumption,
+        );
+
+        assumption.constness.satisfies(goal.constness)
+            && self.can_eq(param_env, goal.trait_ref, assumption.trait_ref)
+    }
+
+    fn as_host_effect_clause(
+        predicate: ty::Predicate<'tcx>,
+    ) -> Option<ty::Binder<'tcx, ty::HostEffectPredicate<'tcx>>> {
+        predicate.as_clause().and_then(|clause| match clause.kind().skip_binder() {
+            ty::ClauseKind::HostEffect(pred) => Some(clause.kind().rebind(pred)),
+            _ => None,
+        })
+    }
+
     fn can_match_projection(
         &self,
         param_env: ty::ParamEnv<'tcx>,
@@ -1484,6 +1509,12 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     .filter_map(|implied| implied.as_trait_clause())
                     .any(|implied| self.can_match_trait(param_env, error, implied))
             })
+        } else if let Some(error) = Self::as_host_effect_clause(error.predicate) {
+            self.enter_forall(error, |error| {
+                elaborate(self.tcx, std::iter::once(cond.predicate))
+                    .filter_map(Self::as_host_effect_clause)
+                    .any(|implied| self.can_match_host_effect(param_env, error, implied))
+            })
         } else if let Some(error) = error.predicate.as_projection_clause() {
             self.enter_forall(error, |error| {
                 elaborate(self.tcx, std::iter::once(cond.predicate))

tests/ui/consts/issue-94675.rs

@@ -10,7 +10,6 @@ impl<'a> Foo<'a> {
     const fn spam(&mut self, baz: &mut Vec<u32>) {
         self.bar[0] = baz.len();
         //~^ ERROR: `Vec<usize>: [const] Index<_>` is not satisfied
-        //~| ERROR: `Vec<usize>: [const] Index<usize>` is not satisfied
         //~| ERROR: `Vec<usize>: [const] IndexMut<usize>` is not satisfied
     }
 }

tests/ui/consts/issue-94675.stderr

@@ -16,17 +16,6 @@ LL |         self.bar[0] = baz.len();
 note: trait `IndexMut` is implemented but not `const`
   --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 
-error[E0277]: the trait bound `Vec<usize>: [const] Index<usize>` is not satisfied
-  --> $DIR/issue-94675.rs:11:9
-   |
-LL |         self.bar[0] = baz.len();
-   |         ^^^^^^^^^^^
-   |
-note: trait `Index` is implemented but not `const`
-  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
-note: required by a bound in `std::ops::IndexMut::index_mut`
-  --> $SRC_DIR/core/src/ops/index.rs:LL:COL
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/const-traits/call.rs

@@ -6,7 +6,6 @@
 const _: () = {
     assert!((const || true)());
     //~^ ERROR }: [const] Fn()` is not satisfied
-    //~| ERROR }: [const] FnMut()` is not satisfied
 };
 
 fn main() {}

tests/ui/traits/const-traits/call.stderr

@@ -4,15 +4,6 @@ error[E0277]: the trait bound `{closure@$DIR/call.rs:7:14: 7:22}: [const] Fn()`
 LL |     assert!((const || true)());
    |             ^^^^^^^^^^^^^^^^^
 
-error[E0277]: the trait bound `{closure@$DIR/call.rs:7:14: 7:22}: [const] FnMut()` is not satisfied
-  --> $DIR/call.rs:7:13
-   |
-LL |     assert!((const || true)());
-   |             ^^^^^^^^^^^^^^^^^
-   |
-note: required by a bound in `call`
-  --> $SRC_DIR/core/src/ops/function.rs:LL:COL
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/const-traits/const_closure-const_trait_impl-ice-113381.stderr

@@ -4,15 +4,6 @@ error[E0277]: the trait bound `{closure@$DIR/const_closure-const_trait_impl-ice-
 LL |     (const || (()).foo())();
    |     ^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0277]: the trait bound `{closure@$DIR/const_closure-const_trait_impl-ice-113381.rs:15:6: 15:14}: [const] FnMut()` is not satisfied
-  --> $DIR/const_closure-const_trait_impl-ice-113381.rs:15:5
-   |
-LL |     (const || (()).foo())();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: required by a bound in `call`
-  --> $SRC_DIR/core/src/ops/function.rs:LL:COL
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/const-traits/non-const-op-const-closure-non-const-outer.rs

@@ -10,7 +10,8 @@ impl Foo for () {
 }
 
 fn main() {
+    // #150052 deduplicate diagnostics for const trait supertraits
+    // so we only get one error here
     (const || { (()).foo() })();
     //~^ ERROR: }: [const] Fn()` is not satisfied
-    //~| ERROR: }: [const] FnMut()` is not satisfied
 }

tests/ui/traits/const-traits/non-const-op-const-closure-non-const-outer.stderr

@@ -1,18 +1,9 @@
-error[E0277]: the trait bound `{closure@$DIR/non-const-op-const-closure-non-const-outer.rs:13:6: 13:14}: [const] Fn()` is not satisfied
-  --> $DIR/non-const-op-const-closure-non-const-outer.rs:13:5
+error[E0277]: the trait bound `{closure@$DIR/non-const-op-const-closure-non-const-outer.rs:15:6: 15:14}: [const] Fn()` is not satisfied
+  --> $DIR/non-const-op-const-closure-non-const-outer.rs:15:5
    |
 LL |     (const || { (()).foo() })();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0277]: the trait bound `{closure@$DIR/non-const-op-const-closure-non-const-outer.rs:13:6: 13:14}: [const] FnMut()` is not satisfied
-  --> $DIR/non-const-op-const-closure-non-const-outer.rs:13:5
-   |
-LL |     (const || { (()).foo() })();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: required by a bound in `call`
-  --> $SRC_DIR/core/src/ops/function.rs:LL:COL
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0277`.