Note binding obligation causes for const equate errors

compiler/rustc_middle/src/traits/mod.rs

@@ -469,6 +469,13 @@ impl<'tcx> ObligationCauseCode<'tcx> {
             _ => None,
         }
     }
+
+    pub fn peel_match_impls(&self) -> &Self {
+        match self {
+            MatchImpl(cause, _) => cause.code(),
+            _ => self,
+        }
+    }
 }
 
 // `ObligationCauseCode` is used a lot. Make sure it doesn't unintentionally get bigger.

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

@@ -1506,13 +1506,26 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 .emit();
             }
             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
-                self.report_mismatched_consts(
+                let mut diag = self.report_mismatched_consts(
                     &error.obligation.cause,
                     expected_found.expected,
                     expected_found.found,
                     err.clone(),
-                )
+                );
-                .emit();
+                let code = error.obligation.cause.code().peel_derives().peel_match_impls();
+                if let ObligationCauseCode::BindingObligation(..)
+                | ObligationCauseCode::ItemObligation(..) = code
+                {
+                    self.note_obligation_cause_code(
+                        &mut diag,
+                        &error.obligation.predicate,
+                        error.obligation.param_env,
+                        code,
+                        &mut vec![],
+                        &mut Default::default(),
+                    );
+                }
+                diag.emit();
             }
         }
     }

src/test/ui/const-generics/generic_const_exprs/abstract-const-as-cast-3.stderr

@@ -24,6 +24,11 @@ LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
    |
    = note: expected constant `{ N as u128 }`
               found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error: unconstrained generic constant
   --> $DIR/abstract-const-as-cast-3.rs:20:19
@@ -51,6 +56,11 @@ LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
    |
    = note: expected constant `{ N as _ }`
               found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:23:5
@@ -60,6 +70,11 @@ LL |     assert_impl::<HasCastInTraitImpl<13, { 12 as u128 }>>();
    |
    = note: expected constant `12`
               found constant `13`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:25:5
@@ -69,6 +84,11 @@ LL |     assert_impl::<HasCastInTraitImpl<14, 13>>();
    |
    = note: expected constant `13`
               found constant `14`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error: unconstrained generic constant
   --> $DIR/abstract-const-as-cast-3.rs:35:19
@@ -96,6 +116,11 @@ LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
    |
    = note: expected constant `{ N as u128 }`
               found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error: unconstrained generic constant
   --> $DIR/abstract-const-as-cast-3.rs:38:19
@@ -123,6 +148,11 @@ LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
    |
    = note: expected constant `{ N as _ }`
               found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:41:5
@@ -132,6 +162,11 @@ LL |     assert_impl::<HasCastInTraitImpl<13, { 12 as u128 }>>();
    |
    = note: expected constant `12`
               found constant `13`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:43:5
@@ -141,6 +176,11 @@ LL |     assert_impl::<HasCastInTraitImpl<14, 13>>();
    |
    = note: expected constant `13`
               found constant `14`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error: aborting due to 12 previous errors
 

src/test/ui/const-generics/generic_const_exprs/issue-72819-generic-in-const-eval.full.stderr

@@ -6,6 +6,13 @@ LL |     let x: Arr<{usize::MAX}> = Arr {};
    |
    = note: expected constant `false`
               found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-72819-generic-in-const-eval.rs:8:39
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                       ^^^^^^ required by this bound in `Arr`
 
 error[E0308]: mismatched types
   --> $DIR/issue-72819-generic-in-const-eval.rs:20:32
@@ -15,6 +22,13 @@ LL |     let x: Arr<{usize::MAX}> = Arr {};
    |
    = note: expected constant `false`
               found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-72819-generic-in-const-eval.rs:8:39
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                       ^^^^^^ required by this bound in `Arr`
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-generics/issues/issue-73260.stderr

@@ -6,6 +6,14 @@ LL |     let x: Arr<{usize::MAX}> = Arr {};
    |
    = note: expected constant `false`
               found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-73260.rs:6:37
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where
+LL |     Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                     ^^^^^^ required by this bound in `Arr`
 
 error[E0308]: mismatched types
   --> $DIR/issue-73260.rs:16:32
@@ -15,6 +23,14 @@ LL |     let x: Arr<{usize::MAX}> = Arr {};
    |
    = note: expected constant `false`
               found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-73260.rs:6:37
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where
+LL |     Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                     ^^^^^^ required by this bound in `Arr`
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-generics/issues/issue-79674.stderr

@@ -6,6 +6,14 @@ LL |     requires_distinct("str", 12);
    |
    = note: expected constant `true`
               found constant `false`
+note: required by a bound in `requires_distinct`
+  --> $DIR/issue-79674.rs:23:37
+   |
+LL | fn requires_distinct<A, B>(_a: A, _b: B) where
+   |    ----------------- required by a bound in this
+LL |     A: MiniTypeId, B: MiniTypeId,
+LL |     Lift<{is_same_type::<A, B>()}>: IsFalse {}
+   |                                     ^^^^^^^ required by this bound in `requires_distinct`
 
 error: aborting due to previous error