Do not retain Normalization goal errors in nested goals for BestObligationVisitor:: non_trivial_candidates

compiler/rustc_trait_selection/src/solve/fulfill/derive_errors.rs

@@ -214,20 +214,16 @@ impl<'tcx> BestObligation<'tcx> {
                 // We always handle rigid alias candidates separately as we may not add them for
                 // aliases whose trait bound doesn't hold.
                 candidates.retain(|c| !matches!(c.kind(), inspect::ProbeKind::RigidAlias { .. }));
-                // If we have >1 candidate, one may still be due to "boring" reasons, like
-                // an alias-relate that failed to hold when deeply evaluated. We really
-                // don't care about reasons like this.
+                // If there are multiple candidates, we can still recurse into a single one if all
+                // others fail due to boring reasons. Currently that's if we fail to normalize
+                // while considering the candidate.
                 if candidates.len() > 1 {
                     candidates.retain(|candidate| {
                         goal.infcx().probe(|_| {
-                            candidate.instantiate_nested_goals(self.span()).iter().any(
+                            !candidate.instantiate_nested_goals(self.span()).iter().any(
                                 |nested_goal| {
-                                    matches!(
-                                        nested_goal.source(),
-                                        GoalSource::ImplWhereBound
-                                            | GoalSource::AliasBoundConstCondition
-                                            | GoalSource::AliasWellFormed
-                                    ) && nested_goal.result().is_err()
+                                    nested_goal.source() == GoalSource::Normalization
+                                        && nested_goal.result().is_err()
                                 },
                             )
                         })

tests/ui/traits/next-solver/confusing-diagnostic-due-to-best-obligation-visitor.rs

@@ -0,0 +1,27 @@
+//@compile-flags: -Znext-solver=globally
+//@check-fail
+
+// See issue https://github.com/rust-lang/rust/issues/161882, test makes sure we don't report
+// irrelevant and not implemented bounds when we can't proof an obligation.
+
+struct MyError;
+
+trait MaybeFallible {
+    type Error: MaybeError; //~ ERROR: the trait bound `MyError: From<<Self as MaybeFallible>::Error>` is not satisfied
+}
+
+trait MaybeError: Sized
+where
+    MyError: From<Self>,
+{
+    type ComposeWithOther: MaybeError; //~ ERROR: the trait bound `MyError: From<<Self as MaybeError>::ComposeWithOther>` is not satisfied
+}
+
+fn compose<A: MaybeFallible, B>() -> Result<(), ()>
+where
+    <A::Error as MaybeError>::ComposeWithOther: From<B>, //~ ERROR: the trait bound `MyError: From<<A as MaybeFallible>::Error>` is not satisfied
+{
+    todo!()
+}
+
+fn main() {}

tests/ui/traits/next-solver/confusing-diagnostic-due-to-best-obligation-visitor.stderr

@@ -0,0 +1,64 @@
+error[E0277]: the trait bound `MyError: From<<A as MaybeFallible>::Error>` is not satisfied
+  --> $DIR/confusing-diagnostic-due-to-best-obligation-visitor.rs:22:49
+   |
+LL |     <A::Error as MaybeError>::ComposeWithOther: From<B>,
+   |                                                 ^^^^^^^ unsatisfied trait bound
+   |
+help: the trait `From<<A as MaybeFallible>::Error>` is not implemented for `MyError`
+  --> $DIR/confusing-diagnostic-due-to-best-obligation-visitor.rs:7:1
+   |
+LL | struct MyError;
+   | ^^^^^^^^^^^^^^
+note: required by a bound in `MaybeError`
+  --> $DIR/confusing-diagnostic-due-to-best-obligation-visitor.rs:15:14
+   |
+LL | trait MaybeError: Sized
+LL | where
+LL |     MyError: From<Self>,
+   |              ^^^^^^^^^^ required by this bound in `MaybeError`
+help: consider extending the `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL |     <A::Error as MaybeError>::ComposeWithOther: From<B>, MyError: From<<A as MaybeFallible>::Error>
+   |                                                          ++++++++++++++++++++++++++++++++++++++++++
+
+error[E0277]: the trait bound `MyError: From<<Self as MaybeFallible>::Error>` is not satisfied
+  --> $DIR/confusing-diagnostic-due-to-best-obligation-visitor.rs:10:17
+   |
+LL |     type Error: MaybeError;
+   |                 ^^^^^^^^^^ unsatisfied trait bound
+   |
+help: the trait `From<<Self as MaybeFallible>::Error>` is not implemented for `MyError`
+  --> $DIR/confusing-diagnostic-due-to-best-obligation-visitor.rs:7:1
+   |
+LL | struct MyError;
+   | ^^^^^^^^^^^^^^
+note: required by a bound in `MaybeError`
+  --> $DIR/confusing-diagnostic-due-to-best-obligation-visitor.rs:15:14
+   |
+LL | trait MaybeError: Sized
+LL | where
+LL |     MyError: From<Self>,
+   |              ^^^^^^^^^^ required by this bound in `MaybeError`
+
+error[E0277]: the trait bound `MyError: From<<Self as MaybeError>::ComposeWithOther>` is not satisfied
+  --> $DIR/confusing-diagnostic-due-to-best-obligation-visitor.rs:17:28
+   |
+LL |     type ComposeWithOther: MaybeError;
+   |                            ^^^^^^^^^^ unsatisfied trait bound
+   |
+help: the trait `From<<Self as MaybeError>::ComposeWithOther>` is not implemented for `MyError`
+  --> $DIR/confusing-diagnostic-due-to-best-obligation-visitor.rs:7:1
+   |
+LL | struct MyError;
+   | ^^^^^^^^^^^^^^
+note: required by a bound in `MaybeError`
+  --> $DIR/confusing-diagnostic-due-to-best-obligation-visitor.rs:15:14
+   |
+LL | trait MaybeError: Sized
+LL | where
+LL |     MyError: From<Self>,
+   |              ^^^^^^^^^^ required by this bound in `MaybeError`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.