Use a proof tree visitor to refine the Obligation for error reporting

compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs

@@ -453,7 +453,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         } else {
             self.infcx.enter_forall(kind, |kind| {
                 let goal = goal.with(self.tcx(), ty::Binder::dummy(kind));
-                self.add_goal(GoalSource::Misc, goal);
+                self.add_goal(GoalSource::ImplWhereBound, goal);
                 self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
             })
         }

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -1,15 +1,19 @@
 use std::mem;
+use std::ops::ControlFlow;
 
 use rustc_infer::infer::InferCtxt;
-use rustc_infer::traits::solve::MaybeCause;
+use rustc_infer::traits::query::NoSolution;
+use rustc_infer::traits::solve::inspect::ProbeKind;
+use rustc_infer::traits::solve::{CandidateSource, GoalSource, MaybeCause};
 use rustc_infer::traits::{
-    query::NoSolution, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes,
+    self, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes, Obligation,
     PredicateObligation, SelectionError, TraitEngine,
 };
 use rustc_middle::ty;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 
 use super::eval_ctxt::GenerateProofTree;
+use super::inspect::{ProofTreeInferCtxtExt, ProofTreeVisitor};
 use super::{Certainty, InferCtxtEvalExt};
 
 /// A trait engine using the new trait solver.
@@ -133,9 +137,9 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
             .collect();
 
         errors.extend(self.obligations.overflowed.drain(..).map(|obligation| FulfillmentError {
-            root_obligation: obligation.clone(),
+            obligation: find_best_leaf_obligation(infcx, &obligation),
             code: FulfillmentErrorCode::Ambiguity { overflow: Some(true) },
-            obligation,
+            root_obligation: obligation,
         }));
 
         errors
@@ -192,8 +196,10 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
 
 fn fulfillment_error_for_no_solution<'tcx>(
     infcx: &InferCtxt<'tcx>,
-    obligation: PredicateObligation<'tcx>,
+    root_obligation: PredicateObligation<'tcx>,
 ) -> FulfillmentError<'tcx> {
+    let obligation = find_best_leaf_obligation(infcx, &root_obligation);
+
     let code = match obligation.predicate.kind().skip_binder() {
         ty::PredicateKind::Clause(ty::ClauseKind::Projection(_)) => {
             FulfillmentErrorCode::ProjectionError(
@@ -213,14 +219,14 @@ fn fulfillment_error_for_no_solution<'tcx>(
         }
         ty::PredicateKind::Subtype(pred) => {
             let (a, b) = infcx.enter_forall_and_leak_universe(
-                obligation.predicate.kind().rebind((pred.a, pred.b)),
+                root_obligation.predicate.kind().rebind((pred.a, pred.b)),
             );
             let expected_found = ExpectedFound::new(true, a, b);
             FulfillmentErrorCode::SubtypeError(expected_found, TypeError::Sorts(expected_found))
         }
         ty::PredicateKind::Coerce(pred) => {
             let (a, b) = infcx.enter_forall_and_leak_universe(
-                obligation.predicate.kind().rebind((pred.a, pred.b)),
+                root_obligation.predicate.kind().rebind((pred.a, pred.b)),
             );
             let expected_found = ExpectedFound::new(false, a, b);
             FulfillmentErrorCode::SubtypeError(expected_found, TypeError::Sorts(expected_found))
@@ -234,7 +240,8 @@ fn fulfillment_error_for_no_solution<'tcx>(
             bug!("unexpected goal: {obligation:?}")
         }
     };
-    FulfillmentError { root_obligation: obligation.clone(), code, obligation }
+
+    FulfillmentError { obligation, code, root_obligation }
 }
 
 fn fulfillment_error_for_stalled<'tcx>(
@@ -258,5 +265,135 @@ fn fulfillment_error_for_stalled<'tcx>(
         }
     });
 
-    FulfillmentError { obligation: obligation.clone(), code, root_obligation: obligation }
+    FulfillmentError {
+        obligation: find_best_leaf_obligation(infcx, &obligation),
+        code,
+        root_obligation: obligation,
+    }
+}
+
+struct BestObligation<'tcx> {
+    obligation: PredicateObligation<'tcx>,
+}
+
+impl<'tcx> BestObligation<'tcx> {
+    fn with_derived_obligation(
+        &mut self,
+        derive_obligation: impl FnOnce(&mut Self) -> PredicateObligation<'tcx>,
+        and_then: impl FnOnce(&mut Self) -> <Self as ProofTreeVisitor<'tcx>>::Result,
+    ) -> <Self as ProofTreeVisitor<'tcx>>::Result {
+        let derived_obligation = derive_obligation(self);
+        let old_obligation = std::mem::replace(&mut self.obligation, derived_obligation);
+        let res = and_then(self);
+        self.obligation = old_obligation;
+        res
+    }
+}
+
+impl<'tcx> ProofTreeVisitor<'tcx> for BestObligation<'tcx> {
+    type Result = ControlFlow<PredicateObligation<'tcx>>;
+
+    fn span(&self) -> rustc_span::Span {
+        self.obligation.cause.span
+    }
+
+    fn visit_goal(&mut self, goal: &super::inspect::InspectGoal<'_, 'tcx>) -> Self::Result {
+        let candidates = goal.candidates();
+        // FIXME: Throw out candidates that have no failing WC and >1 failing misc goal.
+
+        // HACK:
+        if self.obligation.recursion_depth > 3 {
+            return ControlFlow::Break(self.obligation.clone());
+        }
+
+        let [candidate] = candidates.as_slice() else {
+            return ControlFlow::Break(self.obligation.clone());
+        };
+
+        // FIXME: Could we extract a trait ref from a projection here too?
+        // FIXME: Also, what about considering >1 layer up the stack? May be necessary
+        // for normalizes-to.
+        let Some(parent_trait_pred) = goal.goal().predicate.to_opt_poly_trait_pred() else {
+            return ControlFlow::Break(self.obligation.clone());
+        };
+
+        let tcx = goal.infcx().tcx;
+        let mut impl_where_bound_count = 0;
+        for nested_goal in candidate.instantiate_nested_goals(self.span()) {
+            if matches!(nested_goal.source(), GoalSource::ImplWhereBound) {
+                impl_where_bound_count += 1;
+            } else {
+                continue;
+            }
+
+            // Skip nested goals that hold.
+            if matches!(nested_goal.result(), Ok(Certainty::Yes)) {
+                continue;
+            }
+
+            self.with_derived_obligation(
+                |self_| {
+                    let mut cause = self_.obligation.cause.clone();
+                    cause = match candidate.kind() {
+                        ProbeKind::TraitCandidate {
+                            source: CandidateSource::Impl(impl_def_id),
+                            result: _,
+                        } => {
+                            let idx = impl_where_bound_count - 1;
+                            if let Some((_, span)) = tcx
+                                .predicates_of(impl_def_id)
+                                .instantiate_identity(tcx)
+                                .iter()
+                                .nth(idx)
+                            {
+                                cause.derived_cause(parent_trait_pred, |derived| {
+                                    traits::ImplDerivedObligation(Box::new(
+                                        traits::ImplDerivedObligationCause {
+                                            derived,
+                                            impl_or_alias_def_id: impl_def_id,
+                                            impl_def_predicate_index: Some(idx),
+                                            span,
+                                        },
+                                    ))
+                                })
+                            } else {
+                                cause
+                            }
+                        }
+                        ProbeKind::TraitCandidate {
+                            source: CandidateSource::BuiltinImpl(..),
+                            result: _,
+                        } => {
+                            cause.derived_cause(parent_trait_pred, traits::BuiltinDerivedObligation)
+                        }
+                        _ => cause,
+                    };
+
+                    Obligation {
+                        cause,
+                        param_env: nested_goal.goal().param_env,
+                        predicate: nested_goal.goal().predicate,
+                        recursion_depth: self_.obligation.recursion_depth + 1,
+                    }
+                },
+                |self_| self_.visit_goal(&nested_goal),
+            )?;
+        }
+
+        ControlFlow::Break(self.obligation.clone())
+    }
+}
+
+fn find_best_leaf_obligation<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    obligation: &PredicateObligation<'tcx>,
+) -> PredicateObligation<'tcx> {
+    let obligation = infcx.resolve_vars_if_possible(obligation.clone());
+    infcx
+        .visit_proof_tree(
+            obligation.clone().into(),
+            &mut BestObligation { obligation: obligation.clone() },
+        )
+        .break_value()
+        .unwrap_or(obligation)
 }

tests/ui/coherence/occurs-check/opaques.next.stderr

@@ -11,7 +11,7 @@ error[E0282]: type annotations needed
   --> $DIR/opaques.rs:13:20
    |
 LL |     pub fn cast<T>(x: Container<Alias<T>, T>) -> Container<T, T> {
-   |                    ^ cannot infer type for struct `Container<T, T>`
+   |                    ^ cannot infer type for associated type `<T as Trait<T>>::Assoc`
 
 error: aborting due to 2 previous errors
 

tests/ui/for/issue-20605.next.stderr

@@ -2,9 +2,14 @@ error[E0277]: `dyn Iterator<Item = &'a mut u8>` is not an iterator
   --> $DIR/issue-20605.rs:6:17
    |
 LL |     for item in *things { *item = 0 }
-   |                 ^^^^^^^ `dyn Iterator<Item = &'a mut u8>` is not an iterator
+   |                 ^^^^^^^ the trait `IntoIterator` is not implemented for `dyn Iterator<Item = &'a mut u8>`
    |
-   = help: the trait `IntoIterator` is not implemented for `dyn Iterator<Item = &'a mut u8>`
+   = note: the trait bound `dyn Iterator<Item = &'a mut u8>: IntoIterator` is not satisfied
+   = note: required for `dyn Iterator<Item = &'a mut u8>` to implement `IntoIterator`
+help: consider mutably borrowing here
+   |
+LL |     for item in &mut *things { *item = 0 }
+   |                 ++++
 
 error: the type `<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter` is not well-formed
   --> $DIR/issue-20605.rs:6:17

tests/ui/higher-ranked/leak-check/leak-check-in-selection-3.next.stderr

@@ -23,7 +23,20 @@ error[E0283]: type annotations needed
 LL |     impls_indirect_leak::<Box<_>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the function `impls_indirect_leak`
    |
-   = note: cannot satisfy `for<'a> Box<_>: IndirectLeak<'a>`
+note: multiple `impl`s satisfying `for<'a> Box<_>: Leak<'a>` found
+  --> $DIR/leak-check-in-selection-3.rs:9:1
+   |
+LL | impl Leak<'_> for Box<u32> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | impl Leak<'static> for Box<u16> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required for `Box<_>` to implement `for<'a> IndirectLeak<'a>`
+  --> $DIR/leak-check-in-selection-3.rs:23:23
+   |
+LL | impl<'a, T: Leak<'a>> IndirectLeak<'a> for T {}
+   |             --------  ^^^^^^^^^^^^^^^^     ^
+   |             |
+   |             unsatisfied trait bound introduced here
 note: required by a bound in `impls_indirect_leak`
   --> $DIR/leak-check-in-selection-3.rs:25:27
    |

tests/ui/traits/next-solver/auto-with-drop_tracking_mir.fail.stderr

@@ -1,18 +1,26 @@
-error[E0277]: `impl Future<Output = ()>` cannot be sent between threads safely
+error: future cannot be sent between threads safely
   --> $DIR/auto-with-drop_tracking_mir.rs:25:13
    |
 LL |     is_send(foo());
-   |     ------- ^^^^^ `impl Future<Output = ()>` cannot be sent between threads safely
-   |     |
-   |     required by a bound introduced by this call
+   |             ^^^^^ future returned by `foo` is not `Send`
    |
-   = help: the trait `Send` is not implemented for `impl Future<Output = ()>`
+   = help: the trait `Sync` is not implemented for `impl Future<Output = ()>`, which is required by `impl Future<Output = ()>: Send`
+note: future is not `Send` as this value is used across an await
+  --> $DIR/auto-with-drop_tracking_mir.rs:16:11
+   |
+LL |     let x = &NotSync;
+   |         - has type `&NotSync` which is not `Send`
+LL |     bar().await;
+   |           ^^^^^ await occurs here, with `x` maybe used later
 note: required by a bound in `is_send`
   --> $DIR/auto-with-drop_tracking_mir.rs:24:24
    |
 LL |     fn is_send(_: impl Send) {}
    |                        ^^^^ required by this bound in `is_send`
+help: consider dereferencing here
+   |
+LL |     is_send(*foo());
+   |             +
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/next-solver/auto-with-drop_tracking_mir.rs

@@ -23,5 +23,5 @@ async fn bar() {}
 fn main() {
     fn is_send(_: impl Send) {}
     is_send(foo());
-    //[fail]~^ ERROR `impl Future<Output = ()>` cannot be sent between threads safely
+    //[fail]~^ ERROR future cannot be sent between threads safely
 }

tests/ui/traits/next-solver/builtin-fn-must-return-sized.rs

@@ -13,5 +13,5 @@ fn foo<F: Fn<T>, T: Tuple>(f: Option<F>, t: T) {
 
 fn main() {
     foo::<fn() -> str, _>(None, ());
-    //~^ expected a `Fn<_>` closure, found `fn() -> str`
+    //~^ the size for values of type `str` cannot be known at compilation time
 }

tests/ui/traits/next-solver/builtin-fn-must-return-sized.stderr

@@ -1,10 +1,11 @@
-error[E0277]: expected a `Fn<_>` closure, found `fn() -> str`
+error[E0277]: the size for values of type `str` cannot be known at compilation time
   --> $DIR/builtin-fn-must-return-sized.rs:15:11
    |
 LL |     foo::<fn() -> str, _>(None, ());
-   |           ^^^^^^^^^^^ expected an `Fn<_>` closure, found `fn() -> str`
+   |           ^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = help: the trait `Fn<_>` is not implemented for `fn() -> str`
+   = help: within `fn() -> str`, the trait `Sized` is not implemented for `str`, which is required by `fn() -> str: Fn<_>`
+   = note: required because it appears within the type `fn() -> str`
 note: required by a bound in `foo`
   --> $DIR/builtin-fn-must-return-sized.rs:10:11
    |

tests/ui/traits/next-solver/coherence/coherence-fulfill-overflow.stderr

@@ -1,12 +1,12 @@
-error[E0119]: conflicting implementations of trait `Trait` for type `W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<_>>>>>>>>>>>>>>>>>>>>>`
+error[E0119]: conflicting implementations of trait `Trait` for type `W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<_>>>>>>>>>>>>>>>>>>>>>>`
   --> $DIR/coherence-fulfill-overflow.rs:12:1
    |
 LL | impl<T: ?Sized + TwoW> Trait for W<T> {}
    | ------------------------------------- first implementation here
 LL | impl<T: ?Sized + TwoW> Trait for T {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<_>>>>>>>>>>>>>>>>>>>>>`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<_>>>>>>>>>>>>>>>>>>>>>>`
    |
-   = note: overflow evaluating the requirement `W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<_>>>>>>>>>>>>>>>>>>>>>: TwoW`
+   = note: overflow evaluating the requirement `W<W<W<W<W<W<W<W<W<W<W<W<W<W<_>>>>>>>>>>>>>>: TwoW`
    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "20"]` attribute to your crate (`coherence_fulfill_overflow`)
 
 error: aborting due to 1 previous error

tests/ui/traits/next-solver/cycles/coinduction/fixpoint-exponential-growth.stderr

@@ -1,9 +1,16 @@
-error[E0275]: overflow evaluating the requirement `W<_>: Trait`
+error[E0275]: overflow evaluating the requirement `_: Sized`
   --> $DIR/fixpoint-exponential-growth.rs:33:13
    |
 LL |     impls::<W<_>>();
    |             ^^^^
    |
+note: required for `W<(W<_>, W<_>)>` to implement `Trait`
+  --> $DIR/fixpoint-exponential-growth.rs:23:12
+   |
+LL | impl<T, U> Trait for W<(W<T>, W<U>)>
+   |      -     ^^^^^     ^^^^^^^^^^^^^^^
+   |      |
+   |      unsatisfied trait bound introduced here
 note: required by a bound in `impls`
   --> $DIR/fixpoint-exponential-growth.rs:30:13
    |

tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.rs

@@ -61,7 +61,7 @@ where
     // entering the cycle from `A` fails, but would work if we were to use the cache
     // result of `B<X>`.
     impls_trait::<A<X>, _, _, _>();
-    //~^ ERROR the trait bound `A<X>: Trait<_, _, _>` is not satisfied
+    //~^ ERROR the trait bound `X: IncompleteGuidance<_, _>` is not satisfied
 }
 
 fn main() {

tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.stderr

@@ -1,10 +1,21 @@
-error[E0277]: the trait bound `A<X>: Trait<_, _, _>` is not satisfied
+error[E0277]: the trait bound `X: IncompleteGuidance<_, _>` is not satisfied
   --> $DIR/incompleteness-unstable-result.rs:63:19
    |
 LL |     impls_trait::<A<X>, _, _, _>();
-   |                   ^^^^ the trait `Trait<_, _, _>` is not implemented for `A<X>`
+   |                   ^^^^ the trait `IncompleteGuidance<_, _>` is not implemented for `X`, which is required by `A<X>: Trait<_, _, _>`
    |
-   = help: the trait `Trait<U, V, D>` is implemented for `A<T>`
+   = help: the following other types implement trait `IncompleteGuidance<T, V>`:
+             <T as IncompleteGuidance<U, i16>>
+             <T as IncompleteGuidance<U, i8>>
+             <T as IncompleteGuidance<U, u8>>
+note: required for `A<X>` to implement `Trait<_, _, u8>`
+  --> $DIR/incompleteness-unstable-result.rs:32:50
+   |
+LL | impl<T: ?Sized, U: ?Sized, V: ?Sized, D: ?Sized> Trait<U, V, D> for A<T>
+   |                                                  ^^^^^^^^^^^^^^     ^^^^
+LL | where
+LL |     T: IncompleteGuidance<U, V>,
+   |        ------------------------ unsatisfied trait bound introduced here
 note: required by a bound in `impls_trait`
   --> $DIR/incompleteness-unstable-result.rs:51:28
    |