Improve diagnostics for HostEffectPredicate in the new solver

compiler/rustc_middle/src/traits/mod.rs

@@ -126,6 +126,15 @@ impl<'tcx> ObligationCause<'tcx> {
         self
     }
 
+    pub fn derived_host_cause(
+        mut self,
+        parent_host_pred: ty::Binder<'tcx, ty::HostEffectPredicate<'tcx>>,
+        variant: impl FnOnce(DerivedHostCause<'tcx>) -> ObligationCauseCode<'tcx>,
+    ) -> ObligationCause<'tcx> {
+        self.code = variant(DerivedHostCause { parent_host_pred, parent_code: self.code }).into();
+        self
+    }
+
     pub fn to_constraint_category(&self) -> ConstraintCategory<'tcx> {
         match self.code() {
             ObligationCauseCode::MatchImpl(cause, _) => cause.to_constraint_category(),
@@ -279,6 +288,14 @@ pub enum ObligationCauseCode<'tcx> {
     /// Derived obligation for WF goals.
     WellFormedDerived(DerivedCause<'tcx>),
 
+    /// Derived obligation (i.e. `where` clause) on an user-provided impl
+    /// or a trait alias.
+    ImplDerivedHost(Box<ImplDerivedHostCause<'tcx>>),
+
+    /// Derived obligation (i.e. `where` clause) on an user-provided impl
+    /// or a trait alias.
+    BuiltinDerivedHost(DerivedHostCause<'tcx>),
+
     /// Derived obligation refined to point at a specific argument in
     /// a call or method expression.
     FunctionArg {
@@ -438,36 +455,38 @@ pub enum WellFormedLoc {
     },
 }
 
-#[derive(Clone, Debug, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
-#[derive(TypeVisitable, TypeFoldable)]
-pub struct ImplDerivedCause<'tcx> {
-    pub derived: DerivedCause<'tcx>,
-    /// The `DefId` of the `impl` that gave rise to the `derived` obligation.
-    /// If the `derived` obligation arose from a trait alias, which conceptually has a synthetic impl,
-    /// then this will be the `DefId` of that trait alias. Care should therefore be taken to handle
-    /// that exceptional case where appropriate.
-    pub impl_or_alias_def_id: DefId,
-    /// The index of the derived predicate in the parent impl's predicates.
-    pub impl_def_predicate_index: Option<usize>,
-    pub span: Span,
-}
-
 impl<'tcx> ObligationCauseCode<'tcx> {
     /// Returns the base obligation, ignoring derived obligations.
     pub fn peel_derives(&self) -> &Self {
         let mut base_cause = self;
-        while let Some((parent_code, _)) = base_cause.parent() {
+        while let Some(parent_code) = base_cause.parent() {
             base_cause = parent_code;
         }
         base_cause
     }
 
+    pub fn parent(&self) -> Option<&Self> {
+        match self {
+            ObligationCauseCode::FunctionArg { parent_code, .. } => Some(parent_code),
+            ObligationCauseCode::BuiltinDerived(derived)
+            | ObligationCauseCode::WellFormedDerived(derived)
+            | ObligationCauseCode::ImplDerived(box ImplDerivedCause { derived, .. }) => {
+                Some(&derived.parent_code)
+            }
+            ObligationCauseCode::BuiltinDerivedHost(derived)
+            | ObligationCauseCode::ImplDerivedHost(box ImplDerivedHostCause { derived, .. }) => {
+                Some(&derived.parent_code)
+            }
+            _ => None,
+        }
+    }
+
     /// Returns the base obligation and the base trait predicate, if any, ignoring
     /// derived obligations.
     pub fn peel_derives_with_predicate(&self) -> (&Self, Option<ty::PolyTraitPredicate<'tcx>>) {
         let mut base_cause = self;
         let mut base_trait_pred = None;
-        while let Some((parent_code, parent_pred)) = base_cause.parent() {
+        while let Some((parent_code, parent_pred)) = base_cause.parent_with_predicate() {
             base_cause = parent_code;
             if let Some(parent_pred) = parent_pred {
                 base_trait_pred = Some(parent_pred);
@@ -477,7 +496,7 @@ impl<'tcx> ObligationCauseCode<'tcx> {
         (base_cause, base_trait_pred)
     }
 
-    pub fn parent(&self) -> Option<(&Self, Option<ty::PolyTraitPredicate<'tcx>>)> {
+    pub fn parent_with_predicate(&self) -> Option<(&Self, Option<ty::PolyTraitPredicate<'tcx>>)> {
         match self {
             ObligationCauseCode::FunctionArg { parent_code, .. } => Some((parent_code, None)),
             ObligationCauseCode::BuiltinDerived(derived)
@@ -555,6 +574,42 @@ pub struct DerivedCause<'tcx> {
     pub parent_code: InternedObligationCauseCode<'tcx>,
 }
 
+#[derive(Clone, Debug, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+#[derive(TypeVisitable, TypeFoldable)]
+pub struct ImplDerivedCause<'tcx> {
+    pub derived: DerivedCause<'tcx>,
+    /// The `DefId` of the `impl` that gave rise to the `derived` obligation.
+    /// If the `derived` obligation arose from a trait alias, which conceptually has a synthetic impl,
+    /// then this will be the `DefId` of that trait alias. Care should therefore be taken to handle
+    /// that exceptional case where appropriate.
+    pub impl_or_alias_def_id: DefId,
+    /// The index of the derived predicate in the parent impl's predicates.
+    pub impl_def_predicate_index: Option<usize>,
+    pub span: Span,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+#[derive(TypeVisitable, TypeFoldable)]
+pub struct DerivedHostCause<'tcx> {
+    /// The trait predicate of the parent obligation that led to the
+    /// current obligation. Note that only trait obligations lead to
+    /// derived obligations, so we just store the trait predicate here
+    /// directly.
+    pub parent_host_pred: ty::Binder<'tcx, ty::HostEffectPredicate<'tcx>>,
+
+    /// The parent trait had this cause.
+    pub parent_code: InternedObligationCauseCode<'tcx>,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+#[derive(TypeVisitable, TypeFoldable)]
+pub struct ImplDerivedHostCause<'tcx> {
+    pub derived: DerivedHostCause<'tcx>,
+    /// The `DefId` of the `impl` that gave rise to the `derived` obligation.
+    pub impl_def_id: DefId,
+    pub span: Span,
+}
+
 #[derive(Clone, Debug, PartialEq, Eq, TypeVisitable)]
 pub enum SelectionError<'tcx> {
     /// The trait is not implemented.

compiler/rustc_middle/src/ty/predicate.rs

@@ -634,6 +634,28 @@ impl<'tcx> UpcastFrom<TyCtxt<'tcx>, PolyProjectionPredicate<'tcx>> for Clause<'t
     }
 }
 
+impl<'tcx> UpcastFrom<TyCtxt<'tcx>, ty::Binder<'tcx, ty::HostEffectPredicate<'tcx>>>
+    for Predicate<'tcx>
+{
+    fn upcast_from(
+        from: ty::Binder<'tcx, ty::HostEffectPredicate<'tcx>>,
+        tcx: TyCtxt<'tcx>,
+    ) -> Self {
+        from.map_bound(ty::ClauseKind::HostEffect).upcast(tcx)
+    }
+}
+
+impl<'tcx> UpcastFrom<TyCtxt<'tcx>, ty::Binder<'tcx, ty::HostEffectPredicate<'tcx>>>
+    for Clause<'tcx>
+{
+    fn upcast_from(
+        from: ty::Binder<'tcx, ty::HostEffectPredicate<'tcx>>,
+        tcx: TyCtxt<'tcx>,
+    ) -> Self {
+        from.map_bound(ty::ClauseKind::HostEffect).upcast(tcx)
+    }
+}
+
 impl<'tcx> UpcastFrom<TyCtxt<'tcx>, NormalizesTo<'tcx>> for Predicate<'tcx> {
     fn upcast_from(from: NormalizesTo<'tcx>, tcx: TyCtxt<'tcx>) -> Self {
         PredicateKind::NormalizesTo(from).upcast(tcx)

compiler/rustc_next_trait_solver/src/solve/effect_goals.rs

@@ -352,7 +352,7 @@ where
 
         ecx.probe_builtin_trait_candidate(BuiltinImplSource::Misc).enter(|ecx| {
             ecx.add_goals(
-                GoalSource::Misc,
+                GoalSource::ImplWhereBound,
                 const_conditions.into_iter().map(|trait_ref| {
                     goal.with(
                         cx,

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

@@ -754,7 +754,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     applied_do_not_recommend = true;
                 }
             }
-            if let Some((parent_cause, _parent_pred)) = base_cause.parent() {
+            if let Some(parent_cause) = base_cause.parent() {
                 base_cause = parent_cause.clone();
             } else {
                 break;
@@ -779,7 +779,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 trait_ref.skip_binder().args.type_at(1).to_opt_closure_kind()
             && !found_kind.extends(expected_kind)
         {
-            if let Some((_, Some(parent))) = obligation.cause.code().parent() {
+            if let Some((_, Some(parent))) = obligation.cause.code().parent_with_predicate() {
                 // If we have a derived obligation, then the parent will be a `AsyncFn*` goal.
                 trait_ref = parent.to_poly_trait_ref();
             } else if let &ObligationCauseCode::FunctionArg { arg_hir_id, .. } =
@@ -925,7 +925,8 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         let Some(typeck) = &self.typeck_results else {
             return false;
         };
-        let Some((ObligationCauseCode::QuestionMark, Some(y))) = obligation.cause.code().parent()
+        let Some((ObligationCauseCode::QuestionMark, Some(y))) =
+            obligation.cause.code().parent_with_predicate()
         else {
             return false;
         };
@@ -1178,7 +1179,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
 
         let mut code = obligation.cause.code();
         let mut pred = obligation.predicate.as_trait_clause();
-        while let Some((next_code, next_pred)) = code.parent() {
+        while let Some((next_code, next_pred)) = code.parent_with_predicate() {
             if let Some(pred) = pred {
                 self.enter_forall(pred, |pred| {
                     diag.note(format!(
@@ -2093,7 +2094,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         let mut code = obligation.cause.code();
         let mut trait_pred = trait_predicate;
         let mut peeled = false;
-        while let Some((parent_code, parent_trait_pred)) = code.parent() {
+        while let Some((parent_code, parent_trait_pred)) = code.parent_with_predicate() {
             code = parent_code;
             if let Some(parent_trait_pred) = parent_trait_pred {
                 trait_pred = parent_trait_pred;

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

@@ -460,7 +460,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         {
             // Suggest dereferencing the argument to a function/method call if possible
             let mut real_trait_pred = trait_pred;
-            while let Some((parent_code, parent_trait_pred)) = code.parent() {
+            while let Some((parent_code, parent_trait_pred)) = code.parent_with_predicate() {
                 code = parent_code;
                 if let Some(parent_trait_pred) = parent_trait_pred {
                     real_trait_pred = parent_trait_pred;
@@ -1476,7 +1476,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         let mut span = obligation.cause.span;
         let mut trait_pred = trait_pred;
         let mut code = obligation.cause.code();
-        while let Some((c, Some(parent_trait_pred))) = code.parent() {
+        while let Some((c, Some(parent_trait_pred))) = code.parent_with_predicate() {
             // We want the root obligation, in order to detect properly handle
             // `for _ in &mut &mut vec![] {}`.
             code = c;
@@ -3501,6 +3501,59 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     )
                 });
             }
+            ObligationCauseCode::ImplDerivedHost(ref data) => {
+                let self_ty =
+                    self.resolve_vars_if_possible(data.derived.parent_host_pred.self_ty());
+                let msg = format!(
+                    "required for `{self_ty}` to implement `{} {}`",
+                    data.derived.parent_host_pred.skip_binder().constness,
+                    data.derived
+                        .parent_host_pred
+                        .map_bound(|pred| pred.trait_ref)
+                        .print_only_trait_path(),
+                );
+                match tcx.hir().get_if_local(data.impl_def_id) {
+                    Some(Node::Item(hir::Item {
+                        kind: hir::ItemKind::Impl(hir::Impl { of_trait, self_ty, .. }),
+                        ..
+                    })) => {
+                        let mut spans = vec![self_ty.span];
+                        spans.extend(of_trait.as_ref().map(|t| t.path.span));
+                        let mut spans: MultiSpan = spans.into();
+                        spans.push_span_label(data.span, "unsatisfied trait bound introduced here");
+                        err.span_note(spans, msg);
+                    }
+                    _ => {
+                        err.note(msg);
+                    }
+                }
+                ensure_sufficient_stack(|| {
+                    self.note_obligation_cause_code(
+                        body_id,
+                        err,
+                        data.derived.parent_host_pred,
+                        param_env,
+                        &data.derived.parent_code,
+                        obligated_types,
+                        seen_requirements,
+                        long_ty_file,
+                    )
+                });
+            }
+            ObligationCauseCode::BuiltinDerivedHost(ref data) => {
+                ensure_sufficient_stack(|| {
+                    self.note_obligation_cause_code(
+                        body_id,
+                        err,
+                        data.parent_host_pred,
+                        param_env,
+                        &data.parent_code,
+                        obligated_types,
+                        seen_requirements,
+                        long_ty_file,
+                    )
+                });
+            }
             ObligationCauseCode::WellFormedDerived(ref data) => {
                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
                 let parent_predicate = parent_trait_ref;