Make TraitEngines generic over error

23 files changed, 253 insertions, 157 deletions

diff --git a/compiler/rustc_borrowck/src/type_check/constraint_conversion.rs b/compiler/rustc_borrowck/src/type_check/constraint_conversion.rs
index b23ad2e1584..4037f04d0bc 100644
--- a/compiler/rustc_borrowck/src/type_check/constraint_conversion.rs
+++ b/compiler/rustc_borrowck/src/type_check/constraint_conversion.rs
@@ -13,6 +13,7 @@ use rustc_span::Span;
 use rustc_trait_selection::solve::deeply_normalize;
 use rustc_trait_selection::traits::query::type_op::custom::CustomTypeOp;
 use rustc_trait_selection::traits::query::type_op::{TypeOp, TypeOpOutput};
+use rustc_trait_selection::traits::FulfillmentError;
 
 use crate::{
     constraints::OutlivesConstraint,
@@ -286,7 +287,7 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
                     ocx.infcx.at(&ObligationCause::dummy_with_span(self.span), self.param_env),
                     ty,
                 )
-                .map_err(|_| NoSolution)
+                .map_err(|_: Vec<FulfillmentError<'tcx>>| NoSolution)
             },
             "normalize type outlives obligation",
         )
diff --git a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
index db749ef3f81..17db1c07383 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
@@ -10,7 +10,7 @@ use rustc_hir::intravisit;
 use rustc_hir::{GenericParamKind, ImplItemKind};
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
 use rustc_infer::infer::{self, InferCtxt, TyCtxtInferExt};
-use rustc_infer::traits::{util, FulfillmentError};
+use rustc_infer::traits::util;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::util::ExplicitSelf;
@@ -25,7 +25,7 @@ use rustc_trait_selection::regions::InferCtxtRegionExt;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
 use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
 use rustc_trait_selection::traits::{
-    self, ObligationCause, ObligationCauseCode, ObligationCtxt, Reveal,
+    self, FulfillmentError, ObligationCause, ObligationCauseCode, ObligationCtxt, Reveal,
 };
 use std::borrow::Cow;
 use std::iter;
diff --git a/compiler/rustc_hir_analysis/src/coherence/orphan.rs b/compiler/rustc_hir_analysis/src/coherence/orphan.rs
index bdac0d9b0b4..a4818b25f98 100644
--- a/compiler/rustc_hir_analysis/src/coherence/orphan.rs
+++ b/compiler/rustc_hir_analysis/src/coherence/orphan.rs
@@ -13,8 +13,8 @@ use rustc_middle::ty::{TypeSuperVisitable, TypeVisitable, TypeVisitableExt, Type
 use rustc_middle::{bug, span_bug};
 use rustc_span::def_id::{DefId, LocalDefId};
 use rustc_trait_selection::traits::{self, IsFirstInputType, UncoveredTyParams};
+use rustc_trait_selection::traits::{FulfillmentError, StructurallyNormalizeExt, TraitEngineExt};
 use rustc_trait_selection::traits::{OrphanCheckErr, OrphanCheckMode};
-use rustc_trait_selection::traits::{StructurallyNormalizeExt, TraitEngineExt};
 
 #[instrument(level = "debug", skip(tcx))]
 pub(crate) fn orphan_check_impl(
@@ -317,7 +317,8 @@ fn orphan_check<'tcx>(
         }
 
         let ty = if infcx.next_trait_solver() {
-            let mut fulfill_cx = <dyn traits::TraitEngine<'_>>::new(&infcx);
+            let mut fulfill_cx =
+                <dyn traits::TraitEngine<'tcx, FulfillmentError<'tcx>>>::new(&infcx);
             infcx
                 .at(&cause, ty::ParamEnv::empty())
                 .structurally_normalize(ty, &mut *fulfill_cx)
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
index 821c5653040..f646e7de26b 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs
@@ -15,7 +15,6 @@ use rustc_errors::{
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_infer::traits::FulfillmentError;
 use rustc_middle::bug;
 use rustc_middle::query::Key;
 use rustc_middle::ty::print::{PrintPolyTraitRefExt as _, PrintTraitRefExt as _};
@@ -28,6 +27,7 @@ use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::BytePos;
 use rustc_span::{Span, Symbol, DUMMY_SP};
+use rustc_trait_selection::traits::FulfillmentError;
 use rustc_trait_selection::traits::{
     object_safety_violations_for_assoc_item, TraitAliasExpansionInfo,
 };
diff --git a/compiler/rustc_hir_typeck/src/typeck_root_ctxt.rs b/compiler/rustc_hir_typeck/src/typeck_root_ctxt.rs
index 19d6481cc1b..0dfc5408f30 100644
--- a/compiler/rustc_hir_typeck/src/typeck_root_ctxt.rs
+++ b/compiler/rustc_hir_typeck/src/typeck_root_ctxt.rs
@@ -11,7 +11,9 @@ use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::def_id::LocalDefIdMap;
 use rustc_span::Span;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
-use rustc_trait_selection::traits::{self, PredicateObligation, TraitEngine, TraitEngineExt as _};
+use rustc_trait_selection::traits::{
+    self, FulfillmentError, PredicateObligation, TraitEngine, TraitEngineExt as _,
+};
 
 use std::cell::RefCell;
 use std::ops::Deref;
@@ -34,7 +36,7 @@ pub(crate) struct TypeckRootCtxt<'tcx> {
 
     pub(super) locals: RefCell<HirIdMap<Ty<'tcx>>>,
 
-    pub(super) fulfillment_cx: RefCell<Box<dyn TraitEngine<'tcx>>>,
+    pub(super) fulfillment_cx: RefCell<Box<dyn TraitEngine<'tcx, FulfillmentError<'tcx>>>>,
 
     /// Some additional `Sized` obligations badly affect type inference.
     /// These obligations are added in a later stage of typeck.
@@ -83,7 +85,7 @@ impl<'tcx> TypeckRootCtxt<'tcx> {
 
         TypeckRootCtxt {
             typeck_results,
-            fulfillment_cx: RefCell::new(<dyn TraitEngine<'_>>::new(&infcx)),
+            fulfillment_cx: RefCell::new(<dyn TraitEngine<'_, _>>::new(&infcx)),
             infcx,
             locals: RefCell::new(Default::default()),
             deferred_sized_obligations: RefCell::new(Vec::new()),
diff --git a/compiler/rustc_infer/src/infer/canonical/query_response.rs b/compiler/rustc_infer/src/infer/canonical/query_response.rs
index 1dbc1ae7ddd..4917f936f4e 100644
--- a/compiler/rustc_infer/src/infer/canonical/query_response.rs
+++ b/compiler/rustc_infer/src/infer/canonical/query_response.rs
@@ -15,7 +15,7 @@ use crate::infer::canonical::{
 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
 use crate::infer::{DefineOpaqueTypes, InferCtxt, InferOk, InferResult};
 use crate::traits::query::NoSolution;
-use crate::traits::TraitEngine;
+use crate::traits::{FulfillmentErrorLike, TraitEngine};
 use crate::traits::{Obligation, ObligationCause, PredicateObligation};
 use rustc_data_structures::captures::Captures;
 use rustc_index::Idx;
@@ -50,11 +50,11 @@ impl<'tcx> InferCtxt<'tcx> {
     /// - Finally, if any of the obligations result in a hard error,
     ///   then `Err(NoSolution)` is returned.
     #[instrument(skip(self, inference_vars, answer, fulfill_cx), level = "trace")]
-    pub fn make_canonicalized_query_response<T>(
+    pub fn make_canonicalized_query_response<T, E: FulfillmentErrorLike<'tcx>>(
         &self,
         inference_vars: CanonicalVarValues<'tcx>,
         answer: T,
-        fulfill_cx: &mut dyn TraitEngine<'tcx>,
+        fulfill_cx: &mut dyn TraitEngine<'tcx, E>,
     ) -> Result<CanonicalQueryResponse<'tcx, T>, NoSolution>
     where
         T: Debug + TypeFoldable<TyCtxt<'tcx>>,
@@ -97,11 +97,11 @@ impl<'tcx> InferCtxt<'tcx> {
     /// Helper for `make_canonicalized_query_response` that does
     /// everything up until the final canonicalization.
     #[instrument(skip(self, fulfill_cx), level = "debug")]
-    fn make_query_response<T>(
+    fn make_query_response<T, E: FulfillmentErrorLike<'tcx>>(
         &self,
         inference_vars: CanonicalVarValues<'tcx>,
         answer: T,
-        fulfill_cx: &mut dyn TraitEngine<'tcx>,
+        fulfill_cx: &mut dyn TraitEngine<'tcx, E>,
     ) -> Result<QueryResponse<'tcx, T>, NoSolution>
     where
         T: Debug + TypeFoldable<TyCtxt<'tcx>>,
@@ -109,19 +109,13 @@ impl<'tcx> InferCtxt<'tcx> {
         let tcx = self.tcx;
 
         // Select everything, returning errors.
-        let true_errors = fulfill_cx.select_where_possible(self);
-        debug!("true_errors = {:#?}", true_errors);
+        let errors = fulfill_cx.select_all_or_error(self);
 
-        if !true_errors.is_empty() {
-            // FIXME -- we don't indicate *why* we failed to solve
-            debug!("make_query_response: true_errors={:#?}", true_errors);
+        // True error!
+        if errors.iter().any(|e| e.is_true_error()) {
             return Err(NoSolution);
         }
 
-        // Anything left unselected *now* must be an ambiguity.
-        let ambig_errors = fulfill_cx.select_all_or_error(self);
-        debug!("ambig_errors = {:#?}", ambig_errors);
-
         let region_obligations = self.take_registered_region_obligations();
         debug!(?region_obligations);
         let region_constraints = self.with_region_constraints(|region_constraints| {
@@ -135,8 +129,7 @@ impl<'tcx> InferCtxt<'tcx> {
         });
         debug!(?region_constraints);
 
-        let certainty =
-            if ambig_errors.is_empty() { Certainty::Proven } else { Certainty::Ambiguous };
+        let certainty = if errors.is_empty() { Certainty::Proven } else { Certainty::Ambiguous };
 
         let opaque_types = self.take_opaque_types_for_query_response();
 
diff --git a/compiler/rustc_infer/src/infer/mod.rs b/compiler/rustc_infer/src/infer/mod.rs
index 617e6a245ed..5183d9e2d6e 100644
--- a/compiler/rustc_infer/src/infer/mod.rs
+++ b/compiler/rustc_infer/src/infer/mod.rs
@@ -12,7 +12,8 @@ pub use SubregionOrigin::*;
 pub use ValuePairs::*;
 
 use crate::traits::{
-    self, ObligationCause, ObligationInspector, PredicateObligations, TraitEngine,
+    self, FulfillmentErrorLike, ObligationCause, ObligationInspector, PredicateObligations,
+    TraitEngine,
 };
 use error_reporting::TypeErrCtxt;
 use free_regions::RegionRelations;
@@ -737,10 +738,10 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
 
 impl<'tcx, T> InferOk<'tcx, T> {
     /// Extracts `value`, registering any obligations into `fulfill_cx`.
-    pub fn into_value_registering_obligations(
+    pub fn into_value_registering_obligations<E: FulfillmentErrorLike<'tcx>>(
         self,
         infcx: &InferCtxt<'tcx>,
-        fulfill_cx: &mut dyn TraitEngine<'tcx>,
+        fulfill_cx: &mut dyn TraitEngine<'tcx, E>,
     ) -> T {
         let InferOk { value, obligations } = self;
         fulfill_cx.register_predicate_obligations(infcx, obligations);
diff --git a/compiler/rustc_infer/src/traits/engine.rs b/compiler/rustc_infer/src/traits/engine.rs
index a4bcd9d2c8a..1d6875d0f8e 100644
--- a/compiler/rustc_infer/src/traits/engine.rs
+++ b/compiler/rustc_infer/src/traits/engine.rs
@@ -1,12 +1,13 @@
+use std::fmt::Debug;
+
 use crate::infer::InferCtxt;
 use crate::traits::Obligation;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::{self, Ty, Upcast};
 
-use super::FulfillmentError;
 use super::{ObligationCause, PredicateObligation};
 
-pub trait TraitEngine<'tcx>: 'tcx {
+pub trait TraitEngine<'tcx, E: FulfillmentErrorLike<'tcx>>: 'tcx {
     /// Requires that `ty` must implement the trait with `def_id` in
     /// the given environment. This trait must not have any type
     /// parameters (except for `Self`).
@@ -47,12 +48,12 @@ pub trait TraitEngine<'tcx>: 'tcx {
     }
 
     #[must_use]
-    fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>>;
+    fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E>;
 
-    fn collect_remaining_errors(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>>;
+    fn collect_remaining_errors(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E>;
 
     #[must_use]
-    fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
+    fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E> {
         let errors = self.select_where_possible(infcx);
         if !errors.is_empty() {
             return errors;
@@ -71,3 +72,11 @@ pub trait TraitEngine<'tcx>: 'tcx {
         infcx: &InferCtxt<'tcx>,
     ) -> Vec<PredicateObligation<'tcx>>;
 }
+
+pub trait FulfillmentErrorLike<'tcx>: Debug + 'tcx {
+    fn is_true_error(&self) -> bool;
+}
+
+pub trait FromSolverError<'tcx, E>: FulfillmentErrorLike<'tcx> {
+    fn from_solver_error(infcx: &InferCtxt<'tcx>, error: E) -> Self;
+}
diff --git a/compiler/rustc_infer/src/traits/mod.rs b/compiler/rustc_infer/src/traits/mod.rs
index 8304fa3c8a2..26c931876db 100644
--- a/compiler/rustc_infer/src/traits/mod.rs
+++ b/compiler/rustc_infer/src/traits/mod.rs
@@ -23,7 +23,7 @@ pub use self::ImplSource::*;
 pub use self::SelectionError::*;
 use crate::infer::InferCtxt;
 
-pub use self::engine::TraitEngine;
+pub use self::engine::{FromSolverError, FulfillmentErrorLike, TraitEngine};
 pub use self::project::MismatchedProjectionTypes;
 pub(crate) use self::project::UndoLog;
 pub use self::project::{
@@ -124,15 +124,7 @@ pub type Selection<'tcx> = ImplSource<'tcx, PredicateObligation<'tcx>>;
 pub type ObligationInspector<'tcx> =
     fn(&InferCtxt<'tcx>, &PredicateObligation<'tcx>, Result<Certainty, NoSolution>);
 
-pub struct FulfillmentError<'tcx> {
-    pub obligation: PredicateObligation<'tcx>,
-    pub code: FulfillmentErrorCode<'tcx>,
-    /// Diagnostics only: the 'root' obligation which resulted in
-    /// the failure to process `obligation`. This is the obligation
-    /// that was initially passed to `register_predicate_obligation`
-    pub root_obligation: PredicateObligation<'tcx>,
-}
-
+// TODO: Pull this down too
 #[derive(Clone)]
 pub enum FulfillmentErrorCode<'tcx> {
     /// Inherently impossible to fulfill; this trait is implemented if and only
@@ -198,28 +190,6 @@ impl<'tcx, O> Obligation<'tcx, O> {
     }
 }
 
-impl<'tcx> FulfillmentError<'tcx> {
-    pub fn new(
-        obligation: PredicateObligation<'tcx>,
-        code: FulfillmentErrorCode<'tcx>,
-        root_obligation: PredicateObligation<'tcx>,
-    ) -> FulfillmentError<'tcx> {
-        FulfillmentError { obligation, code, root_obligation }
-    }
-
-    pub fn is_true_error(&self) -> bool {
-        match self.code {
-            FulfillmentErrorCode::Select(_)
-            | FulfillmentErrorCode::Project(_)
-            | FulfillmentErrorCode::Subtype(_, _)
-            | FulfillmentErrorCode::ConstEquate(_, _) => true,
-            FulfillmentErrorCode::Cycle(_) | FulfillmentErrorCode::Ambiguity { overflow: _ } => {
-                false
-            }
-        }
-    }
-}
-
 impl<'tcx> PolyTraitObligation<'tcx> {
     pub fn polarity(&self) -> ty::PredicatePolarity {
         self.predicate.skip_binder().polarity
diff --git a/compiler/rustc_infer/src/traits/structural_impls.rs b/compiler/rustc_infer/src/traits/structural_impls.rs
index b616d37e5b5..00abf939807 100644
--- a/compiler/rustc_infer/src/traits/structural_impls.rs
+++ b/compiler/rustc_infer/src/traits/structural_impls.rs
@@ -29,12 +29,6 @@ impl<'tcx, O: fmt::Debug> fmt::Debug for traits::Obligation<'tcx, O> {
     }
 }
 
-impl<'tcx> fmt::Debug for traits::FulfillmentError<'tcx> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "FulfillmentError({:?},{:?})", self.obligation, self.code)
-    }
-}
-
 impl<'tcx> fmt::Debug for traits::FulfillmentErrorCode<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         use traits::FulfillmentErrorCode::*;
diff --git a/compiler/rustc_trait_selection/src/regions.rs b/compiler/rustc_trait_selection/src/regions.rs
index 5e0d7da4f06..cca5bce03e1 100644
--- a/compiler/rustc_trait_selection/src/regions.rs
+++ b/compiler/rustc_trait_selection/src/regions.rs
@@ -1,3 +1,4 @@
+use crate::traits::FulfillmentError;
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
 use rustc_infer::infer::{InferCtxt, RegionResolutionError};
 use rustc_macros::extension;
@@ -27,7 +28,8 @@ impl<'tcx> InferCtxt<'tcx> {
                     ),
                     ty,
                 )
-                .map_err(|_| NoSolution)
+                // TODO:
+                .map_err(|_: Vec<FulfillmentError<'tcx>>| NoSolution)
             } else {
                 Ok(ty)
             }
diff --git a/compiler/rustc_trait_selection/src/solve/fulfill.rs b/compiler/rustc_trait_selection/src/solve/fulfill.rs
index d28cf834032..040bab1dfae 100644
--- a/compiler/rustc_trait_selection/src/solve/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/solve/fulfill.rs
@@ -1,3 +1,4 @@
+use std::marker::PhantomData;
 use std::mem;
 use std::ops::ControlFlow;
 
@@ -5,14 +6,17 @@ use rustc_infer::infer::InferCtxt;
 use rustc_infer::traits::query::NoSolution;
 use rustc_infer::traits::solve::{CandidateSource, GoalSource, MaybeCause};
 use rustc_infer::traits::{
-    self, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes, Obligation,
-    ObligationCause, ObligationCauseCode, PredicateObligation, SelectionError, TraitEngine,
+    self, FromSolverError, FulfillmentErrorCode, FulfillmentErrorLike, MismatchedProjectionTypes,
+    Obligation, ObligationCause, ObligationCauseCode, PredicateObligation, SelectionError,
+    TraitEngine,
 };
 use rustc_middle::bug;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_span::symbol::sym;
 
+use crate::traits::FulfillmentError;
+
 use super::eval_ctxt::GenerateProofTree;
 use super::inspect::{self, ProofTreeInferCtxtExt, ProofTreeVisitor};
 use super::{Certainty, InferCtxtEvalExt};
@@ -28,7 +32,7 @@ use super::{Certainty, InferCtxtEvalExt};
 ///
 /// It is also likely that we want to use slightly different datastructures
 /// here as this will have to deal with far more root goals than `evaluate_all`.
-pub struct FulfillmentCtxt<'tcx> {
+pub struct FulfillmentCtxt<'tcx, E: FulfillmentErrorLike<'tcx>> {
     obligations: ObligationStorage<'tcx>,
 
     /// The snapshot in which this context was created. Using the context
@@ -36,6 +40,7 @@ pub struct FulfillmentCtxt<'tcx> {
     /// gets rolled back. Because of this we explicitly check that we only
     /// use the context in exactly this snapshot.
     usable_in_snapshot: usize,
+    _errors: PhantomData<E>,
 }
 
 #[derive(Default)]
@@ -89,8 +94,8 @@ impl<'tcx> ObligationStorage<'tcx> {
     }
 }
 
-impl<'tcx> FulfillmentCtxt<'tcx> {
-    pub fn new(infcx: &InferCtxt<'tcx>) -> FulfillmentCtxt<'tcx> {
+impl<'tcx, E: FulfillmentErrorLike<'tcx>> FulfillmentCtxt<'tcx, E> {
+    pub fn new(infcx: &InferCtxt<'tcx>) -> FulfillmentCtxt<'tcx, E> {
         assert!(
             infcx.next_trait_solver(),
             "new trait solver fulfillment context created when \
@@ -99,6 +104,7 @@ impl<'tcx> FulfillmentCtxt<'tcx> {
         FulfillmentCtxt {
             obligations: Default::default(),
             usable_in_snapshot: infcx.num_open_snapshots(),
+            _errors: PhantomData,
         }
     }
 
@@ -118,7 +124,9 @@ impl<'tcx> FulfillmentCtxt<'tcx> {
     }
 }
 
-impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
+impl<'tcx, E: FromSolverError<'tcx, NextSolverError<'tcx>>> TraitEngine<'tcx, E>
+    for FulfillmentCtxt<'tcx, E>
+{
     #[instrument(level = "trace", skip(self, infcx))]
     fn register_predicate_obligation(
         &mut self,
@@ -129,24 +137,22 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
         self.obligations.register(obligation);
     }
 
-    fn collect_remaining_errors(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
-        let mut errors: Vec<_> = self
-            .obligations
+    fn collect_remaining_errors(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E> {
+        self.obligations
             .pending
             .drain(..)
-            .map(|obligation| fulfillment_error_for_stalled(infcx, obligation))
-            .collect();
-
-        errors.extend(self.obligations.overflowed.drain(..).map(|obligation| FulfillmentError {
-            obligation: find_best_leaf_obligation(infcx, &obligation, true),
-            code: FulfillmentErrorCode::Ambiguity { overflow: Some(true) },
-            root_obligation: obligation,
-        }));
-
-        errors
+            .map(|obligation| NextSolverError::Ambiguity(obligation))
+            .chain(
+                self.obligations
+                    .overflowed
+                    .drain(..)
+                    .map(|obligation| NextSolverError::Overflow(obligation)),
+            )
+            .map(|e| E::from_solver_error(infcx, e))
+            .collect()
     }
 
-    fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
+    fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E> {
         assert_eq!(self.usable_in_snapshot, infcx.num_open_snapshots());
         let mut errors = Vec::new();
         for i in 0.. {
@@ -164,7 +170,10 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
                 let (changed, certainty) = match result {
                     Ok(result) => result,
                     Err(NoSolution) => {
-                        errors.push(fulfillment_error_for_no_solution(infcx, obligation));
+                        errors.push(E::from_solver_error(
+                            infcx,
+                            NextSolverError::TrueError(obligation),
+                        ));
                         continue;
                     }
                 };
@@ -195,6 +204,28 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
     }
 }
 
+pub enum NextSolverError<'tcx> {
+    TrueError(PredicateObligation<'tcx>),
+    Ambiguity(PredicateObligation<'tcx>),
+    Overflow(PredicateObligation<'tcx>),
+}
+
+impl<'tcx> FromSolverError<'tcx, NextSolverError<'tcx>> for FulfillmentError<'tcx> {
+    fn from_solver_error(infcx: &InferCtxt<'tcx>, error: NextSolverError<'tcx>) -> Self {
+        match error {
+            NextSolverError::TrueError(obligation) => {
+                fulfillment_error_for_no_solution(infcx, obligation)
+            }
+            NextSolverError::Ambiguity(obligation) => {
+                fulfillment_error_for_stalled(infcx, obligation)
+            }
+            NextSolverError::Overflow(obligation) => {
+                fulfillment_error_for_overflow(infcx, obligation)
+            }
+        }
+    }
+}
+
 fn fulfillment_error_for_no_solution<'tcx>(
     infcx: &InferCtxt<'tcx>,
     root_obligation: PredicateObligation<'tcx>,
@@ -280,6 +311,17 @@ fn fulfillment_error_for_stalled<'tcx>(
     }
 }
 
+fn fulfillment_error_for_overflow<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    root_obligation: PredicateObligation<'tcx>,
+) -> FulfillmentError<'tcx> {
+    FulfillmentError {
+        obligation: find_best_leaf_obligation(infcx, &root_obligation, true),
+        code: FulfillmentErrorCode::Ambiguity { overflow: Some(true) },
+        root_obligation,
+    }
+}
+
 fn find_best_leaf_obligation<'tcx>(
     infcx: &InferCtxt<'tcx>,
     obligation: &PredicateObligation<'tcx>,
diff --git a/compiler/rustc_trait_selection/src/solve/mod.rs b/compiler/rustc_trait_selection/src/solve/mod.rs
index f9febd290fe..ffeafd0e89f 100644
--- a/compiler/rustc_trait_selection/src/solve/mod.rs
+++ b/compiler/rustc_trait_selection/src/solve/mod.rs
@@ -40,7 +40,7 @@ mod search_graph;
 mod trait_goals;
 
 pub use eval_ctxt::{EvalCtxt, GenerateProofTree, InferCtxtEvalExt, InferCtxtSelectExt};
-pub use fulfill::FulfillmentCtxt;
+pub use fulfill::{FulfillmentCtxt, NextSolverError};
 pub(crate) use normalize::deeply_normalize_for_diagnostics;
 pub use normalize::{deeply_normalize, deeply_normalize_with_skipped_universes};
 
diff --git a/compiler/rustc_trait_selection/src/solve/normalize.rs b/compiler/rustc_trait_selection/src/solve/normalize.rs
index 970848544c5..f85e5d8746f 100644
--- a/compiler/rustc_trait_selection/src/solve/normalize.rs
+++ b/compiler/rustc_trait_selection/src/solve/normalize.rs
@@ -1,23 +1,29 @@
+use std::marker::PhantomData;
+
 use crate::traits::error_reporting::{OverflowCause, TypeErrCtxtExt};
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
-use crate::traits::{BoundVarReplacer, PlaceholderReplacer};
+use crate::traits::{BoundVarReplacer, FulfillmentError, PlaceholderReplacer};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_infer::infer::at::At;
 use rustc_infer::infer::InferCtxt;
-use rustc_infer::traits::{FulfillmentError, Obligation, TraitEngine};
+use rustc_infer::traits::{FromSolverError, FulfillmentErrorLike, Obligation, TraitEngine};
 use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::{self, Ty, TyCtxt, UniverseIndex};
 use rustc_middle::ty::{FallibleTypeFolder, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::{TypeFoldable, TypeVisitableExt};
 
-use super::FulfillmentCtxt;
+use super::{FulfillmentCtxt, NextSolverError};
 
 /// Deeply normalize all aliases in `value`. This does not handle inference and expects
 /// its input to be already fully resolved.
-pub fn deeply_normalize<'tcx, T: TypeFoldable<TyCtxt<'tcx>>>(
+pub fn deeply_normalize<
+    'tcx,
+    T: TypeFoldable<TyCtxt<'tcx>>,
+    E: FromSolverError<'tcx, NextSolverError<'tcx>>,
+>(
     at: At<'_, 'tcx>,
     value: T,
-) -> Result<T, Vec<FulfillmentError<'tcx>>> {
+) -> Result<T, Vec<E>> {
     assert!(!value.has_escaping_bound_vars());
     deeply_normalize_with_skipped_universes(at, value, vec![])
 }
@@ -28,29 +34,32 @@ pub fn deeply_normalize<'tcx, T: TypeFoldable<TyCtxt<'tcx>>>(
 /// Additionally takes a list of universes which represents the binders which have been
 /// entered before passing `value` to the function. This is currently needed for
 /// `normalize_erasing_regions`, which skips binders as it walks through a type.
-pub fn deeply_normalize_with_skipped_universes<'tcx, T: TypeFoldable<TyCtxt<'tcx>>>(
+pub fn deeply_normalize_with_skipped_universes<
+    'tcx,
+    T: TypeFoldable<TyCtxt<'tcx>>,
+    E: FromSolverError<'tcx, NextSolverError<'tcx>>,
+>(
     at: At<'_, 'tcx>,
     value: T,
     universes: Vec<Option<UniverseIndex>>,
-) -> Result<T, Vec<FulfillmentError<'tcx>>> {
+) -> Result<T, Vec<E>> {
     let fulfill_cx = FulfillmentCtxt::new(at.infcx);
-    let mut folder = NormalizationFolder { at, fulfill_cx, depth: 0, universes };
+    let mut folder =
+        NormalizationFolder { at, fulfill_cx, depth: 0, universes, _errors: PhantomData };
 
     value.try_fold_with(&mut folder)
 }
 
-struct NormalizationFolder<'me, 'tcx> {
+struct NormalizationFolder<'me, 'tcx, E: FulfillmentErrorLike<'tcx>> {
     at: At<'me, 'tcx>,
-    fulfill_cx: FulfillmentCtxt<'tcx>,
+    fulfill_cx: FulfillmentCtxt<'tcx, E>,
     depth: usize,
     universes: Vec<Option<UniverseIndex>>,
+    _errors: PhantomData<E>,
 }
 
-impl<'tcx> NormalizationFolder<'_, 'tcx> {
-    fn normalize_alias_ty(
-        &mut self,
-        alias_ty: Ty<'tcx>,
-    ) -> Result<Ty<'tcx>, Vec<FulfillmentError<'tcx>>> {
+impl<'tcx, E: FromSolverError<'tcx, NextSolverError<'tcx>>> NormalizationFolder<'_, 'tcx, E> {
+    fn normalize_alias_ty(&mut self, alias_ty: Ty<'tcx>) -> Result<Ty<'tcx>, Vec<E>> {
         assert!(matches!(alias_ty.kind(), ty::Alias(..)));
 
         let infcx = self.at.infcx;
@@ -101,7 +110,7 @@ impl<'tcx> NormalizationFolder<'_, 'tcx> {
         &mut self,
         ty: Ty<'tcx>,
         uv: ty::UnevaluatedConst<'tcx>,
-    ) -> Result<ty::Const<'tcx>, Vec<FulfillmentError<'tcx>>> {
+    ) -> Result<ty::Const<'tcx>, Vec<E>> {
         let infcx = self.at.infcx;
         let tcx = infcx.tcx;
         let recursion_limit = tcx.recursion_limit();
@@ -141,8 +150,10 @@ impl<'tcx> NormalizationFolder<'_, 'tcx> {
     }
 }
 
-impl<'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for NormalizationFolder<'_, 'tcx> {
-    type Error = Vec<FulfillmentError<'tcx>>;
+impl<'tcx, E: FromSolverError<'tcx, NextSolverError<'tcx>>> FallibleTypeFolder<TyCtxt<'tcx>>
+    for NormalizationFolder<'_, 'tcx, E>
+{
+    type Error = Vec<E>;
 
     fn interner(&self) -> TyCtxt<'tcx> {
         self.at.infcx.tcx
@@ -242,7 +253,8 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for DeeplyNormalizeForDiagnosticsFolder<'_,
             ty,
             vec![None; ty.outer_exclusive_binder().as_usize()],
         )
-        .unwrap_or_else(|_| ty.super_fold_with(self))
+        // TODO:
+        .unwrap_or_else(|_: Vec<FulfillmentError<'tcx>>| ty.super_fold_with(self))
     }
 
     fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
@@ -251,6 +263,7 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for DeeplyNormalizeForDiagnosticsFolder<'_,
             ct,
             vec![None; ct.outer_exclusive_binder().as_usize()],
         )
-        .unwrap_or_else(|_| ct.super_fold_with(self))
+        // TODO:
+        .unwrap_or_else(|_: Vec<FulfillmentError<'tcx>>| ct.super_fold_with(self))
     }
 }
diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs
index ebdb032dc0e..33eca159912 100644
--- a/compiler/rustc_trait_selection/src/traits/coherence.rs
+++ b/compiler/rustc_trait_selection/src/traits/coherence.rs
@@ -11,6 +11,7 @@ use crate::solve::{deeply_normalize_for_diagnostics, inspect};
 use crate::traits::select::IntercrateAmbiguityCause;
 use crate::traits::NormalizeExt;
 use crate::traits::SkipLeakCheck;
+use crate::traits::{util, FulfillmentErrorCode};
 use crate::traits::{
     Obligation, ObligationCause, PredicateObligation, PredicateObligations, SelectionContext,
 };
@@ -19,7 +20,6 @@ use rustc_errors::{Diag, EmissionGuarantee};
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::{DefineOpaqueTypes, InferCtxt, TyCtxtInferExt};
-use rustc_infer::traits::{util, FulfillmentErrorCode};
 use rustc_middle::bug;
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::traits::solve::{CandidateSource, Certainty, Goal};
diff --git a/compiler/rustc_trait_selection/src/traits/engine.rs b/compiler/rustc_trait_selection/src/traits/engine.rs
index 6fed24c49e0..b62cd345e51 100644
--- a/compiler/rustc_trait_selection/src/traits/engine.rs
+++ b/compiler/rustc_trait_selection/src/traits/engine.rs
@@ -2,12 +2,15 @@ use std::cell::RefCell;
 use std::fmt::Debug;
 
 use super::FulfillmentContext;
-use super::TraitEngine;
+use super::{FromSolverError, TraitEngine};
 use crate::regions::InferCtxtRegionExt;
 use crate::solve::FulfillmentCtxt as NextFulfillmentCtxt;
+use crate::solve::NextSolverError;
 use crate::traits::error_reporting::TypeErrCtxtExt;
+use crate::traits::fulfill::OldSolverError;
 use crate::traits::NormalizeExt;
 use crate::traits::StructurallyNormalizeExt;
+use crate::traits::{FulfillmentError, Obligation, ObligationCause, PredicateObligation};
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def_id::{DefId, LocalDefId};
@@ -18,7 +21,6 @@ use rustc_infer::infer::canonical::{
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
 use rustc_infer::infer::RegionResolutionError;
 use rustc_infer::infer::{DefineOpaqueTypes, InferCtxt, InferOk};
-use rustc_infer::traits::{FulfillmentError, Obligation, ObligationCause, PredicateObligation};
 use rustc_macros::extension;
 use rustc_middle::arena::ArenaAllocatable;
 use rustc_middle::traits::query::NoSolution;
@@ -28,8 +30,12 @@ use rustc_middle::ty::Upcast;
 use rustc_middle::ty::Variance;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 
-#[extension(pub trait TraitEngineExt<'tcx>)]
-impl<'tcx> dyn TraitEngine<'tcx> {
+#[extension(pub trait TraitEngineExt<'tcx, E>)]
+impl<
+    'tcx,
+    E: FromSolverError<'tcx, NextSolverError<'tcx>> + FromSolverError<'tcx, OldSolverError<'tcx>>,
+> dyn TraitEngine<'tcx, E>
+{
     fn new(infcx: &InferCtxt<'tcx>) -> Box<Self> {
         if infcx.next_trait_solver() {
             Box::new(NextFulfillmentCtxt::new(infcx))
@@ -49,12 +55,16 @@ impl<'tcx> dyn TraitEngine<'tcx> {
 /// with obligations outside of hir or mir typeck.
 pub struct ObligationCtxt<'a, 'tcx> {
     pub infcx: &'a InferCtxt<'tcx>,
-    engine: RefCell<Box<dyn TraitEngine<'tcx>>>,
+    engine: RefCell<Box<dyn TraitEngine<'tcx, FulfillmentError<'tcx>>>>,
 }
 
 impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
     pub fn new(infcx: &'a InferCtxt<'tcx>) -> Self {
-        Self { infcx, engine: RefCell::new(<dyn TraitEngine<'_>>::new(infcx)) }
+        // TODO:
+        Self {
+            infcx,
+            engine: RefCell::new(<dyn TraitEngine<'tcx, FulfillmentError<'tcx>>>::new(infcx)),
+        }
     }
 
     pub fn register_obligation(&self, obligation: PredicateObligation<'tcx>) {
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index e1afc2a3529..09fe0196fcf 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -6,7 +6,7 @@ use rustc_data_structures::obligation_forest::ProcessResult;
 use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome};
 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
 use rustc_infer::infer::DefineOpaqueTypes;
-use rustc_infer::traits::ProjectionCacheKey;
+use rustc_infer::traits::{FromSolverError, FulfillmentErrorLike, ProjectionCacheKey};
 use rustc_infer::traits::{PolyTraitObligation, SelectionError, TraitEngine};
 use rustc_middle::bug;
 use rustc_middle::mir::interpret::ErrorHandled;
@@ -50,7 +50,7 @@ impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
 /// along. Once all type inference constraints have been generated, the
 /// method `select_all_or_error` can be used to report any remaining
 /// ambiguous cases as errors.
-pub struct FulfillmentContext<'tcx> {
+pub struct FulfillmentContext<'tcx, E: FulfillmentErrorLike<'tcx>> {
     /// A list of all obligations that have been registered with this
     /// fulfillment context.
     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
@@ -60,6 +60,8 @@ pub struct FulfillmentContext<'tcx> {
     /// gets rolled back. Because of this we explicitly check that we only
     /// use the context in exactly this snapshot.
     usable_in_snapshot: usize,
+
+    _errors: PhantomData<E>,
 }
 
 #[derive(Clone, Debug)]
@@ -76,9 +78,9 @@ pub struct PendingPredicateObligation<'tcx> {
 #[cfg(target_pointer_width = "64")]
 rustc_data_structures::static_assert_size!(PendingPredicateObligation<'_>, 72);
 
-impl<'tcx> FulfillmentContext<'tcx> {
+impl<'tcx, E: FromSolverError<'tcx, OldSolverError<'tcx>>> FulfillmentContext<'tcx, E> {
     /// Creates a new fulfillment context.
-    pub(super) fn new(infcx: &InferCtxt<'tcx>) -> FulfillmentContext<'tcx> {
+    pub(super) fn new(infcx: &InferCtxt<'tcx>) -> FulfillmentContext<'tcx, E> {
         assert!(
             !infcx.next_trait_solver(),
             "old trait solver fulfillment context created when \
@@ -87,13 +89,15 @@ impl<'tcx> FulfillmentContext<'tcx> {
         FulfillmentContext {
             predicates: ObligationForest::new(),
             usable_in_snapshot: infcx.num_open_snapshots(),
+            _errors: PhantomData,
         }
     }
 
     /// Attempts to select obligations using `selcx`.
-    fn select(&mut self, selcx: SelectionContext<'_, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
+    fn select(&mut self, selcx: SelectionContext<'_, 'tcx>) -> Vec<E> {
         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
         let _enter = span.enter();
+        let infcx = selcx.infcx;
 
         // Process pending obligations.
         let outcome: Outcome<_, _> =
@@ -102,8 +106,8 @@ impl<'tcx> FulfillmentContext<'tcx> {
         // FIXME: if we kept the original cache key, we could mark projection
         // obligations as complete for the projection cache here.
 
-        let errors: Vec<FulfillmentError<'tcx>> =
-            outcome.errors.into_iter().map(to_fulfillment_error).collect();
+        let errors: Vec<E> =
+            outcome.errors.into_iter().map(|err| E::from_solver_error(infcx, err)).collect();
 
         debug!(
             "select({} predicates remaining, {} errors) done",
@@ -115,7 +119,9 @@ impl<'tcx> FulfillmentContext<'tcx> {
     }
 }
 
-impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
+impl<'tcx, E: FromSolverError<'tcx, OldSolverError<'tcx>>> TraitEngine<'tcx, E>
+    for FulfillmentContext<'tcx, E>
+{
     #[inline]
     fn register_predicate_obligation(
         &mut self,
@@ -134,18 +140,15 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
     }
 
-    fn collect_remaining_errors(
-        &mut self,
-        _infcx: &InferCtxt<'tcx>,
-    ) -> Vec<FulfillmentError<'tcx>> {
+    fn collect_remaining_errors(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E> {
         self.predicates
             .to_errors(FulfillmentErrorCode::Ambiguity { overflow: None })
             .into_iter()
-            .map(to_fulfillment_error)
+            .map(|err| E::from_solver_error(infcx, err))
             .collect()
     }
 
-    fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
+    fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E> {
         let selcx = SelectionContext::new(infcx);
         self.select(selcx)
     }
@@ -840,13 +843,15 @@ fn args_infer_vars<'a, 'tcx>(
         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 }
 
-fn to_fulfillment_error<'tcx>(
-    error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
-) -> FulfillmentError<'tcx> {
-    let mut iter = error.backtrace.into_iter();
-    let obligation = iter.next().unwrap().obligation;
-    // The root obligation is the last item in the backtrace - if there's only
-    // one item, then it's the same as the main obligation
-    let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
-    FulfillmentError::new(obligation, error.error, root_obligation)
+pub type OldSolverError<'tcx> = Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>;
+
+impl<'tcx> FromSolverError<'tcx, OldSolverError<'tcx>> for FulfillmentError<'tcx> {
+    fn from_solver_error(_infcx: &InferCtxt<'tcx>, error: OldSolverError<'tcx>) -> Self {
+        let mut iter = error.backtrace.into_iter();
+        let obligation = iter.next().unwrap().obligation;
+        // The root obligation is the last item in the backtrace - if there's only
+        // one item, then it's the same as the main obligation
+        let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
+        FulfillmentError::new(obligation, error.error, root_obligation)
+    }
 }
diff --git a/compiler/rustc_trait_selection/src/traits/misc.rs b/compiler/rustc_trait_selection/src/traits/misc.rs
index a1094d98276..8b459492647 100644
--- a/compiler/rustc_trait_selection/src/traits/misc.rs
+++ b/compiler/rustc_trait_selection/src/traits/misc.rs
@@ -1,13 +1,13 @@
 //! Miscellaneous type-system utilities that are too small to deserve their own modules.
 
 use crate::regions::InferCtxtRegionExt;
-use crate::traits::{self, ObligationCause};
+use crate::traits::{self, FulfillmentError, ObligationCause};
 
 use hir::LangItem;
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_hir as hir;
+use rustc_infer::infer::outlives::env::OutlivesEnvironment;
 use rustc_infer::infer::{RegionResolutionError, TyCtxtInferExt};
-use rustc_infer::{infer::outlives::env::OutlivesEnvironment, traits::FulfillmentError};
 use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt, TypeVisitableExt};
 
 use super::outlives_bounds::InferCtxtExt;
diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index 786ab091e91..990a88f748e 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -70,6 +70,49 @@ pub use self::util::{with_replaced_escaping_bound_vars, BoundVarReplacer, Placeh
 
 pub use rustc_infer::traits::*;
 
+pub struct FulfillmentError<'tcx> {
+    pub obligation: PredicateObligation<'tcx>,
+    pub code: FulfillmentErrorCode<'tcx>,
+    /// Diagnostics only: the 'root' obligation which resulted in
+    /// the failure to process `obligation`. This is the obligation
+    /// that was initially passed to `register_predicate_obligation`
+    pub root_obligation: PredicateObligation<'tcx>,
+}
+
+impl<'tcx> FulfillmentError<'tcx> {
+    pub fn new(
+        obligation: PredicateObligation<'tcx>,
+        code: FulfillmentErrorCode<'tcx>,
+        root_obligation: PredicateObligation<'tcx>,
+    ) -> FulfillmentError<'tcx> {
+        FulfillmentError { obligation, code, root_obligation }
+    }
+
+    pub fn is_true_error(&self) -> bool {
+        match self.code {
+            FulfillmentErrorCode::Select(_)
+            | FulfillmentErrorCode::Project(_)
+            | FulfillmentErrorCode::Subtype(_, _)
+            | FulfillmentErrorCode::ConstEquate(_, _) => true,
+            FulfillmentErrorCode::Cycle(_) | FulfillmentErrorCode::Ambiguity { overflow: _ } => {
+                false
+            }
+        }
+    }
+}
+
+impl<'tcx> FulfillmentErrorLike<'tcx> for FulfillmentError<'tcx> {
+    fn is_true_error(&self) -> bool {
+        self.is_true_error()
+    }
+}
+
+impl<'tcx> Debug for FulfillmentError<'tcx> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "FulfillmentError({:?},{:?})", self.obligation, self.code)
+    }
+}
+
 /// Whether to skip the leak check, as part of a future compatibility warning step.
 ///
 /// The "default" for skip-leak-check corresponds to the current
diff --git a/compiler/rustc_trait_selection/src/traits/normalize.rs b/compiler/rustc_trait_selection/src/traits/normalize.rs
index d10aee2d4e2..4e5ca2ee92e 100644
--- a/compiler/rustc_trait_selection/src/traits/normalize.rs
+++ b/compiler/rustc_trait_selection/src/traits/normalize.rs
@@ -3,11 +3,13 @@ use super::error_reporting::OverflowCause;
 use super::error_reporting::TypeErrCtxtExt;
 use super::SelectionContext;
 use super::{project, with_replaced_escaping_bound_vars, BoundVarReplacer, PlaceholderReplacer};
+use crate::solve::NextSolverError;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_infer::infer::at::At;
 use rustc_infer::infer::InferOk;
+use rustc_infer::traits::FromSolverError;
 use rustc_infer::traits::PredicateObligation;
-use rustc_infer::traits::{FulfillmentError, Normalized, Obligation, TraitEngine};
+use rustc_infer::traits::{Normalized, Obligation, TraitEngine};
 use rustc_macros::extension;
 use rustc_middle::traits::{ObligationCause, ObligationCauseCode, Reveal};
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeFolder};
@@ -44,11 +46,14 @@ impl<'tcx> At<'_, 'tcx> {
     /// existing fulfillment context in the old solver. Once we also eagerly prove goals with
     /// the old solver or have removed the old solver, remove `traits::fully_normalize` and
     /// rename this function to `At::fully_normalize`.
-    fn deeply_normalize<T: TypeFoldable<TyCtxt<'tcx>>>(
+    fn deeply_normalize<
+        T: TypeFoldable<TyCtxt<'tcx>>,
+        E: FromSolverError<'tcx, NextSolverError<'tcx>>,
+    >(
         self,
         value: T,
-        fulfill_cx: &mut dyn TraitEngine<'tcx>,
-    ) -> Result<T, Vec<FulfillmentError<'tcx>>> {
+        fulfill_cx: &mut dyn TraitEngine<'tcx, E>,
+    ) -> Result<T, Vec<E>> {
         if self.infcx.next_trait_solver() {
             crate::solve::deeply_normalize(self, value)
         } else {
diff --git a/compiler/rustc_trait_selection/src/traits/query/normalize.rs b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
index 1b5ffeebc01..2c0103e40df 100644
--- a/compiler/rustc_trait_selection/src/traits/query/normalize.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
@@ -9,10 +9,10 @@ use crate::traits::error_reporting::OverflowCause;
 use crate::traits::error_reporting::TypeErrCtxtExt;
 use crate::traits::normalize::needs_normalization;
 use crate::traits::{BoundVarReplacer, PlaceholderReplacer};
+use crate::traits::{FulfillmentError, Normalized};
 use crate::traits::{ObligationCause, PredicateObligation, Reveal};
 use rustc_data_structures::sso::SsoHashMap;
 use rustc_data_structures::stack::ensure_sufficient_stack;
-use rustc_infer::traits::Normalized;
 use rustc_macros::extension;
 use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeSuperFoldable};
 use rustc_middle::ty::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitableExt};
@@ -76,7 +76,10 @@ impl<'cx, 'tcx> At<'cx, 'tcx> {
         };
 
         if self.infcx.next_trait_solver() {
-            match crate::solve::deeply_normalize_with_skipped_universes(self, value, universes) {
+            // TODO:
+            match crate::solve::deeply_normalize_with_skipped_universes::<_, FulfillmentError<'tcx>>(
+                self, value, universes,
+            ) {
                 Ok(value) => return Ok(Normalized { value, obligations: vec![] }),
                 Err(_errors) => {
                     return Err(NoSolution);
diff --git a/compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs b/compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs
index 00cc77e71e7..a1bbd21f829 100644
--- a/compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs
@@ -1,6 +1,7 @@
 use crate::solve;
 use crate::traits::query::NoSolution;
 use crate::traits::wf;
+use crate::traits::FulfillmentError;
 use crate::traits::ObligationCtxt;
 
 use rustc_infer::infer::canonical::Canonical;
@@ -266,7 +267,8 @@ pub fn compute_implied_outlives_bounds_compat_inner<'tcx>(
                         ocx.infcx.at(&ObligationCause::dummy(), param_env),
                         ty_a,
                     )
-                    .map_err(|_errs| NoSolution)?;
+                    // TODO:
+                    .map_err(|_errs: Vec<FulfillmentError<'tcx>>| NoSolution)?;
                 }
                 let mut components = smallvec![];
                 push_outlives_components(tcx, ty_a, &mut components);
diff --git a/compiler/rustc_trait_selection/src/traits/structural_normalize.rs b/compiler/rustc_trait_selection/src/traits/structural_normalize.rs
index 96a06e0c169..a160fc033a2 100644
--- a/compiler/rustc_trait_selection/src/traits/structural_normalize.rs
+++ b/compiler/rustc_trait_selection/src/traits/structural_normalize.rs
@@ -1,5 +1,5 @@
 use rustc_infer::infer::at::At;
-use rustc_infer::traits::{FulfillmentError, TraitEngine};
+use rustc_infer::traits::{FulfillmentErrorLike, TraitEngine};
 use rustc_macros::extension;
 use rustc_middle::ty::{self, Ty};
 
@@ -7,11 +7,11 @@ use crate::traits::{NormalizeExt, Obligation};
 
 #[extension(pub trait StructurallyNormalizeExt<'tcx>)]
 impl<'tcx> At<'_, 'tcx> {
-    fn structurally_normalize(
+    fn structurally_normalize<E: FulfillmentErrorLike<'tcx>>(
         &self,
         ty: Ty<'tcx>,
-        fulfill_cx: &mut dyn TraitEngine<'tcx>,
-    ) -> Result<Ty<'tcx>, Vec<FulfillmentError<'tcx>>> {
+        fulfill_cx: &mut dyn TraitEngine<'tcx, E>,
+    ) -> Result<Ty<'tcx>, Vec<E>> {
         assert!(!ty.is_ty_var(), "should have resolved vars before calling");
 
         if self.infcx.next_trait_solver() {