Clean up derived obligation creation

compiler/rustc_infer/src/traits/mod.rs

@@ -69,14 +69,21 @@ impl<'tcx> PredicateObligation<'tcx> {
     }
 }
 
-impl TraitObligation<'_> {
+impl<'tcx> TraitObligation<'tcx> {
     /// Returns `true` if the trait predicate is considered `const` in its ParamEnv.
     pub fn is_const(&self) -> bool {
         match (self.predicate.skip_binder().constness, self.param_env.constness()) {
             (ty::BoundConstness::ConstIfConst, hir::Constness::Const) => true,
             _ => false,
         }
     }
+
+    pub fn derived_cause(
+        &self,
+        variant: impl FnOnce(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
+    ) -> ObligationCause<'tcx> {
+        self.cause.clone().derived_cause(self.predicate, variant)
+    }
 }
 
 // `PredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.

compiler/rustc_middle/src/traits/mod.rs

@@ -97,9 +97,7 @@ pub struct ObligationCause<'tcx> {
     /// information.
     pub body_id: hir::HirId,
 
-    /// `None` for `MISC_OBLIGATION_CAUSE_CODE` (a common case, occurs ~60% of
-    /// the time). `Some` otherwise.
-    code: Option<Lrc<ObligationCauseCode<'tcx>>>,
+    code: InternedObligationCauseCode<'tcx>,
 }
 
 // This custom hash function speeds up hashing for `Obligation` deduplication
@@ -123,11 +121,7 @@ impl<'tcx> ObligationCause<'tcx> {
         body_id: hir::HirId,
         code: ObligationCauseCode<'tcx>,
     ) -> ObligationCause<'tcx> {
-        ObligationCause {
-            span,
-            body_id,
-            code: if code == MISC_OBLIGATION_CAUSE_CODE { None } else { Some(Lrc::new(code)) },
-        }
+        ObligationCause { span, body_id, code: code.into() }
     }
 
     pub fn misc(span: Span, body_id: hir::HirId) -> ObligationCause<'tcx> {
@@ -136,15 +130,12 @@ impl<'tcx> ObligationCause<'tcx> {
 
     #[inline(always)]
     pub fn dummy() -> ObligationCause<'tcx> {
-        ObligationCause { span: DUMMY_SP, body_id: hir::CRATE_HIR_ID, code: None }
+        ObligationCause::dummy_with_span(DUMMY_SP)
     }
 
+    #[inline(always)]
     pub fn dummy_with_span(span: Span) -> ObligationCause<'tcx> {
-        ObligationCause { span, body_id: hir::CRATE_HIR_ID, code: None }
-    }
-
-    pub fn make_mut_code(&mut self) -> &mut ObligationCauseCode<'tcx> {
-        Lrc::make_mut(self.code.get_or_insert_with(|| Lrc::new(MISC_OBLIGATION_CAUSE_CODE)))
+        ObligationCause { span, body_id: hir::CRATE_HIR_ID, code: Default::default() }
     }
 
     pub fn span(&self, tcx: TyCtxt<'tcx>) -> Span {
@@ -164,14 +155,37 @@ impl<'tcx> ObligationCause<'tcx> {
 
     #[inline]
     pub fn code(&self) -> &ObligationCauseCode<'tcx> {
-        self.code.as_deref().unwrap_or(&MISC_OBLIGATION_CAUSE_CODE)
+        &self.code
     }
 
-    pub fn clone_code(&self) -> Lrc<ObligationCauseCode<'tcx>> {
-        match &self.code {
-            Some(code) => code.clone(),
-            None => Lrc::new(MISC_OBLIGATION_CAUSE_CODE),
-        }
+    pub fn map_code(
+        &mut self,
+        f: impl FnOnce(InternedObligationCauseCode<'tcx>) -> ObligationCauseCode<'tcx>,
+    ) {
+        self.code = f(std::mem::take(&mut self.code)).into();
+    }
+
+    pub fn derived_cause(
+        mut self,
+        parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
+        variant: impl FnOnce(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
+    ) -> ObligationCause<'tcx> {
+        /*!
+         * Creates a cause for obligations that are derived from
+         * `obligation` by a recursive search (e.g., for a builtin
+         * bound, or eventually a `auto trait Foo`). If `obligation`
+         * is itself a derived obligation, this is just a clone, but
+         * otherwise we create a "derived obligation" cause so as to
+         * keep track of the original root obligation for error
+         * reporting.
+         */
+
+        // NOTE(flaper87): As of now, it keeps track of the whole error
+        // chain. Ideally, we should have a way to configure this either
+        // by using -Z verbose or just a CLI argument.
+        self.code =
+            variant(DerivedObligationCause { parent_trait_pred, parent_code: self.code }).into();
+        self
     }
 }
 
@@ -182,6 +196,30 @@ pub struct UnifyReceiverContext<'tcx> {
     pub substs: SubstsRef<'tcx>,
 }
 
+#[derive(Clone, Debug, PartialEq, Eq, Hash, Lift, Default)]
+pub struct InternedObligationCauseCode<'tcx> {
+    /// `None` for `MISC_OBLIGATION_CAUSE_CODE` (a common case, occurs ~60% of
+    /// the time). `Some` otherwise.
+    code: Option<Lrc<ObligationCauseCode<'tcx>>>,
+}
+
+impl<'tcx> ObligationCauseCode<'tcx> {
+    #[inline(always)]
+    fn into(self) -> InternedObligationCauseCode<'tcx> {
+        InternedObligationCauseCode {
+            code: if let MISC_OBLIGATION_CAUSE_CODE = self { None } else { Some(Lrc::new(self)) },
+        }
+    }
+}
+
+impl<'tcx> std::ops::Deref for InternedObligationCauseCode<'tcx> {
+    type Target = ObligationCauseCode<'tcx>;
+
+    fn deref(&self) -> &Self::Target {
+        self.code.as_deref().unwrap_or(&MISC_OBLIGATION_CAUSE_CODE)
+    }
+}
+
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 pub enum ObligationCauseCode<'tcx> {
     /// Not well classified or should be obvious from the span.
@@ -269,7 +307,7 @@ pub enum ObligationCauseCode<'tcx> {
         /// The node of the function call.
         call_hir_id: hir::HirId,
         /// The obligation introduced by this argument.
-        parent_code: Lrc<ObligationCauseCode<'tcx>>,
+        parent_code: InternedObligationCauseCode<'tcx>,
     },
 
     /// Error derived when matching traits/impls; see ObligationCause for more details
@@ -404,25 +442,27 @@ pub struct ImplDerivedObligationCause<'tcx> {
     pub span: Span,
 }
 
-impl ObligationCauseCode<'_> {
+impl<'tcx> ObligationCauseCode<'tcx> {
     // Return the base obligation, ignoring derived obligations.
     pub fn peel_derives(&self) -> &Self {
         let mut base_cause = self;
-        loop {
-            match base_cause {
-                BuiltinDerivedObligation(DerivedObligationCause { parent_code, .. })
-                | DerivedObligation(DerivedObligationCause { parent_code, .. })
-                | FunctionArgumentObligation { parent_code, .. } => {
-                    base_cause = &parent_code;
-                }
-                ImplDerivedObligation(obligation_cause) => {
-                    base_cause = &*obligation_cause.derived.parent_code;
-                }
-                _ => break,
-            }
+        while let Some((parent_code, _)) = base_cause.parent() {
+            base_cause = parent_code;
         }
         base_cause
     }
+
+    pub fn parent(&self) -> Option<(&Self, Option<ty::PolyTraitPredicate<'tcx>>)> {
+        match self {
+            FunctionArgumentObligation { parent_code, .. } => Some((parent_code, None)),
+            BuiltinDerivedObligation(derived)
+            | DerivedObligation(derived)
+            | ImplDerivedObligation(box ImplDerivedObligationCause { derived, .. }) => {
+                Some((&derived.parent_code, Some(derived.parent_trait_pred)))
+            }
+            _ => None,
+        }
+    }
 }
 
 // `ObligationCauseCode` is used a lot. Make sure it doesn't unintentionally get bigger.
@@ -472,7 +512,7 @@ pub struct DerivedObligationCause<'tcx> {
     pub parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
 
     /// The parent trait had this cause.
-    pub parent_code: Lrc<ObligationCauseCode<'tcx>>,
+    pub parent_code: InternedObligationCauseCode<'tcx>,
 }
 
 #[derive(Clone, Debug, TypeFoldable, Lift)]

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

@@ -2,11 +2,10 @@ pub mod on_unimplemented;
 pub mod suggestions;
 
 use super::{
-    DerivedObligationCause, EvaluationResult, FulfillmentContext, FulfillmentError,
-    FulfillmentErrorCode, ImplDerivedObligationCause, MismatchedProjectionTypes, Obligation,
-    ObligationCause, ObligationCauseCode, OnUnimplementedDirective, OnUnimplementedNote,
-    OutputTypeParameterMismatch, Overflow, PredicateObligation, SelectionContext, SelectionError,
-    TraitNotObjectSafe,
+    EvaluationResult, FulfillmentContext, FulfillmentError, FulfillmentErrorCode,
+    MismatchedProjectionTypes, Obligation, ObligationCause, ObligationCauseCode,
+    OnUnimplementedDirective, OnUnimplementedNote, OutputTypeParameterMismatch, Overflow,
+    PredicateObligation, SelectionContext, SelectionError, TraitNotObjectSafe,
 };
 
 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
@@ -684,42 +683,12 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                                 let mut code = obligation.cause.code();
                                 let mut trait_pred = trait_predicate;
                                 let mut peeled = false;
-                                loop {
-                                    match &*code {
-                                        ObligationCauseCode::FunctionArgumentObligation {
-                                            parent_code,
-                                            ..
-                                        } => {
-                                            code = &parent_code;
-                                        }
-                                        ObligationCauseCode::ImplDerivedObligation(
-                                            box ImplDerivedObligationCause {
-                                                derived:
-                                                    DerivedObligationCause {
-                                                        parent_code,
-                                                        parent_trait_pred,
-                                                    },
-                                                ..
-                                            },
-                                        )
-                                        | ObligationCauseCode::BuiltinDerivedObligation(
-                                            DerivedObligationCause {
-                                                parent_code,
-                                                parent_trait_pred,
-                                            },
-                                        )
-                                        | ObligationCauseCode::DerivedObligation(
-                                            DerivedObligationCause {
-                                                parent_code,
-                                                parent_trait_pred,
-                                            },
-                                        ) => {
-                                            peeled = true;
-                                            code = &parent_code;
-                                            trait_pred = *parent_trait_pred;
-                                        }
-                                        _ => break,
-                                    };
+                                while let Some((parent_code, parent_trait_pred)) = code.parent() {
+                                    code = parent_code;
+                                    if let Some(parent_trait_pred) = parent_trait_pred {
+                                        trait_pred = parent_trait_pred;
+                                        peeled = true;
+                                    }
                                 }
                                 let def_id = trait_pred.def_id();
                                 // Mention *all* the `impl`s for the *top most* obligation, the

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

@@ -1,6 +1,6 @@
 use super::{
-    DerivedObligationCause, EvaluationResult, ImplDerivedObligationCause, Obligation,
-    ObligationCause, ObligationCauseCode, PredicateObligation, SelectionContext,
+    EvaluationResult, Obligation, ObligationCause, ObligationCauseCode, PredicateObligation,
+    SelectionContext,
 };
 
 use crate::autoderef::Autoderef;
@@ -623,28 +623,11 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let span = obligation.cause.span;
         let mut real_trait_pred = trait_pred;
         let mut code = obligation.cause.code();
-        loop {
-            match &code {
-                ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
-                    code = &parent_code;
-                }
-                ObligationCauseCode::ImplDerivedObligation(box ImplDerivedObligationCause {
-                    derived: DerivedObligationCause { parent_code, parent_trait_pred },
-                    ..
-                })
-                | ObligationCauseCode::BuiltinDerivedObligation(DerivedObligationCause {
-                    parent_code,
-                    parent_trait_pred,
-                })
-                | ObligationCauseCode::DerivedObligation(DerivedObligationCause {
-                    parent_code,
-                    parent_trait_pred,
-                }) => {
-                    code = &parent_code;
-                    real_trait_pred = *parent_trait_pred;
-                }
-                _ => break,
-            };
+        while let Some((parent_code, parent_trait_pred)) = code.parent() {
+            code = parent_code;
+            if let Some(parent_trait_pred) = parent_trait_pred {
+                real_trait_pred = parent_trait_pred;
+            }
             let Some(real_ty) = real_trait_pred.self_ty().no_bound_vars() else {
                 continue;
             };
@@ -1669,7 +1652,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             debug!("maybe_note_obligation_cause_for_async_await: code={:?}", code);
             match code {
                 ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
-                    next_code = Some(parent_code.as_ref());
+                    next_code = Some(parent_code);
                 }
                 ObligationCauseCode::ImplDerivedObligation(cause) => {
                     let ty = cause.derived.parent_trait_pred.skip_binder().self_ty();
@@ -1700,7 +1683,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         _ => {}
                     }
 
-                    next_code = Some(cause.derived.parent_code.as_ref());
+                    next_code = Some(&cause.derived.parent_code);
                 }
                 ObligationCauseCode::DerivedObligation(derived_obligation)
                 | ObligationCauseCode::BuiltinDerivedObligation(derived_obligation) => {
@@ -1732,7 +1715,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         _ => {}
                     }
 
-                    next_code = Some(derived_obligation.parent_code.as_ref());
+                    next_code = Some(&derived_obligation.parent_code);
                 }
                 _ => break,
             }
@@ -2382,8 +2365,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 let is_upvar_tys_infer_tuple = if !matches!(ty.kind(), ty::Tuple(..)) {
                     false
                 } else {
-                    if let ObligationCauseCode::BuiltinDerivedObligation(ref data) =
-                        *data.parent_code
+                    if let ObligationCauseCode::BuiltinDerivedObligation(data) = &*data.parent_code
                     {
                         let parent_trait_ref =
                             self.resolve_vars_if_possible(data.parent_trait_pred);
@@ -2428,7 +2410,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             err,
                             &parent_predicate,
                             param_env,
-                            &cause_code.peel_derives(),
+                            cause_code.peel_derives(),
                             obligated_types,
                             seen_requirements,
                         )

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -17,17 +17,15 @@ use rustc_middle::ty::{ToPolyTraitRef, ToPredicate};
 use rustc_span::def_id::DefId;
 
 use crate::traits::project::{normalize_with_depth, normalize_with_depth_to};
-use crate::traits::select::TraitObligationExt;
 use crate::traits::util::{self, closure_trait_ref_and_return_type, predicate_for_trait_def};
 use crate::traits::{
-    BuiltinDerivedObligation, DerivedObligationCause, ImplDerivedObligation,
-    ImplDerivedObligationCause, ImplSource, ImplSourceAutoImplData, ImplSourceBuiltinData,
-    ImplSourceClosureData, ImplSourceConstDestructData, ImplSourceDiscriminantKindData,
-    ImplSourceFnPointerData, ImplSourceGeneratorData, ImplSourceObjectData, ImplSourcePointeeData,
-    ImplSourceTraitAliasData, ImplSourceTraitUpcastingData, ImplSourceUserDefinedData, Normalized,
-    ObjectCastObligation, Obligation, ObligationCause, OutputTypeParameterMismatch,
-    PredicateObligation, Selection, SelectionError, TraitNotObjectSafe, TraitObligation,
-    Unimplemented, VtblSegment,
+    BuiltinDerivedObligation, ImplDerivedObligation, ImplDerivedObligationCause, ImplSource,
+    ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData,
+    ImplSourceConstDestructData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
+    ImplSourceGeneratorData, ImplSourceObjectData, ImplSourcePointeeData, ImplSourceTraitAliasData,
+    ImplSourceTraitUpcastingData, ImplSourceUserDefinedData, Normalized, ObjectCastObligation,
+    Obligation, ObligationCause, OutputTypeParameterMismatch, PredicateObligation, Selection,
+    SelectionError, TraitNotObjectSafe, TraitObligation, Unimplemented, VtblSegment,
 };
 
 use super::BuiltinImplConditions;
@@ -1126,21 +1124,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let substs = self.rematch_impl(impl_def_id, &new_obligation);
                 debug!(?substs, "impl substs");
 
-                let derived = DerivedObligationCause {
-                    parent_trait_pred: obligation.predicate,
-                    parent_code: obligation.cause.clone_code(),
-                };
-                let derived_code = ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
-                    derived,
-                    impl_def_id,
-                    span: obligation.cause.span,
-                }));
-
-                let cause = ObligationCause::new(
-                    obligation.cause.span,
-                    obligation.cause.body_id,
-                    derived_code,
-                );
+                let cause = obligation.derived_cause(|derived| {
+                    ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
+                        derived,
+                        impl_def_id,
+                        span: obligation.cause.span,
+                    }))
+                });
                 ensure_sufficient_stack(|| {
                     self.vtable_impl(
                         impl_def_id,

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

@@ -14,9 +14,9 @@ use super::util;
 use super::util::{closure_trait_ref_and_return_type, predicate_for_trait_def};
 use super::wf;
 use super::{
-    DerivedObligationCause, ErrorReporting, ImplDerivedObligation, ImplDerivedObligationCause,
-    Normalized, Obligation, ObligationCause, ObligationCauseCode, Overflow, PredicateObligation,
-    Selection, SelectionError, SelectionResult, TraitObligation, TraitQueryMode,
+    ErrorReporting, ImplDerivedObligation, ImplDerivedObligationCause, Normalized, Obligation,
+    ObligationCause, ObligationCauseCode, Overflow, PredicateObligation, Selection, SelectionError,
+    SelectionResult, TraitObligation, TraitQueryMode,
 };
 
 use crate::infer::{InferCtxt, InferOk, TypeFreshener};
@@ -2316,17 +2316,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         debug!(?predicates);
         assert_eq!(predicates.parent, None);
         let mut obligations = Vec::with_capacity(predicates.predicates.len());
-        let parent_code = cause.clone_code();
         for (predicate, span) in predicates.predicates {
             let span = *span;
-            let derived =
-                DerivedObligationCause { parent_trait_pred, parent_code: parent_code.clone() };
-            let code = ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
-                derived,
-                impl_def_id: def_id,
-                span,
-            }));
-            let cause = ObligationCause::new(cause.span, cause.body_id, code);
+            let cause = cause.clone().derived_cause(parent_trait_pred, |derived| {
+                ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
+                    derived,
+                    impl_def_id: def_id,
+                    span,
+                }))
+            });
             let predicate = normalize_with_depth_to(
                 self,
                 param_env,
@@ -2342,42 +2340,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     }
 }
 
-trait TraitObligationExt<'tcx> {
-    fn derived_cause(
-        &self,
-        variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
-    ) -> ObligationCause<'tcx>;
-}
-
-impl<'tcx> TraitObligationExt<'tcx> for TraitObligation<'tcx> {
-    fn derived_cause(
-        &self,
-        variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
-    ) -> ObligationCause<'tcx> {
-        /*!
-         * Creates a cause for obligations that are derived from
-         * `obligation` by a recursive search (e.g., for a builtin
-         * bound, or eventually a `auto trait Foo`). If `obligation`
-         * is itself a derived obligation, this is just a clone, but
-         * otherwise we create a "derived obligation" cause so as to
-         * keep track of the original root obligation for error
-         * reporting.
-         */
-
-        let obligation = self;
-
-        // NOTE(flaper87): As of now, it keeps track of the whole error
-        // chain. Ideally, we should have a way to configure this either
-        // by using -Z verbose or just a CLI argument.
-        let derived_cause = DerivedObligationCause {
-            parent_trait_pred: obligation.predicate,
-            parent_code: obligation.cause.clone_code(),
-        };
-        let derived_code = variant(derived_cause);
-        ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
-    }
-}
-
 impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
     fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> {
         TraitObligationStackList::with(self)

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -294,30 +294,22 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 
         debug!("compute_trait_ref obligations {:?}", obligations);
-        let cause = self.cause(traits::MiscObligation);
         let param_env = self.param_env;
         let depth = self.recursion_depth;
 
         let item = self.item;
 
-        let extend = |obligation: traits::PredicateObligation<'tcx>| {
-            let mut cause = cause.clone();
-            if let Some(parent_trait_pred) = obligation.predicate.to_opt_poly_trait_pred() {
-                let derived_cause = traits::DerivedObligationCause {
+        let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
+            if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
+                cause = cause.derived_cause(
                     parent_trait_pred,
-                    parent_code: obligation.cause.clone_code(),
-                };
-                *cause.make_mut_code() =
-                    traits::ObligationCauseCode::DerivedObligation(derived_cause);
+                    traits::ObligationCauseCode::DerivedObligation,
+                );
             }
             extend_cause_with_original_assoc_item_obligation(
-                tcx,
-                trait_ref,
-                item,
-                &mut cause,
-                obligation.predicate,
+                tcx, trait_ref, item, &mut cause, predicate,
             );
-            traits::Obligation::with_depth(cause, depth, param_env, obligation.predicate)
+            traits::Obligation::with_depth(cause, depth, param_env, predicate)
         };
 
         if let Elaborate::All = elaborate {
@@ -339,17 +331,17 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
                 })
                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
                 .map(|(i, arg)| {
-                    let mut new_cause = cause.clone();
+                    let mut cause = traits::ObligationCause::misc(self.span, self.body_id);
                     // The first subst is the self ty - use the correct span for it.
                     if i == 0 {
                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
                             item.map(|i| &i.kind)
                         {
-                            new_cause.span = self_ty.span;
+                            cause.span = self_ty.span;
                         }
                     }
                     traits::Obligation::with_depth(
-                        new_cause,
+                        cause,
                         depth,
                         param_env,
                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),

compiler/rustc_typeck/src/check/fn_ctxt/checks.rs

@@ -12,7 +12,6 @@ use crate::check::{
 use crate::structured_errors::StructuredDiagnostic;
 
 use rustc_ast as ast;
-use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, Diagnostic, DiagnosticId, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Res};
@@ -1601,24 +1600,21 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // Peel derived obligation, because it's the type that originally
             // started this inference chain that matters, not the one we wound
             // up with at the end.
-            fn unpeel_to_top(
-                mut code: Lrc<ObligationCauseCode<'_>>,
-            ) -> Lrc<ObligationCauseCode<'_>> {
-                let mut result_code = code.clone();
+            fn unpeel_to_top<'a, 'tcx>(
+                mut code: &'a ObligationCauseCode<'tcx>,
+            ) -> &'a ObligationCauseCode<'tcx> {
+                let mut result_code = code;
                 loop {
-                    let parent = match &*code {
-                        ObligationCauseCode::ImplDerivedObligation(c) => {
-                            c.derived.parent_code.clone()
-                        }
+                    let parent = match code {
+                        ObligationCauseCode::ImplDerivedObligation(c) => &c.derived.parent_code,
                         ObligationCauseCode::BuiltinDerivedObligation(c)
-                        | ObligationCauseCode::DerivedObligation(c) => c.parent_code.clone(),
-                        _ => break,
+                        | ObligationCauseCode::DerivedObligation(c) => &c.parent_code,
+                        _ => break result_code,
                     };
-                    result_code = std::mem::replace(&mut code, parent);
+                    (result_code, code) = (code, parent);
                 }
-                result_code
             }
-            let self_: ty::subst::GenericArg<'_> = match &*unpeel_to_top(error.obligation.cause.clone_code()) {
+            let self_: ty::subst::GenericArg<'_> = match unpeel_to_top(error.obligation.cause.code()) {
                 ObligationCauseCode::BuiltinDerivedObligation(code) |
                 ObligationCauseCode::DerivedObligation(code) => {
                     code.parent_trait_pred.self_ty().skip_binder().into()
@@ -1668,13 +1664,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 // We make sure that only *one* argument matches the obligation failure
                 // and we assign the obligation's span to its expression's.
                 error.obligation.cause.span = args[ref_in].span;
-                let parent_code = error.obligation.cause.clone_code();
-                *error.obligation.cause.make_mut_code() =
+                error.obligation.cause.map_code(|parent_code| {
                     ObligationCauseCode::FunctionArgumentObligation {
                         arg_hir_id: args[ref_in].hir_id,
                         call_hir_id: expr.hir_id,
                         parent_code,
-                    };
+                    }
+                });
             } else if error.obligation.cause.span == call_sp {
                 // Make function calls point at the callee, not the whole thing.
                 if let hir::ExprKind::Call(callee, _) = expr.kind {

src/test/ui/associated-types/hr-associated-type-projection-1.stderr

@@ -2,10 +2,18 @@ error[E0271]: type mismatch resolving `<T as Deref>::Target == T`
   --> $DIR/hr-associated-type-projection-1.rs:13:33
    |
 LL | impl<T: Copy + std::ops::Deref> UnsafeCopy<'_, T> for T {
-   |      - this type parameter      ^^^^^^^^^^^^^^^^^ expected associated type, found type parameter `T`
+   |      - this type parameter      ^^^^^^^^^^^^^^^^^ expected type parameter `T`, found associated type
    |
-   = note: expected associated type `<T as Deref>::Target`
-               found type parameter `T`
+   = note: expected type parameter `T`
+             found associated type `<T as Deref>::Target`
+note: required by a bound in `UnsafeCopy`
+  --> $DIR/hr-associated-type-projection-1.rs:3:64
+   |
+LL | trait UnsafeCopy<'a, T: Copy>
+   |       ---------- required by a bound in this
+LL | where
+LL |     for<'b> <Self as UnsafeCopy<'b, T>>::Item: std::ops::Deref<Target = T>,
+   |                                                                ^^^^^^^^^^ required by this bound in `UnsafeCopy`
 help: consider further restricting this bound
    |
 LL | impl<T: Copy + std::ops::Deref + Deref<Target = T>> UnsafeCopy<'_, T> for T {

src/test/ui/builtin-superkinds/builtin-superkinds-self-type.stderr

@@ -2,8 +2,13 @@ error[E0310]: the parameter type `T` may not live long enough
   --> $DIR/builtin-superkinds-self-type.rs:10:16
    |
 LL | impl <T: Sync> Foo for T { }
-   |                ^^^ ...so that the type `T` will meet its required lifetime bounds
+   |                ^^^ ...so that the type `T` will meet its required lifetime bounds...
    |
+note: ...that is required by this bound
+  --> $DIR/builtin-superkinds-self-type.rs:6:24
+   |
+LL | trait Foo : Sized+Sync+'static {
+   |                        ^^^^^^^
 help: consider adding an explicit lifetime bound...
    |
 LL | impl <T: Sync + 'static> Foo for T { }

src/test/ui/traits/assoc-type-in-superbad.stderr

@@ -2,7 +2,13 @@ error[E0271]: type mismatch resolving `<std::vec::IntoIter<i32> as Iterator>::It
   --> $DIR/assoc-type-in-superbad.rs:12:16
    |
 LL |     type Key = u32;
-   |                ^^^ expected `i32`, found `u32`
+   |                ^^^ expected `u32`, found `i32`
+   |
+note: required by a bound in `Foo`
+  --> $DIR/assoc-type-in-superbad.rs:7:25
+   |
+LL | pub trait Foo: Iterator<Item=<Self as Foo>::Key> {
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Foo`
 
 error: aborting due to previous error