Move an extension trait method onto the type directly and reuse it

compiler/rustc_infer/src/traits/mod.rs

@@ -69,7 +69,7 @@ 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()) {
@@ -77,6 +77,13 @@ impl TraitObligation<'_> {
             _ => false,
         }
     }
+
+    pub fn derived_cause(
+        &self,
+        variant: fn(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

@@ -179,6 +179,30 @@ impl<'tcx> ObligationCause<'tcx> {
             None => Lrc::new(MISC_OBLIGATION_CAUSE_CODE),
         }));
     }
+
+    pub fn derived_cause(
+        mut self,
+        parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
+        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.
+         */
+
+        // 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.map_code(|parent_code| {
+            variant(DerivedObligationCause { parent_trait_pred, parent_code }).into()
+        });
+        self
+    }
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]

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

@@ -17,7 +17,6 @@ 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,

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

@@ -2342,42 +2342,6 @@ 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

@@ -301,14 +301,10 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 
         let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
             if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
-                cause.map_code(|parent_code| {
+                cause = cause.derived_cause(
-                    {
+                    parent_trait_pred,
-                        traits::ObligationCauseCode::DerivedObligation(
+                    traits::ObligationCauseCode::DerivedObligation,
-                            traits::DerivedObligationCause { parent_trait_pred, parent_code },
+                );
-                        )
-                    }
-                    .into()
-                });
             }
             extend_cause_with_original_assoc_item_obligation(
                 tcx, trait_ref, item, &mut cause, predicate,