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

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)]