Make OUT an associated type instead of a generic parameter

This avoids toil when changing other functions in `ObligationForest` to take an `OUT` parameter.

Author: jyn514

compiler/rustc_data_structures/src/obligation_forest/mod.rs

@@ -95,6 +95,10 @@ pub trait ForestObligation: Clone + Debug {
 pub trait ObligationProcessor {
     type Obligation: ForestObligation;
     type Error: Debug;
+    type OUT: OutcomeTrait<
+        Obligation = Self::Obligation,
+        Error = Error<Self::Obligation, Self::Error>,
+    >;
 
     fn needs_process_obligation(&self, obligation: &Self::Obligation) -> bool;
 

compiler/rustc_data_structures/src/obligation_forest/tests.rs

@@ -64,6 +64,7 @@ where
 {
     type Obligation = O;
     type Error = E;
+    type OUT = TestOutcome<O, E>;
 
     fn needs_process_obligation(&self, _obligation: &Self::Obligation) -> bool {
         true

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -224,6 +224,7 @@ fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligatio
 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
     type Obligation = PendingPredicateObligation<'tcx>;
     type Error = FulfillmentErrorCode<'tcx>;
+    type OUT = Outcome<Self::Obligation, Self::Error>;
 
     /// Identifies whether a predicate obligation needs processing.
     ///