Rollup merge of rust-lang#102348 - nnethercote:tweak-FulfillProcessor, r=jackh726

Tweak `FulfillProcessor`.

Avoids some unnecessary references and lifetimes.

r? `@jackh726`

Author: JohnTitor

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -102,7 +102,7 @@ impl<'a, 'tcx> FulfillmentContext<'tcx> {
     }
 
     /// Attempts to select obligations using `selcx`.
-    fn select(&mut self, selcx: &mut SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
+    fn select(&mut self, selcx: SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
         let _enter = span.enter();
 
@@ -197,8 +197,8 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
     ) -> Vec<FulfillmentError<'tcx>> {
-        let mut selcx = SelectionContext::new(infcx);
-        self.select(&mut selcx)
+        let selcx = SelectionContext::new(infcx);
+        self.select(selcx)
     }
 
     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
@@ -210,8 +210,8 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
     }
 }
 
-struct FulfillProcessor<'a, 'b, 'tcx> {
-    selcx: &'a mut SelectionContext<'b, 'tcx>,
+struct FulfillProcessor<'a, 'tcx> {
+    selcx: SelectionContext<'a, 'tcx>,
 }
 
 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
@@ -220,7 +220,7 @@ fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligatio
         .collect()
 }
 
-impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
+impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
     type Obligation = PendingPredicateObligation<'tcx>;
     type Error = FulfillmentErrorCode<'tcx>;
     type OUT = Outcome<Self::Obligation, Self::Error>;
@@ -291,7 +291,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
         if obligation.predicate.has_projections() {
             let mut obligations = Vec::new();
             let predicate = crate::traits::project::try_normalize_with_depth_to(
-                self.selcx,
+                &mut self.selcx,
                 obligation.param_env,
                 obligation.cause.clone(),
                 obligation.recursion_depth + 1,
@@ -608,7 +608,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
     }
 }
 
-impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
+impl<'a, 'tcx> FulfillProcessor<'a, 'tcx> {
     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
     fn process_trait_obligation(
         &mut self,
@@ -643,7 +643,7 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
                 // information about the types in the trait.
                 stalled_on.clear();
                 stalled_on.extend(substs_infer_vars(
-                    self.selcx,
+                    &self.selcx,
                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
                 ));
 
@@ -695,12 +695,12 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
             }
         }
 
-        match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
+        match project::poly_project_and_unify_type(&mut self.selcx, &project_obligation) {
             ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
             ProjectAndUnifyResult::FailedNormalization => {
                 stalled_on.clear();
                 stalled_on.extend(substs_infer_vars(
-                    self.selcx,
+                    &self.selcx,
                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
                 ));
                 ProcessResult::Unchanged
@@ -718,7 +718,7 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 
 /// Returns the set of inference variables contained in `substs`.
 fn substs_infer_vars<'a, 'tcx>(
-    selcx: &mut SelectionContext<'a, 'tcx>,
+    selcx: &SelectionContext<'a, 'tcx>,
     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
     selcx