Rollup merge of #98921 - TaKO8Ki:refactor-fulfillment-context-select, r=Dylan-DPC

Refactor: remove a redundant mutable variable

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -131,8 +131,6 @@ fn select(&mut self, selcx: &mut SelectionContext<'a, 'tcx>) -> Vec<FulfillmentE
         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
         let _enter = span.enter();
 
-        let mut errors = Vec::new();
-
         // Process pending obligations.
         let outcome: Outcome<_, _> = self.predicates.process_obligations(&mut FulfillProcessor {
             selcx,
@@ -142,7 +140,8 @@ fn select(&mut self, selcx: &mut SelectionContext<'a, 'tcx>) -> Vec<FulfillmentE
         // FIXME: if we kept the original cache key, we could mark projection
         // obligations as complete for the projection cache here.
 
-        errors.extend(outcome.errors.into_iter().map(to_fulfillment_error));
+        let errors: Vec<FulfillmentError<'tcx>> =
+            outcome.errors.into_iter().map(to_fulfillment_error).collect();
 
         debug!(
             "select({} predicates remaining, {} errors) done",
@@ -728,7 +727,7 @@ fn process_projection_obligation(
                 }
                 return ProcessResult::Changed(vec![]);
             } else {
-                tracing::debug!("Does NOT hold: {:?}", obligation);
+                debug!("Does NOT hold: {:?}", obligation);
             }
         }