Use Vec extend instead of repeated pushes in several places

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -329,9 +329,7 @@ impl<'a, 'tcx> CoverageSpans<'a, 'tcx> {
     fn mir_to_initial_sorted_coverage_spans(&self) -> Vec<CoverageSpan> {
         let mut initial_spans = Vec::<CoverageSpan>::with_capacity(self.mir_body.num_nodes() * 2);
         for (bcb, bcb_data) in self.basic_coverage_blocks.iter_enumerated() {
-            for coverage_span in self.bcb_to_initial_coverage_spans(bcb, bcb_data) {
+            initial_spans.extend(self.bcb_to_initial_coverage_spans(bcb, bcb_data));
-                initial_spans.push(coverage_span);
-            }
         }
 
         if initial_spans.is_empty() {

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -498,9 +498,7 @@ fn orphan_check_trait_ref<'tcx>(
             return Err(OrphanCheckErr::UncoveredTy(input_ty, local_type));
         }
 
-        for input_ty in non_local_tys {
+        non_local_spans.extend(non_local_tys.into_iter().map(|input_ty| (input_ty, i == 0)));
-            non_local_spans.push((input_ty, i == 0));
-        }
     }
     // If we exit above loop, never found a local type.
     debug!("orphan_check_trait_ref: no local type");

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

@@ -362,9 +362,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             .infcx
             .probe(|_| self.match_projection_obligation_against_definition_bounds(obligation));
 
-        for predicate_index in result {
+        candidates.vec.extend(result.into_iter().map(ProjectionCandidate));
-            candidates.vec.push(ProjectionCandidate(predicate_index));
-        }
     }
 
     /// Given an obligation like `<SomeTrait for T>`, searches the obligations that the caller

compiler/rustc_typeck/src/check/mod.rs

@@ -686,9 +686,8 @@ fn bounds_from_generic_predicates<'tcx>(
     };
     let mut where_clauses = vec![];
     for (ty, bounds) in types {
-        for bound in &bounds {
+        where_clauses
-            where_clauses.push(format!("{}: {}", ty, tcx.def_path_str(*bound)));
+            .extend(bounds.into_iter().map(|bound| format!("{}: {}", ty, tcx.def_path_str(bound))));
-        }
     }
     for projection in &projections {
         let p = projection.skip_binder();

compiler/rustc_typeck/src/check/upvar.rs

@@ -904,10 +904,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) -> MigrationWarningReason {
         let mut reasons = MigrationWarningReason::default();
 
-        for auto_trait in auto_trait_reasons {
+        reasons.auto_traits.extend(auto_trait_reasons);
-            reasons.auto_traits.push(auto_trait);
-        }
-
         reasons.drop_order = drop_order;
 
         reasons