rustc_mir_build: drive-by-cleanup: remove some local mutable state

Author: WaffleLapkin

compiler/rustc_mir_build/src/build/scope.rs

@@ -671,12 +671,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         } else {
             self.scopes.breakable_scopes[break_index].continue_drops.as_mut().unwrap()
         };
-        let mut drop_idx = ROOT_NODE;
-        for scope in &self.scopes.scopes[scope_index + 1..] {
-            for drop in &scope.drops {
-                drop_idx = drops.add_drop(*drop, drop_idx);
-            }
-        }
+
+        let drop_idx = self.scopes.scopes[scope_index + 1..]
+            .iter()
+            .flat_map(|scope| &scope.drops)
+            .fold(ROOT_NODE, |drop_idx, &drop| drops.add_drop(drop, drop_idx));
+
         drops.add_entry(block, drop_idx);
 
         // `build_drop_trees` doesn't have access to our source_info, so we