Pass flow_inits by value.

It's simpler that way, and we don't need the explicit `drop`.

compiler/rustc_borrowck/src/lib.rs

@@ -190,7 +190,7 @@ fn do_mir_borrowck<'tcx>(
         .iter_enumerated()
         .map(|(idx, body)| (idx, MoveData::gather_moves(body, tcx, |_| true)));
 
-    let mut flow_inits = MaybeInitializedPlaces::new(tcx, body, &move_data)
+    let flow_inits = MaybeInitializedPlaces::new(tcx, body, &move_data)
         .iterate_to_fixpoint(tcx, body, Some("borrowck"))
         .into_results_cursor(body);
 
@@ -211,16 +211,12 @@ fn do_mir_borrowck<'tcx>(
         body,
         &promoted,
         &location_table,
-        &mut flow_inits,
+        flow_inits,
         &move_data,
         &borrow_set,
         consumer_options,
     );
 
-    // `flow_inits` is large, so we drop it as soon as possible. This reduces
-    // peak memory usage significantly on some benchmarks.
-    drop(flow_inits);
-
     // Dump MIR results into a file, if that is enabled. This let us
     // write unit-tests, as well as helping with debugging.
     nll::dump_nll_mir(&infcx, body, &regioncx, &opt_closure_req, &borrow_set);