Pass flow_inits by value.

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 flow_inits = MaybeInitializedPlaces::new(tcx, body, &move_data)
+    let mut flow_inits = MaybeInitializedPlaces::new(tcx, body, &move_data)
         .iterate_to_fixpoint(tcx, body, Some("borrowck"))
         .into_results_cursor(body);
 
@@ -211,12 +211,16 @@ fn do_mir_borrowck<'tcx>(
         body,
         &promoted,
         &location_table,
-        flow_inits,
+        &mut 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);

compiler/rustc_borrowck/src/nll.rs

@@ -80,7 +80,7 @@ pub(crate) fn compute_regions<'a, 'tcx>(
     body: &Body<'tcx>,
     promoted: &IndexSlice<Promoted, Body<'tcx>>,
     location_table: &LocationTable,
-    flow_inits: ResultsCursor<'a, 'tcx, MaybeInitializedPlaces<'a, 'tcx>>,
+    flow_inits: &mut ResultsCursor<'a, 'tcx, MaybeInitializedPlaces<'a, 'tcx>>,
     move_data: &MoveData<'tcx>,
     borrow_set: &BorrowSet<'tcx>,
     consumer_options: Option<ConsumerOptions>,

compiler/rustc_borrowck/src/type_check/liveness/mod.rs

@@ -32,7 +32,7 @@ pub(super) fn generate<'a, 'tcx>(
     typeck: &mut TypeChecker<'_, 'tcx>,
     body: &Body<'tcx>,
     elements: &DenseLocationMap,
-    flow_inits: ResultsCursor<'a, 'tcx, MaybeInitializedPlaces<'a, 'tcx>>,
+    flow_inits: &mut ResultsCursor<'a, 'tcx, MaybeInitializedPlaces<'a, 'tcx>>,
     move_data: &MoveData<'tcx>,
 ) {
     debug!("liveness::generate");

compiler/rustc_borrowck/src/type_check/liveness/trace.rs

@@ -38,7 +38,7 @@ pub(super) fn trace<'a, 'tcx>(
     typeck: &mut TypeChecker<'_, 'tcx>,
     body: &Body<'tcx>,
     elements: &DenseLocationMap,
-    flow_inits: ResultsCursor<'a, 'tcx, MaybeInitializedPlaces<'a, 'tcx>>,
+    flow_inits: &mut ResultsCursor<'a, 'tcx, MaybeInitializedPlaces<'a, 'tcx>>,
     move_data: &MoveData<'tcx>,
     relevant_live_locals: Vec<Local>,
     boring_locals: Vec<Local>,
@@ -113,7 +113,7 @@ struct LivenessContext<'a, 'typeck, 'b, 'tcx> {
 
     /// Results of dataflow tracking which variables (and paths) have been
     /// initialized.
-    flow_inits: ResultsCursor<'b, 'tcx, MaybeInitializedPlaces<'b, 'tcx>>,
+    flow_inits: &'a mut ResultsCursor<'b, 'tcx, MaybeInitializedPlaces<'b, 'tcx>>,
 
     /// Index indicating where each variable is assigned, used, or
     /// dropped.

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -124,7 +124,7 @@ pub(crate) fn type_check<'a, 'tcx>(
     location_table: &LocationTable,
     borrow_set: &BorrowSet<'tcx>,
     all_facts: &mut Option<AllFacts>,
-    flow_inits: ResultsCursor<'a, 'tcx, MaybeInitializedPlaces<'a, 'tcx>>,
+    flow_inits: &mut ResultsCursor<'a, 'tcx, MaybeInitializedPlaces<'a, 'tcx>>,
     move_data: &MoveData<'tcx>,
     elements: Rc<DenseLocationMap>,
 ) -> MirTypeckResults<'tcx> {