Use a RefCell in MaybeRequiresStorage.

This will let us make `Analysis` arguments in many other places
immutable, in the next commit.

Author: nnethercote

compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs

@@ -1,4 +1,5 @@
 use std::borrow::Cow;
+use std::cell::RefCell;
 
 use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::visit::{NonMutatingUseContext, PlaceContext, Visitor};
@@ -115,12 +116,12 @@ type BorrowedLocalsResults<'mir, 'tcx> = ResultsCursor<'mir, 'tcx, MaybeBorrowed
 /// Dataflow analysis that determines whether each local requires storage at a
 /// given location; i.e. whether its storage can go away without being observed.
 pub struct MaybeRequiresStorage<'mir, 'tcx> {
-    borrowed_locals: BorrowedLocalsResults<'mir, 'tcx>,
+    borrowed_locals: RefCell<BorrowedLocalsResults<'mir, 'tcx>>,
 }
 
 impl<'mir, 'tcx> MaybeRequiresStorage<'mir, 'tcx> {
     pub fn new(borrowed_locals: BorrowedLocalsResults<'mir, 'tcx>) -> Self {
-        MaybeRequiresStorage { borrowed_locals }
+        MaybeRequiresStorage { borrowed_locals: RefCell::new(borrowed_locals) }
     }
 }
 
@@ -294,9 +295,10 @@ impl<'tcx> Analysis<'tcx> for MaybeRequiresStorage<'_, 'tcx> {
 
 impl<'tcx> MaybeRequiresStorage<'_, 'tcx> {
     /// Kill locals that are fully moved and have not been borrowed.
-    fn check_for_move(&mut self, state: &mut <Self as Analysis<'tcx>>::Domain, loc: Location) {
-        let body = self.borrowed_locals.body();
-        let mut visitor = MoveVisitor { state, borrowed_locals: &mut self.borrowed_locals };
+    fn check_for_move(&self, state: &mut <Self as Analysis<'tcx>>::Domain, loc: Location) {
+        let mut borrowed_locals = self.borrowed_locals.borrow_mut();
+        let body = borrowed_locals.body();
+        let mut visitor = MoveVisitor { state, borrowed_locals: &mut borrowed_locals };
         visitor.visit_location(body, loc);
     }
 }