Compute dominators on demand for borrowck.

Author: cjgillot

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -2382,7 +2382,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         let mut back_edge_stack = Vec::new();
 
         predecessor_locations(self.body, location).for_each(|predecessor| {
-            if location.dominates(predecessor, self.dominators) {
+            if location.dominates(predecessor, self.dominators()) {
                 back_edge_stack.push(predecessor)
             } else {
                 stack.push(predecessor);
@@ -2494,7 +2494,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
             let mut has_predecessor = false;
             predecessor_locations(self.body, location).for_each(|predecessor| {
-                if location.dominates(predecessor, self.dominators) {
+                if location.dominates(predecessor, self.dominators()) {
                     back_edge_stack.push(predecessor)
                 } else {
                     stack.push(predecessor);

compiler/rustc_borrowck/src/lib.rs

@@ -330,7 +330,6 @@ fn do_mir_borrowck<'tcx>(
                 used_mut: Default::default(),
                 used_mut_upvars: SmallVec::new(),
                 borrow_set: Rc::clone(&borrow_set),
-                dominators: promoted_body.basic_blocks.dominators(),
                 upvars: Vec::new(),
                 local_names: IndexVec::from_elem(None, &promoted_body.local_decls),
                 region_names: RefCell::default(),
@@ -359,7 +358,6 @@ fn do_mir_borrowck<'tcx>(
         used_mut: Default::default(),
         used_mut_upvars: SmallVec::new(),
         borrow_set: Rc::clone(&borrow_set),
-        dominators: body.basic_blocks.dominators(),
         upvars,
         local_names,
         region_names: RefCell::default(),
@@ -590,9 +588,6 @@ struct MirBorrowckCtxt<'cx, 'tcx> {
     /// The set of borrows extracted from the MIR
     borrow_set: Rc<BorrowSet<'tcx>>,
 
-    /// Dominators for MIR
-    dominators: &'cx Dominators<BasicBlock>,
-
     /// Information about upvars not necessarily preserved in types or MIR
     upvars: Vec<Upvar<'tcx>>,
 
@@ -1102,7 +1097,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
                 (Read(kind), BorrowKind::Unique | BorrowKind::Mut { .. }) => {
                     // Reading from mere reservations of mutable-borrows is OK.
-                    if !is_active(this.dominators, borrow, location) {
+                    if !is_active(this.dominators(), borrow, location) {
                         assert!(allow_two_phase_borrow(borrow.kind));
                         return Control::Continue;
                     }
@@ -2266,6 +2261,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     fn is_upvar_field_projection(&self, place_ref: PlaceRef<'tcx>) -> Option<FieldIdx> {
         path_utils::is_upvar_field_projection(self.infcx.tcx, &self.upvars, place_ref, self.body())
     }
+
+    fn dominators(&self) -> &Dominators<BasicBlock> {
+        // `BasicBlocks` computes dominators on-demand and caches them.
+        self.body.basic_blocks.dominators()
+    }
 }
 
 mod error {