Lazy dominator tree construction in borrowck

Motivated by the observation that sometimes constructed dominator tree
was never queried.

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -2193,7 +2193,7 @@ 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);
@@ -2305,7 +2305,7 @@ 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

@@ -5,6 +5,7 @@
 #![feature(let_chains)]
 #![feature(min_specialization)]
 #![feature(never_type)]
+#![feature(once_cell)]
 #![feature(rustc_attrs)]
 #![feature(stmt_expr_attributes)]
 #![feature(trusted_step)]
@@ -39,6 +40,7 @@ use rustc_span::{Span, Symbol};
 
 use either::Either;
 use smallvec::SmallVec;
+use std::cell::OnceCell;
 use std::cell::RefCell;
 use std::collections::BTreeMap;
 use std::rc::Rc;
@@ -333,7 +335,7 @@ fn do_mir_borrowck<'tcx>(
                 used_mut: Default::default(),
                 used_mut_upvars: SmallVec::new(),
                 borrow_set: Rc::clone(&borrow_set),
-                dominators: Dominators::dummy(), // not used
+                dominators: Default::default(),
                 upvars: Vec::new(),
                 local_names: IndexVec::from_elem(None, &promoted_body.local_decls),
                 region_names: RefCell::default(),
@@ -346,8 +348,6 @@ fn do_mir_borrowck<'tcx>(
         };
     }
 
-    let dominators = body.basic_blocks.dominators();
-
     let mut mbcx = MirBorrowckCtxt {
         infcx,
         param_env,
@@ -364,7 +364,7 @@ fn do_mir_borrowck<'tcx>(
         used_mut: Default::default(),
         used_mut_upvars: SmallVec::new(),
         borrow_set: Rc::clone(&borrow_set),
-        dominators,
+        dominators: Default::default(),
         upvars,
         local_names,
         region_names: RefCell::default(),
@@ -534,7 +534,7 @@ struct MirBorrowckCtxt<'cx, 'tcx> {
     borrow_set: Rc<BorrowSet<'tcx>>,
 
     /// Dominators for MIR
-    dominators: Dominators<BasicBlock>,
+    dominators: OnceCell<Dominators<BasicBlock>>,
 
     /// Information about upvars not necessarily preserved in types or MIR
     upvars: Vec<Upvar<'tcx>>,
@@ -1051,7 +1051,7 @@ 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;
                     }
@@ -2219,6 +2219,10 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     fn is_upvar_field_projection(&self, place_ref: PlaceRef<'tcx>) -> Option<Field> {
         path_utils::is_upvar_field_projection(self.infcx.tcx, &self.upvars, place_ref, self.body())
     }
+
+    fn dominators(&self) -> &Dominators<BasicBlock> {
+        self.dominators.get_or_init(|| self.body.basic_blocks.dominators())
+    }
 }
 
 mod error {

compiler/rustc_data_structures/src/graph/dominators/mod.rs

@@ -268,10 +268,6 @@ pub struct Dominators<N: Idx> {
 }
 
 impl<Node: Idx> Dominators<Node> {
-    pub fn dummy() -> Self {
-        Self { post_order_rank: IndexVec::new(), immediate_dominators: IndexVec::new() }
-    }
-
     pub fn is_reachable(&self, node: Node) -> bool {
         self.immediate_dominators[node].is_some()
     }