Remove `predecessors_for`

There is no `Arc::map` equivalent to `LockGuard::map`

src/librustc_codegen_ssa/mir/mod.rs

@@ -155,7 +155,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     let cleanup_kinds = analyze::cleanup_kinds(&mir);
     // Allocate a `Block` for every basic block, except
     // the start block, if nothing loops back to it.
-    let reentrant_start_block = !mir.predecessors_for(mir::START_BLOCK).is_empty();
+    let reentrant_start_block = !mir.predecessors()[mir::START_BLOCK].is_empty();
     let block_bxs: IndexVec<mir::BasicBlock, Bx::BasicBlock> = mir
         .basic_blocks()
         .indices()

src/librustc_middle/mir/mod.rs

@@ -23,14 +23,12 @@
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::graph::dominators::{dominators, Dominators};
 use rustc_data_structures::graph::{self, GraphSuccessors};
-use rustc_data_structures::sync::MappedLockGuard;
 use rustc_index::bit_set::BitMatrix;
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_macros::HashStable;
 use rustc_serialize::{Decodable, Encodable};
 use rustc_span::symbol::Symbol;
 use rustc_span::{Span, DUMMY_SP};
-use smallvec::SmallVec;
 use std::borrow::Cow;
 use std::fmt::{self, Debug, Display, Formatter, Write};
 use std::ops::{Index, IndexMut};
@@ -392,15 +390,6 @@ pub fn terminator_loc(&self, bb: BasicBlock) -> Location {
         Location { block: bb, statement_index: self[bb].statements.len() }
     }
 
-    #[inline]
-    pub fn predecessors_for(
-        &self,
-        bb: BasicBlock,
-    ) -> impl std::ops::Deref<Target = SmallVec<[BasicBlock; 4]>> + '_ {
-        let predecessors = self.predecessor_cache.compute(&self.basic_blocks);
-        MappedLockGuard::map(predecessors, |preds| &mut preds[bb])
-    }
-
     #[inline]
     pub fn predecessors(&self) -> impl std::ops::Deref<Target = Predecessors> + '_ {
         self.predecessor_cache.compute(&self.basic_blocks)
@@ -2676,7 +2665,7 @@ impl graph::GraphPredecessors<'graph> for Body<'tcx> {
 impl graph::WithPredecessors for Body<'tcx> {
     #[inline]
     fn predecessors(&self, node: Self::Node) -> <Self as graph::GraphPredecessors<'_>>::Iter {
-        self.predecessors_for(node).clone().into_iter()
+        self.predecessors()[node].clone().into_iter()
     }
 }
 

src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs

@@ -1268,7 +1268,7 @@ fn predecessor_locations(
             location: Location,
         ) -> impl Iterator<Item = Location> + 'a {
             if location.statement_index == 0 {
-                let predecessors = body.predecessors_for(location.block).to_vec();
+                let predecessors = body.predecessors()[location.block].to_vec();
                 Either::Left(predecessors.into_iter().map(move |bb| body.terminator_loc(bb)))
             } else {
                 Either::Right(std::iter::once(Location {

src/librustc_mir/borrow_check/region_infer/values.rs

@@ -89,7 +89,7 @@ impl RegionValueElements {
             // If this is a basic block head, then the predecessors are
             // the terminators of other basic blocks
             stack.extend(
-                body.predecessors_for(block)
+                body.predecessors()[block]
                     .iter()
                     .map(|&pred_bb| body.terminator_loc(pred_bb))
                     .map(|pred_loc| self.point_from_location(pred_loc)),

src/librustc_mir/borrow_check/type_check/liveness/trace.rs

@@ -303,7 +303,7 @@ fn compute_drop_live_points_for_block(&mut self, mpi: MovePathIndex, term_point:
         }
 
         let body = self.cx.body;
-        for &pred_block in body.predecessors_for(block).iter() {
+        for &pred_block in body.predecessors()[block].iter() {
             debug!("compute_drop_live_points_for_block: pred_block = {:?}", pred_block,);
 
             // Check whether the variable is (at least partially)