make traversal::reverse_postorder use RPO cache

compiler/rustc_middle/src/mir/traversal.rs

@@ -295,34 +295,11 @@ pub fn reachable_as_bitset(body: &Body<'_>) -> BitSet<BasicBlock> {
     iter.visited
 }
 
-#[derive(Clone)]
+/// Creates an iterator over the `Body`'s basic blocks, that:
-pub struct ReversePostorderIter<'a, 'tcx> {
+/// - returns basic blocks in a reverse postorder,
+/// - makes use of the `BasicBlocks` CFG cache's reverse postorder.
+pub fn reverse_postorder<'a, 'tcx>(
     body: &'a Body<'tcx>,
-    blocks: &'a [BasicBlock],
+) -> impl Iterator<Item = (BasicBlock, &'a BasicBlockData<'tcx>)> + ExactSizeIterator {
-    idx: usize,
+    body.basic_blocks.reverse_postorder().iter().map(|&bb| (bb, &body.basic_blocks[bb]))
-}
-
-impl<'a, 'tcx> Iterator for ReversePostorderIter<'a, 'tcx> {
-    type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
-
-    fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
-        if self.idx == 0 {
-            return None;
-        }
-        self.idx -= 1;
-
-        self.blocks.get(self.idx).map(|&bb| (bb, &self.body[bb]))
-    }
-
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        (self.idx, Some(self.idx))
-    }
-}
-
-impl<'a, 'tcx> ExactSizeIterator for ReversePostorderIter<'a, 'tcx> {}
-
-pub fn reverse_postorder<'a, 'tcx>(body: &'a Body<'tcx>) -> ReversePostorderIter<'a, 'tcx> {
-    let blocks = body.basic_blocks.postorder();
-    let len = blocks.len();
-    ReversePostorderIter { body, blocks, idx: len }
 }