From 0eec39b67d12d121095a1da4e24109ce4dc41054 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?R=C3=A9my=20Rakic?= <remy.rakic+github@gmail.com>
Date: Wed, 14 Jun 2023 19:59:41 +0000
Subject: [PATCH] make `traversal::reverse_postorder` use RPO cache

---
 compiler/rustc_middle/src/mir/traversal.rs | 35 ++++------------------
 1 file changed, 6 insertions(+), 29 deletions(-)

diff --git a/compiler/rustc_middle/src/mir/traversal.rs b/compiler/rustc_middle/src/mir/traversal.rs
index 99ead14139a..780003f5fa0 100644
--- a/compiler/rustc_middle/src/mir/traversal.rs
+++ b/compiler/rustc_middle/src/mir/traversal.rs
@@ -295,34 +295,11 @@ pub fn reachable_as_bitset(body: &Body<'_>) -> BitSet<BasicBlock> {
     iter.visited
 }
 
-#[derive(Clone)]
-pub struct ReversePostorderIter<'a, 'tcx> {
+/// Creates an iterator over the `Body`'s basic blocks, that:
+/// - 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],
-    idx: usize,
-}
-
-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 }
+) -> impl Iterator<Item = (BasicBlock, &'a BasicBlockData<'tcx>)> + ExactSizeIterator {
+    body.basic_blocks.reverse_postorder().iter().map(|&bb| (bb, &body.basic_blocks[bb]))
 }