From 9d7ec93d2867a3ebd39ba10f237452ae49ff2b34 Mon Sep 17 00:00:00 2001
From: Jim Blandy <jimb@red-bean.com>
Date: Thu, 6 Feb 2025 17:41:55 -0800
Subject: [PATCH] [naga] Simplify iterator construction in
 `type_expression_tandem`. (#7070)

Rather than reversing two iterators and then zipping them, zip them
first and then reverse the result.

However, zipped iterators are only reversible if the inputs implement
`ExactSizeIterator`, so make `UniqueArena::iter` promise that as well.

For consistency, make `Arena::iter` also promise to return an
`ExactSizeIterator`.
---
 naga/src/arena/mod.rs          | 2 +-
 naga/src/arena/unique_arena.rs | 2 +-
 naga/src/compact/mod.rs        | 5 +++--
 3 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/naga/src/arena/mod.rs b/naga/src/arena/mod.rs
index 8bd3fb84cf..014c5167c6 100644
--- a/naga/src/arena/mod.rs
+++ b/naga/src/arena/mod.rs
@@ -94,7 +94,7 @@ impl<T> Arena<T> {
 
     /// Returns an iterator over the items stored in this arena, returning both
     /// the item's handle and a reference to it.
-    pub fn iter(&self) -> impl DoubleEndedIterator<Item = (Handle<T>, &T)> {
+    pub fn iter(&self) -> impl DoubleEndedIterator<Item = (Handle<T>, &T)> + ExactSizeIterator {
         self.data
             .iter()
             .enumerate()
diff --git a/naga/src/arena/unique_arena.rs b/naga/src/arena/unique_arena.rs
index c64bb302eb..9f5e26df11 100644
--- a/naga/src/arena/unique_arena.rs
+++ b/naga/src/arena/unique_arena.rs
@@ -108,7 +108,7 @@ impl<T> Iterator for UniqueArenaDrain<'_, T> {
 impl<T: Eq + hash::Hash> UniqueArena<T> {
     /// Returns an iterator over the items stored in this arena, returning both
     /// the item's handle and a reference to it.
-    pub fn iter(&self) -> impl DoubleEndedIterator<Item = (Handle<T>, &T)> {
+    pub fn iter(&self) -> impl DoubleEndedIterator<Item = (Handle<T>, &T)> + ExactSizeIterator {
         self.set.iter().enumerate().map(|(i, v)| {
             let index = unsafe { Index::new_unchecked(i as u32) };
             (Handle::new(index), v)
diff --git a/naga/src/compact/mod.rs b/naga/src/compact/mod.rs
index ef41a611cb..929888b7e5 100644
--- a/naga/src/compact/mod.rs
+++ b/naga/src/compact/mod.rs
@@ -303,8 +303,9 @@ impl<'module> ModuleTracer<'module> {
         // as used by the time we visit it is genuinely unused, and can be
         // ignored.
         let mut exprs = self.module.global_expressions.iter().rev().peekable();
-        for ((ty_handle, ty), dep) in self.module.types.iter().rev().zip(max_dep.iter().rev()) {
-            while let Some((expr_handle, expr)) = exprs.next_if(|&(h, _)| Some(h) > *dep) {
+
+        for ((ty_handle, ty), dep) in self.module.types.iter().zip(max_dep).rev() {
+            while let Some((expr_handle, expr)) = exprs.next_if(|&(h, _)| Some(h) > dep) {
                 if self.global_expressions_used.contains(expr_handle) {
                     self.as_const_expression().trace_expression(expr);
                 }