From da0fbc19a5b2710d03c9f2d9ccd6bc572c2b2ffa Mon Sep 17 00:00:00 2001
From: Jason Newcomb <jsnewcomb@pm.me>
Date: Fri, 28 Feb 2025 09:10:09 -0500
Subject: Allow the optimizer to elide bounds checks when enumerating
 `IndexSlice`/`IndecVec`.

---
 compiler/rustc_index/src/slice.rs | 6 ++++++
 compiler/rustc_index/src/vec.rs   | 4 ++++
 2 files changed, 10 insertions(+)

(limited to 'compiler/rustc_index')

diff --git a/compiler/rustc_index/src/slice.rs b/compiler/rustc_index/src/slice.rs
index f17ea9e4b59..0f4f885bb3a 100644
--- a/compiler/rustc_index/src/slice.rs
+++ b/compiler/rustc_index/src/slice.rs
@@ -65,6 +65,8 @@ impl<I: Idx, T> IndexSlice<I, T> {
 
     #[inline]
     pub fn iter_enumerated(&self) -> impl DoubleEndedIterator<Item = (I, &T)> + ExactSizeIterator {
+        // Allow the optimizer to elide the bounds checking when creating each index.
+        let _ = I::new(self.len());
         self.raw.iter().enumerate().map(|(n, t)| (I::new(n), t))
     }
 
@@ -72,6 +74,8 @@ impl<I: Idx, T> IndexSlice<I, T> {
     pub fn indices(
         &self,
     ) -> impl DoubleEndedIterator<Item = I> + ExactSizeIterator + Clone + 'static {
+        // Allow the optimizer to elide the bounds checking when creating each index.
+        let _ = I::new(self.len());
         (0..self.len()).map(|n| I::new(n))
     }
 
@@ -84,6 +88,8 @@ impl<I: Idx, T> IndexSlice<I, T> {
     pub fn iter_enumerated_mut(
         &mut self,
     ) -> impl DoubleEndedIterator<Item = (I, &mut T)> + ExactSizeIterator {
+        // Allow the optimizer to elide the bounds checking when creating each index.
+        let _ = I::new(self.len());
         self.raw.iter_mut().enumerate().map(|(n, t)| (I::new(n), t))
     }
 
diff --git a/compiler/rustc_index/src/vec.rs b/compiler/rustc_index/src/vec.rs
index 7f3f3ead5f2..13f0dda180b 100644
--- a/compiler/rustc_index/src/vec.rs
+++ b/compiler/rustc_index/src/vec.rs
@@ -93,6 +93,8 @@ impl<I: Idx, T> IndexVec<I, T> {
     /// be allocated only once, with a capacity of at least `n`.)
     #[inline]
     pub fn from_fn_n(func: impl FnMut(I) -> T, n: usize) -> Self {
+        // Allow the optimizer to elide the bounds checking when creating each index.
+        let _ = I::new(n);
         IndexVec::from_raw((0..n).map(I::new).map(func).collect())
     }
 
@@ -128,6 +130,8 @@ impl<I: Idx, T> IndexVec<I, T> {
     pub fn into_iter_enumerated(
         self,
     ) -> impl DoubleEndedIterator<Item = (I, T)> + ExactSizeIterator {
+        // Allow the optimizer to elide the bounds checking when creating each index.
+        let _ = I::new(self.len());
         self.raw.into_iter().enumerate().map(|(n, t)| (I::new(n), t))
     }
 
-- 
cgit 1.4.1-3-g733a5