Auto merge of #137795 - Jarcho:idx_opt, r=davidtwco

Allow bounds checks when enumerating `IndexSlice` to be elided

Without this hint, each loop iteration has to separately bounds check the index. See https://godbolt.org/z/zrfPY4Ten for an example.

This is technically a behaviour change, but only in cases where the compiler is going to crash anyways.

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))
     }
 

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))
     }