From 4f1b945a4921fc1313bf0e410cf60119927cbbe6 Mon Sep 17 00:00:00 2001
From: Ben Kimock <kimockb@gmail.com>
Date: Sun, 12 Oct 2025 19:12:15 -0400
Subject: [PATCH] Avoid redundant UB check in RangeFrom slice indexing

---
 library/core/src/slice/index.rs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/library/core/src/slice/index.rs b/library/core/src/slice/index.rs
index de220e7e38a4b..a9806060d3d81 100644
--- a/library/core/src/slice/index.rs
+++ b/library/core/src/slice/index.rs
@@ -564,7 +564,10 @@ unsafe impl<T> const SliceIndex<[T]> for ops::RangeFrom<usize> {
             slice_index_fail(self.start, slice.len(), slice.len())
         }
         // SAFETY: `self` is checked to be valid and in bounds above.
-        unsafe { &*self.get_unchecked(slice) }
+        unsafe {
+            let new_len = crate::intrinsics::unchecked_sub(slice.len(), self.start);
+            &*get_offset_len_noubcheck(slice, self.start, new_len)
+        }
     }
 
     #[inline]
@@ -573,7 +576,10 @@ unsafe impl<T> const SliceIndex<[T]> for ops::RangeFrom<usize> {
             slice_index_fail(self.start, slice.len(), slice.len())
         }
         // SAFETY: `self` is checked to be valid and in bounds above.
-        unsafe { &mut *self.get_unchecked_mut(slice) }
+        unsafe {
+            let new_len = crate::intrinsics::unchecked_sub(slice.len(), self.start);
+            &mut *get_offset_len_mut_noubcheck(slice, self.start, new_len)
+        }
     }
 }