diff --git a/library/core/src/fmt/num.rs b/library/core/src/fmt/num.rs
index f1540803f97..d43f25d9fd1 100644
--- a/library/core/src/fmt/num.rs
+++ b/library/core/src/fmt/num.rs
@@ -88,7 +88,10 @@ fn fmt_int<T: DisplayInt>(&self, mut x: T, f: &mut fmt::Formatter<'_>) -> fmt::R
                 };
             }
         }
-        let buf = &buf[curr..];
+        // SAFETY: `curr` is initialized to `buf.len()` and is only decremented, so it can't overflow. It is
+        // decremented exactly once for each digit. Since u128 is the widest fixed width integer format supported,
+        // the maximum number of digits (bits) is 128 for base-2, so `curr` won't underflow as well.
+        let buf = unsafe { buf.get_unchecked(curr..) };
         // SAFETY: The only chars in `buf` are created by `Self::digit` which are assumed to be
         // valid UTF-8
         let buf = unsafe {