diff --git a/library/core/src/slice/iter/macros.rs b/library/core/src/slice/iter/macros.rs
index 04945fa0e82..5434827e5dd 100644
--- a/library/core/src/slice/iter/macros.rs
+++ b/library/core/src/slice/iter/macros.rs
@@ -188,7 +188,7 @@ macro_rules! iterator {
             #[inline]
             fn advance_by(&mut self, n: usize) -> Result<(), usize>{
                 let advance = cmp::min(n, len!(self));
-                // SAFETY: `advance` does not exceed `self.len()` by construction
+                // SAFETY: By construction, `advance` does not exceed `self.len()`.
                 unsafe { self.post_inc_start(advance as isize) };
                 if advance == n { Ok(()) } else { Err(advance) }
             }
@@ -383,7 +383,7 @@ macro_rules! iterator {
             #[inline]
             fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
                 let advance = cmp::min(n, len!(self));
-                // SAFETY: `advance` does not exceed `self.len()` by construction
+                // SAFETY: By construction, `advance` does not exceed `self.len()`.
                 unsafe { self.pre_dec_end(advance as isize) };
                 if advance == n { Ok(()) } else { Err(advance) }
             }
diff --git a/library/core/tests/slice.rs b/library/core/tests/slice.rs
index 3a98cd9d2ee..240b894057c 100644
--- a/library/core/tests/slice.rs
+++ b/library/core/tests/slice.rs
@@ -134,6 +134,48 @@ fn test_partition_point() {
     assert_eq!(b.partition_point(|&x| x < 8), 5);
 }
 
+#[test]
+fn test_iterator_advance_by() {
+    let v = &[0, 1, 2, 3, 4];
+
+    for i in 0..=v.len() {
+        let mut iter = v.iter();
+        iter.advance_by(i).unwrap();
+        assert_eq!(iter.as_slice(), &v[i..]);
+    }
+
+    let mut iter = v.iter();
+    assert_eq!(iter.advance_by(v.len() + 1), Err(v.len()));
+    assert_eq!(iter.as_slice(), &[]);
+
+    let mut iter = v.iter();
+    iter.advance_by(3).unwrap();
+    assert_eq!(iter.as_slice(), &v[3..]);
+    iter.advance_by(2).unwrap();
+    assert_eq!(iter.as_slice(), &[]);
+}
+
+#[test]
+fn test_iterator_advance_back_by() {
+    let v = &[0, 1, 2, 3, 4];
+
+    for i in 0..=v.len() {
+        let mut iter = v.iter();
+        iter.advance_back_by(i).unwrap();
+        assert_eq!(iter.as_slice(), &v[..v.len() - i]);
+    }
+
+    let mut iter = v.iter();
+    assert_eq!(iter.advance_back_by(v.len() + 1), Err(v.len()));
+    assert_eq!(iter.as_slice(), &[]);
+
+    let mut iter = v.iter();
+    iter.advance_back_by(3).unwrap();
+    assert_eq!(iter.as_slice(), &v[..v.len() - 3]);
+    iter.advance_back_by(2).unwrap();
+    assert_eq!(iter.as_slice(), &[]);
+}
+
 #[test]
 fn test_iterator_nth() {
     let v: &[_] = &[0, 1, 2, 3, 4];