diff --git a/library/alloc/src/collections/vec_deque/into_iter.rs b/library/alloc/src/collections/vec_deque/into_iter.rs
index 34bc0ce9177..e2b40f7912e 100644
--- a/library/alloc/src/collections/vec_deque/into_iter.rs
+++ b/library/alloc/src/collections/vec_deque/into_iter.rs
@@ -1,4 +1,5 @@
 use core::iter::{FusedIterator, TrustedLen};
+use core::num::NonZeroUsize;
 use core::{array, fmt, mem::MaybeUninit, ops::Try, ptr};
 
 use crate::alloc::{Allocator, Global};
@@ -54,15 +55,16 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
     }
 
     #[inline]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
-        if self.inner.len < n {
-            let len = self.inner.len;
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        let len = self.inner.len;
+        let rem = if len < n {
             self.inner.clear();
-            Err(len)
+            n - len
         } else {
             self.inner.drain(..n);
-            Ok(())
-        }
+            0
+        };
+        NonZeroUsize::new(rem).map_or(Ok(()), Err)
     }
 
     #[inline]
@@ -182,15 +184,16 @@ impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         let len = self.inner.len;
-        if len >= n {
-            self.inner.truncate(len - n);
-            Ok(())
-        } else {
+        let rem = if len < n {
             self.inner.clear();
-            Err(len)
-        }
+            n - len
+        } else {
+            self.inner.truncate(len - n);
+            0
+        };
+        NonZeroUsize::new(rem).map_or(Ok(()), Err)
     }
 
     fn try_rfold<B, F, R>(&mut self, mut init: B, mut f: F) -> R
diff --git a/library/alloc/src/collections/vec_deque/iter.rs b/library/alloc/src/collections/vec_deque/iter.rs
index d9f3937144d..646a2a991e7 100644
--- a/library/alloc/src/collections/vec_deque/iter.rs
+++ b/library/alloc/src/collections/vec_deque/iter.rs
@@ -1,4 +1,5 @@
 use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
+use core::num::NonZeroUsize;
 use core::ops::Try;
 use core::{fmt, mem, slice};
 
@@ -55,13 +56,15 @@ impl<'a, T> Iterator for Iter<'a, T> {
         }
     }
 
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
-        let m = match self.i1.advance_by(n) {
-            Ok(_) => return Ok(()),
-            Err(m) => m,
-        };
-        mem::swap(&mut self.i1, &mut self.i2);
-        self.i1.advance_by(n - m).map_err(|o| o + m)
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        let remaining = self.i1.advance_by(n);
+        match remaining {
+            Ok(()) => return Ok(()),
+            Err(n) => {
+                mem::swap(&mut self.i1, &mut self.i2);
+                self.i1.advance_by(n.get())
+            }
+        }
     }
 
     #[inline]
@@ -125,14 +128,14 @@ impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
         }
     }
 
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
-        let m = match self.i2.advance_back_by(n) {
-            Ok(_) => return Ok(()),
-            Err(m) => m,
-        };
-
-        mem::swap(&mut self.i1, &mut self.i2);
-        self.i2.advance_back_by(n - m).map_err(|o| m + o)
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        match self.i2.advance_back_by(n) {
+            Ok(()) => return Ok(()),
+            Err(n) => {
+                mem::swap(&mut self.i1, &mut self.i2);
+                self.i2.advance_back_by(n.get())
+            }
+        }
     }
 
     fn rfold<Acc, F>(self, accum: Acc, mut f: F) -> Acc
diff --git a/library/alloc/src/collections/vec_deque/iter_mut.rs b/library/alloc/src/collections/vec_deque/iter_mut.rs
index 2c59d95cd53..7defbb1090f 100644
--- a/library/alloc/src/collections/vec_deque/iter_mut.rs
+++ b/library/alloc/src/collections/vec_deque/iter_mut.rs
@@ -1,4 +1,5 @@
 use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
+use core::num::NonZeroUsize;
 use core::ops::Try;
 use core::{fmt, mem, slice};
 
@@ -47,13 +48,14 @@ impl<'a, T> Iterator for IterMut<'a, T> {
         }
     }
 
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
-        let m = match self.i1.advance_by(n) {
-            Ok(_) => return Ok(()),
-            Err(m) => m,
-        };
-        mem::swap(&mut self.i1, &mut self.i2);
-        self.i1.advance_by(n - m).map_err(|o| o + m)
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        match self.i1.advance_by(n) {
+            Ok(()) => return Ok(()),
+            Err(remaining) => {
+                mem::swap(&mut self.i1, &mut self.i2);
+                self.i1.advance_by(remaining.get())
+            }
+        }
     }
 
     #[inline]
@@ -117,14 +119,14 @@ impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
         }
     }
 
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
-        let m = match self.i2.advance_back_by(n) {
-            Ok(_) => return Ok(()),
-            Err(m) => m,
-        };
-
-        mem::swap(&mut self.i1, &mut self.i2);
-        self.i2.advance_back_by(n - m).map_err(|o| m + o)
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        match self.i2.advance_back_by(n) {
+            Ok(()) => return Ok(()),
+            Err(remaining) => {
+                mem::swap(&mut self.i1, &mut self.i2);
+                self.i2.advance_back_by(remaining.get())
+            }
+        }
     }
 
     fn rfold<Acc, F>(self, accum: Acc, mut f: F) -> Acc
diff --git a/library/alloc/src/vec/into_iter.rs b/library/alloc/src/vec/into_iter.rs
index f6525eb9003..6a05f70e437 100644
--- a/library/alloc/src/vec/into_iter.rs
+++ b/library/alloc/src/vec/into_iter.rs
@@ -11,6 +11,7 @@ use core::iter::{
 };
 use core::marker::PhantomData;
 use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
+use core::num::NonZeroUsize;
 #[cfg(not(no_global_oom_handling))]
 use core::ops::Deref;
 use core::ptr::{self, NonNull};
@@ -213,7 +214,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
     }
 
     #[inline]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         let step_size = self.len().min(n);
         let to_drop = ptr::slice_from_raw_parts_mut(self.ptr as *mut T, step_size);
         if T::IS_ZST {
@@ -227,10 +228,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         unsafe {
             ptr::drop_in_place(to_drop);
         }
-        if step_size < n {
-            return Err(step_size);
-        }
-        Ok(())
+        NonZeroUsize::new(n - step_size).map_or(Ok(()), Err)
     }
 
     #[inline]
@@ -313,7 +311,7 @@ impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         let step_size = self.len().min(n);
         if T::IS_ZST {
             // SAFETY: same as for advance_by()
@@ -327,10 +325,7 @@ impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
         unsafe {
             ptr::drop_in_place(to_drop);
         }
-        if step_size < n {
-            return Err(step_size);
-        }
-        Ok(())
+        NonZeroUsize::new(n - step_size).map_or(Ok(()), Err)
     }
 }
 
diff --git a/library/alloc/tests/vec.rs b/library/alloc/tests/vec.rs
index 2f07c2911a5..3ee16f04e92 100644
--- a/library/alloc/tests/vec.rs
+++ b/library/alloc/tests/vec.rs
@@ -1,5 +1,7 @@
 use core::alloc::{Allocator, Layout};
+use core::assert_eq;
 use core::iter::IntoIterator;
+use core::num::NonZeroUsize;
 use core::ptr::NonNull;
 use std::alloc::System;
 use std::assert_matches::assert_matches;
@@ -1062,21 +1064,21 @@ fn test_into_iter_leak() {
 
 #[test]
 fn test_into_iter_advance_by() {
-    let mut i = [1, 2, 3, 4, 5].into_iter();
-    i.advance_by(0).unwrap();
-    i.advance_back_by(0).unwrap();
+    let mut i = vec![1, 2, 3, 4, 5].into_iter();
+    assert_eq!(i.advance_by(0), Ok(()));
+    assert_eq!(i.advance_back_by(0), Ok(()));
     assert_eq!(i.as_slice(), [1, 2, 3, 4, 5]);
 
-    i.advance_by(1).unwrap();
-    i.advance_back_by(1).unwrap();
+    assert_eq!(i.advance_by(1), Ok(()));
+    assert_eq!(i.advance_back_by(1), Ok(()));
     assert_eq!(i.as_slice(), [2, 3, 4]);
 
-    assert_eq!(i.advance_back_by(usize::MAX), Err(3));
+    assert_eq!(i.advance_back_by(usize::MAX), Err(NonZeroUsize::new(usize::MAX - 3).unwrap()));
 
-    assert_eq!(i.advance_by(usize::MAX), Err(0));
+    assert_eq!(i.advance_by(usize::MAX), Err(NonZeroUsize::new(usize::MAX).unwrap()));
 
-    i.advance_by(0).unwrap();
-    i.advance_back_by(0).unwrap();
+    assert_eq!(i.advance_by(0), Ok(()));
+    assert_eq!(i.advance_back_by(0), Ok(()));
 
     assert_eq!(i.len(), 0);
 }
@@ -1124,7 +1126,7 @@ fn test_into_iter_zst() {
     for _ in vec![C; 5].into_iter().rev() {}
 
     let mut it = vec![C, C].into_iter();
-    it.advance_by(1).unwrap();
+    assert_eq!(it.advance_by(1), Ok(()));
     drop(it);
 
     let mut it = vec![C, C].into_iter();
diff --git a/library/alloc/tests/vec_deque.rs b/library/alloc/tests/vec_deque.rs
index 5a0b852e8d5..ddc27e34ed9 100644
--- a/library/alloc/tests/vec_deque.rs
+++ b/library/alloc/tests/vec_deque.rs
@@ -1,3 +1,4 @@
+use core::num::NonZeroUsize;
 use std::assert_matches::assert_matches;
 use std::collections::TryReserveErrorKind::*;
 use std::collections::{vec_deque::Drain, VecDeque};
@@ -426,6 +427,28 @@ fn test_into_iter() {
         assert_eq!(it.next(), Some(7));
         assert_eq!(it.size_hint(), (5, Some(5)));
     }
+
+    // advance_by
+    {
+        let mut d = VecDeque::new();
+        for i in 0..=4 {
+            d.push_back(i);
+        }
+        for i in 6..=8 {
+            d.push_front(i);
+        }
+
+        let mut it = d.into_iter();
+        assert_eq!(it.advance_by(1), Ok(()));
+        assert_eq!(it.next(), Some(7));
+        assert_eq!(it.advance_back_by(1), Ok(()));
+        assert_eq!(it.next_back(), Some(3));
+
+        let mut it = VecDeque::from(vec![1, 2, 3, 4, 5]).into_iter();
+        assert_eq!(it.advance_by(10), Err(NonZeroUsize::new(5).unwrap()));
+        let mut it = VecDeque::from(vec![1, 2, 3, 4, 5]).into_iter();
+        assert_eq!(it.advance_back_by(10), Err(NonZeroUsize::new(5).unwrap()));
+    }
 }
 
 #[test]
diff --git a/library/core/src/array/iter.rs b/library/core/src/array/iter.rs
index 8259c087d22..73e2c2cfbbe 100644
--- a/library/core/src/array/iter.rs
+++ b/library/core/src/array/iter.rs
@@ -1,5 +1,6 @@
 //! Defines the `IntoIter` owned iterator for arrays.
 
+use crate::num::NonZeroUsize;
 use crate::{
     fmt,
     iter::{self, ExactSizeIterator, FusedIterator, TrustedLen},
@@ -284,12 +285,11 @@ impl<T, const N: usize> Iterator for IntoIter<T, N> {
         self.next_back()
     }
 
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
-        let original_len = self.len();
-
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         // This also moves the start, which marks them as conceptually "dropped",
         // so if anything goes bad then our drop impl won't double-free them.
         let range_to_drop = self.alive.take_prefix(n);
+        let remaining = n - range_to_drop.len();
 
         // SAFETY: These elements are currently initialized, so it's fine to drop them.
         unsafe {
@@ -297,7 +297,7 @@ impl<T, const N: usize> Iterator for IntoIter<T, N> {
             ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(slice));
         }
 
-        if n > original_len { Err(original_len) } else { Ok(()) }
+        NonZeroUsize::new(remaining).map_or(Ok(()), Err)
     }
 }
 
@@ -334,12 +334,11 @@ impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, N> {
         })
     }
 
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
-        let original_len = self.len();
-
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         // This also moves the end, which marks them as conceptually "dropped",
         // so if anything goes bad then our drop impl won't double-free them.
         let range_to_drop = self.alive.take_suffix(n);
+        let remaining = n - range_to_drop.len();
 
         // SAFETY: These elements are currently initialized, so it's fine to drop them.
         unsafe {
@@ -347,7 +346,7 @@ impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, N> {
             ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(slice));
         }
 
-        if n > original_len { Err(original_len) } else { Ok(()) }
+        NonZeroUsize::new(remaining).map_or(Ok(()), Err)
     }
 }
 
diff --git a/library/core/src/iter/adapters/by_ref_sized.rs b/library/core/src/iter/adapters/by_ref_sized.rs
index 477e7117c3e..4e0e19ddc78 100644
--- a/library/core/src/iter/adapters/by_ref_sized.rs
+++ b/library/core/src/iter/adapters/by_ref_sized.rs
@@ -1,3 +1,4 @@
+use crate::num::NonZeroUsize;
 use crate::ops::{NeverShortCircuit, Try};
 
 /// Like `Iterator::by_ref`, but requiring `Sized` so it can forward generics.
@@ -26,7 +27,7 @@ impl<I: Iterator> Iterator for ByRefSized<'_, I> {
     }
 
     #[inline]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         I::advance_by(self.0, n)
     }
 
@@ -62,7 +63,7 @@ impl<I: DoubleEndedIterator> DoubleEndedIterator for ByRefSized<'_, I> {
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         I::advance_back_by(self.0, n)
     }
 
diff --git a/library/core/src/iter/adapters/chain.rs b/library/core/src/iter/adapters/chain.rs
index d4b2640e81d..2046b70c9c6 100644
--- a/library/core/src/iter/adapters/chain.rs
+++ b/library/core/src/iter/adapters/chain.rs
@@ -1,4 +1,5 @@
 use crate::iter::{DoubleEndedIterator, FusedIterator, Iterator, TrustedLen};
+use crate::num::NonZeroUsize;
 use crate::ops::Try;
 
 /// An iterator that links two iterators together, in a chain.
@@ -95,38 +96,33 @@ where
     }
 
     #[inline]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
-        let mut rem = n;
-
+    fn advance_by(&mut self, mut n: usize) -> Result<(), NonZeroUsize> {
         if let Some(ref mut a) = self.a {
-            match a.advance_by(rem) {
+            n = match a.advance_by(n) {
                 Ok(()) => return Ok(()),
-                Err(k) => rem -= k,
-            }
+                Err(k) => k.get(),
+            };
             self.a = None;
         }
 
         if let Some(ref mut b) = self.b {
-            match b.advance_by(rem) {
-                Ok(()) => return Ok(()),
-                Err(k) => rem -= k,
-            }
+            return b.advance_by(n);
             // we don't fuse the second iterator
         }
 
-        if rem == 0 { Ok(()) } else { Err(n - rem) }
+        NonZeroUsize::new(n).map_or(Ok(()), Err)
     }
 
     #[inline]
     fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
         if let Some(ref mut a) = self.a {
-            match a.advance_by(n) {
+            n = match a.advance_by(n) {
                 Ok(()) => match a.next() {
-                    None => n = 0,
+                    None => 0,
                     x => return x,
                 },
-                Err(k) => n -= k,
-            }
+                Err(k) => k.get(),
+            };
 
             self.a = None;
         }
@@ -186,38 +182,33 @@ where
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
-        let mut rem = n;
-
+    fn advance_back_by(&mut self, mut n: usize) -> Result<(), NonZeroUsize> {
         if let Some(ref mut b) = self.b {
-            match b.advance_back_by(rem) {
+            n = match b.advance_back_by(n) {
                 Ok(()) => return Ok(()),
-                Err(k) => rem -= k,
-            }
+                Err(k) => k.get(),
+            };
             self.b = None;
         }
 
         if let Some(ref mut a) = self.a {
-            match a.advance_back_by(rem) {
-                Ok(()) => return Ok(()),
-                Err(k) => rem -= k,
-            }
+            return a.advance_back_by(n);
             // we don't fuse the second iterator
         }
 
-        if rem == 0 { Ok(()) } else { Err(n - rem) }
+        NonZeroUsize::new(n).map_or(Ok(()), Err)
     }
 
     #[inline]
     fn nth_back(&mut self, mut n: usize) -> Option<Self::Item> {
         if let Some(ref mut b) = self.b {
-            match b.advance_back_by(n) {
+            n = match b.advance_back_by(n) {
                 Ok(()) => match b.next_back() {
-                    None => n = 0,
+                    None => 0,
                     x => return x,
                 },
-                Err(k) => n -= k,
-            }
+                Err(k) => k.get(),
+            };
 
             self.b = None;
         }
diff --git a/library/core/src/iter/adapters/copied.rs b/library/core/src/iter/adapters/copied.rs
index a076ab925e3..2289025d0a7 100644
--- a/library/core/src/iter/adapters/copied.rs
+++ b/library/core/src/iter/adapters/copied.rs
@@ -4,6 +4,7 @@ use crate::iter::adapters::{
 use crate::iter::{FusedIterator, TrustedLen};
 use crate::mem::MaybeUninit;
 use crate::mem::SizedTypeProperties;
+use crate::num::NonZeroUsize;
 use crate::ops::Try;
 use crate::{array, ptr};
 
@@ -89,7 +90,7 @@ where
     }
 
     #[inline]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         self.it.advance_by(n)
     }
 
@@ -130,7 +131,7 @@ where
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         self.it.advance_back_by(n)
     }
 }
diff --git a/library/core/src/iter/adapters/cycle.rs b/library/core/src/iter/adapters/cycle.rs
index 02b5939072e..51bd09b6eff 100644
--- a/library/core/src/iter/adapters/cycle.rs
+++ b/library/core/src/iter/adapters/cycle.rs
@@ -1,3 +1,4 @@
+use crate::num::NonZeroUsize;
 use crate::{iter::FusedIterator, ops::Try};
 
 /// An iterator that repeats endlessly.
@@ -81,23 +82,22 @@ where
 
     #[inline]
     #[rustc_inherit_overflow_checks]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
-        let mut rem = n;
-        match self.iter.advance_by(rem) {
-            ret @ Ok(_) => return ret,
-            Err(advanced) => rem -= advanced,
-        }
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        let mut n = match self.iter.advance_by(n) {
+            Ok(()) => return Ok(()),
+            Err(rem) => rem.get(),
+        };
 
-        while rem > 0 {
+        while n > 0 {
             self.iter = self.orig.clone();
-            match self.iter.advance_by(rem) {
-                ret @ Ok(_) => return ret,
-                Err(0) => return Err(n - rem),
-                Err(advanced) => rem -= advanced,
-            }
+            n = match self.iter.advance_by(n) {
+                Ok(()) => return Ok(()),
+                e @ Err(rem) if rem.get() == n => return e,
+                Err(rem) => rem.get(),
+            };
         }
 
-        Ok(())
+        NonZeroUsize::new(n).map_or(Ok(()), Err)
     }
 
     // No `fold` override, because `fold` doesn't make much sense for `Cycle`,
diff --git a/library/core/src/iter/adapters/enumerate.rs b/library/core/src/iter/adapters/enumerate.rs
index 8c32a35a12f..479ea6d83c7 100644
--- a/library/core/src/iter/adapters/enumerate.rs
+++ b/library/core/src/iter/adapters/enumerate.rs
@@ -2,6 +2,7 @@ use crate::iter::adapters::{
     zip::try_get_unchecked, SourceIter, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
 };
 use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen};
+use crate::num::NonZeroUsize;
 use crate::ops::Try;
 
 /// An iterator that yields the current count and the element during iteration.
@@ -114,17 +115,14 @@ where
 
     #[inline]
     #[rustc_inherit_overflow_checks]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
-        match self.iter.advance_by(n) {
-            ret @ Ok(_) => {
-                self.count += n;
-                ret
-            }
-            ret @ Err(advanced) => {
-                self.count += advanced;
-                ret
-            }
-        }
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        let remaining = self.iter.advance_by(n);
+        let advanced = match remaining {
+            Ok(()) => n,
+            Err(rem) => n - rem.get(),
+        };
+        self.count += advanced;
+        remaining
     }
 
     #[rustc_inherit_overflow_checks]
@@ -208,7 +206,7 @@ where
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         // we do not need to update the count since that only tallies the number of items
         // consumed from the front. consuming items from the back can never reduce that.
         self.iter.advance_back_by(n)
diff --git a/library/core/src/iter/adapters/flatten.rs b/library/core/src/iter/adapters/flatten.rs
index e4020c45f71..e0308e3360f 100644
--- a/library/core/src/iter/adapters/flatten.rs
+++ b/library/core/src/iter/adapters/flatten.rs
@@ -1,5 +1,6 @@
 use crate::fmt;
 use crate::iter::{DoubleEndedIterator, Fuse, FusedIterator, Iterator, Map, TrustedLen};
+use crate::num::NonZeroUsize;
 use crate::ops::{ControlFlow, Try};
 
 /// An iterator that maps each element to an iterator, and yields the elements
@@ -75,7 +76,7 @@ where
     }
 
     #[inline]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         self.inner.advance_by(n)
     }
 
@@ -120,7 +121,7 @@ where
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         self.inner.advance_back_by(n)
     }
 }
@@ -236,7 +237,7 @@ where
     }
 
     #[inline]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         self.inner.advance_by(n)
     }
 
@@ -281,7 +282,7 @@ where
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         self.inner.advance_back_by(n)
     }
 }
@@ -552,18 +553,18 @@ where
 
     #[inline]
     #[rustc_inherit_overflow_checks]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         #[inline]
         #[rustc_inherit_overflow_checks]
         fn advance<U: Iterator>(n: usize, iter: &mut U) -> ControlFlow<(), usize> {
             match iter.advance_by(n) {
                 Ok(()) => ControlFlow::Break(()),
-                Err(advanced) => ControlFlow::Continue(n - advanced),
+                Err(remaining) => ControlFlow::Continue(remaining.get()),
             }
         }
 
         match self.iter_try_fold(n, advance) {
-            ControlFlow::Continue(remaining) if remaining > 0 => Err(n - remaining),
+            ControlFlow::Continue(remaining) => NonZeroUsize::new(remaining).map_or(Ok(()), Err),
             _ => Ok(()),
         }
     }
@@ -642,18 +643,18 @@ where
 
     #[inline]
     #[rustc_inherit_overflow_checks]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         #[inline]
         #[rustc_inherit_overflow_checks]
         fn advance<U: DoubleEndedIterator>(n: usize, iter: &mut U) -> ControlFlow<(), usize> {
             match iter.advance_back_by(n) {
                 Ok(()) => ControlFlow::Break(()),
-                Err(advanced) => ControlFlow::Continue(n - advanced),
+                Err(remaining) => ControlFlow::Continue(remaining.get()),
             }
         }
 
         match self.iter_try_rfold(n, advance) {
-            ControlFlow::Continue(remaining) if remaining > 0 => Err(n - remaining),
+            ControlFlow::Continue(remaining) => NonZeroUsize::new(remaining).map_or(Ok(()), Err),
             _ => Ok(()),
         }
     }
diff --git a/library/core/src/iter/adapters/rev.rs b/library/core/src/iter/adapters/rev.rs
index 8ae6d96fde4..1d882087f69 100644
--- a/library/core/src/iter/adapters/rev.rs
+++ b/library/core/src/iter/adapters/rev.rs
@@ -1,4 +1,5 @@
 use crate::iter::{FusedIterator, TrustedLen};
+use crate::num::NonZeroUsize;
 use crate::ops::Try;
 
 /// A double-ended iterator with the direction inverted.
@@ -38,7 +39,7 @@ where
     }
 
     #[inline]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         self.iter.advance_back_by(n)
     }
 
@@ -83,7 +84,7 @@ where
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         self.iter.advance_by(n)
     }
 
diff --git a/library/core/src/iter/adapters/skip.rs b/library/core/src/iter/adapters/skip.rs
index c6334880db5..306338bc7cc 100644
--- a/library/core/src/iter/adapters/skip.rs
+++ b/library/core/src/iter/adapters/skip.rs
@@ -1,5 +1,6 @@
 use crate::intrinsics::unlikely;
 use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable};
+use crate::num::NonZeroUsize;
 use crate::ops::{ControlFlow, Try};
 
 /// An iterator that skips over `n` elements of `iter`.
@@ -128,34 +129,27 @@ where
 
     #[inline]
     #[rustc_inherit_overflow_checks]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
-        let mut rem = n;
-        let step_one = self.n.saturating_add(rem);
+    fn advance_by(&mut self, mut n: usize) -> Result<(), NonZeroUsize> {
+        let skip_inner = self.n;
+        let skip_and_advance = skip_inner.saturating_add(n);
 
-        match self.iter.advance_by(step_one) {
-            Ok(_) => {
-                rem -= step_one - self.n;
-                self.n = 0;
-            }
-            Err(advanced) => {
-                let advanced_without_skip = advanced.saturating_sub(self.n);
-                self.n = self.n.saturating_sub(advanced);
-                return if n == 0 { Ok(()) } else { Err(advanced_without_skip) };
+        let remainder = match self.iter.advance_by(skip_and_advance) {
+            Ok(()) => 0,
+            Err(n) => n.get(),
+        };
+        let advanced_inner = skip_and_advance - remainder;
+        n -= advanced_inner.saturating_sub(skip_inner);
+        self.n = self.n.saturating_sub(advanced_inner);
+
+        // skip_and_advance may have saturated
+        if unlikely(remainder == 0 && n > 0) {
+            n = match self.iter.advance_by(n) {
+                Ok(()) => 0,
+                Err(n) => n.get(),
             }
         }
 
-        // step_one calculation may have saturated
-        if unlikely(rem > 0) {
-            return match self.iter.advance_by(rem) {
-                ret @ Ok(_) => ret,
-                Err(advanced) => {
-                    rem -= advanced;
-                    Err(n - rem)
-                }
-            };
-        }
-
-        Ok(())
+        NonZeroUsize::new(n).map_or(Ok(()), Err)
     }
 }
 
@@ -209,13 +203,11 @@ where
     impl_fold_via_try_fold! { rfold -> try_rfold }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         let min = crate::cmp::min(self.len(), n);
-        return match self.iter.advance_back_by(min) {
-            ret @ Ok(_) if n <= min => ret,
-            Ok(_) => Err(min),
-            _ => panic!("ExactSizeIterator contract violation"),
-        };
+        let rem = self.iter.advance_back_by(min);
+        assert!(rem.is_ok(), "ExactSizeIterator contract violation");
+        NonZeroUsize::new(n - min).map_or(Ok(()), Err)
     }
 }
 
diff --git a/library/core/src/iter/adapters/take.rs b/library/core/src/iter/adapters/take.rs
index d947c7b0e30..ce18bffe714 100644
--- a/library/core/src/iter/adapters/take.rs
+++ b/library/core/src/iter/adapters/take.rs
@@ -1,5 +1,6 @@
 use crate::cmp;
 use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedLen};
+use crate::num::NonZeroUsize;
 use crate::ops::{ControlFlow, Try};
 
 /// An iterator that only iterates over the first `n` iterations of `iter`.
@@ -121,18 +122,15 @@ where
 
     #[inline]
     #[rustc_inherit_overflow_checks]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         let min = self.n.min(n);
-        match self.iter.advance_by(min) {
-            Ok(_) => {
-                self.n -= min;
-                if min < n { Err(min) } else { Ok(()) }
-            }
-            ret @ Err(advanced) => {
-                self.n -= advanced;
-                ret
-            }
-        }
+        let rem = match self.iter.advance_by(min) {
+            Ok(()) => 0,
+            Err(rem) => rem.get(),
+        };
+        let advanced = min - rem;
+        self.n -= advanced;
+        NonZeroUsize::new(n - advanced).map_or(Ok(()), Err)
     }
 }
 
@@ -223,7 +221,7 @@ where
 
     #[inline]
     #[rustc_inherit_overflow_checks]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         // The amount by which the inner iterator needs to be shortened for it to be
         // at most as long as the take() amount.
         let trim_inner = self.iter.len().saturating_sub(self.n);
@@ -232,12 +230,14 @@ where
         // about having to advance more than usize::MAX here.
         let advance_by = trim_inner.saturating_add(n);
 
-        let advanced = match self.iter.advance_back_by(advance_by) {
-            Ok(_) => advance_by - trim_inner,
-            Err(advanced) => advanced - trim_inner,
+        let remainder = match self.iter.advance_back_by(advance_by) {
+            Ok(()) => 0,
+            Err(rem) => rem.get(),
         };
-        self.n -= advanced;
-        return if advanced < n { Err(advanced) } else { Ok(()) };
+        let advanced_by_inner = advance_by - remainder;
+        let advanced_by = advanced_by_inner - trim_inner;
+        self.n -= advanced_by;
+        NonZeroUsize::new(n - advanced_by).map_or(Ok(()), Err)
     }
 }
 
diff --git a/library/core/src/iter/range.rs b/library/core/src/iter/range.rs
index f19636fba5d..1cd71193bd7 100644
--- a/library/core/src/iter/range.rs
+++ b/library/core/src/iter/range.rs
@@ -1,6 +1,7 @@
 use crate::convert::TryFrom;
 use crate::marker::Destruct;
 use crate::mem;
+use crate::num::NonZeroUsize;
 use crate::ops::{self, Try};
 
 use super::{
@@ -530,12 +531,12 @@ trait RangeIteratorImpl {
     // Iterator
     fn spec_next(&mut self) -> Option<Self::Item>;
     fn spec_nth(&mut self, n: usize) -> Option<Self::Item>;
-    fn spec_advance_by(&mut self, n: usize) -> Result<(), usize>;
+    fn spec_advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize>;
 
     // DoubleEndedIterator
     fn spec_next_back(&mut self) -> Option<Self::Item>;
     fn spec_nth_back(&mut self, n: usize) -> Option<Self::Item>;
-    fn spec_advance_back_by(&mut self, n: usize) -> Result<(), usize>;
+    fn spec_advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize>;
 }
 
 impl<A: ~const Step + ~const Destruct> const RangeIteratorImpl for ops::Range<A> {
@@ -567,7 +568,7 @@ impl<A: ~const Step + ~const Destruct> const RangeIteratorImpl for ops::Range<A>
     }
 
     #[inline]
-    default fn spec_advance_by(&mut self, n: usize) -> Result<(), usize> {
+    default fn spec_advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         let available = if self.start <= self.end {
             Step::steps_between(&self.start, &self.end).unwrap_or(usize::MAX)
         } else {
@@ -579,7 +580,7 @@ impl<A: ~const Step + ~const Destruct> const RangeIteratorImpl for ops::Range<A>
         self.start =
             Step::forward_checked(self.start.clone(), taken).expect("`Step` invariants not upheld");
 
-        if taken < n { Err(taken) } else { Ok(()) }
+        NonZeroUsize::new(n - taken).map_or(Ok(()), Err)
     }
 
     #[inline]
@@ -608,7 +609,7 @@ impl<A: ~const Step + ~const Destruct> const RangeIteratorImpl for ops::Range<A>
     }
 
     #[inline]
-    default fn spec_advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    default fn spec_advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         let available = if self.start <= self.end {
             Step::steps_between(&self.start, &self.end).unwrap_or(usize::MAX)
         } else {
@@ -620,7 +621,7 @@ impl<A: ~const Step + ~const Destruct> const RangeIteratorImpl for ops::Range<A>
         self.end =
             Step::backward_checked(self.end.clone(), taken).expect("`Step` invariants not upheld");
 
-        if taken < n { Err(taken) } else { Ok(()) }
+        NonZeroUsize::new(n - taken).map_or(Ok(()), Err)
     }
 }
 
@@ -651,7 +652,7 @@ impl<T: ~const TrustedStep + ~const Destruct> const RangeIteratorImpl for ops::R
     }
 
     #[inline]
-    fn spec_advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn spec_advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         let available = if self.start <= self.end {
             Step::steps_between(&self.start, &self.end).unwrap_or(usize::MAX)
         } else {
@@ -666,7 +667,7 @@ impl<T: ~const TrustedStep + ~const Destruct> const RangeIteratorImpl for ops::R
         // Otherwise 0 is returned which always safe to use.
         self.start = unsafe { Step::forward_unchecked(self.start.clone(), taken) };
 
-        if taken < n { Err(taken) } else { Ok(()) }
+        NonZeroUsize::new(n - taken).map_or(Ok(()), Err)
     }
 
     #[inline]
@@ -695,7 +696,7 @@ impl<T: ~const TrustedStep + ~const Destruct> const RangeIteratorImpl for ops::R
     }
 
     #[inline]
-    fn spec_advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn spec_advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         let available = if self.start <= self.end {
             Step::steps_between(&self.start, &self.end).unwrap_or(usize::MAX)
         } else {
@@ -707,7 +708,7 @@ impl<T: ~const TrustedStep + ~const Destruct> const RangeIteratorImpl for ops::R
         // SAFETY: same as the spec_advance_by() implementation
         self.end = unsafe { Step::backward_unchecked(self.end.clone(), taken) };
 
-        if taken < n { Err(taken) } else { Ok(()) }
+        NonZeroUsize::new(n - taken).map_or(Ok(()), Err)
     }
 }
 
@@ -757,7 +758,7 @@ impl<A: ~const Step + ~const Destruct> const Iterator for ops::Range<A> {
     }
 
     #[inline]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         self.spec_advance_by(n)
     }
 
@@ -836,7 +837,7 @@ impl<A: ~const Step + ~const Destruct> const DoubleEndedIterator for ops::Range<
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         self.spec_advance_back_by(n)
     }
 }
diff --git a/library/core/src/iter/sources/repeat.rs b/library/core/src/iter/sources/repeat.rs
index 733142ed011..67051f6e97b 100644
--- a/library/core/src/iter/sources/repeat.rs
+++ b/library/core/src/iter/sources/repeat.rs
@@ -1,4 +1,5 @@
 use crate::iter::{FusedIterator, TrustedLen};
+use crate::num::NonZeroUsize;
 
 /// Creates a new iterator that endlessly repeats a single element.
 ///
@@ -80,7 +81,7 @@ impl<A: Clone> Iterator for Repeat<A> {
     }
 
     #[inline]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         // Advancing an infinite iterator of a single element is a no-op.
         let _ = n;
         Ok(())
@@ -109,7 +110,7 @@ impl<A: Clone> DoubleEndedIterator for Repeat<A> {
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         // Advancing an infinite iterator of a single element is a no-op.
         let _ = n;
         Ok(())
diff --git a/library/core/src/iter/sources/repeat_n.rs b/library/core/src/iter/sources/repeat_n.rs
index dc61d6065b8..0b0445850bf 100644
--- a/library/core/src/iter/sources/repeat_n.rs
+++ b/library/core/src/iter/sources/repeat_n.rs
@@ -1,5 +1,6 @@
 use crate::iter::{FusedIterator, TrustedLen};
 use crate::mem::ManuallyDrop;
+use crate::num::NonZeroUsize;
 
 /// Creates a new iterator that repeats a single element a given number of times.
 ///
@@ -137,7 +138,7 @@ impl<A: Clone> Iterator for RepeatN<A> {
     }
 
     #[inline]
-    fn advance_by(&mut self, skip: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, skip: usize) -> Result<(), NonZeroUsize> {
         let len = self.count;
 
         if skip >= len {
@@ -145,7 +146,8 @@ impl<A: Clone> Iterator for RepeatN<A> {
         }
 
         if skip > len {
-            Err(len)
+            // SAFETY: we just checked that the difference is positive
+            Err(unsafe { NonZeroUsize::new_unchecked(skip - len) })
         } else {
             self.count = len - skip;
             Ok(())
@@ -178,7 +180,7 @@ impl<A: Clone> DoubleEndedIterator for RepeatN<A> {
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         self.advance_by(n)
     }
 
diff --git a/library/core/src/iter/traits/double_ended.rs b/library/core/src/iter/traits/double_ended.rs
index 7a10dea500a..d82ecb698dd 100644
--- a/library/core/src/iter/traits/double_ended.rs
+++ b/library/core/src/iter/traits/double_ended.rs
@@ -1,4 +1,5 @@
 use crate::marker::Destruct;
+use crate::num::NonZeroUsize;
 use crate::ops::{ControlFlow, Try};
 
 /// An iterator able to yield elements from both ends.
@@ -100,10 +101,11 @@ pub trait DoubleEndedIterator: Iterator {
     /// eagerly skip `n` elements starting from the back by calling [`next_back`] up
     /// to `n` times until [`None`] is encountered.
     ///
-    /// `advance_back_by(n)` will return [`Ok(())`] if the iterator successfully advances by
-    /// `n` elements, or [`Err(k)`] if [`None`] is encountered, where `k` is the number of
-    /// elements the iterator is advanced by before running out of elements (i.e. the length
-    /// of the iterator). Note that `k` is always less than `n`.
+    /// `advance_back_by(n)` will return `Ok(())` if the iterator successfully advances by
+    /// `n` elements, or a `Err(NonZeroUsize)` with value `k` if [`None`] is encountered, where `k`
+    /// is remaining number of steps that could not be advanced because the iterator ran out.
+    /// If `self` is empty and `n` is non-zero, then this returns `Err(n)`.
+    /// Otherwise, `k` is always less than `n`.
     ///
     /// Calling `advance_back_by(0)` can do meaningful work, for example [`Flatten`] can advance its
     /// outer iterator until it finds an inner iterator that is not empty, which then often
@@ -120,25 +122,29 @@ pub trait DoubleEndedIterator: Iterator {
     /// ```
     /// #![feature(iter_advance_by)]
     ///
+    /// use std::num::NonZeroUsize;
     /// let a = [3, 4, 5, 6];
     /// let mut iter = a.iter();
     ///
     /// assert_eq!(iter.advance_back_by(2), Ok(()));
     /// assert_eq!(iter.next_back(), Some(&4));
     /// assert_eq!(iter.advance_back_by(0), Ok(()));
-    /// assert_eq!(iter.advance_back_by(100), Err(1)); // only `&3` was skipped
+    /// assert_eq!(iter.advance_back_by(100), Err(NonZeroUsize::new(99).unwrap())); // only `&3` was skipped
     /// ```
     ///
     /// [`Ok(())`]: Ok
     /// [`Err(k)`]: Err
     #[inline]
     #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize>
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize>
     where
         Self::Item: ~const Destruct,
     {
         for i in 0..n {
-            self.next_back().ok_or(i)?;
+            if self.next_back().is_none() {
+                // SAFETY: `i` is always less than `n`.
+                return Err(unsafe { NonZeroUsize::new_unchecked(n - i) });
+            }
         }
         Ok(())
     }
@@ -188,7 +194,9 @@ pub trait DoubleEndedIterator: Iterator {
     #[stable(feature = "iter_nth_back", since = "1.37.0")]
     #[rustc_do_not_const_check]
     fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
-        self.advance_back_by(n).ok()?;
+        if self.advance_back_by(n).is_err() {
+            return None;
+        }
         self.next_back()
     }
 
@@ -374,7 +382,7 @@ impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I {
     fn next_back(&mut self) -> Option<I::Item> {
         (**self).next_back()
     }
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         (**self).advance_back_by(n)
     }
     fn nth_back(&mut self, n: usize) -> Option<I::Item> {
diff --git a/library/core/src/iter/traits/iterator.rs b/library/core/src/iter/traits/iterator.rs
index 16c9f668b8e..080330fa41e 100644
--- a/library/core/src/iter/traits/iterator.rs
+++ b/library/core/src/iter/traits/iterator.rs
@@ -1,6 +1,7 @@
 use crate::array;
 use crate::cmp::{self, Ordering};
 use crate::marker::Destruct;
+use crate::num::NonZeroUsize;
 use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
 
 use super::super::try_process;
@@ -308,10 +309,11 @@ pub trait Iterator {
     /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
     /// times until [`None`] is encountered.
     ///
-    /// `advance_by(n)` will return [`Ok(())`][Ok] if the iterator successfully advances by
-    /// `n` elements, or [`Err(k)`][Err] if [`None`] is encountered, where `k` is the number
-    /// of elements the iterator is advanced by before running out of elements (i.e. the
-    /// length of the iterator). Note that `k` is always less than `n`.
+    /// `advance_by(n)` will return `Ok(())` if the iterator successfully advances by
+    /// `n` elements, or a `Err(NonZeroUsize)` with value `k` if [`None`] is encountered,
+    /// where `k` is remaining number of steps that could not be advanced because the iterator ran out.
+    /// If `self` is empty and `n` is non-zero, then this returns `Err(n)`.
+    /// Otherwise, `k` is always less than `n`.
     ///
     /// Calling `advance_by(0)` can do meaningful work, for example [`Flatten`]
     /// can advance its outer iterator until it finds an inner iterator that is not empty, which
@@ -327,22 +329,26 @@ pub trait Iterator {
     /// ```
     /// #![feature(iter_advance_by)]
     ///
+    /// use std::num::NonZeroUsize;
     /// let a = [1, 2, 3, 4];
     /// let mut iter = a.iter();
     ///
     /// assert_eq!(iter.advance_by(2), Ok(()));
     /// assert_eq!(iter.next(), Some(&3));
     /// assert_eq!(iter.advance_by(0), Ok(()));
-    /// assert_eq!(iter.advance_by(100), Err(1)); // only `&4` was skipped
+    /// assert_eq!(iter.advance_by(100), Err(NonZeroUsize::new(99).unwrap())); // only `&4` was skipped
     /// ```
     #[inline]
     #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize>
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize>
     where
         Self::Item: ~const Destruct,
     {
         for i in 0..n {
-            self.next().ok_or(i)?;
+            if self.next().is_none() {
+                // SAFETY: `i` is always less than `n`.
+                return Err(unsafe { NonZeroUsize::new_unchecked(n - i) });
+            }
         }
         Ok(())
     }
@@ -4013,7 +4019,7 @@ impl<I: Iterator + ?Sized> Iterator for &mut I {
     fn size_hint(&self) -> (usize, Option<usize>) {
         (**self).size_hint()
     }
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         (**self).advance_by(n)
     }
     fn nth(&mut self, n: usize) -> Option<Self::Item> {
diff --git a/library/core/src/ops/index_range.rs b/library/core/src/ops/index_range.rs
index 3e06776d2c6..265022a394e 100644
--- a/library/core/src/ops/index_range.rs
+++ b/library/core/src/ops/index_range.rs
@@ -1,5 +1,6 @@
 use crate::intrinsics::{assert_unsafe_precondition, unchecked_add, unchecked_sub};
 use crate::iter::{FusedIterator, TrustedLen};
+use crate::num::NonZeroUsize;
 
 /// Like a `Range<usize>`, but with a safety invariant that `start <= end`.
 ///
@@ -132,10 +133,9 @@ impl Iterator for IndexRange {
     }
 
     #[inline]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
-        let original_len = self.len();
-        self.take_prefix(n);
-        if n > original_len { Err(original_len) } else { Ok(()) }
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        let taken = self.take_prefix(n);
+        NonZeroUsize::new(n - taken.len()).map_or(Ok(()), Err)
     }
 }
 
@@ -151,10 +151,9 @@ impl DoubleEndedIterator for IndexRange {
     }
 
     #[inline]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
-        let original_len = self.len();
-        self.take_suffix(n);
-        if n > original_len { Err(original_len) } else { Ok(()) }
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        let taken = self.take_suffix(n);
+        NonZeroUsize::new(n - taken.len()).map_or(Ok(()), Err)
     }
 }
 
diff --git a/library/core/src/slice/iter/macros.rs b/library/core/src/slice/iter/macros.rs
index a800da546b4..b73e35f1e91 100644
--- a/library/core/src/slice/iter/macros.rs
+++ b/library/core/src/slice/iter/macros.rs
@@ -176,11 +176,11 @@ macro_rules! iterator {
             }
 
             #[inline]
-            fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+            fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
                 let advance = cmp::min(len!(self), n);
                 // SAFETY: By construction, `advance` does not exceed `self.len()`.
                 unsafe { self.post_inc_start(advance) };
-                if advance == n { Ok(()) } else { Err(advance) }
+                NonZeroUsize::new(n - advance).map_or(Ok(()), Err)
             }
 
             #[inline]
@@ -371,11 +371,11 @@ macro_rules! iterator {
             }
 
             #[inline]
-            fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+            fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
                 let advance = cmp::min(len!(self), n);
                 // SAFETY: By construction, `advance` does not exceed `self.len()`.
                 unsafe { self.pre_dec_end(advance) };
-                if advance == n { Ok(()) } else { Err(advance) }
+                NonZeroUsize::new(n - advance).map_or(Ok(()), Err)
             }
         }
 
diff --git a/library/core/tests/array.rs b/library/core/tests/array.rs
index 5327e4f8139..0869644c040 100644
--- a/library/core/tests/array.rs
+++ b/library/core/tests/array.rs
@@ -1,5 +1,6 @@
-use core::array;
+use core::{array, assert_eq};
 use core::convert::TryFrom;
+use core::num::NonZeroUsize;
 use core::sync::atomic::{AtomicUsize, Ordering};
 
 #[test]
@@ -557,7 +558,7 @@ fn array_intoiter_advance_by() {
     assert_eq!(counter.get(), 13);
 
     let r = it.advance_by(123456);
-    assert_eq!(r, Err(87));
+    assert_eq!(r, Err(NonZeroUsize::new(123456 - 87).unwrap()));
     assert_eq!(it.len(), 0);
     assert_eq!(counter.get(), 100);
 
@@ -567,7 +568,7 @@ fn array_intoiter_advance_by() {
     assert_eq!(counter.get(), 100);
 
     let r = it.advance_by(10);
-    assert_eq!(r, Err(0));
+    assert_eq!(r, Err(NonZeroUsize::new(10).unwrap()));
     assert_eq!(it.len(), 0);
     assert_eq!(counter.get(), 100);
 }
@@ -610,7 +611,7 @@ fn array_intoiter_advance_back_by() {
     assert_eq!(counter.get(), 13);
 
     let r = it.advance_back_by(123456);
-    assert_eq!(r, Err(87));
+    assert_eq!(r, Err(NonZeroUsize::new(123456 - 87).unwrap()));
     assert_eq!(it.len(), 0);
     assert_eq!(counter.get(), 100);
 
@@ -620,7 +621,7 @@ fn array_intoiter_advance_back_by() {
     assert_eq!(counter.get(), 100);
 
     let r = it.advance_back_by(10);
-    assert_eq!(r, Err(0));
+    assert_eq!(r, Err(NonZeroUsize::new(10).unwrap()));
     assert_eq!(it.len(), 0);
     assert_eq!(counter.get(), 100);
 }
@@ -679,8 +680,8 @@ fn array_into_iter_fold() {
 
     let a = [1, 2, 3, 4, 5, 6];
     let mut it = a.into_iter();
-    it.advance_by(1).unwrap();
-    it.advance_back_by(2).unwrap();
+    assert_eq!(it.advance_by(1), Ok(()));
+    assert_eq!(it.advance_back_by(2), Ok(()));
     let s = it.fold(10, |a, b| 10 * a + b);
     assert_eq!(s, 10234);
 }
@@ -695,8 +696,8 @@ fn array_into_iter_rfold() {
 
     let a = [1, 2, 3, 4, 5, 6];
     let mut it = a.into_iter();
-    it.advance_by(1).unwrap();
-    it.advance_back_by(2).unwrap();
+    assert_eq!(it.advance_by(1), Ok(()));
+    assert_eq!(it.advance_back_by(2), Ok(()));
     let s = it.rfold(10, |a, b| 10 * a + b);
     assert_eq!(s, 10432);
 }
diff --git a/library/core/tests/iter/adapters/chain.rs b/library/core/tests/iter/adapters/chain.rs
index f419f9cec12..175a1b638e1 100644
--- a/library/core/tests/iter/adapters/chain.rs
+++ b/library/core/tests/iter/adapters/chain.rs
@@ -1,5 +1,6 @@
 use super::*;
 use core::iter::*;
+use core::num::NonZeroUsize;
 
 #[test]
 fn test_iterator_chain() {
@@ -31,28 +32,28 @@ fn test_iterator_chain_advance_by() {
 
         for i in 0..xs.len() {
             let mut iter = Unfuse::new(xs).chain(Unfuse::new(ys));
-            iter.advance_by(i).unwrap();
+            assert_eq!(iter.advance_by(i), Ok(()));
             assert_eq!(iter.next(), Some(&xs[i]));
-            assert_eq!(iter.advance_by(100), Err(len - i - 1));
-            iter.advance_by(0).unwrap();
+            assert_eq!(iter.advance_by(100), Err(NonZeroUsize::new(100 - (len - i - 1)).unwrap()));
+            assert_eq!(iter.advance_by(0), Ok(()));
         }
 
         for i in 0..ys.len() {
             let mut iter = Unfuse::new(xs).chain(Unfuse::new(ys));
-            iter.advance_by(xs.len() + i).unwrap();
+            assert_eq!(iter.advance_by(xs.len() + i), Ok(()));
             assert_eq!(iter.next(), Some(&ys[i]));
-            assert_eq!(iter.advance_by(100), Err(ys.len() - i - 1));
-            iter.advance_by(0).unwrap();
+            assert_eq!(iter.advance_by(100), Err(NonZeroUsize::new(100 - (ys.len() - i - 1)).unwrap()));
+            assert_eq!(iter.advance_by(0), Ok(()));
         }
 
         let mut iter = xs.iter().chain(ys);
-        iter.advance_by(len).unwrap();
+        assert_eq!(iter.advance_by(len), Ok(()));
         assert_eq!(iter.next(), None);
-        iter.advance_by(0).unwrap();
+        assert_eq!(iter.advance_by(0), Ok(()));
 
         let mut iter = xs.iter().chain(ys);
-        assert_eq!(iter.advance_by(len + 1), Err(len));
-        iter.advance_by(0).unwrap();
+        assert_eq!(iter.advance_by(len + 1), Err(NonZeroUsize::new(1).unwrap()));
+        assert_eq!(iter.advance_by(0), Ok(()));
     }
 
     test_chain(&[], &[]);
@@ -68,28 +69,28 @@ fn test_iterator_chain_advance_back_by() {
 
         for i in 0..ys.len() {
             let mut iter = Unfuse::new(xs).chain(Unfuse::new(ys));
-            iter.advance_back_by(i).unwrap();
+            assert_eq!(iter.advance_back_by(i), Ok(()));
             assert_eq!(iter.next_back(), Some(&ys[ys.len() - i - 1]));
-            assert_eq!(iter.advance_back_by(100), Err(len - i - 1));
-            iter.advance_back_by(0).unwrap();
+            assert_eq!(iter.advance_back_by(100), Err(NonZeroUsize::new(100 - (len - i - 1)).unwrap()));
+            assert_eq!(iter.advance_back_by(0), Ok(()));
         }
 
         for i in 0..xs.len() {
             let mut iter = Unfuse::new(xs).chain(Unfuse::new(ys));
-            iter.advance_back_by(ys.len() + i).unwrap();
+            assert_eq!(iter.advance_back_by(ys.len() + i), Ok(()));
             assert_eq!(iter.next_back(), Some(&xs[xs.len() - i - 1]));
-            assert_eq!(iter.advance_back_by(100), Err(xs.len() - i - 1));
-            iter.advance_back_by(0).unwrap();
+            assert_eq!(iter.advance_back_by(100), Err(NonZeroUsize::new(100 - (xs.len() - i - 1)).unwrap()));
+            assert_eq!(iter.advance_back_by(0), Ok(()));
         }
 
         let mut iter = xs.iter().chain(ys);
-        iter.advance_back_by(len).unwrap();
+        assert_eq!(iter.advance_back_by(len), Ok(()));
         assert_eq!(iter.next_back(), None);
-        iter.advance_back_by(0).unwrap();
+        assert_eq!(iter.advance_back_by(0), Ok(()));
 
         let mut iter = xs.iter().chain(ys);
-        assert_eq!(iter.advance_back_by(len + 1), Err(len));
-        iter.advance_back_by(0).unwrap();
+        assert_eq!(iter.advance_back_by(len + 1), Err(NonZeroUsize::new(1).unwrap()));
+        assert_eq!(iter.advance_back_by(0), Ok(()));
     }
 
     test_chain(&[], &[]);
diff --git a/library/core/tests/iter/adapters/enumerate.rs b/library/core/tests/iter/adapters/enumerate.rs
index 0e603387847..ff57973a62a 100644
--- a/library/core/tests/iter/adapters/enumerate.rs
+++ b/library/core/tests/iter/adapters/enumerate.rs
@@ -1,4 +1,5 @@
 use core::iter::*;
+use core::num::NonZeroUsize;
 
 #[test]
 fn test_iterator_enumerate() {
@@ -55,6 +56,20 @@ fn test_iterator_enumerate_count() {
     assert_eq!(xs.iter().enumerate().count(), 6);
 }
 
+#[test]
+fn test_iterator_enumerate_advance_by() {
+    let xs = [0, 1, 2, 3, 4, 5];
+    let mut it = xs.iter().enumerate();
+    assert_eq!(it.advance_by(0), Ok(()));
+    assert_eq!(it.next(), Some((0, &0)));
+    assert_eq!(it.advance_by(1), Ok(()));
+    assert_eq!(it.next(), Some((2, &2)));
+    assert_eq!(it.advance_by(2), Ok(()));
+    assert_eq!(it.next(), Some((5, &5)));
+    assert_eq!(it.advance_by(1), Err(NonZeroUsize::new(1).unwrap()));
+    assert_eq!(it.next(), None);
+}
+
 #[test]
 fn test_iterator_enumerate_fold() {
     let xs = [0, 1, 2, 3, 4, 5];
diff --git a/library/core/tests/iter/adapters/flatten.rs b/library/core/tests/iter/adapters/flatten.rs
index 690fd0c2197..91809c9e5fd 100644
--- a/library/core/tests/iter/adapters/flatten.rs
+++ b/library/core/tests/iter/adapters/flatten.rs
@@ -1,5 +1,7 @@
+use core::assert_eq;
 use super::*;
 use core::iter::*;
+use core::num::NonZeroUsize;
 
 #[test]
 fn test_iterator_flatten() {
@@ -61,19 +63,19 @@ fn test_flatten_try_folds() {
 fn test_flatten_advance_by() {
     let mut it = once(0..10).chain(once(10..30)).chain(once(30..40)).flatten();
 
-    it.advance_by(5).unwrap();
+    assert_eq!(it.advance_by(5), Ok(()));
     assert_eq!(it.next(), Some(5));
-    it.advance_by(9).unwrap();
+    assert_eq!(it.advance_by(9), Ok(()));
     assert_eq!(it.next(), Some(15));
-    it.advance_back_by(4).unwrap();
+    assert_eq!(it.advance_back_by(4), Ok(()));
     assert_eq!(it.next_back(), Some(35));
-    it.advance_back_by(9).unwrap();
+    assert_eq!(it.advance_back_by(9), Ok(()));
     assert_eq!(it.next_back(), Some(25));
 
-    assert_eq!(it.advance_by(usize::MAX), Err(9));
-    assert_eq!(it.advance_back_by(usize::MAX), Err(0));
-    it.advance_by(0).unwrap();
-    it.advance_back_by(0).unwrap();
+    assert_eq!(it.advance_by(usize::MAX), Err(NonZeroUsize::new(usize::MAX - 9).unwrap()));
+    assert_eq!(it.advance_back_by(usize::MAX), Err(NonZeroUsize::new(usize::MAX).unwrap()));
+    assert_eq!(it.advance_by(0), Ok(()));
+    assert_eq!(it.advance_back_by(0), Ok(()));
     assert_eq!(it.size_hint(), (0, Some(0)));
 }
 
@@ -174,19 +176,19 @@ fn test_flatten_count() {
     let mut it = once(0..10).chain(once(10..30)).chain(once(30..40)).flatten();
 
     assert_eq!(it.clone().count(), 40);
-    it.advance_by(5).unwrap();
+    assert_eq!(it.advance_by(5), Ok(()));
     assert_eq!(it.clone().count(), 35);
-    it.advance_back_by(5).unwrap();
+    assert_eq!(it.advance_back_by(5), Ok(()));
     assert_eq!(it.clone().count(), 30);
-    it.advance_by(10).unwrap();
+    assert_eq!(it.advance_by(10), Ok(()));
     assert_eq!(it.clone().count(), 20);
-    it.advance_back_by(8).unwrap();
+    assert_eq!(it.advance_back_by(8), Ok(()));
     assert_eq!(it.clone().count(), 12);
-    it.advance_by(4).unwrap();
+    assert_eq!(it.advance_by(4), Ok(()));
     assert_eq!(it.clone().count(), 8);
-    it.advance_back_by(5).unwrap();
+    assert_eq!(it.advance_back_by(5), Ok(()));
     assert_eq!(it.clone().count(), 3);
-    it.advance_by(3).unwrap();
+    assert_eq!(it.advance_by(3), Ok(()));
     assert_eq!(it.clone().count(), 0);
 }
 
@@ -195,18 +197,18 @@ fn test_flatten_last() {
     let mut it = once(0..10).chain(once(10..30)).chain(once(30..40)).flatten();
 
     assert_eq!(it.clone().last(), Some(39));
-    it.advance_by(5).unwrap(); // 5..40
+    assert_eq!(it.advance_by(5), Ok(())); // 5..40
     assert_eq!(it.clone().last(), Some(39));
-    it.advance_back_by(5).unwrap(); // 5..35
+    assert_eq!(it.advance_back_by(5), Ok(())); // 5..35
     assert_eq!(it.clone().last(), Some(34));
-    it.advance_by(10).unwrap(); // 15..35
+    assert_eq!(it.advance_by(10), Ok(())); // 15..35
     assert_eq!(it.clone().last(), Some(34));
-    it.advance_back_by(8).unwrap(); // 15..27
+    assert_eq!(it.advance_back_by(8), Ok(())); // 15..27
     assert_eq!(it.clone().last(), Some(26));
-    it.advance_by(4).unwrap(); // 19..27
+    assert_eq!(it.advance_by(4), Ok(())); // 19..27
     assert_eq!(it.clone().last(), Some(26));
-    it.advance_back_by(5).unwrap(); // 19..22
+    assert_eq!(it.advance_back_by(5), Ok(())); // 19..22
     assert_eq!(it.clone().last(), Some(21));
-    it.advance_by(3).unwrap(); // 22..22
+    assert_eq!(it.advance_by(3), Ok(())); // 22..22
     assert_eq!(it.clone().last(), None);
 }
diff --git a/library/core/tests/iter/adapters/skip.rs b/library/core/tests/iter/adapters/skip.rs
index 754641834e8..e3e88a84fad 100644
--- a/library/core/tests/iter/adapters/skip.rs
+++ b/library/core/tests/iter/adapters/skip.rs
@@ -1,4 +1,5 @@
 use core::iter::*;
+use core::num::NonZeroUsize;
 
 use super::Unfuse;
 
@@ -74,11 +75,14 @@ fn test_iterator_skip_nth() {
 #[test]
 fn test_skip_advance_by() {
     assert_eq!((0..0).skip(10).advance_by(0), Ok(()));
-    assert_eq!((0..0).skip(10).advance_by(1), Err(0));
-    assert_eq!((0u128..(usize::MAX as u128) + 1).skip(usize::MAX).advance_by(usize::MAX), Err(1));
-    assert_eq!((0u128..u128::MAX).skip(usize::MAX).advance_by(1), Ok(()));
+    assert_eq!((0..0).skip(10).advance_by(1), Err(NonZeroUsize::new(1).unwrap()));
+    assert_eq!(
+        (0u128..(usize::MAX as u128) + 1).skip(usize::MAX - 10).advance_by(usize::MAX - 5),
+        Err(NonZeroUsize::new(usize::MAX - 16).unwrap())
+    );
+    assert_eq!((0u128..u128::MAX).skip(usize::MAX - 10).advance_by(20), Ok(()));
 
-    assert_eq!((0..2).skip(1).advance_back_by(10), Err(1));
+    assert_eq!((0..2).skip(1).advance_back_by(10), Err(NonZeroUsize::new(9).unwrap()));
     assert_eq!((0..0).skip(1).advance_back_by(0), Ok(()));
 }
 
diff --git a/library/core/tests/iter/adapters/take.rs b/library/core/tests/iter/adapters/take.rs
index 3e26b43a2ed..3cad47c06de 100644
--- a/library/core/tests/iter/adapters/take.rs
+++ b/library/core/tests/iter/adapters/take.rs
@@ -1,4 +1,5 @@
 use core::iter::*;
+use core::num::NonZeroUsize;
 
 #[test]
 fn test_iterator_take() {
@@ -78,21 +79,21 @@ fn test_take_advance_by() {
     let mut take = (0..10).take(3);
     assert_eq!(take.advance_by(2), Ok(()));
     assert_eq!(take.next(), Some(2));
-    assert_eq!(take.advance_by(1), Err(0));
+    assert_eq!(take.advance_by(1), Err(NonZeroUsize::new(1).unwrap()));
 
     assert_eq!((0..0).take(10).advance_by(0), Ok(()));
-    assert_eq!((0..0).take(10).advance_by(1), Err(0));
-    assert_eq!((0..10).take(4).advance_by(5), Err(4));
+    assert_eq!((0..0).take(10).advance_by(1), Err(NonZeroUsize::new(1).unwrap()));
+    assert_eq!((0..10).take(4).advance_by(5), Err(NonZeroUsize::new(1).unwrap()));
 
     let mut take = (0..10).take(3);
     assert_eq!(take.advance_back_by(2), Ok(()));
     assert_eq!(take.next(), Some(0));
-    assert_eq!(take.advance_back_by(1), Err(0));
+    assert_eq!(take.advance_back_by(1), Err(NonZeroUsize::new(1).unwrap()));
 
-    assert_eq!((0..2).take(1).advance_back_by(10), Err(1));
-    assert_eq!((0..0).take(1).advance_back_by(1), Err(0));
+    assert_eq!((0..2).take(1).advance_back_by(10), Err(NonZeroUsize::new(9).unwrap()));
+    assert_eq!((0..0).take(1).advance_back_by(1), Err(NonZeroUsize::new(1).unwrap()));
     assert_eq!((0..0).take(1).advance_back_by(0), Ok(()));
-    assert_eq!((0..usize::MAX).take(100).advance_back_by(usize::MAX), Err(100));
+    assert_eq!((0..usize::MAX).take(100).advance_back_by(usize::MAX), Err(NonZeroUsize::new(usize::MAX - 100).unwrap()));
 }
 
 #[test]
diff --git a/library/core/tests/iter/range.rs b/library/core/tests/iter/range.rs
index 0f91ffe2dfc..0a77ecddb84 100644
--- a/library/core/tests/iter/range.rs
+++ b/library/core/tests/iter/range.rs
@@ -1,3 +1,4 @@
+use core::num::NonZeroUsize;
 use super::*;
 
 #[test]
@@ -287,25 +288,25 @@ fn test_range_step() {
 #[test]
 fn test_range_advance_by() {
     let mut r = 0..usize::MAX;
-    r.advance_by(0).unwrap();
-    r.advance_back_by(0).unwrap();
+    assert_eq!(Ok(()), r.advance_by(0));
+    assert_eq!(Ok(()), r.advance_back_by(0));
 
     assert_eq!(r.len(), usize::MAX);
 
-    r.advance_by(1).unwrap();
-    r.advance_back_by(1).unwrap();
+    assert_eq!(Ok(()), r.advance_by(1));
+    assert_eq!(Ok(()), r.advance_back_by(1));
 
     assert_eq!((r.start, r.end), (1, usize::MAX - 1));
 
-    assert_eq!(r.advance_by(usize::MAX), Err(usize::MAX - 2));
+    assert_eq!(Err(NonZeroUsize::new(2).unwrap()), r.advance_by(usize::MAX));
 
-    r.advance_by(0).unwrap();
-    r.advance_back_by(0).unwrap();
+    assert_eq!(Ok(()), r.advance_by(0));
+    assert_eq!(Ok(()), r.advance_back_by(0));
 
     let mut r = 0u128..u128::MAX;
 
-    r.advance_by(usize::MAX).unwrap();
-    r.advance_back_by(usize::MAX).unwrap();
+    assert_eq!(Ok(()), r.advance_by(usize::MAX));
+    assert_eq!(Ok(()), r.advance_back_by(usize::MAX));
 
     assert_eq!((r.start, r.end), (0u128 + usize::MAX as u128, u128::MAX - usize::MAX as u128));
 }
diff --git a/library/core/tests/iter/traits/iterator.rs b/library/core/tests/iter/traits/iterator.rs
index 62566a9502d..9eebfb1f1f3 100644
--- a/library/core/tests/iter/traits/iterator.rs
+++ b/library/core/tests/iter/traits/iterator.rs
@@ -1,3 +1,5 @@
+use core::num::NonZeroUsize;
+
 /// A wrapper struct that implements `Eq` and `Ord` based on the wrapped
 /// integer modulo 3. Used to test that `Iterator::max` and `Iterator::min`
 /// return the correct element if some of them are equal.
@@ -150,11 +152,11 @@ fn test_iterator_advance_by() {
         let mut iter = v.iter();
         assert_eq!(iter.advance_by(i), Ok(()));
         assert_eq!(iter.next().unwrap(), &v[i]);
-        assert_eq!(iter.advance_by(100), Err(v.len() - 1 - i));
+        assert_eq!(iter.advance_by(100), Err(NonZeroUsize::new(100 - (v.len() - 1 - i)).unwrap()));
     }
 
     assert_eq!(v.iter().advance_by(v.len()), Ok(()));
-    assert_eq!(v.iter().advance_by(100), Err(v.len()));
+    assert_eq!(v.iter().advance_by(100), Err(NonZeroUsize::new(100 - v.len()).unwrap()));
 }
 
 #[test]
@@ -165,11 +167,11 @@ fn test_iterator_advance_back_by() {
         let mut iter = v.iter();
         assert_eq!(iter.advance_back_by(i), Ok(()));
         assert_eq!(iter.next_back().unwrap(), &v[v.len() - 1 - i]);
-        assert_eq!(iter.advance_back_by(100), Err(v.len() - 1 - i));
+        assert_eq!(iter.advance_back_by(100), Err(NonZeroUsize::new(100 - (v.len() - 1 - i)).unwrap()));
     }
 
     assert_eq!(v.iter().advance_back_by(v.len()), Ok(()));
-    assert_eq!(v.iter().advance_back_by(100), Err(v.len()));
+    assert_eq!(v.iter().advance_back_by(100), Err(NonZeroUsize::new(100 - v.len()).unwrap()));
 }
 
 #[test]
@@ -180,11 +182,11 @@ fn test_iterator_rev_advance_back_by() {
         let mut iter = v.iter().rev();
         assert_eq!(iter.advance_back_by(i), Ok(()));
         assert_eq!(iter.next_back().unwrap(), &v[i]);
-        assert_eq!(iter.advance_back_by(100), Err(v.len() - 1 - i));
+        assert_eq!(iter.advance_back_by(100), Err(NonZeroUsize::new(100 - (v.len() - 1 - i)).unwrap()));
     }
 
     assert_eq!(v.iter().rev().advance_back_by(v.len()), Ok(()));
-    assert_eq!(v.iter().rev().advance_back_by(100), Err(v.len()));
+    assert_eq!(v.iter().rev().advance_back_by(100), Err(NonZeroUsize::new(100 - v.len()).unwrap()));
 }
 
 #[test]
@@ -424,11 +426,11 @@ fn test_iterator_rev_advance_by() {
         let mut iter = v.iter().rev();
         assert_eq!(iter.advance_by(i), Ok(()));
         assert_eq!(iter.next().unwrap(), &v[v.len() - 1 - i]);
-        assert_eq!(iter.advance_by(100), Err(v.len() - 1 - i));
+        assert_eq!(iter.advance_by(100), Err(NonZeroUsize::new(100 - (v.len() - 1 - i)).unwrap()));
     }
 
     assert_eq!(v.iter().rev().advance_by(v.len()), Ok(()));
-    assert_eq!(v.iter().rev().advance_by(100), Err(v.len()));
+    assert_eq!(v.iter().rev().advance_by(100), Err(NonZeroUsize::new(100 - v.len()).unwrap()));
 }
 
 #[test]
diff --git a/library/core/tests/slice.rs b/library/core/tests/slice.rs
index 39559cdbb5e..88f54591bb4 100644
--- a/library/core/tests/slice.rs
+++ b/library/core/tests/slice.rs
@@ -1,6 +1,7 @@
 use core::cell::Cell;
 use core::cmp::Ordering;
 use core::mem::MaybeUninit;
+use core::num::NonZeroUsize;
 use core::result::Result::{Err, Ok};
 use core::slice;
 
@@ -142,20 +143,20 @@ fn test_iterator_advance_by() {
 
     for i in 0..=v.len() {
         let mut iter = v.iter();
-        iter.advance_by(i).unwrap();
+        assert_eq!(iter.advance_by(i), Ok(()));
         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.advance_by(v.len() + 1), Err(NonZeroUsize::new(1).unwrap()));
     assert_eq!(iter.as_slice(), &[]);
 
     let mut iter = v.iter();
-    iter.advance_by(3).unwrap();
+    assert_eq!(iter.advance_by(3), Ok(()));
     assert_eq!(iter.as_slice(), &v[3..]);
-    iter.advance_by(2).unwrap();
+    assert_eq!(iter.advance_by(2), Ok(()));
     assert_eq!(iter.as_slice(), &[]);
-    iter.advance_by(0).unwrap();
+    assert_eq!(iter.advance_by(0), Ok(()));
 }
 
 #[test]
@@ -164,20 +165,20 @@ fn test_iterator_advance_back_by() {
 
     for i in 0..=v.len() {
         let mut iter = v.iter();
-        iter.advance_back_by(i).unwrap();
+        assert_eq!(iter.advance_back_by(i), Ok(()));
         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.advance_back_by(v.len() + 1), Err(NonZeroUsize::new(1).unwrap()));
     assert_eq!(iter.as_slice(), &[]);
 
     let mut iter = v.iter();
-    iter.advance_back_by(3).unwrap();
+    assert_eq!(iter.advance_back_by(3), Ok(()));
     assert_eq!(iter.as_slice(), &v[..v.len() - 3]);
-    iter.advance_back_by(2).unwrap();
+    assert_eq!(iter.advance_back_by(2), Ok(()));
     assert_eq!(iter.as_slice(), &[]);
-    iter.advance_back_by(0).unwrap();
+    assert_eq!(iter.advance_back_by(0), Ok(()));
 }
 
 #[test]