diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs index 00dbe7104d1a4..72a54c5ce82d0 100644 --- a/src/libcore/slice/mod.rs +++ b/src/libcore/slice/mod.rs @@ -4155,15 +4155,32 @@ impl<'a, T> DoubleEndedIterator for Chunks<'a, T> { #[inline] fn nth_back(&mut self, n: usize) -> Option { - let (end, overflow) = self.v.len().overflowing_sub(n * self.chunk_size); + let remainder = match self.v.len().checked_rem(self.chunk_size) { + Some(res) => res, + None => 0, + }; + + let sub_chunk_size = if remainder != 0 { remainder } else { self.chunk_size }; + + let safe_sub = match n.checked_mul(sub_chunk_size) { + Some(res) => res, + None => 0, + }; + + let (end, overflow) = self.v.len().overflowing_sub(safe_sub); if overflow { - self.v = &mut []; + self.v= &[]; None } else { - let start = match end.checked_sub(self.chunk_size) { - Some(res) => cmp::min(self.v.len(), res), - None => 0, + let start = if n == 0 { + self.v.len() - sub_chunk_size + } else { + match end.checked_sub(self.chunk_size) { + Some(res) => res, + None => 0, + } }; + let nth_back = &self.v[start..end]; self.v = &self.v[..start]; Some(nth_back) diff --git a/src/libcore/tests/slice.rs b/src/libcore/tests/slice.rs index edea405fad791..cf2602f3210f1 100644 --- a/src/libcore/tests/slice.rs +++ b/src/libcore/tests/slice.rs @@ -144,7 +144,7 @@ fn test_chunks_nth_back() { let v2: &[i32] = &[0, 1, 2, 3, 4]; let mut c2 = v2.chunks(3); - assert_eq!(c2.nth_back(1).unwrap(), &[0, 1]); + assert_eq!(c2.nth_back(1).unwrap(), &[0, 1, 2]); assert_eq!(c2.next(), None); assert_eq!(c2.next_back(), None); @@ -152,6 +152,10 @@ fn test_chunks_nth_back() { let mut c3 = v3.chunks(10); assert_eq!(c3.nth_back(0).unwrap(), &[0, 1, 2, 3, 4]); assert_eq!(c3.next(), None); + + let v4: &[i32] = &[0, 1, 2]; + let mut c4 = v4.chunks(10); + assert_eq!(c4.nth_back(1_000_000_000usize), None); } #[test]