From 3f9b26dc64a2068d30027fd29ffbbfe07663419f Mon Sep 17 00:00:00 2001 From: The8472 Date: Thu, 14 Oct 2021 12:10:18 +0200 Subject: [PATCH] Fix Iterator::advance_by contract inconsistency The `advance_by(n)` docs state that in the error case `Err(k)` that k is always less than n. It also states that `advance_by(0)` may return `Err(0)` to indicate an exhausted iterator. These statements are inconsistent. Since only one implementation (Skip) actually made use of that I changed it to return Ok(()) in that case too. While adding some tests I also found a bug in `Take::advance_back_by`. --- library/alloc/tests/vec.rs | 3 ++ library/core/src/iter/adapters/skip.rs | 4 +-- library/core/src/iter/adapters/take.rs | 29 ++++++++++---------- library/core/src/iter/traits/double_ended.rs | 3 -- library/core/src/iter/traits/iterator.rs | 3 -- library/core/tests/iter/adapters/chain.rs | 8 ++++++ library/core/tests/iter/adapters/flatten.rs | 3 ++ library/core/tests/iter/adapters/skip.rs | 11 ++++++++ library/core/tests/iter/adapters/take.rs | 22 +++++++++++++++ library/core/tests/iter/range.rs | 3 ++ library/core/tests/slice.rs | 2 ++ 11 files changed, 69 insertions(+), 22 deletions(-) diff --git a/library/alloc/tests/vec.rs b/library/alloc/tests/vec.rs index 00a878c0794..77314282532 100644 --- a/library/alloc/tests/vec.rs +++ b/library/alloc/tests/vec.rs @@ -985,6 +985,9 @@ fn test_into_iter_advance_by() { assert_eq!(i.advance_by(usize::MAX), Err(0)); + i.advance_by(0).unwrap(); + i.advance_back_by(0).unwrap(); + assert_eq!(i.len(), 0); } diff --git a/library/core/src/iter/adapters/skip.rs b/library/core/src/iter/adapters/skip.rs index 565fc224f53..ea1da8ba434 100644 --- a/library/core/src/iter/adapters/skip.rs +++ b/library/core/src/iter/adapters/skip.rs @@ -119,8 +119,8 @@ where #[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); + match self.iter.advance_by(step_one) { Ok(_) => { rem -= step_one - self.n; @@ -129,7 +129,7 @@ where Err(advanced) => { let advanced_without_skip = advanced.saturating_sub(self.n); self.n = self.n.saturating_sub(advanced); - return Err(advanced_without_skip); + return if n == 0 { Ok(()) } else { Err(advanced_without_skip) }; } } diff --git a/library/core/src/iter/adapters/take.rs b/library/core/src/iter/adapters/take.rs index 81f6c294fac..2962e0104d1 100644 --- a/library/core/src/iter/adapters/take.rs +++ b/library/core/src/iter/adapters/take.rs @@ -215,21 +215,22 @@ where } #[inline] + #[rustc_inherit_overflow_checks] fn advance_back_by(&mut self, n: usize) -> Result<(), usize> { - let inner_len = self.iter.len(); - let len = self.n; - let remainder = len.saturating_sub(n); - let to_advance = inner_len - remainder; - match self.iter.advance_back_by(to_advance) { - Ok(_) => { - self.n = remainder; - if n > len { - return Err(len); - } - return Ok(()); - } - _ => panic!("ExactSizeIterator contract violation"), - } + // 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); + // The amount we need to advance inner to fulfill the caller's request. + // take(), advance_by() and len() all can be at most usize, so we don't have to worry + // 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, + }; + self.n -= advanced; + return if advanced < n { Err(advanced) } else { Ok(()) }; } } diff --git a/library/core/src/iter/traits/double_ended.rs b/library/core/src/iter/traits/double_ended.rs index 9a589c1f3b5..a6aed6d210b 100644 --- a/library/core/src/iter/traits/double_ended.rs +++ b/library/core/src/iter/traits/double_ended.rs @@ -106,9 +106,6 @@ pub trait DoubleEndedIterator: Iterator { /// 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 /// allows it to return a more accurate `size_hint()` than in its initial state. - /// `advance_back_by(0)` may either return `Ok()` or `Err(0)`. The former conveys no information - /// whether the iterator is or is not exhausted, the latter can be treated as if [`next_back`] - /// had returned `None`. Replacing a `Err(0)` with `Ok` is only correct for `n = 0`. /// /// [`advance_by`]: Iterator::advance_by /// [`Flatten`]: crate::iter::Flatten diff --git a/library/core/src/iter/traits/iterator.rs b/library/core/src/iter/traits/iterator.rs index d957a7527cf..588758fed9c 100644 --- a/library/core/src/iter/traits/iterator.rs +++ b/library/core/src/iter/traits/iterator.rs @@ -249,9 +249,6 @@ pub trait Iterator { /// 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 /// then often allows it to return a more accurate `size_hint()` than in its initial state. - /// `advance_by(0)` may either return `Ok()` or `Err(0)`. The former conveys no information - /// whether the iterator is or is not exhausted, the latter can be treated as if [`next`] - /// had returned `None`. Replacing a `Err(0)` with `Ok` is only correct for `n = 0`. /// /// [`Flatten`]: crate::iter::Flatten /// [`next`]: Iterator::next diff --git a/library/core/tests/iter/adapters/chain.rs b/library/core/tests/iter/adapters/chain.rs index 4cd79687b53..f419f9cec12 100644 --- a/library/core/tests/iter/adapters/chain.rs +++ b/library/core/tests/iter/adapters/chain.rs @@ -34,6 +34,7 @@ fn test_iterator_chain_advance_by() { iter.advance_by(i).unwrap(); assert_eq!(iter.next(), Some(&xs[i])); assert_eq!(iter.advance_by(100), Err(len - i - 1)); + iter.advance_by(0).unwrap(); } for i in 0..ys.len() { @@ -41,14 +42,17 @@ fn test_iterator_chain_advance_by() { iter.advance_by(xs.len() + i).unwrap(); assert_eq!(iter.next(), Some(&ys[i])); assert_eq!(iter.advance_by(100), Err(ys.len() - i - 1)); + iter.advance_by(0).unwrap(); } let mut iter = xs.iter().chain(ys); iter.advance_by(len).unwrap(); assert_eq!(iter.next(), None); + iter.advance_by(0).unwrap(); let mut iter = xs.iter().chain(ys); assert_eq!(iter.advance_by(len + 1), Err(len)); + iter.advance_by(0).unwrap(); } test_chain(&[], &[]); @@ -67,6 +71,7 @@ fn test_iterator_chain_advance_back_by() { iter.advance_back_by(i).unwrap(); 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(); } for i in 0..xs.len() { @@ -74,14 +79,17 @@ fn test_iterator_chain_advance_back_by() { iter.advance_back_by(ys.len() + i).unwrap(); 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(); } let mut iter = xs.iter().chain(ys); iter.advance_back_by(len).unwrap(); assert_eq!(iter.next_back(), None); + iter.advance_back_by(0).unwrap(); let mut iter = xs.iter().chain(ys); assert_eq!(iter.advance_back_by(len + 1), Err(len)); + iter.advance_back_by(0).unwrap(); } test_chain(&[], &[]); diff --git a/library/core/tests/iter/adapters/flatten.rs b/library/core/tests/iter/adapters/flatten.rs index 4ae50a2f066..cd6513327f0 100644 --- a/library/core/tests/iter/adapters/flatten.rs +++ b/library/core/tests/iter/adapters/flatten.rs @@ -61,6 +61,7 @@ fn test_flatten_try_folds() { #[test] 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.next(), Some(5)); it.advance_by(9).unwrap(); @@ -72,6 +73,8 @@ fn test_flatten_advance_by() { 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.size_hint(), (0, Some(0))); } diff --git a/library/core/tests/iter/adapters/skip.rs b/library/core/tests/iter/adapters/skip.rs index cf60057a164..0c464bdd03a 100644 --- a/library/core/tests/iter/adapters/skip.rs +++ b/library/core/tests/iter/adapters/skip.rs @@ -69,6 +69,17 @@ fn test_iterator_skip_nth() { assert_eq!(it.nth(0), None); } +#[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..2).skip(1).advance_back_by(10), Err(1)); + assert_eq!((0..0).skip(1).advance_back_by(0), Ok(())); +} + #[test] fn test_iterator_skip_count() { let xs = [0, 1, 2, 3, 5, 13, 15, 16, 17, 19, 20, 30]; diff --git a/library/core/tests/iter/adapters/take.rs b/library/core/tests/iter/adapters/take.rs index 89f9cb1e2ed..bfb659f0a83 100644 --- a/library/core/tests/iter/adapters/take.rs +++ b/library/core/tests/iter/adapters/take.rs @@ -73,6 +73,28 @@ fn test_iterator_take_nth_back() { assert_eq!(it.nth_back(1), None); } +#[test] +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!((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)); + + 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!((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..0).take(1).advance_back_by(0), Ok(())); + assert_eq!((0..usize::MAX).take(100).advance_back_by(usize::MAX), Err(100)); +} + #[test] fn test_iterator_take_short() { let xs = [0, 1, 2, 3]; diff --git a/library/core/tests/iter/range.rs b/library/core/tests/iter/range.rs index 6b4cf33efe1..84498a8eae5 100644 --- a/library/core/tests/iter/range.rs +++ b/library/core/tests/iter/range.rs @@ -300,6 +300,9 @@ fn test_range_advance_by() { assert_eq!(r.advance_by(usize::MAX), Err(usize::MAX - 2)); + r.advance_by(0).unwrap(); + r.advance_back_by(0).unwrap(); + let mut r = 0u128..u128::MAX; r.advance_by(usize::MAX).unwrap(); diff --git a/library/core/tests/slice.rs b/library/core/tests/slice.rs index 8d05e47edf4..66f03c68cbe 100644 --- a/library/core/tests/slice.rs +++ b/library/core/tests/slice.rs @@ -154,6 +154,7 @@ fn test_iterator_advance_by() { assert_eq!(iter.as_slice(), &v[3..]); iter.advance_by(2).unwrap(); assert_eq!(iter.as_slice(), &[]); + iter.advance_by(0).unwrap(); } #[test] @@ -175,6 +176,7 @@ fn test_iterator_advance_back_by() { assert_eq!(iter.as_slice(), &v[..v.len() - 3]); iter.advance_back_by(2).unwrap(); assert_eq!(iter.as_slice(), &[]); + iter.advance_back_by(0).unwrap(); } #[test]