Add Vec::drain.

Author: reitermarkus

CHANGELOG.md

@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `IntoIterator` implementation for `LinearMap`
 - Added `Deque::{get, get_mut, get_unchecked, get_unchecked_mut}`.
 - Added `serde::Serialize` and `serde::Deserialize` implementations to `HistoryBuffer`.
+- Added `Vec::drain`.
 
 ### Changed
 

src/lib.rs

@@ -96,6 +96,7 @@ pub use indexmap::{
 pub use indexset::{FnvIndexSet, IndexSet, Iter as IndexSetIter};
 pub use linear_map::LinearMap;
 pub use string::String;
+
 pub use vec::{Vec, VecView};
 
 #[macro_use]
@@ -107,6 +108,7 @@ mod histbuf;
 mod indexmap;
 mod indexset;
 mod linear_map;
+mod slice;
 pub mod storage;
 pub mod string;
 pub mod vec;

src/slice.rs

@@ -0,0 +1,38 @@
+use core::ops;
+
+// FIXME: Remove when `slice_range` feature is stable.
+#[track_caller]
+#[must_use]
+pub fn range<R>(range: R, bounds: ops::RangeTo<usize>) -> ops::Range<usize>
+where
+    R: ops::RangeBounds<usize>,
+{
+    let len = bounds.end;
+
+    let start: ops::Bound<&usize> = range.start_bound();
+    let start = match start {
+        ops::Bound::Included(&start) => start,
+        ops::Bound::Excluded(start) => start
+            .checked_add(1)
+            .unwrap_or_else(|| panic!("attempted to index slice from after maximum usize")),
+        ops::Bound::Unbounded => 0,
+    };
+
+    let end: ops::Bound<&usize> = range.end_bound();
+    let end = match end {
+        ops::Bound::Included(end) => end
+            .checked_add(1)
+            .unwrap_or_else(|| panic!("attempted to index slice up to maximum usize")),
+        ops::Bound::Excluded(&end) => end,
+        ops::Bound::Unbounded => len,
+    };
+
+    if start > end {
+        panic!("slice index starts at {start} but ends at {end}");
+    }
+    if end > len {
+        panic!("range end index {end} out of range for slice of length {len}");
+    }
+
+    ops::Range { start, end }
+}

src/vec/drain.rs

@@ -0,0 +1,222 @@
+use core::{
+    fmt,
+    iter::FusedIterator,
+    mem::{self, size_of},
+    ptr::{self, NonNull},
+    slice,
+};
+
+use super::VecView;
+
+/// A draining iterator for [`Vec`](super::Vec).
+///
+/// This `struct` is created by [`Vec::drain`](super::Vec::drain).
+/// See its documentation for more.
+///
+/// # Example
+///
+/// ```
+/// use heapless::{vec, Vec};
+///
+/// let mut v = Vec::<_, 4>::from_array([0, 1, 2]);
+/// let iter: vec::Drain<'_, _> = v.drain(..);
+/// ```
+pub struct Drain<'a, T: 'a> {
+    /// Index of tail to preserve
+    pub(super) tail_start: usize,
+    /// Length of tail
+    pub(super) tail_len: usize,
+    /// Current remaining range to remove
+    pub(super) iter: slice::Iter<'a, T>,
+    pub(super) vec: NonNull<VecView<T>>,
+}
+
+impl<T: fmt::Debug> fmt::Debug for Drain<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("Drain").field(&self.iter.as_slice()).finish()
+    }
+}
+
+impl<'a, T> Drain<'a, T> {
+    /// Returns the remaining items of this iterator as a slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use heapless::{vec, Vec};
+    ///
+    /// let mut vec = Vec::<_, 3>::from_array(['a', 'b', 'c']);
+    /// let mut drain = vec.drain(..);
+    /// assert_eq!(drain.as_slice(), &['a', 'b', 'c']);
+    /// let _ = drain.next().unwrap();
+    /// assert_eq!(drain.as_slice(), &['b', 'c']);
+    /// ```
+    #[must_use]
+    pub fn as_slice(&self) -> &[T] {
+        self.iter.as_slice()
+    }
+}
+
+impl<'a, T> AsRef<[T]> for Drain<'a, T> {
+    fn as_ref(&self) -> &[T] {
+        self.as_slice()
+    }
+}
+
+unsafe impl<T: Sync> Sync for Drain<'_, T> {}
+unsafe impl<T: Send> Send for Drain<'_, T> {}
+
+impl<T> Iterator for Drain<'_, T> {
+    type Item = T;
+
+    #[inline]
+    fn next(&mut self) -> Option<T> {
+        self.iter
+            .next()
+            .map(|elt| unsafe { ptr::read(elt as *const _) })
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
+
+impl<T> DoubleEndedIterator for Drain<'_, T> {
+    #[inline]
+    fn next_back(&mut self) -> Option<T> {
+        self.iter
+            .next_back()
+            .map(|elt| unsafe { ptr::read(elt as *const _) })
+    }
+}
+
+impl<T> Drop for Drain<'_, T> {
+    fn drop(&mut self) {
+        /// Moves back the un-`Drain`ed elements to restore the original `Vec`.
+        struct DropGuard<'r, 'a, T>(&'r mut Drain<'a, T>);
+
+        impl<'r, 'a, T> Drop for DropGuard<'r, 'a, T> {
+            fn drop(&mut self) {
+                if self.0.tail_len > 0 {
+                    unsafe {
+                        let source_vec = self.0.vec.as_mut();
+                        // memmove back untouched tail, update to new length
+                        let start = source_vec.len();
+                        let tail = self.0.tail_start;
+                        if tail != start {
+                            let dst = source_vec.as_mut_ptr().add(start);
+                            let src = source_vec.as_ptr().add(tail);
+                            ptr::copy(src, dst, self.0.tail_len);
+                        }
+                        source_vec.set_len(start + self.0.tail_len);
+                    }
+                }
+            }
+        }
+
+        let iter = mem::take(&mut self.iter);
+        let drop_len = iter.len();
+
+        let mut vec = self.vec;
+
+        if size_of::<T>() == 0 {
+            // ZSTs have no identity, so we don't need to move them around, we only need to drop the correct amount.
+            // this can be achieved by manipulating the `Vec` length instead of moving values out from `iter`.
+            unsafe {
+                let vec = vec.as_mut();
+                let old_len = vec.len();
+                vec.set_len(old_len + drop_len + self.tail_len);
+                vec.truncate(old_len + self.tail_len);
+            }
+
+            return;
+        }
+
+        // ensure elements are moved back into their appropriate places, even when drop_in_place panics
+        let _guard = DropGuard(self);
+
+        if drop_len == 0 {
+            return;
+        }
+
+        // as_slice() must only be called when iter.len() is > 0 because
+        // it also gets touched by vec::Splice which may turn it into a dangling pointer
+        // which would make it and the vec pointer point to different allocations which would
+        // lead to invalid pointer arithmetic below.
+        let drop_ptr = iter.as_slice().as_ptr();
+
+        unsafe {
+            // drop_ptr comes from a slice::Iter which only gives us a &[T] but for drop_in_place
+            // a pointer with mutable provenance is necessary. Therefore we must reconstruct
+            // it from the original vec but also avoid creating a &mut to the front since that could
+            // invalidate raw pointers to it which some unsafe code might rely on.
+            let vec_ptr = vec.as_mut().as_mut_ptr();
+            // FIXME: Replace with `sub_ptr` once stable.
+            let drop_offset = (drop_ptr as usize - vec_ptr as usize) / size_of::<T>();
+            let to_drop = ptr::slice_from_raw_parts_mut(vec_ptr.add(drop_offset), drop_len);
+            ptr::drop_in_place(to_drop);
+        }
+    }
+}
+
+impl<T> ExactSizeIterator for Drain<'_, T> {}
+
+impl<T> FusedIterator for Drain<'_, T> {}
+
+#[cfg(test)]
+mod tests {
+    use super::super::Vec;
+
+    #[test]
+    fn drain_front() {
+        let mut vec = Vec::<_, 8>::from_array([1, 2, 3, 4]);
+        let mut it = vec.drain(..1);
+        assert_eq!(it.next(), Some(1));
+        drop(it);
+        assert_eq!(vec, &[2, 3, 4]);
+    }
+
+    #[test]
+    fn drain_middle() {
+        let mut vec = Vec::<_, 8>::from_array([1, 2, 3, 4]);
+        let mut it = vec.drain(1..3);
+        assert_eq!(it.next(), Some(2));
+        assert_eq!(it.next(), Some(3));
+        drop(it);
+        assert_eq!(vec, &[1, 4]);
+    }
+
+    #[test]
+    fn drain_end() {
+        let mut vec = Vec::<_, 8>::from_array([1, 2, 3, 4]);
+        let mut it = vec.drain(3..);
+        assert_eq!(it.next(), Some(4));
+        drop(it);
+        assert_eq!(vec, &[1, 2, 3]);
+    }
+
+    #[test]
+    fn drain_drop_rest() {
+        droppable!();
+
+        let mut vec = Vec::<_, 8>::from_array([
+            Droppable::new(),
+            Droppable::new(),
+            Droppable::new(),
+            Droppable::new(),
+        ]);
+        assert_eq!(Droppable::count(), 4);
+
+        let mut iter = vec.drain(2..);
+        assert_eq!(iter.next().unwrap().0, 3);
+        drop(iter);
+        assert_eq!(Droppable::count(), 2);
+
+        assert_eq!(vec.len(), 2);
+        assert_eq!(vec.remove(0).0, 1);
+        assert_eq!(Droppable::count(), 1);
+
+        drop(vec);
+        assert_eq!(Droppable::count(), 0);
+    }
+}