Repository: rust-ndarray/ndarray (Rust)

Issue:
move semantics for multislice!()
I'm trying to mutably borrow two elements of an `ArrayViewMut` and return them. On `0.12` I've been using `split_at` creatively to get multiple mutable references, but it's unwieldy and probably slower than it needs to be.

On `master`, `multislice!` makes it all a lot more ergonomic and this almost works:
```rust
use ndarray::{multislice, Array2, ArrayViewMut, Dim, IndexLonger, Ix, Ix0};

fn get_mut2<'a, A>(
    mut arr: ArrayViewMut<'a, A, Dim<[Ix; 2]>>,
    [x, y]: [usize; 2],
    [x2, y2]: [usize; 2],
) -> (&'a mut A, &'a mut A) {
    let (a, b) = multislice!(arr, mut [x, y], mut [x2, y2]);
    (a.index(Ix0()), b.index(Ix0()))
}
```
This fails to compile because it attempts to return the two mutable references which are still bound to the lifetime of the borrow of `arr` by `multislice!`, and going through `IndexLonger` doesn't help.

From what I can tell, this  could be fixed if `multislice!` was allowed to use the semantics of `slice_move()` through some keyword (similarly to how `mut` is used to mark mutable slices), or perhaps a `multislice_move!` variant that applies move semantics to every returned slice, since it doesn't sound possible to have a both move and non-move semantics in the same `multislice` call.

Any thoughts?


Discussion:
What do you think of #692?



Patch:
diff --git a/src/dimension/mod.rs b/src/dimension/mod.rs
index 2a0ec4b0d..28d2e9b2c 100644
--- a/src/dimension/mod.rs
+++ b/src/dimension/mod.rs
@@ -544,7 +544,6 @@ fn slice_min_max(axis_len: usize, slice: Slice) -> Option<(usize, usize)> {
 }
 
 /// Returns `true` iff the slices intersect.
-#[allow(dead_code)]
 pub fn slices_intersect<D: Dimension>(
     dim: &D,
     indices1: &D::SliceArg,
diff --git a/src/impl_methods.rs b/src/impl_methods.rs
index 97193de36..027f5a8af 100644
--- a/src/impl_methods.rs
+++ b/src/impl_methods.rs
@@ -28,6 +28,7 @@ use crate::iter::{
     AxisChunksIter, AxisChunksIterMut, AxisIter, AxisIterMut, ExactChunks, ExactChunksMut,
     IndexedIter, IndexedIterMut, Iter, IterMut, Lanes, LanesMut, Windows,
 };
+use crate::slice::MultiSlice;
 use crate::stacking::stack;
 use crate::{NdIndex, Slice, SliceInfo, SliceOrIndex};
 
@@ -350,6 +351,39 @@ where
         self.view_mut().slice_move(info)
     }
 
+    /// Return multiple disjoint, sliced, mutable views of the array.
+    ///
+    /// See [*Slicing*](#slicing) for full documentation.
+    /// See also [`SliceInfo`] and [`D::SliceArg`].
+    ///
+    /// [`SliceInfo`]: struct.SliceInfo.html
+    /// [`D::SliceArg`]: trait.Dimension.html#associatedtype.SliceArg
+    ///
+    /// **Panics** if any of the following occur:
+    ///
+    /// * if any of the views would intersect (i.e. if any element would appear in multiple slices)
+    /// * if an index is out of bounds or step size is zero
+    /// * if `D` is `IxDyn` and `info` does not match the number of array axes
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use ndarray::{arr2, s};
+    ///
+    /// let mut a = arr2(&[[1, 2, 3], [4, 5, 6]]);
+    /// let (mut edges, mut middle) = a.multi_slice_mut((s![.., ..;2], s![.., 1]));
+    /// edges.fill(1);
+    /// middle.fill(0);
+    /// assert_eq!(a, arr2(&[[1, 0, 1], [1, 0, 1]]));
+    /// ```
+    pub fn multi_slice_mut<'a, M>(&'a mut self, info: M) -> M::Output
+    where
+        M: MultiSlice<'a, A, D>,
+        S: DataMut,
+    {
+        info.multi_slice_move(self.view_mut())
+    }
+
     /// Slice the array, possibly changing the number of dimensions.
     ///
     /// See [*Slicing*](#slicing) for full documentation.
diff --git a/src/impl_views/splitting.rs b/src/impl_views/splitting.rs
index e9fcfcb64..dcfb04b86 100644
--- a/src/impl_views/splitting.rs
+++ b/src/impl_views/splitting.rs
@@ -7,6 +7,7 @@
 // except according to those terms.
 
 use crate::imp_prelude::*;
+use crate::slice::MultiSlice;
 
 /// Methods for read-only array views.
 impl<'a, A, D> ArrayView<'a, A, D>
@@ -109,4 +110,29 @@ where
             (left.deref_into_view_mut(), right.deref_into_view_mut())
         }
     }
+
+    /// Split the view into multiple disjoint slices.
+    ///
+    /// This is similar to [`.multi_slice_mut()`], but `.multi_slice_move()`
+    /// consumes `self` and produces views with lifetimes matching that of
+    /// `self`.
+    ///
+    /// See [*Slicing*](#slicing) for full documentation.
+    /// See also [`SliceInfo`] and [`D::SliceArg`].
+    ///
+    /// [`.multi_slice_mut()`]: struct.ArrayBase.html#method.multi_slice_mut
+    /// [`SliceInfo`]: struct.SliceInfo.html
+    /// [`D::SliceArg`]: trait.Dimension.html#associatedtype.SliceArg
+    ///
+    /// **Panics** if any of the following occur:
+    ///
+    /// * if any of the views would intersect (i.e. if any element would appear in multiple slices)
+    /// * if an index is out of bounds or step size is zero
+    /// * if `D` is `IxDyn` and `info` does not match the number of array axes
+    pub fn multi_slice_move<M>(self, info: M) -> M::Output
+    where
+        M: MultiSlice<'a, A, D>,
+    {
+        info.multi_slice_move(self)
+    }
 }
diff --git a/src/lib.rs b/src/lib.rs
index 35d1e1aab..680dc8557 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -475,6 +475,13 @@ pub type Ixs = isize;
 /// [`.slice_move()`]: #method.slice_move
 /// [`.slice_collapse()`]: #method.slice_collapse
 ///
+/// It's possible to take multiple simultaneous *mutable* slices with
+/// [`.multi_slice_mut()`] or (for [`ArrayViewMut`] only)
+/// [`.multi_slice_move()`].
+///
+/// [`.multi_slice_mut()`]: #method.multi_slice_mut
+/// [`.multi_slice_move()`]: type.ArrayViewMut.html#method.multi_slice_move
+///
 /// ```
 /// extern crate ndarray;
 ///
@@ -525,6 +532,20 @@ pub type Ixs = isize;
 ///                [12, 11, 10]]);
 /// assert_eq!(f, g);
 /// assert_eq!(f.shape(), &[2, 3]);
+///
+/// // Let's take two disjoint, mutable slices of a matrix with
+/// //
+/// // - One containing all the even-index columns in the matrix
+/// // - One containing all the odd-index columns in the matrix
+/// let mut h = arr2(&[[0, 1, 2, 3],
+///                    [4, 5, 6, 7]]);
+/// let (s0, s1) = h.multi_slice_mut((s![.., ..;2], s![.., 1..;2]));
+/// let i = arr2(&[[0, 2],
+///                [4, 6]]);
+/// let j = arr2(&[[1, 3],
+///                [5, 7]]);
+/// assert_eq!(s0, i);
+/// assert_eq!(s1, j);
 /// }
 /// ```
 ///
diff --git a/src/slice.rs b/src/slice.rs
index 1d0dfa2b0..58bff48b5 100644
--- a/src/slice.rs
+++ b/src/slice.rs
@@ -5,8 +5,9 @@
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
+use crate::dimension::slices_intersect;
 use crate::error::{ErrorKind, ShapeError};
-use crate::Dimension;
+use crate::{ArrayViewMut, Dimension};
 use std::fmt;
 use std::marker::PhantomData;
 use std::ops::{Deref, Range, RangeFrom, RangeFull, RangeInclusive, RangeTo, RangeToInclusive};
@@ -629,3 +630,103 @@ macro_rules! s(
         &*&$crate::s![@parse ::std::marker::PhantomData::<$crate::Ix0>, [] $($t)*]
     };
 );
+
+/// Slicing information describing multiple mutable, disjoint slices.
+///
+/// It's unfortunate that we need `'a` and `A` to be parameters of the trait,
+/// but they're necessary until Rust supports generic associated types.
+pub trait MultiSlice<'a, A, D>
+where
+    A: 'a,
+    D: Dimension,
+{
+    /// The type of the slices created by `.multi_slice_move()`.
+    type Output;
+
+    /// Split the view into multiple disjoint slices.
+    ///
+    /// **Panics** if performing any individual slice panics or if the slices
+    /// are not disjoint (i.e. if they intersect).
+    fn multi_slice_move(&self, view: ArrayViewMut<'a, A, D>) -> Self::Output;
+}
+
+impl<'a, A, D> MultiSlice<'a, A, D> for ()
+where
+    A: 'a,
+    D: Dimension,
+{
+    type Output = ();
+
+    fn multi_slice_move(&self, _view: ArrayViewMut<'a, A, D>) -> Self::Output {}
+}
+
+impl<'a, A, D, Do0> MultiSlice<'a, A, D> for (&SliceInfo<D::SliceArg, Do0>,)
+where
+    A: 'a,
+    D: Dimension,
+    Do0: Dimension,
+{
+    type Output = (ArrayViewMut<'a, A, Do0>,);
+
+    fn multi_slice_move(&self, view: ArrayViewMut<'a, A, D>) -> Self::Output {
+        (view.slice_move(self.0),)
+    }
+}
+
+macro_rules! impl_multislice_tuple {
+    ([$($but_last:ident)*] $last:ident) => {
+        impl_multislice_tuple!(@def_impl ($($but_last,)* $last,), [$($but_last)*] $last);
+    };
+    (@def_impl ($($all:ident,)*), [$($but_last:ident)*] $last:ident) => {
+        impl<'a, A, D, $($all,)*> MultiSlice<'a, A, D> for ($(&SliceInfo<D::SliceArg, $all>,)*)
+        where
+            A: 'a,
+            D: Dimension,
+            $($all: Dimension,)*
+        {
+            type Output = ($(ArrayViewMut<'a, A, $all>,)*);
+
+            fn multi_slice_move(&self, view: ArrayViewMut<'a, A, D>) -> Self::Output {
+                #[allow(non_snake_case)]
+                let ($($all,)*) = self;
+
+                let shape = view.raw_dim();
+                assert!(!impl_multislice_tuple!(@intersects_self &shape, ($($all,)*)));
+
+                let raw_view = view.into_raw_view_mut();
+                unsafe {
+                    (
+                        $(raw_view.clone().slice_move($but_last).deref_into_view_mut(),)*
+                        raw_view.slice_move($last).deref_into_view_mut(),
+                    )
+                }
+            }
+        }
+    };
+    (@intersects_self $shape:expr, ($head:expr,)) => {
+        false
+    };
+    (@intersects_self $shape:expr, ($head:expr, $($tail:expr,)*)) => {
+        $(slices_intersect($shape, $head, $tail)) ||*
+            || impl_multislice_tuple!(@intersects_self $shape, ($($tail,)*))
+    };
+}
+
+impl_multislice_tuple!([Do0] Do1);
+impl_multislice_tuple!([Do0 Do1] Do2);
+impl_multislice_tuple!([Do0 Do1 Do2] Do3);
+impl_multislice_tuple!([Do0 Do1 Do2 Do3] Do4);
+impl_multislice_tuple!([Do0 Do1 Do2 Do3 Do4] Do5);
+
+impl<'a, A, D, T> MultiSlice<'a, A, D> for &T
+where
+    A: 'a,
+    D: Dimension,
+    T: MultiSlice<'a, A, D>,
+{
+    type Output = T::Output;
+
+    fn multi_slice_move(&self, view: ArrayViewMut<'a, A, D>) -> Self::Output {
+        T::multi_slice_move(self, view)
+    }
+}


Test patch:
diff --git a/tests/array.rs b/tests/array.rs
index ea374bc7c..db54b7e5f 100644
--- a/tests/array.rs
+++ b/tests/array.rs
@@ -15,6 +15,20 @@ use ndarray::{arr3, rcarr2};
 use ndarray::{Slice, SliceInfo, SliceOrIndex};
 use std::iter::FromIterator;
 
+macro_rules! assert_panics {
+    ($body:expr) => {
+        if let Ok(v) = ::std::panic::catch_unwind(|| $body) {
+            panic!("assertion failed: should_panic; \
+            non-panicking result: {:?}", v);
+        }
+    };
+    ($body:expr, $($arg:tt)*) => {
+        if let Ok(_) = ::std::panic::catch_unwind(|| $body) {
+            panic!($($arg)*);
+        }
+    };
+}
+
 #[test]
 fn test_matmul_arcarray() {
     let mut A = ArcArray::<usize, _>::zeros((2, 3));
@@ -328,6 +342,82 @@ fn test_slice_collapse_with_indices() {
     assert_eq!(vi, Array3::from_elem((1, 1, 1), elem));
 }
 
+#[test]
+fn test_multislice() {
+    macro_rules! do_test {
+        ($arr:expr, $($s:expr),*) => {
+            {
+                let arr = $arr;
+                let copy = arr.clone();
+                assert_eq!(
+                    arr.multi_slice_mut(($($s,)*)),
+                    ($(copy.clone().slice_mut($s),)*)
+                );
+            }
+        };
+    }
+
+    let mut arr = Array1::from_iter(0..48).into_shape((8, 6)).unwrap();
+
+    assert_eq!(
+        (arr.clone().view_mut(),),
+        arr.multi_slice_mut((s![.., ..],)),
+    );
+    assert_eq!(arr.multi_slice_mut(()), ());
+    do_test!(&mut arr, s![0, ..]);
+    do_test!(&mut arr, s![0, ..], s![1, ..]);
+    do_test!(&mut arr, s![0, ..], s![-1, ..]);
+    do_test!(&mut arr, s![0, ..], s![1.., ..]);
+    do_test!(&mut arr, s![1, ..], s![..;2, ..]);
+    do_test!(&mut arr, s![..2, ..], s![2.., ..]);
+    do_test!(&mut arr, s![1..;2, ..], s![..;2, ..]);
+    do_test!(&mut arr, s![..;-2, ..], s![..;2, ..]);
+    do_test!(&mut arr, s![..;12, ..], s![3..;3, ..]);
+    do_test!(&mut arr, s![3, ..], s![..-1;-2, ..]);
+    do_test!(&mut arr, s![0, ..], s![1, ..], s![2, ..]);
+    do_test!(&mut arr, s![0, ..], s![1, ..], s![2, ..], s![3, ..]);
+}
+
+#[test]
+fn test_multislice_intersecting() {
+    assert_panics!({
+        let mut arr = Array2::<u8>::zeros((8, 6));
+        arr.multi_slice_mut((s![3, ..], s![3, ..]));
+    });
+    assert_panics!({
+        let mut arr = Array2::<u8>::zeros((8, 6));
+        arr.multi_slice_mut((s![3, ..], s![3.., ..]));
+    });
+    assert_panics!({
+        let mut arr = Array2::<u8>::zeros((8, 6));
+        arr.multi_slice_mut((s![3, ..], s![..;3, ..]));
+    });
+    assert_panics!({
+        let mut arr = Array2::<u8>::zeros((8, 6));
+        arr.multi_slice_mut((s![..;6, ..], s![3..;3, ..]));
+    });
+    assert_panics!({
+        let mut arr = Array2::<u8>::zeros((8, 6));
+        arr.multi_slice_mut((s![2, ..], s![..-1;-2, ..]));
+    });
+    assert_panics!({
+        let mut arr = Array2::<u8>::zeros((8, 6));
+        arr.multi_slice_mut((s![4, ..], s![3, ..], s![3, ..]));
+    });
+    assert_panics!({
+        let mut arr = Array2::<u8>::zeros((8, 6));
+        arr.multi_slice_mut((s![3, ..], s![4, ..], s![3, ..]));
+    });
+    assert_panics!({
+        let mut arr = Array2::<u8>::zeros((8, 6));
+        arr.multi_slice_mut((s![3, ..], s![3, ..], s![4, ..]));
+    });
+    assert_panics!({
+        let mut arr = Array2::<u8>::zeros((8, 6));
+        arr.multi_slice_mut((s![3, ..], s![3, ..], s![4, ..], s![3, ..]));
+    });
+}
+
 #[should_panic]
 #[test]
 fn index_out_of_bounds() {