Fix duplicate mutable borrow soundness in buffer

This gets rid of multiple simultaneous &mut references to the
vector (which should be fine, I think), and also replaces the
`.iter_mut()` that just resets the index (which definitely leads to
soundness issues) with an `.iter_mut()` and a `.into_iter()` that don't
let you have concurrent mutable borrows to the same sample data.

src/buffer.rs

@@ -26,9 +26,9 @@ impl<'a> Buffer<'a> {
     }
 
     /// Iterate over the samples, returning a channel iterator for each sample.
-    pub fn iter_mut(&mut self) -> Samples<'_, 'a> {
+    pub fn iter_mut(&mut self) -> Samples<'a> {
         Samples {
-            buffers: &mut self.output_slices,
+            buffers: self.output_slices.as_mut_slice(),
             current_sample: 0,
         }
     }
@@ -49,24 +49,20 @@ impl<'a> Buffer<'a> {
 
 /// An iterator over all samples in the buffer, yielding iterators over each channel for every
 /// sample. This iteration order offers good cache locality for per-sample access.
-pub struct Samples<'outer, 'inner> {
+pub struct Samples<'a> {
     /// The raw output buffers.
-    pub(self) buffers: &'outer mut [&'inner mut [f32]],
+    pub(self) buffers: *mut [&'a mut [f32]],
     pub(self) current_sample: usize,
 }
 
-impl<'outer, 'inner> Iterator for Samples<'outer, 'inner> {
-    type Item = Channels<'outer, 'inner>;
+impl<'a> Iterator for Samples<'a> {
+    type Item = Channels<'a>;
 
     fn next(&mut self) -> Option<Self::Item> {
-        if self.current_sample < self.buffers[0].len() {
-            // SAFETY: We guarantee that each sample is only mutably borrowed once in the channels
-            // iterator
-            let buffers: &'outer mut _ = unsafe { &mut *(self.buffers as *mut _) };
+        if self.current_sample < self.len() {
             let channels = Channels {
-                buffers,
+                buffers: self.buffers,
                 current_sample: self.current_sample,
-                current_channel: 0,
             };
 
             self.current_sample += 1;
@@ -78,35 +74,56 @@ impl<'outer, 'inner> Iterator for Samples<'outer, 'inner> {
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let remaining = self.buffers[0].len() - self.current_sample;
+        let remaining = unsafe { (*self.buffers)[0].len() } - self.current_sample;
         (remaining, Some(remaining))
     }
 }
 
-impl<'outer, 'inner> ExactSizeIterator for Samples<'outer, 'inner> {}
+impl<'a> ExactSizeIterator for Samples<'a> {}
 
-/// An iterator over the channel data for a sample, yielded by [Samples].
-pub struct Channels<'outer, 'inner> {
+/// Can construct iterators over actual iterator over the channel data for a sample, yielded by
+/// [Samples].
+pub struct Channels<'a> {
     /// The raw output buffers.
-    pub(self) buffers: &'outer mut [&'inner mut [f32]],
+    pub(self) buffers: *mut [&'a mut [f32]],
+    pub(self) current_sample: usize,
+}
+
+/// The actual iterator over the channel data for a sample, yielded by [Channels].
+pub struct ChannelsIter<'a> {
+    /// The raw output buffers.
+    pub(self) buffers: *mut [&'a mut [f32]],
     pub(self) current_sample: usize,
     pub(self) current_channel: usize,
 }
 
-impl<'outer, 'inner> Iterator for Channels<'outer, 'inner> {
-    type Item = &'inner mut f32;
+impl<'a> IntoIterator for Channels<'a> {
+    type Item = &'a mut f32;
+    type IntoIter = ChannelsIter<'a>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        ChannelsIter {
+            buffers: self.buffers,
+            current_sample: self.current_sample,
+            current_channel: 0,
+        }
+    }
+}
+
+impl<'a> Iterator for ChannelsIter<'a> {
+    type Item = &'a mut f32;
 
     fn next(&mut self) -> Option<Self::Item> {
-        if self.current_channel < self.buffers.len() {
+        if self.current_channel < self.len() {
             // SAFETY: These bounds have already been checked
             let sample = unsafe {
-                self.buffers
+                (*self.buffers)
                     .get_unchecked_mut(self.current_channel)
                     .get_unchecked_mut(self.current_sample)
             };
             // SAFETY: It is not possible to have multiple mutable references to the same sample at
             // the same time
-            let sample: &'inner mut f32 = unsafe { &mut *(sample as *mut f32) };
+            let sample: &'a mut f32 = unsafe { &mut *(sample as *mut f32) };
 
             self.current_channel += 1;
 
@@ -117,19 +134,29 @@ impl<'outer, 'inner> Iterator for Channels<'outer, 'inner> {
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let remaining = self.buffers.len() - self.current_channel;
+        let remaining = unsafe { (*self.buffers).len() } - self.current_channel;
         (remaining, Some(remaining))
     }
 }
 
-impl<'outer, 'inner> ExactSizeIterator for Channels<'outer, 'inner> {}
+impl<'a> ExactSizeIterator for ChannelsIter<'a> {}
+
+impl Channels<'_> {
+    /// Get the number of channels.
+    pub fn len(&self) -> usize {
+        unsafe { (*self.buffers).len() }
+    }
 
-impl<'outer, 'inner> Channels<'outer, 'inner> {
     /// A resetting iterator. This lets you iterate over the same channels multiple times. Otherwise
     /// you don't need to use this function as [Channels] already implements [Iterator].
-    pub fn iter_mut(&mut self) -> &mut Self {
-        self.current_channel = 0;
-        self
+    pub fn iter_mut<'a: 'b, 'b>(&'a mut self) -> ChannelsIter<'b> {
+        // SAFETY: No two [ChannelIters] can exist at a time
+        let buffers: *mut [&'b mut [f32]] = unsafe { std::mem::transmute(self.buffers) };
+        ChannelsIter {
+            buffers,
+            current_sample: self.current_sample,
+            current_channel: 0,
+        }
     }
 
     /// Access a sample by index. Useful when you would otehrwise iterate over this 'Channels'
@@ -139,7 +166,7 @@ impl<'outer, 'inner> Channels<'outer, 'inner> {
         // SAFETY: The channel bound has already been checked
         unsafe {
             Some(
-                self.buffers
+                (*self.buffers)
                     .get_mut(channel_index)?
                     .get_unchecked_mut(self.current_sample),
             )
@@ -153,7 +180,7 @@ impl<'outer, 'inner> Channels<'outer, 'inner> {
     /// `channel_index` must be in the range `0..Self::len()`.
     #[inline]
     pub unsafe fn get_unchecked_mut(&mut self, channel_index: usize) -> &mut f32 {
-        self.buffers
+        (*self.buffers)
             .get_unchecked_mut(channel_index)
             .get_unchecked_mut(self.current_sample)
     }