Use higher stride on some platforms when cfg(debug_assertions)

To help with debugging incorrect stride/width calculations, it helps
that there are more common backends (than Android) that have
`stride != width * 4`.

Author: madsmtm

src/backends/cg.rs

@@ -259,8 +259,9 @@ impl<D: HasDisplayHandle, W: HasWindowHandle> SurfaceInterface<D, W> for CGImpl<
     }
 
     fn buffer_mut(&mut self) -> Result<BufferImpl<'_>, SoftBufferError> {
+        let buffer_size = util::byte_stride(self.width as u32) as usize * self.height / 4;
         Ok(BufferImpl {
-            buffer: util::PixelBuffer(vec![Pixel::default(); self.width * self.height]),
+            buffer: util::PixelBuffer(vec![Pixel::default(); buffer_size]),
             width: self.width,
             height: self.height,
             color_space: &self.color_space,
@@ -280,7 +281,7 @@ pub struct BufferImpl<'a> {
 
 impl BufferInterface for BufferImpl<'_> {
     fn byte_stride(&self) -> NonZeroU32 {
-        NonZeroU32::new(self.width().get() * 4).unwrap()
+        NonZeroU32::new(util::byte_stride(self.width as u32)).unwrap()
     }
 
     fn width(&self) -> NonZeroU32 {
@@ -342,7 +343,7 @@ impl BufferInterface for BufferImpl<'_> {
                 self.height,
                 8,
                 32,
-                self.width * 4,
+                util::byte_stride(self.width as u32) as usize,
                 Some(self.color_space),
                 bitmap_info,
                 Some(&data_provider),

src/backends/wayland/buffer.rs

@@ -15,6 +15,7 @@ use wayland_client::{
 };
 
 use super::State;
+use crate::util;
 use crate::Pixel;
 
 #[cfg(any(target_os = "linux", target_os = "freebsd"))]
@@ -99,7 +100,7 @@ impl WaylandBuffer {
             0,
             width,
             height,
-            width * 4,
+            util::byte_stride(width as u32) as i32,
             // This is documented as `0xXXRRGGBB` on a little-endian machine, which means a byte
             // order of `[B, G, R, X]`.
             wl_shm::Format::Xrgb8888,
@@ -141,7 +142,7 @@ impl WaylandBuffer {
                 0,
                 width,
                 height,
-                width * 4,
+                util::byte_stride(width as u32) as i32,
                 wl_shm::Format::Xrgb8888,
                 &self.qh,
                 self.released.clone(),
@@ -161,7 +162,7 @@ impl WaylandBuffer {
     }
 
     fn len(&self) -> usize {
-        self.width as usize * self.height as usize
+        util::byte_stride(self.width as u32) as usize * self.height as usize / 4
     }
 
     #[inline]

src/backends/wayland/mod.rs

@@ -1,7 +1,7 @@
 use crate::{
     backend_interface::*,
     error::{InitError, SwResultExt},
-    Pixel, Rect, SoftBufferError,
+    util, Pixel, Rect, SoftBufferError,
 };
 use raw_window_handle::{HasDisplayHandle, HasWindowHandle, RawDisplayHandle, RawWindowHandle};
 use std::{
@@ -220,7 +220,7 @@ pub struct BufferImpl<'a> {
 
 impl BufferInterface for BufferImpl<'_> {
     fn byte_stride(&self) -> NonZeroU32 {
-        NonZeroU32::new(self.width as u32 * 4).unwrap()
+        NonZeroU32::new(util::byte_stride(self.width as u32)).unwrap()
     }
 
     fn width(&self) -> NonZeroU32 {

src/backends/win32.rs

@@ -8,7 +8,7 @@ use raw_window_handle::{HasDisplayHandle, HasWindowHandle, RawWindowHandle};
 
 use std::io;
 use std::marker::PhantomData;
-use std::mem;
+use std::mem::size_of;
 use std::num::{NonZeroI32, NonZeroU32};
 use std::ptr::{self, NonNull};
 use std::slice;
@@ -55,7 +55,7 @@ impl Buffer {
         // Create a new bitmap info struct.
         let bitmap_info = BitmapInfo {
             bmi_header: Gdi::BITMAPINFOHEADER {
-                biSize: mem::size_of::<Gdi::BITMAPINFOHEADER>() as u32,
+                biSize: size_of::<Gdi::BITMAPINFOHEADER>() as u32,
                 biWidth: width.get(),
                 biHeight: -height.get(),
                 biPlanes: 1,
@@ -116,12 +116,9 @@ impl Buffer {
 
     #[inline]
     fn pixels_mut(&mut self) -> &mut [Pixel] {
-        unsafe {
-            slice::from_raw_parts_mut(
-                self.pixels.as_ptr(),
-                i32::from(self.width) as usize * i32::from(self.height) as usize,
-            )
-        }
+        let num_bytes =
+            byte_stride(self.width.get() as u32, 32) as usize * self.height.get() as usize;
+        unsafe { slice::from_raw_parts_mut(self.pixels.as_ptr(), num_bytes / size_of::<Pixel>()) }
     }
 }
 
@@ -252,11 +249,7 @@ pub struct BufferImpl<'a> {
 impl BufferInterface for BufferImpl<'_> {
     fn byte_stride(&self) -> NonZeroU32 {
         let width = self.buffer.width.get() as u32;
-        let bit_count = 32;
-        // https://learn.microsoft.com/en-us/windows/win32/api/wingdi/ns-wingdi-bitmapinfoheader#calculating-surface-stride
-        // When `bit_count == 32`, this is always just equal to `width * 4`.
-        let stride = ((width * bit_count + 31) & !31) >> 3;
-        NonZeroU32::new(stride).unwrap()
+        NonZeroU32::new(byte_stride(width, 32)).unwrap()
     }
 
     fn width(&self) -> NonZeroU32 {
@@ -476,3 +469,10 @@ impl<T> From<T> for OnlyUsedFromOrigin<T> {
         Self(t)
     }
 }
+
+#[inline]
+fn byte_stride(width: u32, bit_count: u32) -> u32 {
+    // <https://learn.microsoft.com/en-us/windows/win32/api/wingdi/ns-wingdi-bitmapinfoheader#calculating-surface-stride>
+    // When `bit_count == 32`, this is always just equal to `width * 4`.
+    ((width * bit_count + 31) & !31) >> 3
+}

src/util.rs

@@ -65,3 +65,19 @@ impl ops::DerefMut for PixelBuffer {
         &mut self.0
     }
 }
+
+/// Compute the byte stride desired by Softbuffer when a platform can use any stride.
+///
+/// TODO(madsmtm): This should take the pixel format / bit depth as input after:
+/// <https://github.com/rust-windowing/softbuffer/issues/98>
+#[inline]
+pub(crate) fn byte_stride(width: u32) -> u32 {
+    let row_alignment = if cfg!(debug_assertions) {
+        16 // Use a higher alignment to help users catch issues with their stride calculations.
+    } else {
+        4 // At least 4 is necessary for `Buffer` to return `&mut [u32]`.
+    };
+    // TODO: Use `next_multiple_of` when in MSRV.
+    let mask = row_alignment * 4 - 1;
+    ((width * 32 + mask) & !mask) >> 3
+}