Clumsy workaround for rustc complaint

rust-lang/rust#93367

Author: kornelski

Diff for: src/capi.rs

@@ -11,6 +11,7 @@ use crate::Image;
 use crate::rows::RowCallback;
 use crate::seacow::RowBitmapMut;
 use crate::seacow::SeaCow;
+use crate::seacow::Pointer;
 use std::mem::MaybeUninit;
 
 pub const LIQ_VERSION: u32 = 40000;
@@ -20,18 +21,18 @@ pub fn liq_get_palette_impl(r: &mut QuantizationResult) -> &Palette {
 }
 
 pub unsafe fn liq_image_create_rgba_rows_impl<'rows>(attr: &Attributes, rows: &'rows [*const RGBA], width: u32, height: u32, gamma: f64) -> Option<crate::image::Image<'rows>> {
-    let rows = SeaCow::borrowed(rows);
+    let rows = SeaCow::borrowed(std::mem::transmute::<&'rows [*const RGBA], &'rows [Pointer<RGBA>]>(rows));
     let rows_slice = rows.as_slice();
-    if rows_slice.iter().any(|r| r.is_null()) {
+    if rows_slice.iter().any(|r| r.0.is_null()) {
         return None;
     }
     crate::image::Image::new_internal(attr, crate::rows::PixelsSource::Pixels { rows, pixels: None }, width, height, gamma).ok()
 }
 
 pub unsafe fn liq_image_create_rgba_bitmap_impl<'rows>(attr: &Attributes, rows: Box<[*const RGBA]>, width: u32, height: u32, gamma: f64) -> Option<crate::image::Image<'rows>> {
-    let rows = SeaCow::boxed(rows);
+    let rows = SeaCow::boxed(std::mem::transmute::<Box<[*const RGBA]>, Box<[Pointer<RGBA>]>>(rows));
     let rows_slice = rows.as_slice();
-    if rows_slice.iter().any(|r| r.is_null()) {
+    if rows_slice.iter().any(|r| r.0.is_null()) {
         return None;
     }
     crate::image::Image::new_internal(attr, crate::rows::PixelsSource::Pixels { rows, pixels: None }, width, height, gamma).ok()

Diff for: src/image.rs

@@ -6,6 +6,7 @@ use crate::remap::DitherMapMode;
 use crate::rows::{DynamicRows, PixelsSource};
 use crate::seacow::RowBitmap;
 use crate::seacow::SeaCow;
+use crate::seacow::Pointer;
 use crate::LIQ_HIGH_MEMORY_LIMIT;
 use rgb::ComponentMap;
 use std::mem::MaybeUninit;
@@ -329,7 +330,7 @@ impl<'pixels> Image<'pixels> {
             return Err(BufferTooSmall);
         }
 
-        let rows = SeaCow::boxed(slice.chunks(stride).map(|row| row.as_ptr()).take(height).collect());
+        let rows = SeaCow::boxed(slice.chunks(stride).map(|row| Pointer(row.as_ptr())).take(height).collect());
         Image::new_internal(attr, PixelsSource::Pixels { rows, pixels: Some(pixels) }, width as u32, height as u32, gamma)
     }
 }

Diff for: src/rows.rs

@@ -3,13 +3,14 @@ use crate::pal::{f_pixel, gamma_lut, RGBA};
 use crate::seacow::SeaCow;
 use crate::LIQ_HIGH_MEMORY_LIMIT;
 use std::mem::MaybeUninit;
+use crate::seacow::Pointer;
 
 pub(crate) type RowCallback<'a> = dyn Fn(&mut [MaybeUninit<RGBA>], usize) + Send + Sync + 'a;
 
 pub(crate) enum PixelsSource<'pixels, 'rows> {
     /// The `pixels` field is never read, but it is used to store the rows.
     #[allow(dead_code)]
-    Pixels { rows: SeaCow<'rows, *const RGBA>, pixels: Option<SeaCow<'pixels, RGBA>> },
+    Pixels { rows: SeaCow<'rows, Pointer<RGBA>>, pixels: Option<SeaCow<'pixels, RGBA>> },
     Callback(Box<RowCallback<'rows>>),
 }
 
@@ -71,7 +72,7 @@ impl<'pixels,'rows> DynamicRows<'pixels,'rows> {
     fn row_rgba<'px>(&'px self, temp_row: &'px mut [MaybeUninit<RGBA>], row: usize) -> &[RGBA] {
         match &self.pixels {
             PixelsSource::Pixels { rows, .. } => unsafe {
-                std::slice::from_raw_parts(rows.as_slice()[row], self.width())
+                std::slice::from_raw_parts(rows.as_slice()[row].0, self.width())
             },
             PixelsSource::Callback(cb) => {
                 cb(temp_row, row);
@@ -168,7 +169,7 @@ impl<'pixels,'rows> DynamicRows<'pixels,'rows> {
                 PixelsSource::Pixels { pixels: Some(pixels), .. } => pixels.make_owned(free_fn),
                 PixelsSource::Pixels { pixels, rows } => {
                     // the row with the lowest address is assumed to be at the start of the bitmap
-                    let ptr = rows.as_slice().iter().copied().min().ok_or(Error::Unsupported)?;
+                    let ptr = rows.as_slice().iter().map(|p| p.0).min().ok_or(Error::Unsupported)?;
                     *pixels = Some(SeaCow::c_owned(ptr as *mut _, len, free_fn));
                 },
                 PixelsSource::Callback(_) => return Err(Error::ValueOutOfRange),

Diff for: src/seacow.rs

@@ -9,8 +9,20 @@ pub struct SeaCow<'a, T> {
 unsafe impl<T: Send> Send for SeaCowInner<'_, T> {}
 unsafe impl<T: Sync> Sync for SeaCowInner<'_, T> {}
 
-unsafe impl<T: Send> Send for SeaCow<'_, *const T> {}
-unsafe impl<T: Sync> Sync for SeaCow<'_, *const T> {}
+/// Rust assumes `*const T` is never `Send`/`Sync`, but it can be.
+/// This is fudge for https://github.com/rust-lang/rust/issues/93367
+#[repr(transparent)]
+#[derive(Copy, Clone)]
+pub(crate) struct Pointer<T>(pub *const T);
+
+#[derive(Copy, Clone)]
+#[repr(transparent)]
+pub(crate) struct PointerMut<T>(pub *mut T);
+
+unsafe impl<T: Send + Sync> Send for Pointer<T> {}
+unsafe impl<T: Send + Sync> Sync for Pointer<T> {}
+unsafe impl<T: Send + Sync> Send for PointerMut<T> {}
+unsafe impl<T: Send + Sync> Sync for PointerMut<T> {}
 
 impl<'a, T> SeaCow<'a, T> {
     #[inline]
@@ -78,13 +90,13 @@ impl<'a, T> SeaCow<'a, T> {
 }
 
 pub(crate) struct RowBitmap<'a, T> {
-    rows: &'a [*const T],
+    rows: &'a [Pointer<T>],
     width: usize,
 }
 unsafe impl<T: Send + Sync> Send for RowBitmap<'_, T> {}
 
 pub(crate) struct RowBitmapMut<'a, T> {
-    rows: MutCow<'a, [*mut T]>,
+    rows: MutCow<'a, [PointerMut<T>]>,
     width: usize,
 }
 unsafe impl<T: Send + Sync> Send for RowBitmapMut<'_, T> {}
@@ -94,16 +106,16 @@ impl<'a, T> RowBitmapMut<'a, MaybeUninit<T>> {
     pub(crate) unsafe fn assume_init<'maybeowned>(&'maybeowned mut self) -> RowBitmap<'maybeowned, T> {
         RowBitmap {
             width: self.width,
-            rows: std::mem::transmute::<&'maybeowned [*mut MaybeUninit<T>], &'maybeowned [*const T]>(self.rows.borrow()),
+            rows: std::mem::transmute::<&'maybeowned [PointerMut<MaybeUninit<T>>], &'maybeowned [Pointer<T>]>(self.rows.borrow()),
         }
     }
 }
 
 impl<'a, T> RowBitmap<'a, T> {
     pub fn rows(&self) -> impl Iterator<Item = &[T]> {
         let width = self.width;
-        self.rows.iter().map(move |&row| {
-            unsafe { std::slice::from_raw_parts(row, width) }
+        self.rows.iter().map(move |row| {
+            unsafe { std::slice::from_raw_parts(row.0, width) }
         })
     }
 }
@@ -127,7 +139,7 @@ impl<'a, T: Sync + Send + Copy + 'static> RowBitmapMut<'a, T> {
     #[inline]
     pub fn new_contiguous(data: &mut [T], width: usize) -> Self {
         Self {
-            rows: MutCow::Owned(data.chunks_exact_mut(width).map(|r| r.as_mut_ptr()).collect()),
+            rows: MutCow::Owned(data.chunks_exact_mut(width).map(|r| PointerMut(r.as_mut_ptr())).collect()),
             width,
         }
     }
@@ -137,18 +149,14 @@ impl<'a, T: Sync + Send + Copy + 'static> RowBitmapMut<'a, T> {
     #[cfg(feature = "_internal_c_ffi")]
     pub unsafe fn new(rows: &'a mut [*mut T], width: usize) -> Self {
         Self {
-            rows: MutCow::Borrowed(rows),
+            rows: MutCow::Borrowed(std::mem::transmute::<&'a mut [*mut T], &'a mut [PointerMut<T>]>(rows)),
             width,
         }
     }
 
     pub fn rows_mut(&mut self) -> impl Iterator<Item = &mut [T]> + Send {
         let width = self.width;
-        // Rust is pessimistic about `*mut` pointers
-        struct ItIsSync<T>(*mut T);
-        unsafe impl<T: Send + Sync> Sync for ItIsSync<T> {}
-        let send_slice = unsafe { std::mem::transmute::<&mut [*mut T], &mut [ItIsSync<T>]>(self.rows.borrow()) };
-        send_slice.iter().map(move |row| {
+        self.rows.borrow().iter().map(move |row| {
             unsafe { std::slice::from_raw_parts_mut(row.0, width) }
         })
     }