Make the unsafe part of zeroing buffers in map_buffer simpler.

wgpu-core/src/device/mod.rs

@@ -190,17 +190,17 @@ fn map_buffer<A: hal::Api>(
     //
     // If this is a write mapping zeroing out the memory here is the only reasonable way as all data is pushed to GPU anyways.
     let zero_init_needs_flush_now = mapping.is_coherent && buffer.sync_mapped_writes.is_none(); // No need to flush if it is flushed later anyways.
-    for uninitialized_range in buffer.initialization_status.drain(offset..(size + offset)) {
-        let num_bytes = uninitialized_range.end - uninitialized_range.start;
+    let mapped = unsafe { std::slice::from_raw_parts_mut(mapping.ptr.as_ptr(), size as usize) };
+
+    for uninitialized in buffer.initialization_status.drain(offset..(size + offset)) {
         // The mapping's pointer is already offset, however we track the uninitialized range relative to the buffer's start.
-        let start = (uninitialized_range.start - offset) as isize;
-        unsafe { ptr::write_bytes(mapping.ptr.as_ptr().offset(start), 0, num_bytes as usize) };
+        let fill_range =
+            (uninitialized.start - offset) as usize..(uninitialized.end - offset) as usize;
+        mapped[fill_range].fill(0);
+
         if zero_init_needs_flush_now {
             unsafe {
-                raw.flush_mapped_ranges(
-                    buffer.raw.as_ref().unwrap(),
-                    iter::once(uninitialized_range),
-                )
+                raw.flush_mapped_ranges(buffer.raw.as_ref().unwrap(), iter::once(uninitialized))
             };
         }
     }