Add the static lifetime bound to async buffer mapping callbacks

These callbacks are executed outside of the mapping function and could
previously reference data that had been dropped. This fixes the
soundness issues described in #95, but not the buggy mapping
behavior.

examples/hello_compute_rust/main.rs

@@ -88,13 +88,16 @@ fn main() {
     }
     encoder.copy_buffer_to_buffer(&storage_buffer, 0, &staging_buffer, 0, size);
 
+    device.get_queue().submit(&[encoder.finish()]);
+
     staging_buffer.map_read_async(0, size, |result: wgpu::BufferMapAsyncResult<&[u32]>| {
         if let wgpu::BufferMapAsyncResult::Success(data) = result {
             println!("Times: {:?}", data);
         }
 
-        staging_buffer.unmap();
+        // staging_buffer.unmap(); // TODO
     });
 
+    let encoder = device.create_command_encoder(&wgpu::CommandEncoderDescriptor { todo: 0 });
     device.get_queue().submit(&[encoder.finish()]);
 }

wgpu-rs/src/lib.rs

@@ -530,7 +530,7 @@ impl Buffer {
     pub fn map_read_async<T, F>(&self, start: u32, size: u32, callback: F)
     where
         T: 'static + Copy,
-        F: FnOnce(BufferMapAsyncResult<&[T]>),
+        F: FnOnce(BufferMapAsyncResult<&[T]>) + 'static,
     {
         let type_size = std::mem::size_of::<T>() as u32;
         assert_ne!(type_size, 0);
@@ -575,7 +575,7 @@ impl Buffer {
     pub fn map_write_async<T, F>(&self, start: u32, size: u32, callback: F)
     where
         T: 'static + Copy,
-        F: FnOnce(BufferMapAsyncResult<&mut [T]>),
+        F: FnOnce(BufferMapAsyncResult<&mut [T]>) + 'static,
     {
         let type_size = std::mem::size_of::<T>() as u32;
         assert_ne!(type_size, 0);