Add Buffer::size() and Buffer::usage(). (gfx-rs#2923)

CHANGELOG.md

@@ -56,6 +56,10 @@ the same every time it is rendered, we now warn if it is missing.
 +fn vert_main(v_in: VertexInput) -> @builtin(position) @invariant vec4<f32> {...}
 ```
 
+### Added/New Features
+
+- Add `Buffer::size()` and `Buffer::usage()`; by @kpreid in [#2923](https://github.com/gfx-rs/wgpu/pull/2923)
+
 ### Bug Fixes
 
 #### General

wgpu/src/lib.rs

@@ -659,6 +659,7 @@ pub struct Buffer {
     context: Arc<C>,
     id: <C as Context>::BufferId,
     map_context: Mutex<MapContext>,
+    size: wgt::BufferAddress,
     usage: BufferUsages,
 }
 
@@ -2129,6 +2130,7 @@ impl Device {
             context: Arc::clone(&self.context),
             id: Context::device_create_buffer(&*self.context, &self.id, desc),
             map_context: Mutex::new(map_context),
+            size: desc.size,
             usage: desc.usage,
         }
     }
@@ -2426,6 +2428,20 @@ impl Buffer {
     pub fn destroy(&self) {
         Context::buffer_destroy(&*self.context, &self.id);
     }
+
+    /// Returns the length of the buffer allocation in bytes.
+    ///
+    /// This is always equal to the `size` that was specified when creating the buffer.
+    pub fn size(&self) -> wgt::BufferAddress {
+        self.size
+    }
+
+    /// Returns the allowed usages for this `Buffer`.
+    ///
+    /// This is always equal to the `usage` that was specified when creating the buffer.
+    pub fn usage(&self) -> BufferUsages {
+        self.usage
+    }
 }
 
 impl<'a> BufferSlice<'a> {

wgpu/tests/resource_descriptor_accessor.rs

@@ -0,0 +1,20 @@
+use crate::common::{initialize_test, TestParameters};
+
+/// Buffer's size and usage can be read back.
+#[test]
+fn buffer_size_and_usage() {
+    initialize_test(TestParameters::default(), |ctx| {
+        let buffer = ctx.device.create_buffer(&wgpu::BufferDescriptor {
+            label: None,
+            size: 1234,
+            usage: wgpu::BufferUsages::COPY_SRC | wgpu::BufferUsages::COPY_DST,
+            mapped_at_creation: false,
+        });
+
+        assert_eq!(buffer.size(), 1234);
+        assert_eq!(
+            buffer.usage(),
+            wgpu::BufferUsages::COPY_SRC | wgpu::BufferUsages::COPY_DST
+        );
+    })
+}

wgpu/tests/root.rs

@@ -6,6 +6,7 @@ mod device;
 mod example_wgsl;
 mod instance;
 mod poll;
+mod resource_descriptor_accessor;
 mod shader_primitive_index;
 mod vertex_indices;
 mod zero_init_texture_after_discard;