add headless graphics api

openxr/src/graphics/headless.rs

@@ -0,0 +1,62 @@
+use sys::platform::*;
+
+use crate::*;
+
+/// Used to create a session without graphics.
+///
+/// See [`XR_MND_headless`] for safety details.
+///
+/// [`XR_MND_headless`]: https://www.khronos.org/registry/OpenXR/specs/1.1/html/xrspec.html#XR_MND_headless
+pub enum Headless {}
+
+impl Graphics for Headless {
+    type Requirements = Requirements;
+    type SessionCreateInfo = SessionCreateInfo;
+    type SwapchainImage = u32;
+    type Format = i64;
+
+    fn raise_format(x: i64) -> Self::Format {
+        x
+    }
+    fn lower_format(x: Self::Format) -> i64 {
+        x
+    }
+
+    fn requirements(_instance: &Instance, _system: SystemId) -> Result<Requirements> {
+        Ok(Requirements {})
+    }
+
+    unsafe fn create_session(
+        instance: &Instance,
+        system: SystemId,
+        _info: &SessionCreateInfo,
+    ) -> Result<sys::Session> {
+        let info = sys::SessionCreateInfo {
+            ty: sys::SessionCreateInfo::TYPE,
+            next: std::ptr::null(),
+            create_flags: Default::default(),
+            system_id: system,
+        };
+        let mut out = sys::Session::NULL;
+        cvt((instance.fp().create_session)(
+            instance.as_raw(),
+            &info,
+            &mut out,
+        ))?;
+        Ok(out)
+    }
+
+    fn enumerate_swapchain_images(
+        _swapchain: &Swapchain<Self>,
+    ) -> Result<Vec<Self::SwapchainImage>> {
+        // in a MND_headless session, xrEnumerateSwapchainFormats will always
+        // enumerate 0 formats, and so it's not possible to create a swapchain
+        unreachable!();
+    }
+}
+
+#[derive(Clone, Copy)]
+pub struct Requirements {}
+
+#[derive(Clone, Copy)]
+pub struct SessionCreateInfo {}

openxr/src/graphics/mod.rs

@@ -49,3 +49,6 @@ pub use opengl::OpenGL;
 
 pub mod opengles;
 pub use opengles::OpenGlEs;
+
+pub mod headless;
+pub use headless::Headless;