extensions: Only call assume_init() when Vulkan returns SUCCESS

It is undefined behaviour to construct a safe object by calling
`MaybeUninit::assume_init()` when the object in question hasn't been
initialized by anything (in this case the underlying Vulkan call) at
all, even if the object is never "used".  Postpone the `assume_init()`
call until after checking if `vk::Result::SUCCESS` has been returned by
the native implementation, by introducing a new
`assume_initialized_on_success()` helper that takes a `MaybeUninit<T>`.

ash/src/extensions/ext/acquire_drm_display.rs

@@ -40,7 +40,7 @@ impl AcquireDrmDisplay {
     ) -> VkResult<vk::DisplayKHR> {
         let mut display = mem::MaybeUninit::uninit();
         (self.fp.get_drm_display_ext)(physical_device, drm_fd, connector_id, display.as_mut_ptr())
-            .result_with_success(display.assume_init())
+            .assume_initialized_on_success(display)
     }
 
     #[inline]

ash/src/extensions/khr/display.rs

@@ -88,7 +88,7 @@ impl Display {
             allocation_callbacks.as_raw_ptr(),
             display_mode.as_mut_ptr(),
         )
-        .result_with_success(display_mode.assume_init())
+        .assume_initialized_on_success(display_mode)
     }
 
     /// <https://www.khronos.org/registry/vulkan/specs/1.3-extensions/man/html/vkGetDisplayPlaneCapabilitiesKHR.html>
@@ -106,7 +106,7 @@ impl Display {
             plane_index,
             display_plane_capabilities.as_mut_ptr(),
         )
-        .result_with_success(display_plane_capabilities.assume_init())
+        .assume_initialized_on_success(display_plane_capabilities)
     }
 
     /// <https://www.khronos.org/registry/vulkan/specs/1.3-extensions/man/html/vkCreateDisplayPlaneSurfaceKHR.html>
@@ -123,7 +123,7 @@ impl Display {
             allocation_callbacks.as_raw_ptr(),
             surface.as_mut_ptr(),
         )
-        .result_with_success(surface.assume_init())
+        .assume_initialized_on_success(surface)
     }
 
     #[inline]

ash/src/prelude.rs

@@ -1,6 +1,7 @@
 use std::convert::TryInto;
 #[cfg(feature = "debug")]
 use std::fmt;
+use std::mem;
 
 use crate::vk;
 pub type VkResult<T> = Result<T, vk::Result>;
@@ -18,6 +19,11 @@ impl vk::Result {
             _ => Err(self),
         }
     }
+
+    #[inline]
+    pub unsafe fn assume_initialized_on_success<T>(self, v: mem::MaybeUninit<T>) -> VkResult<T> {
+        self.result().map(move |()| v.assume_init())
+    }
 }
 
 /// Repeatedly calls `f` until it does not return [`vk::Result::INCOMPLETE`] anymore,