egl/display: Pass *pointer to* output variable when querying `DEVICE_…

…EXT`

By casting a `null_mut()` ptr and passing it directly _by value_ to
`QueryDisplayAttribEXT()`, the function ignores writing the output as
it didn't receive an address (it received `NULL`) to write the device
to.  Instead we should take the _pointer address of this `null_mut()`
pointer_ and provide that to the function instead so that it can
_overwrite_ it with a pointer to the requested `eglDevice`.

This is even more clear when allocating a `MaybeUninit` in which the
pointer will be returned, which has a convenient `.as_mut_ptr()` member
to pass its pointer value directly into `QueryDisplayAttribEXT()`.

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 - Fixed EGL's `Device::query_devices()` being too strict about required extensions
 - Fixed crash in `EglGetProcAddress` on Win32-x86 platform due to wrong calling convention
+- Fixed EGL's `Display::device()` always returning an error due to invalid pointer-argument passing inside
 
 # Version 0.32.0
 

glutin/src/api/egl/display.rs

@@ -2,11 +2,11 @@
 
 use std::collections::HashSet;
 use std::ffi::{self, CStr};
+use std::fmt;
 use std::mem::MaybeUninit;
 use std::ops::Deref;
 use std::os::raw::c_char;
 use std::sync::Arc;
-use std::{fmt, ptr};
 
 use glutin_egl_sys::egl;
 use glutin_egl_sys::egl::types::{EGLAttrib, EGLDisplay, EGLint};
@@ -193,12 +193,12 @@ impl Display {
             .into());
         }
 
-        let device = ptr::null_mut();
+        let mut device = MaybeUninit::uninit();
         if unsafe {
             self.inner.egl.QueryDisplayAttribEXT(
                 *self.inner.raw,
                 egl::DEVICE_EXT as EGLint,
-                device as *mut _,
+                device.as_mut_ptr(),
             )
         } == egl::FALSE
         {
@@ -211,6 +211,13 @@ impl Display {
             }));
         }
 
+        let device = unsafe { device.assume_init() } as egl::types::EGLDeviceEXT;
+        debug_assert_ne!(
+            device,
+            egl::NO_DEVICE_EXT,
+            "eglQueryDisplayAttribEXT(EGL_DEVICE_EXT) should never return EGL_NO_DEVICE_EXT on \
+             success"
+        );
         Device::from_ptr(self.inner.egl, device)
     }