Remove assertions from Windows dark mode code (#1459)

* Remove assertions from Windows dark mode code

In general, winit should never assert on anything unless it means that
it is impossible to continue the execution of the program. There are
several assertions in the Windows dark mode code where this is not the
case.

Based on surface level inspection, all existing assertions could be
easily replaced with just simple conditional checks, allowing the
execution of the program to proceed with sane default values.

Fixes #1458.

* Add changelog entry

* Format code

* Pass dark mode by mutable reference

* Format code

* Return bool instead of mutable reference

* Fix dark mode success reply

Co-Authored-By: daxpedda <daxpedda@gmail.com>

* Fix dark mode success reply

* Replace magic integers with constants

Co-authored-by: daxpedda <daxpedda@gmail.com>

CHANGELOG.md

@@ -11,6 +11,7 @@
 - On Wayland, Hide CSD for fullscreen windows.
 - On Windows, ignore spurious mouse move messages.
 - **Breaking:** Move `ModifiersChanged` variant from `DeviceEvent` to `WindowEvent`.
+- On Windows, fix crash at startup on systems that do not properly support Windows' Dark Mode
 
 # 0.21.0 (2020-02-04)
 

src/platform_impl/windows/dark_mode.rs

@@ -6,9 +6,10 @@ use std::os::windows::ffi::OsStrExt;
 use winapi::{
     shared::{
         basetsd::SIZE_T,
-        minwindef::{BOOL, DWORD, UINT, ULONG, WORD},
+        minwindef::{BOOL, DWORD, FALSE, UINT, ULONG, WORD},
         ntdef::{LPSTR, NTSTATUS, NT_SUCCESS, PVOID, WCHAR},
         windef::HWND,
+        winerror::S_OK,
     },
     um::{libloaderapi, uxtheme, winuser},
 };
@@ -44,9 +45,8 @@ lazy_static! {
                 };
 
                 let status = (rtl_get_version)(&mut vi as _);
-                assert!(NT_SUCCESS(status));
 
-                if vi.dwMajorVersion == 10 && vi.dwMinorVersion == 0 {
+                if NT_SUCCESS(status) && vi.dwMajorVersion == 10 && vi.dwMinorVersion == 0 {
                     Some(vi.dwBuildNumber)
                 } else {
                     None
@@ -82,22 +82,15 @@ pub fn try_dark_mode(hwnd: HWND) -> bool {
             LIGHT_THEME_NAME.as_ptr()
         };
 
-        unsafe {
-            assert_eq!(
-                0,
-                uxtheme::SetWindowTheme(hwnd, theme_name as _, std::ptr::null())
-            );
+        let status = unsafe { uxtheme::SetWindowTheme(hwnd, theme_name as _, std::ptr::null()) };
 
-            set_dark_mode_for_window(hwnd, is_dark_mode)
-        }
-
-        is_dark_mode
+        status == S_OK && set_dark_mode_for_window(hwnd, is_dark_mode)
     } else {
         false
     }
 }
 
-fn set_dark_mode_for_window(hwnd: HWND, is_dark_mode: bool) {
+fn set_dark_mode_for_window(hwnd: HWND, is_dark_mode: bool) -> bool {
     // Uses Windows undocumented API SetWindowCompositionAttribute,
     // as seen in win32-darkmode example linked at top of file.
 
@@ -132,11 +125,12 @@ fn set_dark_mode_for_window(hwnd: HWND, is_dark_mode: bool) {
                 cbData: std::mem::size_of_val(&is_dark_mode_bigbool) as _,
             };
 
-            assert_eq!(
-                1,
-                set_window_composition_attribute(hwnd, &mut data as *mut _)
-            );
+            let status = set_window_composition_attribute(hwnd, &mut data as *mut _);
+
+            status != FALSE
         }
+    } else {
+        false
     }
 }
 
@@ -205,7 +199,7 @@ fn is_high_contrast() -> bool {
         )
     };
 
-    (ok > 0) && ((HCF_HIGHCONTRASTON & hc.dwFlags) == 1)
+    ok != FALSE && (HCF_HIGHCONTRASTON & hc.dwFlags) == 1
 }
 
 fn widestring(src: &'static str) -> Vec<u16> {