Add ability to assign a menu when creating a window on Windows

The primary reason for doing this here instead of just assigning a menu later on with `SetMenu` is to avoid that `with_inner_size` breaks, since the menu takes up some space that is not taken into account, which means the user would have to use `set_inner_size` again after adding the menu.

Notes:
- Child windows cannot have a menu, so perhaps this should be represented typewise?
- Dark mode doesn't look good with win32 menus, unfortunately not fixable, see ysc3839/win32-darkmode#1

Author: madsmtm

FEATURES.md

@@ -116,6 +116,7 @@ If your PR makes notable changes to Winit's features, please update this section
 ### Windows
 * Setting the taskbar icon
 * Setting the parent window
+* Setting a menu bar
 * `WS_EX_NOREDIRECTIONBITMAP` support
 * Theme the title bar according to Windows 10 Dark Mode setting or set a preferred theme
 

src/platform/windows.rs

@@ -5,7 +5,7 @@ use std::path::Path;
 
 use libc;
 use winapi::shared::minwindef::WORD;
-use winapi::shared::windef::HWND;
+use winapi::shared::windef::{HMENU, HWND};
 
 use crate::{
     dpi::PhysicalSize,
@@ -112,6 +112,14 @@ pub trait WindowBuilderExtWindows {
     /// Sets a parent to the window to be created.
     fn with_parent_window(self, parent: HWND) -> WindowBuilder;
 
+    /// Sets a menu on the window to be created.
+    ///
+    /// The menu must have been manually created beforehand with [`winapi::um::winuser::CreateMenu`] or similar.
+    ///
+    /// Note: Dark mode cannot be supported for win32 menus, it's simply not possible to change how the menus look.
+    /// If you use this, it is recommended that you combine it with `with_theme(Some(Theme::Light))` to avoid a jarring effect.
+    fn with_menu(self, menu: HMENU) -> WindowBuilder;
+
     /// This sets `ICON_BIG`. A good ceiling here is 256x256.
     fn with_taskbar_icon(self, taskbar_icon: Option<Icon>) -> WindowBuilder;
 
@@ -137,6 +145,12 @@ impl WindowBuilderExtWindows for WindowBuilder {
         self
     }
 
+    #[inline]
+    fn with_menu(mut self, menu: HMENU) -> WindowBuilder {
+        self.platform_specific.menu = Some(menu);
+        self
+    }
+
     #[inline]
     fn with_taskbar_icon(mut self, taskbar_icon: Option<Icon>) -> WindowBuilder {
         self.platform_specific.taskbar_icon = taskbar_icon;

src/platform_impl/windows/mod.rs

@@ -1,6 +1,6 @@
 #![cfg(target_os = "windows")]
 
-use winapi::{self, shared::windef::HWND};
+use winapi::{self, shared::windef::HMENU, shared::windef::HWND};
 
 pub use self::{
     event_loop::{EventLoop, EventLoopProxy, EventLoopWindowTarget},
@@ -17,7 +17,9 @@ use crate::window::Theme;
 
 #[derive(Clone)]
 pub struct PlatformSpecificWindowBuilderAttributes {
+    // TODO: parent and menu are mutually exclusive; a child window cannot have a menu!
     pub parent: Option<HWND>,
+    pub menu: Option<HMENU>,
     pub taskbar_icon: Option<Icon>,
     pub no_redirection_bitmap: bool,
     pub drag_and_drop: bool,
@@ -28,6 +30,7 @@ impl Default for PlatformSpecificWindowBuilderAttributes {
     fn default() -> Self {
         Self {
             parent: None,
+            menu: None,
             taskbar_icon: None,
             no_redirection_bitmap: false,
             drag_and_drop: true,

src/platform_impl/windows/window.rs

@@ -718,7 +718,7 @@ unsafe fn init<T: 'static>(
             winuser::CW_USEDEFAULT,
             winuser::CW_USEDEFAULT,
             pl_attribs.parent.unwrap_or(ptr::null_mut()),
-            ptr::null_mut(),
+            pl_attribs.menu.unwrap_or(ptr::null_mut()),
             libloaderapi::GetModuleHandleW(ptr::null()),
             ptr::null_mut(),
         );