feat: Add AnyThread wrapper

This type can be used to ignore thread affinity for types that already
take `HasWindowHandle`.

Signed-off-by: John Nunley <dev@notgull.net>

src/platform/windows.rs

@@ -2,7 +2,7 @@
 //!
 //! The supported OS version is Windows 7 or higher, though Windows 10 is
 //! tested regularly.
-use std::{ffi::c_void, path::Path};
+use std::{borrow::Borrow, ffi::c_void, path::Path};
 
 use crate::{
     dpi::PhysicalSize,
@@ -105,6 +105,62 @@ pub enum CornerPreference {
     RoundSmall = 3,
 }
 
+/// A wrapper around a [`Window`] that ignores thread-specific window handle limitations.
+/// 
+/// See [`WindowBorrowExtWindows::any_thread`] for more information.
+#[derive(Debug)]
+pub struct AnyThread<W>(W);
+
+impl<W: Borrow<Window>> AnyThread<W> {
+    /// Get a reference to the inner window.
+    #[inline]
+    pub fn get_ref(&self) -> &Window {
+        self.0.borrow()
+    }
+
+    /// Get a reference to the inner object.
+    #[inline]
+    pub fn inner(&self) -> &W {
+        &self.0
+    }
+
+    /// Unwrap and get the inner window.
+    #[inline]
+    pub fn into_inner(self) -> W {
+        self.0
+    }
+}
+
+impl<W: Borrow<Window>> AsRef<Window> for AnyThread<W> {
+    fn as_ref(&self) -> &Window {
+        self.get_ref()
+    }
+}
+
+impl<W: Borrow<Window>> Borrow<Window> for AnyThread<W> {
+    fn borrow(&self) -> &Window {
+        self.get_ref()
+    }
+}
+
+impl<W: Borrow<Window>> std::ops::Deref for AnyThread<W> {
+    type Target = Window;
+
+    fn deref(&self) -> &Self::Target {
+        self.get_ref()
+    }
+}
+
+#[cfg(feature = "rwh_06")]
+impl<W: Borrow<Window>> rwh_06::HasWindowHandle for AnyThread<W> {
+    fn window_handle(&self) -> Result<rwh_06::WindowHandle<'_>, rwh_06::HandleError> {
+        // SAFETY: The top level user has asserted this is only used safely.
+        unsafe {
+            self.get_ref().window_handle_any_thread()
+        }
+    }
+}
+
 /// Additional methods on `EventLoop` that are specific to Windows.
 pub trait EventLoopBuilderExtWindows {
     /// Whether to allow the event loop to be created off of the main thread.
@@ -265,6 +321,11 @@ pub trait WindowExtWindows {
     /// However in some cases you may already know that you are using the window handle for
     /// operations that are guaranteed to be thread-safe. In which case this function aims
     /// to provide an escape hatch so these functions are still accessible from other threads.
+    /// 
+    /// # Safety
+    /// 
+    /// It is the responsibility of the user to only pass the window handle into thread-safe
+    /// Win32 APIs.
     ///
     /// [`SetWindowSubclass`]: https://learn.microsoft.com/en-us/windows/win32/api/commctrl/nf-commctrl-setwindowsubclass
     /// [`GetDC`]: https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getdc
@@ -363,6 +424,35 @@ impl WindowExtWindows for Window {
     }
 }
 
+/// Additional methods for anything that dereference to [`Window`].
+/// 
+/// [`Window`]: crate::window::Window
+pub trait WindowBorrowExtWindows : Borrow<Window> + Sized {
+    /// Create an object that allows accessing the inner window handle in a thread-unsafe way.
+    /// 
+    /// It is possible to call [`window_handle_any_thread`] to get around Windows's thread
+    /// affinity limitations. However, it may be desired to pass the [`Window`] into something
+    /// that requires the [`HasWindowHandle`] trait, while ignoring thread affinity limitations.
+    /// 
+    /// This function wraps anything that implements `Borrow<Window>` into a structure that
+    /// uses the inner window handle as a mean of implementing [`HasWindowHandle`]. It wraps
+    /// `Window`, `&Window`, `Arc<Window>`, and other reference types.
+    /// 
+    /// # Safety
+    /// 
+    /// It is the responsibility of the user to only pass the window handle into thread-safe
+    /// Win32 APIs.
+    /// 
+    /// [`window_handle_any_thread`]: WindowExtWindows::window_handle_any_thread
+    /// [`Window`]: crate::window::Window
+    /// [`HasWindowHandle`]: rwh_06::HasWindowHandle
+    unsafe fn any_thread(self) -> AnyThread<Self> {
+        AnyThread(self) 
+    }
+}
+
+impl<W: Borrow<Window> + Sized> WindowBorrowExtWindows for W {}
+
 /// Additional methods on `WindowAttributes` that are specific to Windows.
 #[allow(rustdoc::broken_intra_doc_links)]
 pub trait WindowAttributesExtWindows {