Make with_parent_window safe

[madsmtm]

Alternative to #3136. EDIT: We'd still need some more work to make Fullscreen Send + Sync on all platforms.

Builds upon #3126 to remove the unsafe from WindowBuilder::with_parent_window, by taking the lifetime inherent in that, and extending it on the platforms where it is relevant.

TODO:

• Merge the aforementioned PR.
• Test on all platforms changed
• Add entry to CHANGELOG.md

[kchibisov]

Alternative to #3136.

This is not an alternative since it doesn't achieve what said PR achieves.

However doing something like that is fine.

[kchibisov]

After looking into it, I don't think it's a good idea as of now to accept owned handler if winit can only provide Raw handles.

This update should be a part of the winit safe handles work.

[daxpedda]

That should actually be good to go now, right?

[daxpedda]

After looking at this a bit more closely, I question how thread safety is achieved here exactly. WindowBuilder is Send, but with_parent() can be called on any thread, but WindowHandle isn't Send. So a WindowHandle that has been passed into with_parent() that would be valid on a thread that is not the main thread, is suddenly being used in the main thread.

[madsmtm]

thread safety

I think this works on platforms with a "main-thread only" concept, unsure about platforms where a handle may be bound to a specific thread (Windows?)

My reasoning for main thread only being safe on macOS/iOS:

• WindowHandle<'_> is !Send + !Sync, because it is bound to the main thread.
• If we get that in with_parent_window, we hence know we're running on the main thread (because of the yet-to-be-documented contract on WindowHandle<'_>).
• This we put it into OwnedWindowHandle, which contains a MainThreadBound.
• So now, even though WindowBuilder is moved, we're guaranteed to only access the parent window on the main thread.

[daxpedda]

If the only valid thread is the main thread, I agree there is no problem, because WindowBuilder only gets consumed in the main thread, which is checked by Winit.

Summarizing remaining problems:

• How to correctly implement OwnedWindowHandle on all platforms if at all possible, alternatively we have to discuss adding a lifetime to WindowBuilder.
• Determine thread safety for all platforms. See Make more handles !Send and !Sync, and document main thread safety raw-window-handle#154 for documenting this in raw-window-handle.