Bug: Pointer types differ in mutability

[balatamoghna]

Which crate is this about?

windows

Crate version

0.30.0

Summary

use windows::{core::*, Win32::Foundation::*, Win32::Graphics::Gdi::ValidateRect, Win32::System::LibraryLoader::GetModuleHandleA, Win32::UI::WindowsAndMessaging::*};

fn main() -> Result<()> {
    unsafe {
        let instance = GetModuleHandleA(None);
        debug_assert!(instance.0 != 0);

        let window_class = "window";

        let wc = WNDCLASSA {
            hCursor: LoadCursorW(None, IDC_ARROW),
            hInstance: instance,
            lpszClassName: PSTR(b"window\0".as_ptr()),

            style: CS_HREDRAW | CS_VREDRAW,
            lpfnWndProc: Some(wndproc),
            ..Default::default()
        };

        let atom = RegisterClassA(&wc);
        debug_assert!(atom != 0);

        CreateWindowExA(Default::default(), window_class, "This is a sample window", WS_OVERLAPPEDWINDOW | WS_VISIBLE, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, None, None, instance, std::ptr::null_mut());

        let mut message = MSG::default();

        while GetMessageA(&mut message, HWND(0), 0, 0).into() {
            DispatchMessageA(&message);
        }

        Ok(())
    }
}

extern "system" fn wndproc(window: HWND, message: u32, wparam: WPARAM, lparam: LPARAM) -> LRESULT {
    unsafe {
        match message as u32 {
            WM_PAINT => {
                println!("WM_PAINT");
                ValidateRect(window, std::ptr::null());
                LRESULT(0)
            }
            WM_DESTROY => {
                println!("WM_DESTROY");
                PostQuitMessage(0);
                LRESULT(0)
            }
            _ => DefWindowProcA(window, message, wparam, lparam),
        }
    }
}

Running the basic sample for create-window gives the error:
"expected raw pointer *mut u8
found raw pointer *const u8"

This seems to be the offending line: lpszClassName: PSTR(b"window\0".as_ptr()),
The function .as_ptr() gives a *const u8 but the expected type for lpszClassName is *mut u8.

My Cargo.toml file has:
[dependencies.windows]
features = [
"alloc",
"Win32_Foundation",
"Win32_Graphics_Gdi",
"Win32_System_LibraryLoader",
"Win32_UI_WindowsAndMessaging",
]

Expected behavior

I expected the sample create-windows to work

Actual behavior

I got a compiler error: error[E0308]: mismatched types

Additional comments

No response

[balatamoghna]

Update: You can still cast the *const u8 pointer to *mut u8 ptr and it'll work.

Is this the solution for the bug? Is there any unintended behavior expected if we do this?

[tim-weis]

This seems to be the same issue as #777. In that case the upcoming release of 0.31.0 will address this. The recently imported metadata version 17.0.9 provides the appropriate [Const] annotations for the respective fields in the WNDCLASSA structure.

[balatamoghna]

Thank you!

[kennykerr]

Yes, this was fixed with #1470.