Why does a LoadCursorA call give a build error?

Question

Why does a LoadCursorA generate 'trait implementation' build errors when a LoadCursorW call does not when using the rust Crate for Windows?

#[allow(dead_code)]
use windows::Win32::UI::WindowsAndMessaging::LoadCursorA;
use windows::Win32::UI::WindowsAndMessaging::IDC_ARROW;

fn main() {
    unsafe {
        let _h_cursor = LoadCursorA(None, IDC_ARROW);
        //let _h_cursor = LoadCursorA(None, &IDC_ARROW);
    };
}

error[E0277]: the trait bound PWSTR: IntoParam<'_, PSTR> is not satisfied

Because [`IDC_ARROW` is a `PWSTR`](https://microsoft.github.io/windows-docs-rs/doc/windows/Win32/UI/WindowsAndMessaging/constant.IDC_ARROW.html) and so cannot be used with the `…A` functions, which require a `PSTR`. — Jmb, Nov 19 '21 at 13:45
Can the LoadCursorA call be modified to build and if so how? Thanks. — MRahilly, Nov 19 '21 at 16:48
`IDC_*` constants are not really pointers but short integer constants disguised as pointers, so you should be safe doing `IDC_ARROW as PSTR`. — rodrigo, Nov 19 '21 at 18:58
Trying IDC_ARROW as PSTR gives error[E0605] non-primitive cast: `PWSTR` as `PSTR` — MRahilly, Nov 20 '21 at 11:07

Jmb · Answer 1 · 2021-11-22T14:06:28.363

The reason IDC_ARROW doesn't work with LoadCursorA is because it is a PWSTR, so it can only work with …W functions. Functions in …A need a PSTR instead.

However IDC_ARROW is not really a PWSTR (ie. a "pointer to wide-character string"). It's just a small constant masquerading as a PWSTR, so we can convert it to a PSTR and pass that to LoadCursorA:

#[allow(dead_code)]
use windows::Win32::UI::WindowsAndMessaging::LoadCursorA;
use windows::Win32::UI::WindowsAndMessaging::IDC_ARROW;

fn main() {
    unsafe {
        let _h_cursor = LoadCursorA(None, PSTR(IDC_ARROW.0 as _));
    };
}

Of course another option is just to use LoadCursorW instead:

#[allow(dead_code)]
use windows::Win32::UI::WindowsAndMessaging::LoadCursorW;
use windows::Win32::UI::WindowsAndMessaging::IDC_ARROW;

fn main() {
    unsafe {
        let _h_cursor = LoadCursorW(None, IDC_ARROW);
    };
}

Why does a LoadCursorA call give a build error?

1 Answers1