IDWriteTextRenderer::DrawGlyphRun takes &self and *const c_void. No pointer to mutable state present.

[david7a68]

Summary

When implementing IDWriteTextRenderer, it makes sense that one would need mutable state somewhere, if only to record glyphs. However, the function signature for IDWriteTextRenderer::DrawGlyphRun does not allow mutable access to self or the context pointer.

This doesn't prevent an implementation from working, but it certainly is an ergonomic papercut.

Crate manifest

windows-core = "0.56"

[dependencies.windows]
version = "0.56"
features = [
    "implement",
    "Win32_Graphics_Direct2D_Common",
    "Win32_Graphics_DirectWrite",
    "Win32_System_Com",
]

Crate code

No response

[riverar]

If I understood correctly, you're implementing an interface; &mut self isn't possible with COM where references can linger around. Instead, try using something like RefCellor RwLock to coordinate access to mutable state.

Some examples:

• Feature request: COM _Impl types are not mutable #2304 (comment)
• https://github.com/microsoft/windows-rs/blob/master/crates/tests/implement/tests/vector.rs

[kennykerr]

Right, &mut self implies exclusive access which is not compatible with COM where you may have multiple references accessing the same object simultaneously. Using &self forces you to be deliberate about this and avoids undefined behavior.

[david7a68]

I see. Closing this issue.

Thanks for the clarification!