Why does the PNG surface's write_func (and cairo_write_func_t) use mutable data?

[inklesspen]

gtk-rs-core/cairo/sys/src/lib.rs

Lines 264 to 265 in 94c3909

	pub type cairo_write_func_t =
	Option<unsafe extern "C" fn(*mut c_void, *mut c_uchar, c_uint) -> cairo_status_t>;

gtk-rs-core/cairo/src/surface_png.rs

Lines 58 to 62 in 94c3909

	unsafe extern "C" fn write_func<W: Write>(
	closure: *mut c_void,
	data: *mut u8,
	len: c_uint,
	) -> crate::ffi::cairo_status_t {

The corresponding C type takes a const unsigned char *data.

My guess is this was a copy-paste error when adding both read_func and write_func, since read_func does need to take a mutable char pointer.

[sdroege]

Yes that looks like a copy&paste mistake. Do you want to provide a PR for this @inklesspen?

[inklesspen]

I think a similar change is required in stream.rs's write_callback, but I don't think I entirely understand what's going on there.

I'll send the PR anyway.