object_subclass macro creates mutable reference to mutable static

[felinira]

I stumbled upon this bit:

gtk-rs-core/glib-macros/src/object_impl_attributes/subclass.rs

Line 106 in b97554d

unsafe { ::std::ptr::NonNull::from(&mut DATA) }

which throws the following warning (which is then swallowed because it's coming from a macro invocation):

creating a mutable reference to mutable static is discouraged
for more information, see issue #114447 rust-lang/rust#114447
this will be a hard error in the 2024 edition

This will fail to compile in rust 2024 either way, so we need to deal with this before upgrading. However, this is creating a mutable temporary reference that is then converted into a pointer and subsequently dropped. We should use std::ptr::addr_of_mut!() instead, and also make sure we are actually using it correctly here.

[sdroege]

Yes, this code is from before addr_of_mut! was stable :) You make a PR?