I wouldn't call this kind of macro idiomatic in non-GTK code. Reference counting is mostly used only occasionally in Rust code, and so the boilerplate cost of adding another local variable when giving a closure its own reference-counted pointer is outweighed by the maintainability cost of introducing a macro.
gtk-rs is different because it pervasively uses Rust bindings to a non-Rust-based memory management system based on reference counting and interior mutability, kind of like if everything was wrapped in Rc<RefCell<T>>. In that scenario, yeah, adding another local in front of every closure capture sounds like a pain. If most Rust code were like this, I suspect that something like glib::clone would be added as built-in syntax.
gtk-rs is different because it pervasively uses Rust bindings to a non-Rust-based memory management system based on reference counting and interior mutability, kind of like if everything was wrapped in Rc<RefCell<T>>. In that scenario, yeah, adding another local in front of every closure capture sounds like a pain. If most Rust code were like this, I suspect that something like glib::clone would be added as built-in syntax.