glib: Add a Value::downcast_mut() function

[ids1024]

Useful since TypedValue::set* take &mut self.

[sdroege]

Makes sense, but TypedValue was planned to get removed as part of #31 . Do you actually use this in a useful way? If so, how? In my experience keeping Rust values around in the code and calling to_value() where needed is more convenient.

[ids1024]

As far as I can tell, without downcasting to a mutable TypedValue, there isn't a way to safe way to set the value of an existing Value. (SetValue::set_value is an unsafe unchecked way to do this).

I am trying to add a binding for g_settings_bind_with_mapping, which requires a callback to set the Value passed as an argument. I assume there are other APIs requiring something similar.

[sdroege]

You'd assign a new value, that's effectively the same.

[ids1024]

Ok, assigning like that seems to work.

I like the idea having typed versions of things like this, but I guess it's reasonable enough to remove it if no one can find cases where it's especially useful. It does complicate things.

[sdroege]

Yeah I like it too in principle, but I never had use for it or where I could use it, not using it was actually simpler. It only seems to complicate things compared to just passing Values around where needed and elsewhere keeping everything in a proper Rust type.