Argument default should be any expression, not just a literal value

[alekitto]

I'm trying to expose a function with a nullable argument, but I want to make the argument optional (default null).
I've tried to implement it like this:

#[php_function]
#[php(defaults(x = None))]
pub fn test_func(x: Option<String>) -> String {
    // ...
}

but this syntax is invalid.

defaults only accepts literal values (syn::Lit), but should probably accept any expression (syn::Expr maybe?)