Rust bindings for C++ static data members

Public static data members of a class receive bindings. Which kind depends on the member's type and whether its value is a compile-time constant:

• A member of integer or bool type whose value is a compile-time constant (because it is constexpr, or const or inline with a constant initializer) becomes an associated const of the Rust struct. Its value is copied into the bindings, so Rust doesn't need to access the C++ object.
• A member whose value is not a compile-time constant (for example, one that is only declared in the class and is defined in a .cc file) becomes a Rust static that refers to the C++ object, because its value is only known at run time. Rust has no associated statics, so it is placed in the snake-cased module that holds the class's other nested items. If it is mutable, it is a static mut, and, as with other C++ globals, accessing it is unsafe.

For example, given:

struct Counter {
  static constexpr int kMax = 10;
  static const bool kEnabled = true;
  static int count;  // Defined in counter.cc.
};

Crubit generates (simplified):

impl Counter {
    pub const kMax: ffi_11::c_int = ffi_11::new_c_int(10);
    pub const kEnabled: bool = true;
}

pub mod counter {
    unsafe extern "C" {
        pub static mut count: ffi_11::c_int;
    }
}

Compile-time constants of other types, such as floating-point or class types, and static data members of class template specializations don't receive bindings yet.