Attribute Macro ion_c_sys_macros::position_error

#[position_error]

This macro is intended to support the IonCReaderHandle type. When any function returns an IonCError, the error should be populated with the current reader position.

Without this macro, errors could be decorated like so:

fn foo() -> IonCResult<()> {
    fn1().map_err(|err| include_current_position(self.reader, err))?
    // ...
    Ok(())
}

However, functions can fail in many places, so decorating each of these with map_err is tedious and error prone:

fn foo() -> IonCResult<()> {
  fn1()?.map_err(..);
  fn2()?.map_err(..);
}

This macro rewrites the function such that any error gets the position added. It does this by moving the existing implementation into a closure and then calling map_err on the result.

Note: to make the variety of possible functions work (which include mutable borrows, lifetime elision), the closure uses the move keyword. There are some gymnastics (adapted from dtolnay/no-panic) to make that work out.