Macro oerr 

macro_rules! oerr {
    { $var:expr, $e:expr } => { ... };
}

Takes a place (variable or field) holding an Option<T> and an expression that returns T; returns a &T to the value held by the Option, runs the expression and stores the result in the place if it holds None. The name is from what the // operator in Perl (theres also //=which this is really) is called. The expression is executed in the context of theoerrcall, with no additional subroutine wrapper, meaning e.g.?jumps out of theoerr` (it is designed like this on purpose).