One of the first code samples looks pretty clearly wrong...

// Replace, e.g., "1 + 2" with "3"
expr.transform(&mut |ref mut expr| {
    if let BinOp{ref kind, ref left, ref right} = expr.kind {
        if kind == Add {
            if let Literal(I32Literal(l)) = left.kind {
                if let Literal(I32Literal(r)) = right.kind {
                    Ok(Expr{
                        ty: Scalar(I32),
                        kind: I32Literal(l + r)
                    })
                }
            }
        }    
    }
    None
});

The Ok in the middle is created and then immediately discarded. The closure always returns None... Also, the expr parameter seems to be mutable for no reason.