Macro err

macro_rules! err {
    ($v:ident, code: $code:ident, cause: $cause:expr, $($args:expr),+) => { ... };
    ($v:ident, try: $res:expr, $($args:expr),+) => { ... };
    ($v:ident, try: $res:expr) => { ... };
    ($v:ident, code: $code:ident, $($args:expr),+) => { ... };
    ($v:ident, cause: $cause:expr, $($args:expr),+) => { ... };
    ($v:ident, desc: $($args:expr),+) => { ... };
}

Macro to compose Error values.

Here are few possible ways:

err!(InsufficientBytes, desc: "for payload {}", len);

err!(MalformedPacket, code: QoSNotSupported, "qos={}", qos);

err_at!(IOError, cause: err, "from addr:{}", ip_addr);

err_at!(IOError, code: ServerShutdown, cause: err, "reason", reason);