This error indicates that the bindings in a match arm would require a value to be moved into more than one location, thus violating unique ownership. Code like the following is invalid as it requires the entire Option<String> to be moved into a variable called op_string while simultaneously requiring the inner String to be moved into a variable called s.

Erroneous code example:

let x = Some("s".to_string());

match x {
    op_string @ Some(s) => {}, // error: cannot bind by-move with sub-bindings
    None => {},
}

See also the error E0303.