Rust error E0408: variable `y` is not bound in all patterns

The compiler team's explanation

An "or" pattern was used where the variable bindings are not consistently bound across patterns.

Erroneous code example:

match x {
    Some(y) | None => { /* use y */ } // error: variable `y` from pattern #1 is
                                      //        not bound in pattern #2
    _ => ()
}

Here, y is bound to the contents of the Some and can be used within the block corresponding to the match arm. However, in case x is None, we have not specified what y is, and the block will use a nonexistent variable.

To fix this error, either split into multiple match arms:

let x = Some(1);
match x {
    Some(y) => { /* use y */ }
    None => { /* ... */ }
}

or, bind the variable to a field of the same type in all sub-patterns of the or pattern:

let x = (0, 2);
match x {
    (0, y) | (y, 0) => { /* use y */}
    _ => {}
}

In this example, if x matches the pattern (0, _), the second field is set to y. If it matches (_, 0), the first field is set to y; so in all cases y is set to some value.

From rustc --explain E0408 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.