E0027: Non-Exhaustive Match

Your match expression doesn't cover all possible cases.

Example

enum Color {
    Red,
    Green,
    Blue,
}

fn describe(c: Color) -> i64 {
    match c {
        Color::Red => { 0 },
        Color::Green => { 1 },
        // Error: non-exhaustive match, missing: `Color::Blue`
    }
}

How to fix

Either handle all variants explicitly:

fn describe(c: Color) -> i64 {
    match c {
        Color::Red => { 0 },
        Color::Green => { 1 },
        Color::Blue => { 2 },
    }
}

Or use a wildcard pattern to catch remaining cases:

fn describe(c: Color) -> i64 {
    match c {
        Color::Red => { 0 },
        _ => { 1 },
    }
}