The maximum value of an enum was reached, so it cannot be automatically set in the next enum value. Erroneous code example:

#[repr(i64)]
enum Foo {
    X = 0x7fffffffffffffff,
    Y, // error: enum discriminant overflowed on value after
       //        9223372036854775807: i64; set explicitly via
       //        Y = -9223372036854775808 if that is desired outcome
}

To fix this, please set manually the next enum value or put the enum variant with the maximum value at the end of the enum. Examples:

#[repr(i64)]
enum Foo {
    X = 0x7fffffffffffffff,
    Y = 0, // ok!
}

Or:

#[repr(i64)]
enum Foo {
    Y = 0, // ok!
    X = 0x7fffffffffffffff,
}