Exhaustive integer matching

Question

I try to exhaustively match integers like this:

fn main() {
    for test in range(std::u8::MIN, std::u8::MAX) {
        match test {
            0x00..0xff => {},
        }
    }
}

But the compiler complains:

all.rs:3:9: 6:10 error: non-exhaustive patterns: `_` not covered [E0004]
all.rs:3         match test {
all.rs:4             0x00..0xff => {},
all.rs:5         }
error: aborting due to previous error

However, all possible values are covered. A quick check confirms this:

fn main() {
    for test in range(std::u8::MIN, std::u8::MAX) {
        match test {
            0x00..0xff => {},
            _ => fail!("impossible"),
        }
    }
}

And now compile and run it:

$ rustc all.rs
$ ./all
$

All possible values are covered, so why does rustc still want the obviously unreachable _ arm?

Answer 1

Exhaustive integer matching was stabilised in Rust 1.33.0. The original example in the question now works (updated to use the modern syntax for ranges).

fn main() {
    for i in std::u8::MIN..=std::u8::MAX {
        match i {
            0x00..=0xff => {}
        }
    }
}

Answer 2

As discussed in the referenced issue, Rust's exhaustiveness checking is based entirely on enums and never values, so this fails because u8 simply isn't an enum type.