File: guards.rs

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#![deny(unreachable_patterns)]

enum Q { R(Option<usize>) }

pub fn main() {
    match Q::R(None) {
        Q::R(S) if S.is_some() => {}
        _ => {}
    }

    match 0u8 { //~ ERROR non-exhaustive patterns
        0 .. 128 => {}
        128 ..= 255 if true => {}
    }

    match 0u8 {
        0 .. 128 => {}
        128 ..= 255 if false => {}
        128 ..= 255 => {} // ok, because previous arm was guarded
    }
}