This is fine:

checked_captures.push(match capture {
    ByValue(name, span) => CheckedCapture::ByValue(name, span)
    ByReference(name, span) => CheckedCapture::ByReference(name, span)
    ByMutableReference(name, span) => CheckedCapture::ByMutableReference(name, span)
    AllByReference(span) => {
        .compiler.panic("AllByReference capture should not be looked up by name")
    }
})

This is not, and gives us Type mismatch: expected ‘CheckedCapture’, but got ‘never’:

checked_captures.push(match capture {
    ByValue(name, span) => CheckedCapture::ByValue(name, span)
    ByReference(name, span) => CheckedCapture::ByReference(name, span)
    ByMutableReference(name, span) => CheckedCapture::ByMutableReference(name, span)
    AllByReference(span) => .compiler.panic("AllByReference capture should not be looked up by name")
})

This turns out to be intended behaviour: the short => foo syntax can only be used for yielding values. However, a nicer error message when a match has a type mismatch with never, would be very helpful! It should tell you that you need to wrap it in {}.