When matching against a range, the compiler verifies that the range is
non-empty. Range patterns include both end-points, so this is equivalent to
requiring the start of the range to be less than or equal to the end of the
range.
Erroneous code example:
```compile_fail,E0030
match 5u32 {
// This range is ok, albeit pointless.
1 ..= 1 => {}
// This range is empty, and the compiler can tell.
1000 ..= 5 => {}
}
```