1 Cannot mutate place in this match guard.
3 When matching on a variable it cannot be mutated in the match guards, as this
4 could cause the match to be non-exhaustive:
10 Some(_) if { x = None; false } => (),
11 Some(v) => (), // No longer matches
15 Here executing `x = None` would modify the value being matched and require us
16 to go "back in time" to the `None` arm.