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Description
From check_match.rs:
// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html
//
// Whether a vector `v` of patterns is 'useful' in relation to a set of such
// vectors `m` is defined as there being a set of inputs that will match `v`
// but not any of the sets in `m`.
//
// This is used both for reachability checking (if a pattern isn't useful in
// relation to preceding patterns, it is not reachable) and exhaustiveness
// checking (if a wildcard pattern is useful in relation to a matrix, the
// matrix isn't exhaustive).
// Note: is_useful doesn't work on empty types, as the paper notes.
// So it assumes that v is non-empty.
This results in, most prominently (#4499), a match on an empty enum with a wildcard pattern being accepted:
enum X { }
fn x() -> X { unimplemented!() }
match x() {
_ => println!("hi")
}
while if you add a variant to the enum and a corresponding arm to the match:
enum X { X }
fn x() -> X { unimplemented!() }
match x() {
X => println!("hello"),
_ => println!("hi")
}
rustc rejects the program with error: unreachable pattern.
Clearly a trailing wildcard pattern after you've already matched all possible values in preceding arms should always be considered unreachable, and this is true even when the number of potential values, and hence preceding arms, are both zero.
As the comment notes the algorithm we use relies on "no empty types" as an assumption, so this may not be an easy bug, but it's still a bug.