Hm. Sometimes you have to put inout because you're, say, satisfying a protocol requirement. I wouldn't want to add a warning unless we had a way to silence it.

(The most obvious way I can think of is self-assignment, but I feel like normally we'd want to warn on that. The next is to use _ instead of a real param name, but that affects doc comments.)

I think `_` makes sense, and maybe it's a good thing that the documentation would reflect the fact that this conformance does not mutate things.

If we don't want doc comments to be affected, though, self-assignment with parentheses could make it explicit.

func mut<A>(_ a: inout A) {
  (a = a)
}