Avoid unused param warning in unused definition

[som-snytt]

Avoid warning if method is marked unused.

https://github.com/scala/scala/blob/v2.13.14/src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala#L99

Avoid warning if parameter type is singleton.

https://github.com/scala/scala/blob/v2.13.14/src/compiler/scala/tools/nsc/ast/TreeDSL.scala#L132

Problem

Probably all unused warnings should be silenced by unused enclosing definition, much like deprecation.

[som-snytt]

What about

class `t12992 enclosing def is unused` {
  private val n = 42
  @annotation.unused def f() = n + 2 // unused code uses n
}

Is n unused? The PR doesn't look inside f.

[som-snytt]

I like that behavior but it's necessary to consider side effects.

In this example, max is untested but you must inspect the rhs for update of the var.

    var evaluatedCountOfMaxBy = 0

    val max = list.maxBy(x => {
      evaluatedCountOfMaxBy += 1
      x * 10
    })

[som-snytt]

not sure why I wrote this trick, but it is undone.

    type ignore = annotation.unused
    @ignore def ignore: ignore = ???  // ignore that ignore looks unused