warnings when using -Wnonunit-statement

[yanns]

When using the scalac flag -Wnonunit-statement, most of the rules emit the following warning:

unused value of type Boolean (add `: Unit` to discard silently)

My first quick analysis is coming from:

def __advance(): Boolean = ... 
def __updateMaxCursor(): Boolean

used like:

  case class StringMatch(stringTree: Tree) extends OpTree {
    final private val autoExpandMaxStringLength = 8

    def render(wrapped: Boolean): Tree = {
      def unrollUnwrapped(s: String, ix: Int = 0): Tree =
        if (ix < s.length) q"""
          if (cursorChar == ${s charAt ix}) {
            __advance()
            ${unrollUnwrapped(s, ix + 1)}:Boolean
          } else false"""
        else q"true"

In that code, the boolean returned by __advance() is not used.

Possible solutions:

• (1) We could add some : Unit in the code like this:

  case class StringMatch(stringTree: Tree) extends OpTree {
    final private val autoExpandMaxStringLength = 8

    def render(wrapped: Boolean): Tree = {
      def unrollUnwrapped(s: String, ix: Int = 0): Tree =
        if (ix < s.length) q"""
          if (cursorChar == ${s charAt ix}) {
            __advance(): Unit
            ${unrollUnwrapped(s, ix + 1)}:Boolean
          } else false"""
        else q"true"

• (2) we could change methods like __advance() to return Unit. We would then have to return true in some branches.

I'm wondering if there is a speed difference between having a method returning a value that we don't use?
The fact that we return true allows also some code like q"_root_.java.lang.Character.toLowerCase(cursorChar) == $charTree && __advance()" that shortcuts the __advance() if the first predicate does not match.