Changes in one module triggers `~` in another. E.g. changes in `sub2.scala` trigger `~sub1/compile`.

build.properties

# build.properties
sbt.version=1.1.6

build.sbt

lazy val root = project.in(file("."))
  .aggregate(sub1, sub2)

lazy val sub1 = project

lazy val sub2 = project

sub1.scala

// sub1/src/main/scala/sub1.scala
object sub1 {
}

sub2.scala

// sub2/src/main/scala/sub2.scala
object sub2 {
}