// library/Library.scala
package library
object Library {
  @inline def pleaseInlineMe() = 0
  object Nested {
    @inline def pleaseInlineMe() = 0
  }
}

// Application.scala
object Application {
  def main(arguments: Array[String]): Unit = {
    // No warning here
    println(library.Library.pleaseInlineMe())

    // Could not inline required method pleaseInlineMe because bytecode unavailable.
    println(library.Library.Nested.pleaseInlineMe())
  }
}

// build.sbt
lazy val library = Project("library", file("library"))

lazy val application = Project("application", file(".")).dependsOn(library)

scalacOptions += "-Yinline-warnings"

scalacOptions += "-optimise"

$ sbt compile
[info] Loading global plugins from C:\Users\user\.sbt\0.13\plugins
[info] Set current project to application (in build file:/D:/cygwin/home/user/Documents/inline-nested-object/)
[info] Compiling 1 Scala source to D:\cygwin\home\user\Documents\inline-nested-object\library\target\scala-2.10\classes...
[info] Compiling 1 Scala source to D:\cygwin\home\user\Documents\inline-nested-object\target\scala-2.10\classes...
[warn] D:\cygwin\home\user\Documents\inline-nested-object\Application.scala:8: Could not inline required method pleaseInlineMe because bytecode unavailable.
[warn]     println(library.Library.Nested.pleaseInlineMe())
[warn]                                                  ^
[warn] one warning found
[success] Total time: 4 s, completed 2014-4-22 16:13:10