Warn about unused imports.

Hidden behind -Xlint as usual. This commit also includes
further simplification of the symbol lookup logic which I
unearthed on the way to reporting unused imports.  Plus
unusually comprehensive documentation of same.

1 files changed, 125 insertions, 0 deletions

diff --git a/test/files/neg/warn-unused-imports.scala b/test/files/neg/warn-unused-imports.scala
new file mode 100644
index 0000000000..b7a2f1c414
--- /dev/null
+++ b/test/files/neg/warn-unused-imports.scala
@@ -0,0 +1,125 @@
+class Bippo {
+  def length: Int = 123
+  class Tree
+}
+
+package object p1 {
+  class A
+  implicit class B(val s: String) { def bippy = s }
+  val c: Bippo = new Bippo
+  type D = String
+}
+package object p2 {
+  class A
+  implicit class B(val s: String) { def bippy = s }
+  val c: Bippo = new Bippo
+  type D = Int
+}
+
+trait NoWarn {
+  {
+    import p1._ // no warn
+    println("abc".bippy)
+  }
+
+  {
+    import p1._ // no warn
+    println(new A)
+  }
+
+  {
+    import p1.B // no warn
+    println("abc".bippy)
+  }
+
+  {
+    import p1._ // no warn
+    import c._  // no warn
+    println(length)
+  }
+
+  {
+    import p1._ // no warn
+    import c._  // no warn
+    val x: Tree = null
+    println(x)
+  }
+
+  {
+    import p1.D // no warn
+    val x: D = null
+    println(x)
+  }
+}
+
+trait Warn {
+  {
+    import p1.A // warn
+    println(123)
+  }
+
+  {
+    import p1.{ A, B } // warn on A
+    println("abc".bippy)
+  }
+
+  {
+    import p1.{ A, B } // warn on both
+    println(123)
+  }
+
+  {
+    import p1._ // no warn (technically this could warn, but not worth the effort to unroll unusedness transitively)
+    import c._  // warn
+    println(123)
+  }
+
+  {
+    import p1._ // warn
+    println(123)
+  }
+
+  {
+    class Tree
+    import p1._ // no warn
+    import c._  // warn
+    val x: Tree = null
+    println(x)
+  }
+
+  {
+    import p1.c._  // warn
+    println(123)
+  }
+}
+
+trait Nested {
+  {
+    import p1._   // warn
+    trait Warn {  // warn about unused local trait for good measure
+      import p2._
+      println(new A)
+      println("abc".bippy)
+    }
+    println("")
+  }
+
+  {
+    import p1._   // no warn
+    trait NoWarn {
+      import p2.B  // no warn
+      println("abc".bippy)
+      println(new A)
+    }
+    println(new NoWarn { })
+  }
+
+  {
+    import p1.A   // warn
+    trait Warn {
+      import p2.A
+      println(new A)
+    }
+    println(new Warn { })
+  }
+}