Fix #3478: Optimize away unused defs and lazy vals

compiler/src/dotty/tools/dotc/core/Symbols.scala

@@ -743,6 +743,8 @@ object Symbols {
     def keysIterator: Iterator[Symbol] = value.keySet().asScala.iterator
 
     def toMap: Map[Symbol, T] = value.asScala.toMap
+
+    override def toString: String = value.asScala.toString()
   }
 
   @inline def newMutableSymbolMap[T]: MutableSymbolMap[T] =

compiler/src/dotty/tools/dotc/transform/localopt/Devalify.scala

@@ -159,6 +159,12 @@ class Devalify extends Optimisation {
         if (replacements.contains(t.symbol))
           deepReplacer.transform(replacements(t.symbol)).ensureConforms(t.tpe.widen)
         else t
+      case t: DefDef if !t.symbol.owner.isClass =>
+        if (timesUsed.getOrElse(t.symbol, 0) + timesUsedAsType.getOrElse(t.symbol, 0) != 0) t
+        else {
+          simplify.println(s"Dropping definition of ${t.symbol.showFullName} as not used")
+          EmptyTree
+        }
       case tree => tree
     }
 

compiler/test/dotty/tools/dotc/SimplifyTests.scala

@@ -164,6 +164,35 @@ abstract class SimplifyTests(val optimise: Boolean) extends DottyBytecodeTest {
         |print(8)
       """)
 
+  @Test def localDefinitionElimination =
+    check(
+      """
+        |lazy val foo = 1
+        |def bar = 2
+        |val baz = 3
+      """,
+      """
+      """)
+
+  @Test def localDefinitionNoElimination =
+    check(
+      """
+        |val j = 0 // dummy
+        |class Foo {
+        |  lazy val foo = 1
+        |  def bar = 2
+        |  val baz = 3
+        |}
+      """,
+      """
+        |class Foo {
+        |  lazy val foo = 1
+        |  def bar = 2
+        |  val baz = 3
+        |}
+      """)
+
+
   // @Test def listPatmapExample =
   //   check(
   //     """