Make all objects Serializable

To avoid deadlocks when combining objects, lambdas and multi-threading,
lambdas in objects are compiled to instance methods of the module class
instead of static methods (see tests/run/deadlock.scala and
scala/scala-dev#195 for details).

This has worked well for us so far but this is problematic for
serialization: serializing a lambda requires serializing all the values
it captures, if this lambda is in an object, this means serializing the
enclosing object, which fails if the object does not extend
Serializable.

Because serializing objects is basically free since scala#5775, it seems like
the simplest solution is to simply make all objects Serializable, this
certainly seems preferable to deadlocks.

This commit causes a cyclic reference to happen in some cases, we add a
workaround to avoid this in Trees.scala and fix it properly in the
commit.

Author: smarter

test/xsbt/ExtractUsedNamesSpecification.scala

@@ -79,10 +79,10 @@ class ExtractUsedNamesSpecification {
     val usedNames = compilerForTesting.extractUsedNamesFromSrc(srcA, srcB, srcC, srcD)
     val scalaVersion = scala.util.Properties.versionNumberString
     val namesA = standardNames ++ Set("Nothing", "Any")
-    val namesAX = standardNames ++ Set("x", "T", "A", "Nothing", "Any")
+    val namesAX = standardNames ++ objectStandardNames ++ Set("x", "T", "A", "Nothing", "Any", "scala")
     val namesB = Set("A", "Int", "A;init;", "Unit")
-    val namesC = Set("B;init;", "B", "Unit")
-    val namesD = standardNames ++ Set("C", "X", "foo", "Int", "T")
+    val namesC = objectStandardNames ++ Set("B;init;", "B", "Unit")
+    val namesD = standardNames ++ objectStandardNames ++ Set("C", "X", "foo", "Int", "T")
     assertEquals(namesA, usedNames("A"))
     assertEquals(namesAX, usedNames("A.X"))
     assertEquals(namesB, usedNames("B"))
@@ -130,23 +130,30 @@ class ExtractUsedNamesSpecification {
                   |""".stripMargin
     val compilerForTesting = new ScalaCompilerForUnitTesting
     val usedNames = compilerForTesting.extractUsedNamesFromSrc(src1, src2)
-    val expectedNames_lista = standardNames ++ Set("B", "lista", "List", "A")
-    val expectedNames_at = standardNames ++ Set("B", "at", "A", "T", "X0", "X1")
-    val expectedNames_as = standardNames ++ Set("B", "as", "S", "Y")
-    val expectedNames_foo = standardNames ++ Set("B",
-                                                 "foo",
-                                                 "M",
-                                                 "N",
-                                                 "Predef",
-                                                 "???",
-                                                 "Nothing")
-    val expectedNames_bar = standardNames ++ Set("B",
-                                                 "bar",
-                                                 "P1",
-                                                 "P0",
-                                                 "Predef",
-                                                 "???",
-                                                 "Nothing")
+    val expectedNames_lista =
+      standardNames ++ objectStandardNames ++ Set("B", "lista", "List", "A")
+    val expectedNames_at =
+      standardNames ++ objectStandardNames ++  Set("B", "at", "A", "T", "X0", "X1")
+    val expectedNames_as =
+      standardNames ++ objectStandardNames ++  Set("B", "as", "S", "Y")
+    val expectedNames_foo =
+      standardNames ++ objectStandardNames ++
+       Set("B",
+           "foo",
+           "M",
+           "N",
+           "Predef",
+           "???",
+           "Nothing")
+    val expectedNames_bar =
+      standardNames ++ objectStandardNames ++
+       Set("B",
+           "bar",
+           "P1",
+           "P0",
+           "Predef",
+           "???",
+           "Nothing")
     assertEquals(expectedNames_lista, usedNames("Test_lista"))
     assertEquals(expectedNames_at, usedNames("Test_at"))
     assertEquals(expectedNames_as, usedNames("Test_as"))
@@ -167,7 +174,7 @@ class ExtractUsedNamesSpecification {
                     |""".stripMargin
     val compilerForTesting = new ScalaCompilerForUnitTesting
     val usedNames = compilerForTesting.extractUsedNamesFromSrc(srcFoo, srcBar)
-    val expectedNames = standardNames ++ Set("Outer", "TypeInner", "Inner", "Int")
+    val expectedNames = standardNames ++ objectStandardNames ++ Set("Outer", "TypeInner", "Inner", "Int")
     assertEquals(expectedNames, usedNames("Bar"))
   }
 
@@ -302,4 +309,9 @@ class ExtractUsedNamesSpecification {
     // the return type of the default constructor is Unit
     "Unit"
   )
+
+  private val objectStandardNames = Set(
+    // all Dotty objects extend scala.Serializable
+    "scala", "Serializable"
+  )
 }