SI-8941 Deterministic tests for pres. compiler idempotency

A retrospective test case which covers typechecking idempptency which
was introduced in 0b78a01 / 148736c. It also tests the
implicit class handling, which was fixed in the previous commit.

It is difficult to test this using existing presentation compiler
testing infrastructure, as one can't control at which point during
the first typechecking the subesquent work item will be noticed.

Instead, I've created a test with a custom subclass of
`interactive.Global` that allows precise, deterministic control
of when this happens. It overrides `signalDone`, which is called
after each tree is typechecked, and watches for a defintion with
a well known name. At that point, it triggers a targetted typecheck
of the tree marked with a special comment.

It is likely that this approach can be generalized to a reusable
base class down the track. In particular, I expect that some of
the nasty interactive ScalaDoc bugs could use this single-threaded
approach to testing the presentation compiler.

src/interactive/scala/tools/nsc/interactive/Global.scala

@@ -64,6 +64,8 @@ trait InteractiveAnalyzer extends Analyzer {
     // that case the definitions that were already attributed as
     // well as any default parameters of such methods need to be
     // re-entered in the current scope.
+    //
+    // Tested in test/files/presentation/t8941b
     override def enterExistingSym(sym: Symbol, tree: Tree): Context = {
       if (sym != null && sym.owner.isTerm) {
         enterIfNotThere(sym)
@@ -737,7 +739,7 @@ class Global(settings: Settings, _reporter: Reporter, projectName: String = "")
     }
   }
 
-  private def reloadSource(source: SourceFile) {
+  private[interactive] def reloadSource(source: SourceFile) {
     val unit = new RichCompilationUnit(source)
     unitOfFile(source.file) = unit
     toBeRemoved -= source.file
@@ -786,7 +788,7 @@ class Global(settings: Settings, _reporter: Reporter, projectName: String = "")
   }
 
   /** A fully attributed tree located at position `pos` */
-  private def typedTreeAt(pos: Position): Tree = getUnit(pos.source) match {
+  private[interactive] def typedTreeAt(pos: Position): Tree = getUnit(pos.source) match {
     case None =>
       reloadSources(List(pos.source))
       try typedTreeAt(pos)

test/files/presentation/t8941b/IdempotencyTest.scala

@@ -0,0 +1,73 @@
+package scala.tools.nsc
+package interactive
+package tests.core
+
+import reporters.{Reporter => CompilerReporter}
+import scala.tools.nsc.interactive.InteractiveReporter
+import scala.reflect.internal.util.SourceFile
+
+/** Determistically interrupts typechecking of `code` when a defintion named
+  * `MagicInterruptionMarker` is typechecked, and then performs a targetted
+  * typecheck of the tree at the specal comment marker marker
+  */  
+abstract class IdempotencyTest { self =>
+  private val settings = new Settings
+  settings.usejavacp.value = true
+
+  private object Break extends scala.util.control.ControlThrowable
+
+  private val compilerReporter: CompilerReporter = new InteractiveReporter {
+    override def compiler = self.compiler
+  }
+
+  object compiler extends Global(settings, compilerReporter) {
+    override def checkForMoreWork(pos: Position) {
+    }
+    override def signalDone(context: Context, old: Tree, result: Tree) {
+      // println("signalDone: " + old.toString.take(50).replaceAll("\n", "\\n"))
+      if (!interrupted && analyzer.lockedCount == 0 && interruptsEnabled && shouldInterrupt(result)) {
+        interrupted = true
+        val typed = typedTreeAt(markerPosition)
+        checkTypedTree(typed)
+        throw Break
+      }
+      super.signalDone(context, old, result)
+    }
+
+    // we're driving manually using our own thread, disable the check here.
+    override def assertCorrectThread() {}
+  }
+
+  import compiler._
+
+  private var interrupted = false
+
+  // Extension points
+  protected def code: String
+  protected def shouldInterrupt(tree: Tree): Boolean = {
+    tree.symbol != null && tree.symbol.name.toString == "MagicInterruptionMarker"
+  }
+  protected def checkTypedTree(tree: Tree): Unit = {}
+
+
+  private val source: SourceFile = newSourceFile(code)
+  private def markerPosition: Position = source.position(code.indexOf("/*?*/"))
+
+  def assertNoProblems() {
+    val problems = getUnit(source).get.problems
+    assert(problems.isEmpty, problems.mkString("\n"))
+  }
+
+  def show() {
+    reloadSource(source)
+    try {
+      typedTree(source, true)
+      assert(false, "Expected to break out of typechecking.")
+    } catch {
+      case Break => // expected
+    }
+    assertNoProblems()
+  }
+
+  def main(args: Array[String]) { show() }
+}

test/files/presentation/t8941b/Test.scala

@@ -0,0 +1,53 @@
+import scala.tools.nsc.interactive.tests.core.IdempotencyTest
+
+// At the time of writing this test, removing any part of `enterExistingSym` 
+// leads to a failure.
+object Test {
+  def main(args: Array[String]) {
+    test("""
+      object Foo {
+        def term {
+          def foo(c: String = "") = c
+          class MagicInterruptionMarker
+          foo()/*?*/
+        }
+      }
+    """)
+
+    test("""
+      object Foo {
+        def term {
+          def foo = 42
+          class MagicInterruptionMarker
+          foo/*?*/
+        }
+      }
+    """)
+
+    test("""
+      object Foo {
+        def term {
+          lazy val foo = 42
+          class MagicInterruptionMarker
+          foo/*?*/
+        }
+      }
+    """)
+
+    test("""
+      object Foo {
+        implicit class C(val a: String) extends AnyVal
+        class MagicInterruptionMarker
+        ""/*?*/
+      }
+    """)
+  }
+
+  def test(code0: String) {
+    val t = new IdempotencyTest {
+      def code = code0
+    }
+    t.show()
+  }
+}
+