SI-7982 Changed contract of askLoadedType to unload units by default

The rationale for not keeping units loaded by default is that the more
units are loaded, the slower is background compilation. For instance, in
the Scala IDE for Eclipse (which uses the presentation compiler),
typechecking occurs every time the reconciler kicks-in (~500millis after
you stop typing), hence it is important that units are not kept loaded
unless strictly necessary (for some extra information about this, see
https://www.assembla.com/spaces/scala-ide/tickets/1001388)

While I agree that using a boolean argument (`keepLoaded`) for deciding
if a unit should be loaded isn't a great design, other methods in
`CompilerControl` also have a keepLoaded parameter, so at least we have
some consistency. For the future, I'm thinking we should be able to
remove the `keepLoaded` flag altogether, and change the implementation
of `askLoadedType` to preserve the same units loaded in the presentation
compiler before and after its execution. Basically, if you want a unit
to be kept loaded, you should call `askReload` first, and then
`askLoadedType`. However, to reduce impact, I think the changes carried
by this commit will help us estimate if the solution I just outlined is
viable (because `askLoadeType` won't be keeping units loaded by default,
which wasn't the case with the former implementation).

(While the patch was mostly contributed by @huitseeker, @dotta has edited the
commit message to preserve the comments in the PR
https://github.com/scala/scala/pull/3209)

3 files changed, 35 insertions, 0 deletions

diff --git a/test/files/presentation/ide-t1001388.check b/test/files/presentation/ide-t1001388.check
new file mode 100644
index 0000000000..d58f86d6c6
--- /dev/null
+++ b/test/files/presentation/ide-t1001388.check
@@ -0,0 +1 @@
+Test OK
\ No newline at end of file
diff --git a/test/files/presentation/ide-t1001388/Test.scala b/test/files/presentation/ide-t1001388/Test.scala
new file mode 100644
index 0000000000..f6079cf0b2
--- /dev/null
+++ b/test/files/presentation/ide-t1001388/Test.scala
@@ -0,0 +1,28 @@
+import scala.tools.nsc.interactive.tests.InteractiveTest
+import scala.reflect.internal.util.SourceFile
+import scala.tools.nsc.interactive.Response
+
+object Test extends InteractiveTest {
+  override def execute(): Unit = {
+    val sourceA = loadSourceAndWaitUntilTypechecked("A.scala")
+    checkPresent(sourceA)
+  }
+
+  private def loadSourceAndWaitUntilTypechecked(sourceName: String): SourceFile = {
+    val sourceFile = sourceFiles.find(_.file.name == sourceName).head
+    askLoadedTyped(sourceFile).get
+    /* The response to `askLoadedType` may return before `interactive.Global.waitLoadedType`
+     * fully executes. Because this test expects `waitLoadedType` is fully executed before
+     * calling `checkPresent`, with the below no-op presentation compiler request we make
+     * sure this requirement is fulfilled.
+     */
+    compiler.askForResponse(() => ()).get
+    sourceFile
+  }
+
+  private def checkPresent(source: SourceFile): Unit = compiler.getUnitOf(source) match {
+    case Some(unit) => reporter.println("Compilation Unit for " + source.file.name + " still loaded after askLoadedTyped")
+
+    case None => reporter.println("Test OK")
+  }
+}
diff --git a/test/files/presentation/ide-t1001388/src/a/A.scala b/test/files/presentation/ide-t1001388/src/a/A.scala
new file mode 100644
index 0000000000..be09097598
--- /dev/null
+++ b/test/files/presentation/ide-t1001388/src/a/A.scala
@@ -0,0 +1,6 @@
+package a
+
+object A {
+  val tagString = "foo"
+  Seq.empty[Byte].toArray.toSeq
+}