Use the presentation compiler to drive REPL tab completion

The old implementation is still avaiable under a flag, but we'll
remove it in due course.

Design goal:

  - Push as much code in src/interactive as possible to enable reuse
    outside of the REPL
  - Don't entangle the REPL completion with JLine. The enclosed test
    case drives the REPL and autocompletion programatically.
  - Don't hard code UI choices, like how to render symbols or
    how to filter candidates.

When completion is requested, we wrap the entered code into the
same "interpreter wrapper" synthetic code as is done for regular
execution. We then start a throwaway instance of the presentation
compiler, which takes this as its one and only source file, and
has a classpath formed from the REPL's classpath and the REPL's
output directory (by default, this is in memory).

We can then typecheck the tree, and find the position in the synthetic
source corresponding to the cursor location. This is enough to use
the new completion APIs in the presentation compiler to prepare
a list of candidates.

We go to extra lengths to allow completion of partially typed
identifiers that appear to be keywords, e.g `global.def` should offer
`definitions`.

Two secret handshakes are included; move the the end of the line,
type `// print<TAB>` and you'll see the post-typer tree.
`// typeAt 4 6<TAB>` shows the type of the range position within
the buffer.

The enclosed unit test exercises most of the new functionality.

1 files changed, 22 insertions, 18 deletions

diff --git a/src/repl-jline/scala/tools/nsc/interpreter/jline/JLineReader.scala b/src/repl-jline/scala/tools/nsc/interpreter/jline/JLineReader.scala
index f0fce13fe8..5082c99a76 100644
--- a/src/repl-jline/scala/tools/nsc/interpreter/jline/JLineReader.scala
+++ b/src/repl-jline/scala/tools/nsc/interpreter/jline/JLineReader.scala
@@ -15,7 +15,7 @@ import jconsole.history.{History => JHistory}
 
 
 import scala.tools.nsc.interpreter
-import scala.tools.nsc.interpreter.Completion
+import scala.tools.nsc.interpreter.{Completion, JLineCompletion, NoCompletion}
 import scala.tools.nsc.interpreter.Completion.Candidates
 import scala.tools.nsc.interpreter.session.History
 
@@ -121,23 +121,27 @@ private class JLineConsoleReader extends jconsole.ConsoleReader with interpreter
   def initCompletion(completion: Completion): Unit = {
     this setBellEnabled false
 
-    if (completion ne interpreter.NoCompletion) {
-      val jlineCompleter = new ArgumentCompleter(new JLineDelimiter,
-          new Completer {
-            val tc = completion.completer()
-            def complete(_buf: String, cursor: Int, candidates: JList[CharSequence]): Int = {
-              val buf = if (_buf == null) "" else _buf
-              val Candidates(newCursor, newCandidates) = tc.complete(buf, cursor)
-              newCandidates foreach (candidates add _)
-              newCursor
-            }
-          }
-        )
-
-      jlineCompleter setStrict false
-
-      this addCompleter jlineCompleter
-      this setAutoprintThreshold 400 // max completion candidates without warning
+    // adapt the JLine completion interface
+    def completer =
+      new Completer {
+        val tc = completion.completer()
+        def complete(_buf: String, cursor: Int, candidates: JList[CharSequence]): Int = {
+          val buf = if (_buf == null) "" else _buf
+          val Candidates(newCursor, newCandidates) = tc.complete(buf, cursor)
+          newCandidates foreach (candidates add _)
+          newCursor
+        }
+      }
+
+    // a last bit of nastiness: parsing help depending on the flavor of completer (fixme)
+    completion match {
+      case _: JLineCompletion =>
+        val jlineCompleter = new ArgumentCompleter(new JLineDelimiter, completer)
+        jlineCompleter setStrict false
+        this addCompleter jlineCompleter
+      case NoCompletion       => ()
+      case _                  => this addCompleter completer
     }
+    setAutoprintThreshold(400) // max completion candidates without warning
   }
 }