Add a `dotty-interfaces` package

We introduce a new entry point for the compiler in
`dotty.tools.dotc.Driver`:
```
def process(args: Array[String], simple: interfaces.SimpleReporter,
  callback: interfaces.CompilerCallback): interfaces.ReporterResult
```
Except for `args` which is just an array, the argument types and return
type of this method are Java interfaces defined in a new package called
`dotty-interfaces` which has a stable ABI. This means that you can
programmatically run a compiler with a custom reporter and callbacks
without having to recompile it against every version of dotty: you only
need to have `dotty-interfaces` present at compile-time and call the
`process` method using Java reflection.

See `test/test/InterfaceEntryPointTest.scala` for a concrete example.

This design is based on discussions with the IntelliJ IDEA Scala plugin
team. Thanks to Nikolay Tropin for the discussions and his PR
proposal (see #1011).

1 files changed, 7 insertions, 7 deletions

diff --git a/src/dotty/tools/dotc/reporting/Diagnostic.scala b/src/dotty/tools/dotc/reporting/Diagnostic.scala
index ea3ea4112..bcf55e993 100644
--- a/src/dotty/tools/dotc/reporting/Diagnostic.scala
+++ b/src/dotty/tools/dotc/reporting/Diagnostic.scala
@@ -4,22 +4,22 @@ package reporting
 
 import util.SourcePosition
 
-object Diagnostic {
-
-  // Error levels
-  val ERROR = 2
-  val WARNING = 1
-  val INFO = 0
+import java.util.Optional
 
+object Diagnostic {
   val nonSensicalStartTag = "<nonsensical>"
   val nonSensicalEndTag = "</nonsensical>"
 }
 
-class Diagnostic(msgFn: => String, val pos: SourcePosition, val level: Int) extends Exception {
+class Diagnostic(msgFn: => String, val pos: SourcePosition, val level: Int)
+    extends Exception with interfaces.Diagnostic {
   import Diagnostic._
   private var myMsg: String = null
   private var myIsNonSensical: Boolean = false
 
+  override def position: Optional[interfaces.SourcePosition] =
+    if (pos.exists && pos.source.exists) Optional.of(pos) else Optional.empty()
+
   /** The message to report */
   def message: String = {
     if (myMsg == null) {