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, 28 insertions, 0 deletions

diff --git a/interfaces/src/main/java/dotty/tools/dotc/interfaces/CompilerCallback.java b/interfaces/src/main/java/dotty/tools/dotc/interfaces/CompilerCallback.java
new file mode 100644
index 000000000..17d2f6bbd
--- /dev/null
+++ b/interfaces/src/main/java/dotty/tools/dotc/interfaces/CompilerCallback.java
@@ -0,0 +1,28 @@
+package dotty.tools.dotc.interfaces;
+
+/** Set of callbacks called in response to events during the compilation process.
+ *
+ *  You should implement this interface if you want to react to one or more of
+ *  these events.
+ *
+ *  @see the method `process` of `dotty.tools.dotc.Driver` for more information.
+ */
+public interface CompilerCallback {
+  /** Called when a class has been generated.
+   *
+   *  @param source         The source file corresponding to this class.
+   *                        Example: ./src/library/scala/collection/Seq.scala
+   *  @param generatedClass The generated classfile for this class.
+   *                        Example: ./scala/collection/Seq$.class
+   *  @param className      The name of this class.
+   *                        Example: scala.collection.Seq$
+   */
+  default void onClassGenerated(SourceFile source, AbstractFile generatedClass, String className) {};
+
+  /** Called when every class for this file has been generated.
+   *
+   *  @param source         The source file.
+   *                        Example: ./src/library/scala/collection/Seq.scala
+   */
+  default void onSourceCompiled(SourceFile source) {};
+}