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, 26 insertions, 0 deletions

diff --git a/interfaces/src/main/java/dotty/tools/dotc/interfaces/Diagnostic.java b/interfaces/src/main/java/dotty/tools/dotc/interfaces/Diagnostic.java
new file mode 100644
index 000000000..436f9f8ea
--- /dev/null
+++ b/interfaces/src/main/java/dotty/tools/dotc/interfaces/Diagnostic.java
@@ -0,0 +1,26 @@
+package dotty.tools.dotc.interfaces;
+
+import java.util.Optional;
+
+/** A diagnostic is a message emitted during the compilation process.
+ *
+ *  It can either be an error, a warning or an information.
+ *
+ *  User code should not implement this interface, but it may have to
+ *  manipulate objects of this type.
+ */
+public interface Diagnostic {
+  public static final int ERROR = 2;
+  public static final int WARNING = 1;
+  public static final int INFO = 0;
+
+  /** The message to report */
+  String message();
+
+  /** Level of the diagnostic, can be either ERROR, WARNING or INFO */
+  int level();
+
+  /** The position in a source file of the code that caused this diagnostic
+   *  to be emitted. */
+  Optional<SourcePosition> position();
+}