Add defs.bzl with core rules and notice about evolving APIs (#955)

* Add defs.bzl with core rules

* move to unstable directory

scala/unstable/defs.bzl

@@ -0,0 +1,40 @@
+"""
+Starlark rules for building Scala projects.
+
+These are the core rules under active development. Their APIs are
+not guaranteed stable and we anticipate some breaking changes.
+
+We do not recommend using these APIs for production codebases. Instead,
+use the stable rules exported by scala.bzl:
+
+```
+load(
+    "@io_bazel_rules_scala//scala:scala.bzl",
+    "scala_library",
+    "scala_binary",
+    "scala_test"
+)
+```
+
+"""
+
+load(
+    "@io_bazel_rules_scala//scala/private:rules/scala_binary.bzl",
+    _make_scala_binary = "make_scala_binary",
+)
+load(
+    "@io_bazel_rules_scala//scala/private:rules/scala_library.bzl",
+    _make_scala_library = "make_scala_library",
+)
+load(
+    "@io_bazel_rules_scala//scala/private:rules/scala_test.bzl",
+    _make_scala_test = "make_scala_test",
+)
+
+make_scala_library = _make_scala_library
+make_scala_binary = _make_scala_binary
+make_scala_test = _make_scala_test
+
+scala_library = _make_scala_library()
+scala_binary = _make_scala_binary()
+scala_test = _make_scala_test()

test/unstable/BUILD

@@ -0,0 +1,27 @@
+load(
+    "//scala/unstable:defs.bzl",
+    "scala_binary",
+    "scala_library",
+    "scala_test",
+)
+
+scala_binary(
+    name = "binary",
+    srcs = ["binary.scala"],
+    main_class = "test.v2.Binary",
+    deps = [":library"],
+)
+
+scala_library(
+    name = "library",
+    srcs = ["library.scala"],
+    deps = [],
+)
+
+scala_test(
+    name = "test",
+    srcs = ["test.scala"],
+    deps = [
+        ":library",
+    ],
+)

test/unstable/binary.scala

@@ -0,0 +1,7 @@
+package test.v2
+
+object Binary {
+  def main(args: Array[String]): Unit = {
+    println(s"${Library.method1} ${Library.method2}")
+  }
+}

test/unstable/library.scala

@@ -0,0 +1,6 @@
+package test.v2
+
+object Library {
+  def method1(): String = "hello"
+  def method2(): String = "world"
+}

test/unstable/test.scala

@@ -0,0 +1,13 @@
+package test.v2
+
+import org.scalatest.FunSuite
+
+class Test extends FunSuite {
+  test("method1") {
+    assert(Library.method1 == "hello")
+  }
+
+  test("method2") {
+    assert(Library.method2 == "world")
+  }
+}