Add test for scalac dependencies (bazelbuild#998)

* Add test for scalac dependencies

* comment

third_party/dependency_analyzer/src/test/BUILD

@@ -38,6 +38,24 @@ scala_test(
     ],
 )
 
+scala_test(
+    name = "scalac_dependency_test",
+    size = "small",
+    srcs = [
+        "io/bazel/rulesscala/dependencyanalyzer/ScalacDependencyTest.scala",
+    ],
+    jvm_flags = common_jvm_flags,
+    unused_dependency_checker_mode = "off",
+    deps = [
+        "//external:io_bazel_rules_scala/dependency/scala/scala_compiler",
+        "//external:io_bazel_rules_scala/dependency/scala/scala_library",
+        "//external:io_bazel_rules_scala/dependency/scala/scala_reflect",
+        "//third_party/dependency_analyzer/src/main:dependency_analyzer",
+        "//third_party/utils/src/test:test_util",
+        "@scalac_rules_commons_io//jar",
+    ],
+)
+
 scala_test(
     name = "strict_deps_test",
     size = "small",

third_party/dependency_analyzer/src/test/io/bazel/rulesscala/dependencyanalyzer/ScalacDependencyTest.scala

@@ -0,0 +1,109 @@
+package third_party.dependency_analyzer.src.test.io.bazel.rulesscala.dependencyanalyzer
+
+import java.nio.file.Files
+import java.nio.file.Path
+import java.util.UUID
+import org.apache.commons.io.FileUtils
+import org.scalatest._
+import third_party.utils.src.test.io.bazel.rulesscala.utils.JavaCompileUtil
+import third_party.utils.src.test.io.bazel.rulesscala.utils.TestUtil
+
+/**
+ * Test that the scalac compiler behaves how we expect it to around
+ * dependencies. That is, for given scenarios, we want to make sure
+ * that scalac requires the given set of dependencies; no more and
+ * no less.
+ *
+ * To clarify: these tests do not reflect the end result of strict/unused
+ * deps as we are considering alternatives of how to mitigate scalac's
+ * limitations.
+ */
+class ScalacDependencyTest extends FunSuite {
+  private def withSandbox(action: Sandbox => Unit): Unit = {
+    val tmpDir = Files.createTempDirectory("dependency_analyzer_test_temp")
+    val file = tmpDir.toFile
+    try {
+      action(new Sandbox(tmpDir))
+    } finally {
+      FileUtils.deleteDirectory(file)
+    }
+  }
+
+  private class Sandbox(tmpDir: Path) {
+    def compile(
+      code: String
+    ): Unit = {
+      val errors =
+        TestUtil.runCompiler(
+          code = code,
+          extraClasspath = List(tmpDir.toString),
+          outputPathOpt = Some(tmpDir)
+        )
+      assert(errors.isEmpty)
+    }
+
+    def compileJava(
+      className: String,
+      code: String
+    ): Unit = {
+      JavaCompileUtil.compile(
+        tmpDir = tmpDir.toString,
+        className = className,
+        code = code
+      )
+    }
+
+    def checkExactDepsNeeded(
+      code: String,
+      deps: List[String]
+    ): Unit = {
+      def doesCompileSucceed(usedDeps: List[String]): Boolean = {
+        val subdir = tmpDir.resolve(UUID.randomUUID().toString)
+        Files.createDirectory(subdir)
+        usedDeps.foreach { dep =>
+          val name = s"$dep.class"
+          Files.copy(tmpDir.resolve(name), subdir.resolve(name))
+        }
+        val errors =
+          TestUtil.runCompiler(
+            code = code,
+            extraClasspath = List(subdir.toString)
+          )
+        errors.isEmpty
+      }
+
+      assert(doesCompileSucceed(deps), s"Failed to compile with all deps")
+
+      deps.foreach { toSkip =>
+        val remaining = deps.filter(_ != toSkip)
+        // sanity check we removed exactly one item
+        assert(remaining.size + 1 == deps.size)
+        assert(
+          !doesCompileSucceed(remaining),
+          s"Compile succeeded even though $toSkip was missing")
+      }
+    }
+  }
+
+  test("static annotation of superclass not needed") {
+    withSandbox { sandbox =>
+      sandbox.compile("class A extends scala.annotation.StaticAnnotation")
+      sandbox.compile("@A class B")
+      sandbox.checkExactDepsNeeded(
+        code = "class C extends B",
+        deps = List("B")
+      )
+    }
+  }
+
+  test("superclass of superclass needed") {
+    withSandbox { sandbox =>
+      sandbox.compile("class A")
+      sandbox.compile("class B extends A")
+      sandbox.checkExactDepsNeeded(
+        code = "class C extends B",
+        deps = List("A", "B")
+      )
+    }
+  }
+}