Reduce boilerplate in compiler JUnit tests (#5158)

Many JUnit tests share a compiler instance between all
test cases in a class to reduce overhead.

This commit refactors the mechanism to reduce the boilerplate.

In the new scheme:

  - Using the `@ClassRule` hook in JUnit, we create a per-class
    map for each test class.
  - Per-class values are registered from the test class itself
    by calling `cached("someKey", () => mkExpensiveThing)`
  - At the end of the test, the entries in this map are `close()`-ed
    (if they implement `Closable`), and are released for
    garbage collection.)

1 files changed, 1 insertions, 8 deletions

diff --git a/test/junit/scala/tools/nsc/backend/jvm/opt/UnusedLocalVariablesTest.scala b/test/junit/scala/tools/nsc/backend/jvm/opt/UnusedLocalVariablesTest.scala
index 4f71df1822..77e73e64b9 100644
--- a/test/junit/scala/tools/nsc/backend/jvm/opt/UnusedLocalVariablesTest.scala
+++ b/test/junit/scala/tools/nsc/backend/jvm/opt/UnusedLocalVariablesTest.scala
@@ -14,16 +14,9 @@ import scala.tools.partest.ASMConverters
 import ASMConverters._
 import scala.tools.testing.ClearAfterClass
 
-object UnusedLocalVariablesTest extends ClearAfterClass.Clearable {
-  var dceCompiler = newCompiler(extraArgs = "-Yopt:unreachable-code")
-  def clear(): Unit = { dceCompiler = null }
-}
-
 @RunWith(classOf[JUnit4])
 class UnusedLocalVariablesTest extends ClearAfterClass {
-  ClearAfterClass.stateToClear = UnusedLocalVariablesTest
-
-  val dceCompiler = UnusedLocalVariablesTest.dceCompiler
+  val dceCompiler = cached("dceCompiler", () => newCompiler(extraArgs = "-Yopt:unreachable-code"))
 
   @Test
   def removeUnusedVar(): Unit = {