Rewrite TraversableLike.stringPrefix not to blow up code size in Scala.js.

The commit 30876fe2dd8cbe657a6cad6b11bbc34f10c29b36 changed
`TraversableLike.stringPrefix` to report nicer results for inner
classes and method-local classes. The changes included calls to
`String.split()`, `Character.isDigit()` and `Character.isUpperCase()`.
This was particularly bad for Scala.js, because those methods
bring with them huge parts of the JDK (the `java.util.regex.*`
implementation on the one hand, and the Unicode database on the
other hand), which increased generated code size by 6 KB after
minimification and gzip for an application that does not otherwise
use those methods. This sudden increase is tracked in the Scala.js
bug tracker at https://github.com/scala-js/scala-js/issues/2591.

This commit rewrites `TraversableLike.stringPrefix` in a very
imperative way, without resorting to those methods. The behavior
is (mostly) preserved. There can be different results when
`getClass().getName()` contains non-ASCII lowercase letters and/or
digits. Those will now be recognized as user-defined instead of
likely compiler-synthesized (which is a progression). There still
are false positives for ASCII lowercase letters, which cause the
`stringPrefix` to be empty (as before).

Since the new implementation is imperative anyway, at least I made
it not allocate anything but the result `String` in the common
case where the result does not contain any `.`.

1 files changed, 41 insertions, 5 deletions

diff --git a/test/junit/scala/collection/TraversableLikeTest.scala b/test/junit/scala/collection/TraversableLikeTest.scala
index 8588956016..f703abf3e4 100644
--- a/test/junit/scala/collection/TraversableLikeTest.scala
+++ b/test/junit/scala/collection/TraversableLikeTest.scala
@@ -5,29 +5,65 @@ import org.junit.Test
 import org.junit.runner.RunWith
 import org.junit.runners.JUnit4
 
+object TraversableLikeTest {
+  abstract class FakeIndexedSeq[A] extends IndexedSeq[A] {
+    def apply(i: Int): A = ???
+    def length: Int = 0
+  }
+}
+
 @RunWith(classOf[JUnit4])
 class TraversableLikeTest {
+  import TraversableLikeTest._
+
   // For test_SI9019; out here because as of test writing, putting this in a method would crash compiler
   class Baz[@specialized(Int) A]() extends IndexedSeq[A] {
     def apply(i: Int) = ???
     def length: Int = 0
   }
-  
+
   @Test
   def test_SI9019 {
     object Foo {
       def mkBar = () => {
-        class Bar extends IndexedSeq[Int] {
-          def apply(i: Int) = ???
-          def length: Int = 0
-        }
+        class Bar extends FakeIndexedSeq[Int]
         new Bar
       }
+
+      def mkFalsePositiveToSyntheticTest = () => {
+        /* A class whose name tarts with an ASCII lowercase letter.
+         * It will be a false positive to the synthetic-part test.
+         */
+        class falsePositive extends FakeIndexedSeq[Int]
+        new falsePositive
+      }
+
+      def mkFrench = () => {
+        // For non-French speakers, this means "strange class name"
+        class ÉtrangeNomDeClasse extends FakeIndexedSeq[Int]
+        new ÉtrangeNomDeClasse
+      }
+
+      def mkFrenchLowercase = () => {
+        class étrangeNomDeClasseMinuscules extends FakeIndexedSeq[Int]
+        new étrangeNomDeClasseMinuscules
+      }
     }
+
     val bar = Foo.mkBar()
     assertEquals("Bar", bar.stringPrefix)  // Previously would have been outermost class, TraversableLikeTest
 
     val baz = new Baz[Int]()
     assertEquals("TraversableLikeTest.Baz", baz.stringPrefix)  // Make sure we don't see specialization $mcI$sp stuff
+
+    // The false positive unfortunately produces an empty stringPrefix
+    val falsePositive = Foo.mkFalsePositiveToSyntheticTest()
+    assertEquals("", falsePositive.stringPrefix)
+
+    val french = Foo.mkFrench()
+    assertEquals("ÉtrangeNomDeClasse", french.stringPrefix)
+
+    val frenchLowercase = Foo.mkFrenchLowercase()
+    assertEquals("étrangeNomDeClasseMinuscules", frenchLowercase.stringPrefix)
   }
 }