Stabilize order of annotations in the class file

[retronym]

Regressed in #6846, which added support for encoding repeated
annotations.

Test failure before replacing groupBy with LinkedHashMap:

$ sbt junit/testOnly scala.tools.nsc.DeterminismTest
...
java.lang.AssertionError: assertion failed: Difference detected between recompiling List(b.scala, Annot1.java) Run:
jardiff -r /var/folders/tz/p8vd07wn7wxck3b9v54grlzw0000gp/T/reference814657788418452571 /var/folders/tz/p8vd07wn7wxck3b9v54grlzw0000gp/T/recompileOutput4882243280168823330

$ jardiff -r /var/folders/tz/p8vd07wn7wxck3b9v54grlzw0000gp/T/reference814657788418452571 /var/folders/tz/p8vd07wn7wxck3b9v54grlzw0000gp/T/recompileOutput4882243280168823330
diff --git a/Test.class.asm b/Test.class.asm
index 98bfd80..a056f9a 100644
--- a/Test.class.asm
+++ b/Test.class.asm
@@ -4,10 +4,10 @@

   // compiled from: b.scala

-  @LAnnot2;(value=java.lang.Object.class)
-
   @LAnnot1;(value="foo")

+  @LAnnot2;(value=java.lang.Object.class)
+
   @Lscala/reflect/ScalaSignature;(bytes="\u0006\u0001u1AAA\u0002\u0001\r!)Q\u0002\u0001C\u0001\u001d\u0009!A+Z:u\u0015\u0005!\u0011a\u0002\u001ff[B$\u0018PP\u0002\u0001'\u0009\u0001q\u0001\u0005\u0002\u0009\u00175\u0009\u0011BC\u0001\u000b\u0003\u0015\u00198-\u00197b\u0013\u0009a\u0011B\u0001\u0004B]f\u0014VMZ\u0001\u0007y%t\u0017\u000e\u001e \u0015\u0003=\u0001\"\u0001\u0005\u0001\u000e\u0003\rAC\u0001\u0001\n\u0016-A\u0011\u0001cE\u0005\u0003)\r\u0011a!\u00118o_R\u0014\u0014!\u0002<bYV,7%A\u0004)\u0009\u0001ARc\u0007\u0009\u0003!eI!AG\u0002\u0003\r\u0005sgn\u001c;2C\u0005a\u0012a\u00014p_\u0002")

[retronym]

FTR groupBy was the culprit here as it doesn't preserve the original iteration order.

[He-Pin]

hm, So call groupBy on a List Will not gives me a ListMap?

[retronym]

Nope, it gives back a HashMap currently:

scala/src/library/scala/collection/Iterable.scala

Lines 538 to 568 in 2d9e6ac

	/** Partitions this $coll into a map of ${coll}s according to some discriminator function.
	*
	* $willForceEvaluation
	*
	* @param f the discriminator function.
	* @tparam K the type of keys returned by the discriminator function.
	* @return A map from keys to ${coll}s such that the following invariant holds:
	* {{{
	* (xs groupBy f)(k) = xs filter (x => f(x) == k)
	* }}}
	* That is, every key `k` is bound to a $coll of those elements `x`
	* for which `f(x)` equals `k`.
	*
	*/
	def groupBy[K](f: A => K): immutable.Map[K, C] = {
	val m = mutable.Map.empty[K, Builder[A, C]]
	val it = iterator
	while (it.hasNext) {
	val elem = it.next()
	val key = f(elem)
	val bldr = m.getOrElseUpdate(key, newSpecificBuilder)
	bldr += elem
	}
	var result = immutable.HashMap.empty[K, C]
	val mapIt = m.iterator
	while (mapIt.hasNext) {
	val (k, v) = mapIt.next()
	result = result.updated(k, v.result())
	}
	result
	}

[He-Pin]

I just looked into the code too, I think that should change. The groupBy method on LinearSeq should return a ListMap, but which is slow.