Cleanup for experimental SIP-62 implementation

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1804,7 +1804,7 @@ object desugar {
     /** Create tree for for-comprehension `<for (enums) do body>` or
      *   `<for (enums) yield body>` where mapName and flatMapName are chosen
      *  corresponding to whether this is a for-do or a for-yield.
-     *  The creation performs the following rewrite rules:
+     *  If betterFors are enabled, the creation performs the following rewrite rules:
      *
      *  1.
      *
@@ -1872,6 +1872,46 @@ object desugar {
      *    (Where empty for-comprehensions are excluded by the parser)
      *
      *   If the aliases are not followed by a guard, otherwise an error.
+     * 
+     * With betterFors disabled, the translation is as follows:
+     * 
+     * 1.
+     *
+     *    for (P <- G) E   ==>   G.foreach (P => E)
+     *
+     *     Here and in the following (P => E) is interpreted as the function (P => E)
+     *     if P is a variable pattern and as the partial function { case P => E } otherwise.
+     *
+     *  2.
+     *
+     *    for (P <- G) yield E  ==>  G.map (P => E)
+     *
+     *  3.
+     *
+     *    for (P_1 <- G_1; P_2 <- G_2; ...) ...
+     *      ==>
+     *    G_1.flatMap (P_1 => for (P_2 <- G_2; ...) ...)
+     *
+     *  4.
+     *
+     *    for (P <- G; E; ...) ...
+     *      =>
+     *    for (P <- G.filter (P => E); ...) ...
+     *
+     *  5. For any N:
+     *
+     *    for (P_1 <- G; P_2 = E_2; val P_N = E_N; ...)
+     *      ==>
+     *    for (TupleN(P_1, P_2, ... P_N) <-
+     *      for (x_1 @ P_1 <- G) yield {
+     *        val x_2 @ P_2 = E_2
+     *        ...
+     *        val x_N & P_N = E_N
+     *        TupleN(x_1, ..., x_N)
+     *      } ...)
+     *
+     *    If any of the P_i are variable patterns, the corresponding `x_i @ P_i` is not generated
+     *    and the variable constituting P_i is used instead of x_i
      *
      *  @param mapName      The name to be used for maps (either map or foreach)
      *  @param flatMapName  The name to be used for flatMaps (either flatMap or foreach)

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -68,7 +68,8 @@ object Feature:
     (into, "Allow into modifier on parameter types"),
     (namedTuples, "Allow named tuples"),
     (modularity, "Enable experimental modularity features"),
-    (betterMatchTypeExtractors, "Enable better match type extractors")
+    (betterMatchTypeExtractors, "Enable better match type extractors"),
+    (betterFors, "Enable improvements in `for` comprehensions")
   )
 
   // legacy language features from Scala 2 that are no longer supported.

library/src/scala/runtime/stdLibPatches/language.scala

@@ -135,7 +135,7 @@ object language:
 
     /** Experimental support for improvements in `for` comprehensions
      *
-     * @see [[https://dotty.epfl.ch/docs/reference/experimental/better-fors]]
+     * @see [[https://github.com/scala/improvement-proposals/pull/79]]
      */
     @compileTimeOnly("`betterFors` can only be used at compile time in import statements")
     object betterFors

project/MiMaFilters.scala

@@ -8,7 +8,8 @@ object MiMaFilters {
     val ForwardsBreakingChanges: Map[String, Seq[ProblemFilter]] = Map(
       // Additions that require a new minor version of the library
       Build.mimaPreviousDottyVersion -> Seq(
-
+        ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.stdLibPatches.language#experimental.betterFors"),
+        ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$experimental$betterFors$"),
       ),
 
       // Additions since last LTS

tests/run/fors.scala

@@ -6,6 +6,7 @@
 
 import annotation.tailrec
 
+@scala.annotation.experimental
 object Test extends App {
   val xs = List(1, 2, 3)
   val ys = List(Symbol("a"), Symbol("b"), Symbol("c"))