Split forwards and backwards incompatibilities

This makes it evident which changes are backwards incompatible and which
are froward incompatible. Previously it was not clear which changes were
backwards incompatible and which were forwards incompatible.

Author: nicolasstucki

project/Build.scala

@@ -2163,7 +2163,8 @@ object Build {
             (Compile/doc/target).value
           },
           commonMiMaSettings,
-          mimaBinaryIssueFilters ++= MiMaFilters.Library
+          mimaBackwardIssueFilters := MiMaFilters.LibraryBackwards,
+          mimaForwardIssueFilters := MiMaFilters.LibraryForward,
         )
       } else base
     }

project/MiMaFilters.scala

@@ -2,16 +2,24 @@
 import com.typesafe.tools.mima.core._
 
 object MiMaFilters {
-  val Library: Seq[ProblemFilter] = Seq(
-    // This language feature was in 3.4.0-RC1 but will be removed in 3.4.0-RC2
-    ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.stdLibPatches.language#deprecated.ascriptionVarargsUnpacking"),
-    ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$deprecated$ascriptionVarargsUnpacking$"),
-
-    // This language feature is not in 3.4.0-RC1 but will be added in 3.4.0-RC2
-    ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.stdLibPatches.language.3.5-migration"),
-    ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.stdLibPatches.language.3.5"),
-    ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$3$u002E5$"),
-    ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$3$u002E5$minusmigration$"),
+  val LibraryBackwards: Map[String, Seq[ProblemFilter]] = Map(
+    // In general we should never have backwards incompatible changes in the library.
+    // Only exceptional cases should be added here.
+    Build.previousDottyVersion -> Seq(
+      // This language feature was in 3.4.0-RC1 but will be removed in 3.4.0-RC2
+      ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.stdLibPatches.language#deprecated.ascriptionVarargsUnpacking"),
+      ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$deprecated$ascriptionVarargsUnpacking$"),
+    )
+  )
+  val LibraryForward: Map[String, Seq[ProblemFilter]] = Map(
+    // Additions that require a new minor version of the library
+    Build.previousDottyVersion -> Seq(
+      // This language feature is not in 3.4.0-RC1 but will be added in 3.4.0-RC2
+      ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.stdLibPatches.language.3.5-migration"),
+      ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.stdLibPatches.language.3.5"),
+      ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$3$u002E5$"),
+      ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$3$u002E5$minusmigration$"),
+    )
   )
   val TastyCore: Seq[ProblemFilter] = Seq(
   )