Implement Scala 2.13-3.0 sandwich

Fixes https://github.com/sbt/sbt/issues/5369
Ref https://contributors.scala-lang.org/t/roadmap-for-the-tasty-reader-for-scala-2/4231

This implements support for inter-project dependencies between Scala 2.13 and Dotty, and vice versa. Scala 2.13 depending on Dotty would require 2.13.4 and above.

main/src/main/scala/sbt/Defaults.scala

@@ -3450,14 +3450,38 @@ object Classpaths {
   def deliverPattern(outputPath: File): String =
     (outputPath / "[artifact]-[revision](-[classifier]).[ext]").absolutePath
 
+  private[sbt] def isScala2Scala3Sandwich(sbv1: String, sbv2: String): Boolean = {
+    def compare(a: String, b: String): Boolean =
+      a == "2.13" && (b.startsWith("0.") || b.startsWith("3.0"))
+    compare(sbv1, sbv2) || compare(sbv2, sbv1)
+  }
+
   def projectDependenciesTask: Initialize[Task[Seq[ModuleID]]] =
     Def.task {
+      val sbv = scalaBinaryVersion.value
       val ref = thisProjectRef.value
       val data = settingsData.value
       val deps = buildDependencies.value
       deps.classpath(ref) flatMap { dep =>
-        (projectID in dep.project) get data map {
+        val depProjIdOpt = (dep.project / projectID).get(data)
-          _.withConfigurations(dep.configuration).withExplicitArtifacts(Vector.empty)
+        val depSVOpt = (dep.project / scalaVersion).get(data)
+        val depSBVOpt = (dep.project / scalaBinaryVersion).get(data)
+        val depCrossOpt = (dep.project / crossVersion).get(data)
+        (depProjIdOpt, depSVOpt, depSBVOpt, depCrossOpt) match {
+          case (Some(depProjId), Some(depSV), Some(depSBV), Some(depCross)) =>
+            if (sbv == depSBV || depCross != CrossVersion.binary)
+              Some(
+                depProjId.withConfigurations(dep.configuration).withExplicitArtifacts(Vector.empty)
+              )
+            else if (isScala2Scala3Sandwich(sbv, depSBV) && depCross == CrossVersion.binary)
+              Some(
+                depProjId
+                  .withCrossVersion(CrossVersion.constant(depSBV))
+                  .withConfigurations(dep.configuration)
+                  .withExplicitArtifacts(Vector.empty)
+              )
+            else sys.error(s"scalaBinaryVersion mismatch: expected $sbv but found ${depSBV}")
+          case _ => None
         }
       }
     }

sbt/src/sbt-test/plugins/dotty-compiler-plugin/build.sbt

@@ -1,4 +1,4 @@
-lazy val dottyVersion = "0.14.0-RC1"
+ThisBuild / scalaVersion := "0.26.0-RC1"
 
 lazy val plugin = project
   .in(file("plugin"))
@@ -6,7 +6,6 @@ lazy val plugin = project
     name := "dividezero",
     version := "0.0.1",
     organization := "ch.epfl.lamp",
-    scalaVersion := dottyVersion,
 
     scalacOptions ++= Seq(
       "-language:implicitConversions"

sbt/src/sbt-test/plugins/dotty-compiler-plugin/project/plugins.sbt

@@ -1 +1 @@
-addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.3.1")
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.4.1")

sbt/src/sbt-test/plugins/dotty-sandwich/bar-app/D.scala

@@ -0,0 +1,5 @@
+package example
+
+object D {
+  val x = C.x
+}

sbt/src/sbt-test/plugins/dotty-sandwich/bar-core/C.scala

@@ -0,0 +1,5 @@
+package example
+
+object C {
+  val x = 1
+}

sbt/src/sbt-test/plugins/dotty-sandwich/build.sbt

@@ -0,0 +1,24 @@
+ThisBuild / scalaVersion := "0.23.0"
+
+ThisBuild / resolvers += "scala-integration" at "https://scala-ci.typesafe.com/artifactory/scala-integration/"
+// TODO use 2.13.4 when it's out
+lazy val scala213 = "2.13.4-bin-aeee8f0"
+
+lazy val root = (project in file("."))
+  .aggregate(fooApp, fooCore, barApp, barCore)
+
+lazy val fooApp = (project in file("foo-app"))
+  .dependsOn(fooCore)
+
+lazy val fooCore = (project in file("foo-core"))
+  .settings(
+    scalaVersion := scala213
+  )
+
+lazy val barApp = (project in file("bar-app"))
+  .dependsOn(barCore)
+  .settings(
+    scalaVersion := scala213
+  )
+
+lazy val barCore = (project in file("bar-core"))

sbt/src/sbt-test/plugins/dotty-sandwich/foo-app/B.scala

@@ -0,0 +1,5 @@
+package example
+
+object B {
+  val x = A.x
+}

sbt/src/sbt-test/plugins/dotty-sandwich/foo-core/A.scala

@@ -0,0 +1,5 @@
+package example
+
+object A {
+  val x = 1
+}

sbt/src/sbt-test/plugins/dotty-sandwich/project/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.4.1")

sbt/src/sbt-test/plugins/dotty-sandwich/test

@@ -0,0 +1,3 @@
+> fooApp/compile
+
+> barApp/compile

sbt/src/sbt-test/plugins/dotty/build.sbt

@@ -1 +1 @@
-scalaVersion := "0.14.0-RC1"
+ThisBuild / scalaVersion := "0.26.0-RC1"

sbt/src/sbt-test/plugins/dotty/project/plugins.sbt

@@ -1 +1 @@
-addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.3.1")
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.4.1")