Support cross-build for external project ref

main/src/main/scala/sbt/Cross.scala

@@ -118,12 +118,11 @@ object Cross {
   )(command: String): (Seq[ProjectRef], String) = {
     import extracted._
     import DefaultParsers._
-    val parser = (OpOrID <~ charClass(_ == '/', "/")) ~ any.* map {
-      case seg1 ~ cmd => (seg1, cmd.mkString)
-    }
+    val parser = ((('{' ~> URIClass <~ '}').? ~ OpOrID <~ '/') ~ any.*.string)
+      .map { case uri ~ seg1 ~ cmd => (uri, seg1, cmd) }
     Parser.parse(command, parser) match {
-      case Right((seg1, cmd)) =>
-        structure.allProjectRefs.find(_.project == seg1) match {
+      case Right((uri, seg1, cmd)) =>
+        structure.allProjectRefs.find(p => uri.contains(p.build.toString) && seg1 == p.project) match {
           case Some(proj) => (Seq(proj), cmd)
           case _          => (resolveAggregates(extracted), command)
         }
@@ -187,9 +186,10 @@ object Cross {
               keys.toSeq.flatMap { k =>
                 project(k).withFilter(projects.contains).flatMap { p =>
                   if (p == extracted.currentRef || !projects.contains(extracted.currentRef)) {
-                    val parts = project(k).map(_.project) ++ k.scope.config.toOption.map {
-                      case ConfigKey(n) => n.head.toUpper + n.tail
-                    } ++ k.scope.task.toOption.map(_.label) ++ Some(k.key.label)
+                    val parts = project(k).map(p => s"{${p.build}}${p.project}") ++
+                      k.scope.config.toOption.map(c => c.name.capitalize) ++
+                      k.scope.task.toOption.map(_.label) ++
+                      Some(k.key.label)
                     Some(v -> parts.mkString("", "/", fullArgs))
                   } else None
                 }
@@ -257,7 +257,7 @@ object Cross {
             val (aggs, aggCommand) = parseSlashCommand(Project.extract(state))(rawCmd)
             aggs
               .intersect(affectedRefs)
-              .map({ case ProjectRef(_, proj) => s"$proj/$aggCommand" })
+              .map(p => s"{${p.build}}${p.project}/$aggCommand")
               .mkString("all ", " ", "")
           }
         }

sbt-app/src/sbt-test/actions/cross-multiproject/build.sbt

@@ -5,7 +5,7 @@ ThisBuild / crossScalaVersions := Seq(scala212, scala213)
 ThisBuild / scalaVersion       := scala212
 
 lazy val rootProj = (project in file("."))
-  .aggregate(libProj, fooPlugin)
+  .aggregate(libProj, fooPlugin, externalProj)
   .settings(
     crossScalaVersions := Nil,
     addCommandAlias("build", "compile")
@@ -27,3 +27,5 @@ lazy val extrasProj = (project in file("extras"))
   .settings(
     name := "foo-extras",
   )
+
+lazy val externalProj = ProjectRef(file("ref"), "external")

sbt-app/src/sbt-test/actions/cross-multiproject/ref/build.sbt

@@ -0,0 +1,4 @@
+lazy val external = (project in file("."))
+  .settings(
+    scalaVersion := "2.12.18"
+  )

sbt-app/src/sbt-test/actions/cross-multiproject/ref/src/main/scala/C.scala

@@ -0,0 +1,5 @@
+package ref.sbt
+
+object Ref {
+
+}

sbt-app/src/sbt-test/actions/cross-multiproject/test

@@ -18,6 +18,8 @@ $ exists lib/target/scala-2.12
 -$ exists lib/target/scala-2.13
 $ exists sbt-foo/target/scala-2.12
 -$ exists sbt-foo/target/scala-2.13
+$ exists ref/target/scala-2.12
+-$ exists ref/target/scala-2.13
 
 # Test legacy cross build with command support
 # > clean
@@ -39,8 +41,11 @@ $ exists extras/target/scala-2.12
 > ++ 2.13.12 -v compile
 $ exists lib/target/scala-2.13
 -$ exists lib/target/scala-2.12
-# -$ exists sbt-foo/target/scala-2.12
+# sbt-foo and ref do not cross-build 2.13
+-$ exists sbt-foo/target/scala-2.12
 -$ exists sbt-foo/target/scala-2.13
+-$ exists ref/target/scala-2.12
+-$ exists ref/target/scala-2.13
 
 # test wildcard switching (2.12)
 > clean
@@ -49,14 +54,19 @@ $ exists lib/target/scala-2.12
 -$ exists lib/target/scala-2.13
 $ exists sbt-foo/target/scala-2.12
 -$ exists sbt-foo/target/scala-2.13
+$ exists ref/target/scala-2.12
+-$ exists ref/target/scala-2.13
 
 # test wildcard switching (2.13)
 > clean
 > ++ 2.13.x -v compile
 $ exists lib/target/scala-2.13
 -$ exists lib/target/scala-2.12
-# -$ exists sbt-foo/target/scala-2.12
+# sbt-foo and ref do not cross-build 2.13
+-$ exists sbt-foo/target/scala-2.12
 -$ exists sbt-foo/target/scala-2.13
+-$ exists ref/target/scala-2.12
+-$ exists ref/target/scala-2.13
 
 # test wildcard switching (no matches)
 -> ++ 3.*