add crossLibrary

It's a thin layer around custom, but specifically created for cross library building.

Ref #2

src/main/scala/sbt/ProjectMatrix.scala

@@ -62,14 +62,15 @@ sealed trait ProjectMatrix extends CompositeProject {
 
   def jvmPlatform(scalaVersions: Seq[String]): ProjectMatrix
   def jvmPlatform(scalaVersions: Seq[String], settings: Seq[Setting[_]]): ProjectMatrix
-
   def jvm: ProjectFinder
 
   def jsPlatform(scalaVersions: Seq[String]): ProjectMatrix
   def jsPlatform(scalaVersions: Seq[String], settings: Seq[Setting[_]]): ProjectMatrix
-
   def js: ProjectFinder
 
+  def crossLibrary(scalaVersions: Seq[String], suffix: String, settings: Seq[Setting[_]]): ProjectMatrix
+  def crossLib(suffix: String): ProjectFinder
+
   def projectRefs: Seq[ProjectReference]
 }
 
@@ -203,15 +204,27 @@ object ProjectMatrix {
     override def jvmPlatform(scalaVersions: Seq[String], settings: Seq[Setting[_]]): ProjectMatrix =
       custom(jvmIdSuffix, jvmDirectorySuffix, scalaVersions, { _.settings(settings) })
 
-    override def jsPlatform(scalaVersions: Seq[String]): ProjectMatrix =
-      jsPlatform(scalaVersions, Nil)
-    override def jsPlatform(scalaVersions: Seq[String], settings: Seq[Setting[_]]): ProjectMatrix =
-      custom(jsIdSuffix, jsDirectorySuffix, scalaVersions, { _.settings(settings) })
-
     override def jvm: ProjectFinder = new SuffixBaseProjectFinder(jvmIdSuffix)
 
+    override def jsPlatform(scalaVersions: Seq[String]): ProjectMatrix =
+      jsPlatform(scalaVersions, Nil)
+    
+    override def jsPlatform(scalaVersions: Seq[String], settings: Seq[Setting[_]]): ProjectMatrix =
+      custom(jsIdSuffix, jsDirectorySuffix, scalaVersions, { _.settings(settings) })
+    
     override def js: ProjectFinder = new SuffixBaseProjectFinder(jsIdSuffix)
 
+    override def crossLibrary(scalaVersions: Seq[String], suffix: String, settings: Seq[Setting[_]]): ProjectMatrix =
+      custom(suffix.replaceAllLiterally(".", "_"),
+        "-" + suffix.toLowerCase,
+        scalaVersions,
+        { _.settings(
+          Seq(moduleName := name.value + "_" + suffix.toLowerCase) ++ settings
+        ) })
+
+    override def crossLib(suffix: String): ProjectFinder =
+      new SuffixBaseProjectFinder(suffix.replaceAllLiterally(".", "_"))
+
     override def projectRefs: Seq[ProjectReference] =
       componentProjects map { case p => (p: ProjectReference) }
 

src/sbt-test/projectMatrix/crosslib/build.sbt

@@ -0,0 +1,35 @@
+ThisBuild / organization := "com.example"
+ThisBuild / version := "0.1.0-SNAPSHOT"
+ThisBuild / publishMavenStyle := true
+
+ThisBuild / ivyPaths := {
+  val base = (ThisBuild / baseDirectory).value
+  IvyPaths(base, Some(base / "ivy-cache"))
+}
+publish / skip := true
+
+lazy val core = (projectMatrix in file("core"))
+  .settings(
+    name := "core",
+    ivyPaths := (ThisBuild / ivyPaths).value
+  )
+  .crossLibrary(
+    scalaVersions = Seq("2.12.8", "2.11.12"),
+    suffix = "Config1.2",
+    settings = Seq(
+      libraryDependencies += "com.typesafe" % "config" % "1.2.1"
+    )
+  )
+  .crossLibrary(
+    scalaVersions = Seq("2.12.8"),
+    suffix = "Config1.3",
+    settings = Seq(
+      libraryDependencies += "com.typesafe" % "config" % "1.3.3"
+    )
+  )
+
+// to reference project ref
+lazy val coreConfig1_3 = core.crossLib("Config1.3")("2.12.8")
+  .settings(
+    publishMavenStyle := true
+  )

src/sbt-test/projectMatrix/crosslib/project/plugins.sbt

@@ -0,0 +1,5 @@
+sys.props.get("plugin.version") match {
+  case Some(x) => addSbtPlugin("com.eed3si9n" % "sbt-projectmatrix" % x)
+  case _ => sys.error("""|The system property 'plugin.version' is not defined.
+                         |Specify this property using the scriptedLaunchOpts -D.""".stripMargin)
+}

src/sbt-test/projectMatrix/crosslib/test

@@ -0,0 +1,5 @@
+> publishLocal
+
+$ exists ivy-cache/local/com.example/core_config1.2_2.11/0.1.0-SNAPSHOT/poms/core_config1.2_2.11.pom
+$ exists ivy-cache/local/com.example/core_config1.2_2.12/0.1.0-SNAPSHOT/poms/core_config1.2_2.12.pom
+$ exists ivy-cache/local/com.example/core_config1.3_2.12/0.1.0-SNAPSHOT/poms/core_config1.3_2.12.pom