improvement: Use sbt bridge defined by the user

This is needed to fully compile Dotty right now.

backend/src/main/scala/sbt/internal/inc/BloopComponentCompiler.scala

@@ -117,7 +117,13 @@ object BloopComponentCompiler {
     private def compiledBridge(bridgeSources: ModuleID, scalaInstance: ScalaInstance): File = {
       val raw = new RawCompiler(scalaInstance, ClasspathOptionsUtil.auto, logger)
       val bridgeJarsOpt = scalaInstance match {
-        case b: _root_.bloop.ScalaInstance => b.bridgeJarsOpt
+        case b: _root_.bloop.ScalaInstance =>
+          b.bridgeJarsOpt.foreach { jars =>
+            logger.debug(s"Using user defined bridge jars for Scala ${jars.mkString(",")}")(
+              DebugFilter.Compilation
+            )
+          }
+          b.bridgeJarsOpt
         case _ => None
       }
       val zinc = new BloopComponentCompiler(raw, manager, bridgeSources, bridgeJarsOpt, logger)

integrations/sbt-bloop/src/main/scala/bloop/integrations/sbt/SbtBloop.scala

@@ -1025,6 +1025,7 @@ object BloopDefaults {
               val modules = onlyCompilationModules(allModules, classpath).toList
               if (modules.isEmpty) None else Some(Config.Resolution(modules))
             }
+            val bridge = Keys.scalaCompilerBridgeBinaryJar.value.map(file => List(file.toPath()))
 
             // Force source generators on this task manually
             Keys.managedSources.value
@@ -1034,7 +1035,7 @@ object BloopDefaults {
             val c = Keys.classpathOptions.value
             val java = Config.Java(javacOptions)
             val compileSetup = Config.CompileSetup(compileOrder, c.bootLibrary, c.compiler, c.extra, c.autoBoot, c.filterLibrary)
-            val `scala` = Config.Scala(scalaOrg, scalaName, scalaVersion, scalacOptions, allScalaJars, None, Some(compileSetup), None)
+            val `scala` = Config.Scala(scalaOrg, scalaName, scalaVersion, scalacOptions, allScalaJars, None, Some(compileSetup), bridge)
             val resources = Some(bloopResourcesTask.value)
 
             val sbt = None // Written by `postGenerate` instead

integrations/sbt-bloop/src/sbt-test/sbt-bloop/custom-bridge/build.sbt

@@ -0,0 +1,18 @@
+import bloop.integrations.sbt.BloopDefaults
+
+val foo = project
+  .in(file(".") / "foo")
+  .settings(
+    scalaCompilerBridgeBinaryJar := Some(baseDirectory.value / "fake.jar")
+  )
+
+val checkBloopFiles = taskKey[Unit]("Check bloop file contents")
+checkBloopFiles in ThisBuild := {
+  val bloopDir = Keys.baseDirectory.value./(".bloop")
+  val fooConfig = bloopDir./("foo.json")
+  val config = BloopDefaults.unsafeParseConfig(fooConfig.toPath)
+  val bridgeJars = config.project.scala.get.bridgeJars.map(_.map(_.toString))
+  val expectedJars = Some(List((Keys.baseDirectory.value / "foo" / "fake.jar").toString))
+  assert(bridgeJars == expectedJars)
+
+}

integrations/sbt-bloop/src/sbt-test/sbt-bloop/custom-bridge/project/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("ch.epfl.scala" % "sbt-bloop" % sys.props.apply("plugin.version"))

integrations/sbt-bloop/src/sbt-test/sbt-bloop/custom-bridge/test

@@ -0,0 +1,3 @@
+> foo/bloopGenerate
+> foo/test:bloopGenerate
+> checkBloopFiles