Resolve Scala SDK independently for each target

This prepares us for changes in `rules_scala` that will allow customizing the Scala version for each target.
In order to achieve that, we can no longer resolve the Scala SDK globally as the maximal version used. We need to use per-target info.
Scala SDK will be still discovered based on the compiler class path.
Now though we will look into an implicit dependency of each target, the `_scalac`.

This change is backward-compatible, as the mentioned data is already available.
It is also forward-compatible with the anticipated cross-build feature of `rules_scala`.
(see: bazelbuild/rules_scala#1290)

The aspect will produce additional data – namely few compiler classpath jars per Scala target.
Perhaps we could review this change in the future to normalize the data back. Now it's not possible, as the data about alternative configurations (here: of scalac) is discarded in the server.

aspects/rules/scala/scala_info.bzl

@@ -42,6 +42,10 @@ def extract_scala_info(target, ctx, output_groups, **kwargs):
     # check of _scala_toolchain is necessary, because SCALA_TOOLCHAIN will always be present
     if hasattr(ctx.rule.attr, "_scala_toolchain"):
         common_scalac_opts = ctx.toolchains[SCALA_TOOLCHAIN].scalacopts
+        if hasattr(ctx.rule.attr, "_scalac"):
+            compiler_classpath = find_scalac_classpath(ctx.rule.attr._scalac.default_runfiles.files.to_list())
+            if compiler_classpath:
+                scala_info["compiler_classpath"] = map(file_location, compiler_classpath)
     else:
         common_scalac_opts = []
     scala_info["scalac_opts"] = common_scalac_opts + getattr(ctx.rule.attr, "scalacopts", [])

server/src/main/kotlin/org/jetbrains/bsp/bazel/server/sync/BazelProjectMapper.kt

@@ -179,29 +179,27 @@ class BazelProjectMapper(
   private fun calculateScalaLibrariesMapper(targetsToImport: Sequence<TargetInfo>): Map<String, List<Library>> {
     val projectLevelScalaSdkLibraries = calculateProjectLevelScalaLibraries()
     val scalaTargets = targetsToImport.filter { it.hasScalaTargetInfo() }.map { it.id }
-    return projectLevelScalaSdkLibraries
-      ?.let { libraries -> scalaTargets.associateWith { libraries } }
-      .orEmpty()
+    return scalaTargets.associateWith {
+      languagePluginsService.scalaLanguagePlugin.scalaSdks[it]?.compilerJars?.mapNotNull {
+        projectLevelScalaSdkLibraries[it]
+      }.orEmpty()
+    }
   }
 
-  private fun calculateProjectLevelScalaLibraries(): List<Library>? {
-    val scalaSdkLibrariesJars = getProjectLevelScalaSdkLibrariesJars()
-    return if (scalaSdkLibrariesJars.isNotEmpty()) {
-      scalaSdkLibrariesJars.map {
-        Library(
-          label = Paths.get(it).name,
-          outputs = setOf(it),
-          sources = emptySet(),
-          dependencies = emptyList()
-        )
-      }
-    } else null
-  }
+  private fun calculateProjectLevelScalaLibraries(): Map<URI, Library> =
+    getProjectLevelScalaSdkLibrariesJars().associateWith {
+      Library(
+        label = Paths.get(it).name,
+        outputs = setOf(it),
+        sources = emptySet(),
+        dependencies = emptyList()
+      )
+    }
 
   private fun getProjectLevelScalaSdkLibrariesJars(): Set<URI> =
-    languagePluginsService.scalaLanguagePlugin.scalaSdk
-      ?.compilerJars
-      ?.toSet().orEmpty()
+    languagePluginsService.scalaLanguagePlugin.scalaSdks.values.toSet().flatMap {
+      it.compilerJars
+    }.toSet()
 
   private fun calculateAndroidLibrariesMapper(targetsToImport: Sequence<TargetInfo>): Map<String, List<Library>> =
     targetsToImport.mapNotNull { target ->

server/src/main/kotlin/org/jetbrains/bsp/bazel/server/sync/languages/scala/ScalaLanguagePlugin.kt

@@ -26,25 +26,25 @@ class ScalaLanguagePlugin(
     private val bazelPathsResolver: BazelPathsResolver
 ) : LanguagePlugin<ScalaModule>() {
 
-    var scalaSdk: ScalaSdk? = null
+    var scalaSdks: Map<String, ScalaSdk> = emptyMap()
 
     override fun prepareSync(targets: Sequence<BspTargetInfo.TargetInfo>) {
-        scalaSdk = ScalaSdkResolver(bazelPathsResolver).resolve(targets)
+        scalaSdks = targets.associateBy(
+            { it.id },
+            ScalaSdkResolver(bazelPathsResolver)::resolveSdk
+        ).filterValues{ it != null }.mapValues{ it.value!! }
     }
 
     override fun resolveModule(targetInfo: BspTargetInfo.TargetInfo): ScalaModule? {
         if (!targetInfo.hasScalaTargetInfo()) {
             return null
         }
         val scalaTargetInfo = targetInfo.scalaTargetInfo
-        val sdk = getScalaSdkOrThrow()
+        val sdk = scalaSdks[targetInfo.id] ?: return null
         val scalacOpts = scalaTargetInfo.scalacOptsList
         return ScalaModule(sdk, scalacOpts, javaLanguagePlugin.resolveModule(targetInfo))
     }
 
-    private fun getScalaSdkOrThrow(): ScalaSdk =
-        scalaSdk ?: throw RuntimeException("Failed to resolve Scala SDK for project")
-
     override fun dependencySources(
         targetInfo: BspTargetInfo.TargetInfo,
         dependencyGraph: DependencyGraph

server/src/main/kotlin/org/jetbrains/bsp/bazel/server/sync/languages/scala/ScalaSdkResolver.kt

@@ -15,13 +15,13 @@ class ScalaSdkResolver(private val bazelPathsResolver: BazelPathsResolver) {
             .sortedWith(SCALA_VERSION_COMPARATOR)
             .lastOrNull()
 
-    private fun resolveSdk(targetInfo: BspTargetInfo.TargetInfo): ScalaSdk? {
-        if (!targetInfo.hasScalaToolchainInfo()) {
+    fun resolveSdk(targetInfo: BspTargetInfo.TargetInfo): ScalaSdk? {
+        if (!targetInfo.hasScalaTargetInfo()) {
             return null
         }
-        val scalaToolchain = targetInfo.scalaToolchainInfo
+        val scalaTarget = targetInfo.scalaTargetInfo
         val compilerJars =
-            bazelPathsResolver.resolvePaths(scalaToolchain.compilerClasspathList).sorted()
+            bazelPathsResolver.resolvePaths(scalaTarget.compilerClasspathList).sorted()
         val maybeVersions = compilerJars.mapNotNull(::extractVersion)
         if (maybeVersions.none()) {
             return null

server/src/main/kotlin/org/jetbrains/bsp/bazel/server/sync/proto/bsp_target_info.proto

@@ -49,6 +49,7 @@ message JavaRuntimeInfo {
 
 message ScalaTargetInfo {
   repeated string scalac_opts = 1;
+  repeated FileLocation compiler_classpath = 2;
 }
 
 message ScalaToolchainInfo {