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 one of a dep providers of a Scala toolchain, namely the `scala_compile_classpath` – a canonical place to put all compile-related jars.

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.

aspects/rules/scala/scala_info.bzl

@@ -1,3 +1,4 @@
+load("@io_bazel_rules_scala//scala:providers.bzl", "DepsInfo")
 load("//aspects:utils/utils.bzl", "create_proto", "file_location", "is_external", "map", "update_sync_output_groups")
 
 def find_scalac_classpath(runfiles):
@@ -43,8 +44,27 @@ def extract_scala_info(target, ctx, output_groups, **kwargs):
     if hasattr(ctx.rule.attr, "_scala_toolchain"):
         toolchain = ctx.toolchains[SCALA_TOOLCHAIN]
         common_scalac_opts = toolchain.scalacopts
+        dep_providers = toolchain.dep_providers
+        compiler_classpath = _extract_compiler_classpath(dep_providers)
+        if compiler_classpath:
+            scala_info["compiler_classpath"] = compiler_classpath
     else:
         common_scalac_opts = []
     scala_info["scalac_opts"] = common_scalac_opts + getattr(ctx.rule.attr, "scalacopts", [])
 
     return create_proto(target, ctx, struct(**scala_info), "scala_target_info"), None
+
+def _extract_compiler_classpath(dep_providers):
+    for dep_provider in dep_providers:
+        if DepsInfo not in dep_provider:
+            continue
+        deps_info = dep_provider[DepsInfo]
+        if deps_info.deps_id != "scala_compile_classpath":
+            continue
+
+        compile_jars = []
+        for dep in deps_info.deps:
+            compile_jars.extend(dep[JavaInfo].compile_jars.to_list())
+        classpath = find_scalac_classpath(compile_jars)
+        return map(file_location, classpath)
+    return []

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

@@ -28,22 +28,20 @@ class ScalaLanguagePlugin(
 
     var scalaSdk: ScalaSdk? = null
 
-    override fun prepareSync(targets: Sequence<BspTargetInfo.TargetInfo>) {
-        scalaSdk = ScalaSdkResolver(bazelPathsResolver).resolve(targets)
-    }
+    private val scalaSdkResolver = ScalaSdkResolver(bazelPathsResolver)
 
     override fun resolveModule(targetInfo: BspTargetInfo.TargetInfo): ScalaModule? {
         if (!targetInfo.hasScalaTargetInfo()) {
             return null
         }
         val scalaTargetInfo = targetInfo.scalaTargetInfo
-        val sdk = getScalaSdkOrThrow()
+        val sdk = getScalaSdkOrThrow(targetInfo)
         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")
+    private fun getScalaSdkOrThrow(targetInfo: BspTargetInfo.TargetInfo): ScalaSdk =
+        scalaSdkResolver.resolveSdk(targetInfo) ?: throw RuntimeException("Failed to resolve Scala SDK for target")
 
     override fun dependencySources(
         targetInfo: BspTargetInfo.TargetInfo,

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

@@ -8,20 +8,15 @@ import kotlin.math.min
 
 class ScalaSdkResolver(private val bazelPathsResolver: BazelPathsResolver) {
 
-    fun resolve(targets: Sequence<BspTargetInfo.TargetInfo>): ScalaSdk? =
-        targets
-            .mapNotNull(::resolveSdk)
-            .distinct()
-            .sortedWith(SCALA_VERSION_COMPARATOR)
-            .lastOrNull()
+    fun resolve(targets: Sequence<BspTargetInfo.TargetInfo>): ScalaSdk? = null
 
-    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
@@ -58,6 +53,6 @@ class ScalaSdkResolver(private val bazelPathsResolver: BazelPathsResolver) {
             0
         }
         private val VERSION_PATTERN =
-            Pattern.compile("(?:processed_)?scala3?-(?:library|compiler|reflect)(?:_3)?-([.\\d]+)\\.jar")
+            Pattern.compile("(?:processed_)?scala3?-(?:library|compiler|reflect)(?:_3)?-([.\\d]+)(?:-stamped)?\\.jar")
     }
 }

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 {