Prioritize build targets with supported Scala versions.

Previously, if two build targets in the repo were eligible for a source
file, Metals would sometimes prioritize a target with an unsupported
Scala version over a target with a supported Scala version. This
resulted in completions/hover/parameterHints not to work while editing
cross-built sources.

This PR introduces a change to the logic where we select a build target
for a given source file. Now we sort the elibible candidates by two
criteria: the Scala version and the platform (JVM/JS/Native). This
change takes the first step towards
scalameta/metals-feature-requests#13, which is
about making this logic configurable.

metals/src/main/scala/scala/meta/internal/metals/BuildTargets.scala

@@ -7,6 +7,7 @@ import ch.epfl.scala.bsp4j.BuildTarget
 import ch.epfl.scala.bsp4j.BuildTargetIdentifier
 import ch.epfl.scala.bsp4j.ScalacOptionsItem
 import ch.epfl.scala.bsp4j.ScalacOptionsResult
+import ch.epfl.scala.bsp4j.ScalaBuildTarget
 import ch.epfl.scala.bsp4j.WorkspaceBuildTargetsResult
 import java.util.concurrent.ConcurrentLinkedQueue
 import scala.annotation.tailrec
@@ -175,6 +176,10 @@ final class BuildTargets() {
       buildTarget: BuildTargetIdentifier
   ): Option[BuildTarget] =
     buildTargetInfo.get(buildTarget)
+  def scalaInfo(
+      buildTarget: BuildTargetIdentifier
+  ): Option[ScalaBuildTarget] =
+    info(buildTarget).flatMap(_.asScalaBuildTarget)
 
   def scalacOptions(
       buildTarget: BuildTargetIdentifier
@@ -188,11 +193,25 @@ final class BuildTargets() {
       source: AbsolutePath
   ): Option[BuildTargetIdentifier] = {
     val buildTargets = sourceBuildTargets(source)
-    buildTargets // prioritize JVM targets over JS/Native
-      .find(x => scalacOptions(x).exists(_.isJVM))
-      .orElse(buildTargets.headOption)
-      .orElse(tables.flatMap(_.dependencySources.getBuildTarget(source)))
-      .orElse(inferBuildTarget(source))
+    if (buildTargets.isEmpty) {
+      tables
+        .flatMap(_.dependencySources.getBuildTarget(source))
+        .orElse(inferBuildTarget(source))
+    } else {
+      Some(buildTargets.maxBy { t =>
+        var score = 1
+
+        val isSupportedScalaVersion = scalaInfo(t).exists(
+          t => ScalaVersions.isSupportedScalaVersion(t.getScalaVersion())
+        )
+        if (isSupportedScalaVersion) score <<= 2
+
+        val isJVM = scalacOptions(t).exists(_.isJVM)
+        if (isJVM) score <<= 1
+
+        score
+      })
+    }
   }
 
   /**

tests/unit/src/main/scala/tests/QuickBuild.scala

@@ -15,6 +15,7 @@ import java.nio.file.Paths
 import java.security.MessageDigest
 import scala.meta.internal.io.FileIO
 import scala.meta.internal.metals.MetalsEnrichments._
+import scala.meta.internal.metals.ScalaVersions
 import scala.meta.internal.metals.Time
 import scala.meta.internal.metals.Timer
 import scala.meta.internal.metals.{BuildInfo => V}
@@ -89,12 +90,14 @@ case class QuickBuild(
         .resolve(s"scala-$binaryVersion")
         .resolve(s"${testPrefix}classes")
     }
-    val sources = (List(
+    val extraSources =
+      additionalSources.map(relpath => workspace.resolve(relpath).toNIO).toList
+    val sources = extraSources ::: List(
       "src/main/java",
       "src/main/scala",
       s"src/main/scala-$binaryVersion",
       s"src/main/scala-$binaryVersion"
-    ) ++ additionalSources).map(relpath => baseDirectory.resolve(relpath))
+    ).map(relpath => baseDirectory.resolve(relpath))
     val allDependencies = Array(
       s"org.scala-lang:scala-library:$scalaVersion",
       s"org.scala-lang:scala-reflect:$scalaVersion"
@@ -107,7 +110,10 @@ case class QuickBuild(
     )
     val (dependencySources, classpath) =
       allJars.partition(_.getFileName.toString.endsWith("-sources.jar"))
-    val allPlugins = s"org.scalameta:::semanticdb-scalac:${V.semanticdbVersion}" :: compilerPlugins.toList
+    val allPlugins =
+      if (ScalaVersions.isSupportedScalaVersion(scalaVersion))
+        s"org.scalameta:::semanticdb-scalac:${V.semanticdbVersion}" :: compilerPlugins.toList
+      else compilerPlugins.toList
     val pluginDependencies = allPlugins.map(
       plugin =>
         QuickBuild

tests/unit/src/test/scala/tests/BuildTargetsSlowSuite.scala

@@ -0,0 +1,36 @@
+package tests
+
+object BuildTargetsSlowSuite
+    extends BaseSlowSuite("build-targets")
+    with TestHovers {
+
+  testAsync("scala-priority") {
+    for {
+      _ <- server.initialize(
+        s"""/metals.json
+           |{
+           |  "a": {
+           |    "scalaVersion": "2.10.6",
+           |    "libraryDependencies": ["com.lihaoyi::sourcecode:0.1.7"],
+           |    "additionalSources": [ "shared/Main.scala" ]
+           |  },
+           |  "b": {
+           |    "scalaVersion": "${BuildInfo.scalaVersion}",
+           |    "libraryDependencies": ["com.lihaoyi::sourcecode:0.1.7"],
+           |    "additionalSources": [ "shared/Main.scala" ]
+           |  }
+           |}
+        """.stripMargin
+      )
+      // Assert that a supported Scala version target is picked over 2.10.
+      _ <- server.assertHover(
+        "shared/Main.scala",
+        """
+          |object Main {
+          |  sourcecode.Line(1).val@@ue
+          |}""".stripMargin,
+        """val value: Int""".hover
+      )
+    } yield ()
+  }
+}

tests/unit/src/test/scala/tests/DiagnosticsSlowSuite.scala

@@ -304,7 +304,7 @@ object DiagnosticsSlowSuite extends BaseSlowSuite("diagnostics") {
           |/metals.json
           |{
           |  "a": { 
-          |    "additionalSources" : [ "weird/path/A.scala" ] 
+          |    "additionalSources" : [ "a/weird/path/A.scala" ]
           |  }
           |}
           |/a/weird/path/A.scala

tests/unit/src/test/scala/tests/FileWatcherSlowSuite.scala

@@ -24,7 +24,7 @@ object FileWatcherSlowSuite extends BaseSlowSuite("file-watcher") {
         """
           |/metals.json
           |{
-          |  "a": { "additionalSources" : ["weird/path/d/D.scala", "non/existent/one/e/E.scala"] },
+          |  "a": { "additionalSources" : ["a/weird/path/d/D.scala", "non/existent/one/e/E.scala"] },
           |  "b": { },
           |  "c": { "dependsOn": [ "a" ] }
           |}