sbt-dotty: don't pollute classpath with dotty-doc dependencies

This lead to runtime errors when running `scalatestTestDotty/test` in
scalatest, because scalatest depends on a different version of flexmark.
I'm not sure why these jars leaked into the runtime classpath (might be
an sbt bug) but it seems better to limit the compiler classpath to the
minimum needed anyway, so we now only add the dotty-doc dependencies
when running the `doc` task.

Author: smarter

project/Build.scala

@@ -294,7 +294,9 @@ object Build {
         dottyCompiler,
         allJars
       )
-    }
+    },
+    // sbt-dotty defines `scalaInstance in doc` so we need to override it manually
+    scalaInstance in doc := scalaInstance.value,
   )
 
   lazy val commonBenchmarkSettings = Seq(

sbt-dotty/src/dotty/tools/sbtplugin/DottyPlugin.scala

@@ -1,6 +1,7 @@
 package dotty.tools.sbtplugin
 
 import sbt._
+import sbt.Def.Initialize
 import sbt.Keys._
 import sbt.librarymanagement.{
   ivy, DependencyResolution, ScalaModuleInfo, SemanticSelector, UpdateConfiguration, UnresolvedWarningConfiguration,
@@ -172,12 +173,13 @@ object DottyPlugin extends AutoPlugin {
       scalaCompilerBridgeBinaryJar := Def.settingDyn {
         if (isDotty.value) Def.task {
           val dottyBridgeArtifacts = fetchArtifactsOf(
+            scalaOrganization.value % "dotty-sbt-bridge" % scalaVersion.value,
             dependencyResolution.value,
             scalaModuleInfo.value,
             updateConfiguration.value,
             (unresolvedWarningConfiguration in update).value,
             streams.value.log,
-            scalaOrganization.value % "dotty-sbt-bridge" % scalaVersion.value).allFiles
+          ).allFiles
           val jars = dottyBridgeArtifacts.filter(art => art.getName.startsWith("dotty-sbt-bridge") && art.getName.endsWith(".jar")).toArray
           if (jars.size == 0)
             throw new MessageOnlyException("No jar found for dotty-sbt-bridge")
@@ -289,39 +291,20 @@ object DottyPlugin extends AutoPlugin {
       },
       // ... instead, we'll fetch the compiler and its dependencies ourselves.
       scalaInstance := Def.taskDyn {
-        if (isDotty.value) Def.task {
-          val updateReport =
-            fetchArtifactsOf(
-              dependencyResolution.value,
-              scalaModuleInfo.value,
-              updateConfiguration.value,
-              (unresolvedWarningConfiguration in update).value,
-              streams.value.log,
-              scalaOrganization.value %% "dotty-doc" % scalaVersion.value)
-          val scalaLibraryJar = getJar(updateReport,
-            "org.scala-lang", "scala-library", revision = AllPassFilter)
-          val dottyLibraryJar = getJar(updateReport,
-            scalaOrganization.value, s"dotty-library_${scalaBinaryVersion.value}", scalaVersion.value)
-          val compilerJar = getJar(updateReport,
-            scalaOrganization.value, s"dotty-compiler_${scalaBinaryVersion.value}", scalaVersion.value)
-          val allJars =
-            getJars(updateReport, AllPassFilter, AllPassFilter, AllPassFilter)
-
-          makeScalaInstance(
-            state.value,
-            scalaVersion.value,
-            scalaLibraryJar,
-            dottyLibraryJar,
-            compilerJar,
-            allJars
-          )
-        }
+        if (isDotty.value)
+          dottyScalaInstanceTask("dotty-compiler")
         else
-          // This dereferences the Initialize graph, but keeps the Task unevaluated,
-          // so its effect gets fired only when isDotty.value evaluates to false. yay monad.
           Def.valueStrict { scalaInstance.taskValue }
       }.value,
 
+      // We need more stuff on the classpath to run the `doc` task.
+      scalaInstance in doc := Def.taskDyn {
+        if (isDotty.value)
+          dottyScalaInstanceTask("dotty-doc")
+        else
+          Def.valueStrict { (scalaInstance in doc).taskValue }
+      }.value,
+
       // Because managedScalaInstance is false, sbt won't add the standard library to our dependencies for us
       libraryDependencies ++= {
         if (isDotty.value && autoScalaLibrary.value)
@@ -355,6 +338,7 @@ object DottyPlugin extends AutoPlugin {
         old
       }
     }.value,
+
     scalacOptions ++= {
       if (isDotty.value) {
         val projectName =
@@ -369,17 +353,17 @@ object DottyPlugin extends AutoPlugin {
       }
       else
         Seq()
-    }
+    },
   ))
 
   /** Fetch artifacts for moduleID */
   def fetchArtifactsOf(
+    moduleID: ModuleID,
     dependencyRes: DependencyResolution,
     scalaInfo: Option[ScalaModuleInfo],
     updateConfig: UpdateConfiguration,
     warningConfig: UnresolvedWarningConfiguration,
-    log: Logger,
-    moduleID: ModuleID): UpdateReport = {
+    log: Logger): UpdateReport = {
     val descriptor = dependencyRes.wrapDependencyInModule(moduleID, scalaInfo)
 
     dependencyRes.update(descriptor, updateConfig, warningConfig, log) match {
@@ -401,13 +385,43 @@ object DottyPlugin extends AutoPlugin {
   }
 
   /** Get the single jar in updateReport that match the given filter.
-    * If zero or more than one jar match, an exception will be thrown. */
+   *  If zero or more than one jar match, an exception will be thrown.
+   */
   def getJar(updateReport: UpdateReport, organization: NameFilter, name: NameFilter, revision: NameFilter): File = {
     val jars = getJars(updateReport, organization, name, revision)
     assert(jars.size == 1, s"There should only be one $name jar but found: $jars")
     jars.head
   }
 
+  /** Create a scalaInstance task that uses Dotty based on `moduleName`. */
+  def dottyScalaInstanceTask(moduleName: String): Initialize[Task[ScalaInstance]] = Def.task {
+    val updateReport =
+      fetchArtifactsOf(
+        scalaOrganization.value %% moduleName % scalaVersion.value,
+        dependencyResolution.value,
+        scalaModuleInfo.value,
+        updateConfiguration.value,
+        (unresolvedWarningConfiguration in update).value,
+        streams.value.log)
+    val scalaLibraryJar = getJar(updateReport,
+      "org.scala-lang", "scala-library", revision = AllPassFilter)
+    val dottyLibraryJar = getJar(updateReport,
+      scalaOrganization.value, s"dotty-library_${scalaBinaryVersion.value}", scalaVersion.value)
+    val compilerJar = getJar(updateReport,
+      scalaOrganization.value, s"dotty-compiler_${scalaBinaryVersion.value}", scalaVersion.value)
+    val allJars =
+      getJars(updateReport, AllPassFilter, AllPassFilter, AllPassFilter)
+
+    makeScalaInstance(
+      state.value,
+      scalaVersion.value,
+      scalaLibraryJar,
+      dottyLibraryJar,
+      compilerJar,
+      allJars
+    )
+  }
+
   def makeScalaInstance(
     state: State, dottyVersion: String, scalaLibrary: File, dottyLibrary: File, compiler: File, all: Seq[File]
   ): ScalaInstance = {