Fix windows CI; conditionally enable forward compat community build in PRs

Author: prolativ

.github/workflows/ci.yaml

@@ -327,6 +327,12 @@ jobs:
         - ${{ github.workspace }}/../../cache/ivy:/root/.ivy2/cache
         - ${{ github.workspace }}/../../cache/general:/root/.cache
     if: "github.event_name == 'schedule' && github.repository == 'lampepfl/dotty'
+        || (
+           github.event_name == 'pull_request'
+           && !contains(github.event.pull_request.body, '[skip ci]')
+           && !contains(github.event.pull_request.body, '[skip community_build]')
+           && contains(github.event.pull_request.body, '[test_forward_compat]')
+         )
          || (
            github.event_name == 'workflow_dispatch'
            && github.repository == 'lampepfl/dotty'

compiler/test/dotty/tools/vulpix/ParallelTesting.scala

@@ -520,11 +520,12 @@ trait ParallelTesting extends RunnerOrchestration { self =>
       }
 
     protected def compileWithOtherCompiler(compiler: String, files: Array[JFile], flags: TestFlags, targetDir: JFile): TestReporter =
-      val compilerDir = getCompiler(compiler).toString
+      val compilerDir = getCompiler(compiler)
 
       def substituteClasspath(old: String): String =
         old.split(JFile.pathSeparator).map { o =>
-          if JFile(o) == JFile(Properties.dottyLibrary) then s"$compilerDir/lib/scala3-library_3-$compiler.jar"
+          if JFile(o) == JFile(Properties.dottyLibrary) then
+            compilerDir.resolve("lib").resolve(s"scala3-library_3-$compiler.jar").toString
           else o
         }.mkString(JFile.pathSeparator)
 
@@ -535,7 +536,10 @@ trait ParallelTesting extends RunnerOrchestration { self =>
       val dummyStream = new PrintStream(new ByteArrayOutputStream())
       val reporter = TestReporter.reporter(dummyStream, ERROR)
 
-      val command = Array(compilerDir + "/bin/scalac") ++ flags1.all ++ files.map(_.getPath)
+      val isOSWindows = System.getProperty("os.name").toLowerCase.startsWith("windows")
+      val scalacBinaryName = if isOSWindows then "scalac.bat" else "scalac"
+      val scalacPath = compilerDir.resolve("bin").resolve(scalacBinaryName).toString
+      val command = Array(scalacPath) ++ flags1.all ++ files.map(_.getPath)
       val process = Runtime.getRuntime.exec(command)
       val errorsText = Source.fromInputStream(process.getErrorStream).mkString
       if process.waitFor() != 0 then
@@ -1419,10 +1423,10 @@ object ParallelTesting {
   def isTastyFile(f: JFile): Boolean =
     f.getName.endsWith(".tasty")
 
-  def getCompiler(version: String): JFile =
-    val dir = cache.resolve(s"scala3-${version}").toFile
+  def getCompiler(version: String): Path =
+    val dir = cache.resolve(s"scala3-${version}")
     synchronized {
-      if dir.exists then
+      if dir.toFile.exists then
         dir
       else
         import scala.sys.process._