Normalize slashes in paths in TASTy binaries

compiler/src/dotty/tools/dotc/util/SourceFile.scala

@@ -233,7 +233,15 @@ object SourceFile {
         // As we already check that the prefix matches, the special handling for
         // Windows is not needed.
 
-        refPath.relativize(sourcePath).toString
+        // We also consistently use forward slashes as path element separators
+        // for relative paths. If we didn't do that, it'd be impossible to parse
+        // them back, as one would need to know whether they were created on Windows
+        // and use both slashes as separators, or on other OS and use forward slash
+        // as separator, backslash as file name character.
+
+        import scala.jdk.CollectionConverters._
+        val path = refPath.relativize(sourcePath)
+        path.iterator.asScala.mkString("/")
       else
         sourcePath.toString
   }

compiler/test/dotty/tools/dotc/core/tasty/PathPicklingTest.scala

@@ -0,0 +1,76 @@
+package dotty.tools.dotc.core.tasty
+
+import java.io.{File => JFile, ByteArrayOutputStream, IOException}
+import java.nio.file.{Files, NoSuchFileException, Path, Paths}
+
+import scala.sys.process._
+
+import org.junit.Test
+import org.junit.Assert.{assertEquals, assertTrue, assertFalse, fail}
+
+import dotty.tools.dotc.ast.tpd
+import dotty.tools.dotc.ast.tpd.TreeOps
+import dotty.tools.dotc.{Driver, Main}
+import dotty.tools.dotc.decompiler
+import dotty.tools.dotc.core.Comments.CommentsContext
+import dotty.tools.dotc.core.Contexts.Context
+import dotty.tools.dotc.core.Decorators.{toTermName, toTypeName}
+import dotty.tools.dotc.core.Mode
+import dotty.tools.dotc.core.Names.Name
+import dotty.tools.dotc.interfaces.Diagnostic.ERROR
+import dotty.tools.dotc.reporting.TestReporter
+import dotty.tools.io.{Directory, File, Path}
+
+import dotty.tools.vulpix.TestConfiguration
+
+class PathPicklingTest {
+
+  @Test def test(): Unit = {
+    val out = JFile("out/testPathPickling")
+    val cwd = JFile("").getAbsolutePath()
+    delete(out)
+    out.mkdir()
+
+    locally {
+      val ignorantProcessLogger = ProcessLogger(_ => ())
+      val options = TestConfiguration.defaultOptions
+        .and("-d", s"$out/out.jar")
+        .and("-sourceroot", "tests/pos")
+        .and(s"$cwd/tests/pos/i10430/lib.scala", s"$cwd/tests/pos/i10430/app.scala")
+      val reporter = TestReporter.reporter(System.out, logLevel = ERROR)
+      val rep = Main.process(options.all, reporter)
+      assertFalse("Compilation failed.", rep.hasErrors)
+    }
+
+    val decompiled =
+      val outstream = new ByteArrayOutputStream()
+      val options = TestConfiguration.defaultOptions
+        .and("-print-tasty")
+        .and("-color:never")
+        .and(s"$out/out.jar")
+      val reporter = TestReporter.reporter(System.out, logLevel = ERROR)
+      val rep = Console.withOut(outstream) {
+        decompiler.Main.process(options.all, reporter)
+      }
+      assertFalse("Decompilation failed.", rep.hasErrors)
+      new String(outstream.toByteArray(), "UTF-8")
+
+    assertTrue(decompiled.contains(": i10430/lib.scala"))
+    assertTrue(decompiled.contains(": i10430/app.scala"))
+    assertTrue(decompiled.contains("[i10430/lib.scala]"))
+    assertTrue(decompiled.contains("[i10430/app.scala]"))
+
+    assertFalse(decompiled.contains(": i10430\\lib.scala"))
+    assertFalse(decompiled.contains(": i10430\\app.scala"))
+    assertFalse(decompiled.contains("[i10430\\lib.scala]"))
+    assertFalse(decompiled.contains("[i10430\\app.scala]"))
+  }
+
+  private def delete(file: JFile): Unit = {
+    if (file.isDirectory) file.listFiles.foreach(delete)
+    try Files.delete(file.toPath)
+    catch {
+      case _: NoSuchFileException => // already deleted, everything's fine
+    }
+  }
+}