Add :javap to the REPL

Provides bytecode disassembly using the `javap` tool/interface
supplied by the user's JDK.

Adapted from the Scala 2 implementation, which was written by
Paul Phillips and Som Snytt / A. P. Marki

Author: griggt

compiler/src/dotty/tools/dotc/config/PathResolver.scala

@@ -129,6 +129,47 @@ object PathResolver {
       )
   }
 
+  /** Locations discovered by supplemental heuristics.
+   */
+  object SupplementalLocations {
+
+    /** The platform-specific support jar.
+     *
+     *  Usually this is `tools.jar` in the jdk/lib directory of the platform distribution.
+     *
+     *  The file location is determined by probing the lib directory under JDK_HOME or JAVA_HOME,
+     *  if one of those environment variables is set, then the lib directory under java.home,
+     *  and finally the lib directory under the parent of java.home. Or, as a last resort,
+     *  search deeply under those locations (except for the parent of java.home, on the notion
+     *  that if this is not a canonical installation, then that search would have little
+     *  chance of succeeding).
+     */
+    def platformTools: Option[File] = {
+      val jarName = "tools.jar"
+      def jarPath(path: Path) = (path / "lib" / jarName).toFile
+      def jarAt(path: Path) = {
+        val f = jarPath(path)
+        if (f.isFile) Some(f) else None
+      }
+      val jdkDir = {
+        val d = Directory(jdkHome)
+        if (d.isDirectory) Some(d) else None
+      }
+      def deeply(dir: Directory) = dir.deepFiles find (_.name == jarName)
+
+      val home    = envOrSome("JDK_HOME", envOrNone("JAVA_HOME")) map (p => Path(p))
+      val install = Some(Path(javaHome))
+
+      (home flatMap jarAt) orElse (install flatMap jarAt) orElse (install map (_.parent) flatMap jarAt) orElse
+        (jdkDir flatMap deeply)
+    }
+
+    override def toString = s"""
+      |object SupplementalLocations {
+      |  platformTools        = $platformTools
+      |}""".trim.stripMargin
+  }
+
   def fromPathString(path: String)(using Context): ClassPath = {
     val settings = ctx.settings.classpath.update(path)
     inContext(ctx.fresh.setSettings(settings)) {

compiler/src/dotty/tools/dotc/config/Properties.scala

@@ -55,6 +55,7 @@ trait PropertiesTrait {
 
   def envOrElse(name: String, alt: String): String      = Option(System getenv name) getOrElse alt
   def envOrNone(name: String): Option[String]           = Option(System getenv name)
+  def envOrSome(name: String, alt: => Option[String])   = envOrNone(name) orElse alt
 
   // for values based on propFilename
   def scalaPropOrElse(name: String, alt: String): String = scalaProps.getProperty(name, alt)