Report when we could not detect the declaring parent of a target (#2885)

We now report the target name in case we weren't able to detect it's
declaring parent class.

Tested manually with a Mill version before the merge of #2883.

```scala
// /tmp/mill-2844/build.sc
import mill._

object mod extends Module {

  object foo extends Module {
    def bar = T {
      baz()
    }
  }

  private def baz = T { "baz" }
}
```

```
> mill -i dev.run /tmp/mill-2844 -i mod.foo.bar
[1691/1691] dev.run 
[info] compiling 1 Scala source to /tmp/mill-2844/out/mill-build/compile.dest/classes ...
[info] done compiling
Could not detect the parent class of target mod.baz. Please report this at https://github.com/com-lihaoyi/mill/issues/new/choose. As a workaround, you can run Mill with `--disable-callgraph-invalidation` option.
```

Pull request: #2885

build.sc

@@ -31,6 +31,7 @@ object Settings {
  val githubRepo = "mill"
  val projectUrl = s"https://github.com/${githubOrg}/${githubRepo}"
  val changelogUrl = s"${projectUrl}#changelog"
+ val newIssueUrl = s"${projectUrl}/issues/new/choose"
  val docUrl = "https://mill-build.com"
  // the exact branches containing a doc root
  val docBranches = Seq()
@@ -109,7 +110,8 @@ object Deps {
  object Play_3_0 extends Play {
  val playVersion = "3.0.0"
  }
- val play = Seq(Play_3_0, Play_2_9, Play_2_8, Play_2_7, Play_2_6).map(p => (p.playBinVersion, p)).toMap
+ val play =
+ Seq(Play_3_0, Play_2_9, Play_2_8, Play_2_7, Play_2_6).map(p => (p.playBinVersion, p)).toMap
 
  val acyclic = ivy"com.lihaoyi:::acyclic:0.3.9"
  val ammoniteVersion = "3.0.0-M0-53-084f7f4e"
@@ -493,7 +495,12 @@ object main extends MillStableScalaModule with BuildInfo {
  def buildInfoPackageName = "mill.api"
  def buildInfoMembers = Seq(
  BuildInfo.Value("millVersion", millVersion(), "Mill version."),
- BuildInfo.Value("millDocUrl", Settings.docUrl, "Mill documentation url.")
+ BuildInfo.Value("millDocUrl", Settings.docUrl, "Mill documentation url."),
+ BuildInfo.Value(
+ "millReportNewIssueUrl",
+ Settings.newIssueUrl,
+ "URL to create a new issue in Mills issue tracker."
+ )
  )
 
  def ivyDeps = Agg(
@@ -1417,15 +1424,17 @@ object dev extends MillPublishScalaModule {
  case wd0 +: rest =>
  val wd = os.Path(wd0, T.workspace)
  os.makeDir.all(wd)
- try Jvm.runSubprocess(
+ try {
+ Jvm.runSubprocess(
  Seq(launcher().path.toString) ++ rest,
  forkEnv(),
  workingDir = wd
  )
- catch {
- case e: Throwable => () /*ignore to avoid confusing stacktrace and error messages*/
+ mill.api.Result.Success(())
+ } catch {
+ case e: Throwable =>
+ mill.api.Result.Failure(s"dev.run failed with an exception. ${e.getMessage()}")
  }
- mill.api.Result.Success(())
  }
  }
 }

main/eval/src/mill/eval/GroupEvaluator.scala

@@ -108,7 +108,14 @@ private[mill] trait GroupEvaluator {
  m.getName == (c.getName.replace('.', '$') + "$" + encodedTaskName)
  } yield m
 
- val methodClass = methods.head.getDeclaringClass.getName
+ val methodClass = methods
+ .headOption
+ .getOrElse(throw new MillException(
+ s"Could not detect the parent class of target ${namedTask}. " +
+ s"Please report this at ${BuildInfo.millReportNewIssueUrl} . " +
+ s"As a workaround, you can run Mill with `--disable-callgraph-invalidation` option."
+ ))
+ .getDeclaringClass.getName
  val name = namedTask.ctx.segment.pathSegments.last
  val expectedName = methodClass + "#" + name + "()mill.define.Target"
 
@@ -123,15 +130,16 @@ private[mill] trait GroupEvaluator {
  ctx.enclosingModule match {
  case null => None
  case m: mill.define.Module => Some((m, m.millOuterCtx))
- case unknown => sys.error(s"Unknown ctx: $unknown")
+ case unknown =>
+ throw new MillException(s"Unknown ctx of target ${namedTask}: $unknown")
  }
  }
 
  val constructorHashes = allEnclosingModules
  .map(m =>
  constructorHashSignatures.get(m.getClass.getName) match {
  case Some(Seq((singleMethod, hash))) => hash
- case Some(multiple) => sys.error(
+ case Some(multiple) => throw new MillException(
  s"Multiple constructors found for module $m: ${multiple.mkString(",")}"
  )
  case None => 0