No longer crash on NoSymbol.owner.

Historically calling NoSymbol.owner has crashed the compiler.
With this commit, NoSymbol owns itself. This is consistent with
the way ownership chains are handled elsewhere in the compiler
(e.g. NoContext.owner is NoContext, NoSymbol.enclClass is
NoSymbol, and so on) and frees every call site which handles
symbols from having to perform precondition tests against
NoSymbol.

Since calling NoSymbol.owner sometimes (not always) indicates
a bug which we'd like to catch sooner than later, I have
introduced a couple more methods for selected call sites.

  def owner: Symbol       // NoSymbol.owner is self, log if -Xdev
  def safeOwner: Symbol   // NoSymbol.owner is self, ignore
  def assertOwner: Symbol // NoSymbol.owner is fatal

The idea is that everyone can call sym.owner without undue anxiety
or paranoid null-like tests. When compiling under -Xdev calls to
`owner` are logged with a stack trace, so any call sites for which
that is an expected occurrence should call safeOwner instead to
communicate the intention and stay out of the log. Conversely, any
call site where crashing on the owner call was a desirable behavior
can opt into calling assertOwner.

This commit also includes all the safeOwner calls necessary to
give us a silent log when compiling scala.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/reflect/scala/reflect/internal/SymbolTable.scala b/src/reflect/scala/reflect/internal/SymbolTable.scala
index c340670635..f0be0f7d05 100644
--- a/src/reflect/scala/reflect/internal/SymbolTable.scala
+++ b/src/reflect/scala/reflect/internal/SymbolTable.scala
@@ -60,6 +60,7 @@ abstract class SymbolTable extends macros.Universe
 
   def shouldLogAtThisPhase = false
   def isPastTyper = false
+  protected def isDeveloper: Boolean = settings.debug
 
   @deprecated("Give us a reason", "2.10.0")
   def abort(): Nothing = abort("unknown error")
@@ -69,8 +70,12 @@ abstract class SymbolTable extends macros.Universe
 
   /** Override with final implementation for inlining. */
   def debuglog(msg:  => String): Unit = if (settings.debug) log(msg)
-  def devWarning(msg: => String): Unit = if (settings.debug) Console.err.println(msg)
+  def devWarning(msg: => String): Unit = if (isDeveloper) Console.err.println(msg)
   def throwableAsString(t: Throwable): String = "" + t
+  def throwableAsString(t: Throwable, maxFrames: Int): String = t.getStackTrace take maxFrames mkString "\n  at "
+
+  @inline final def devWarningDumpStack(msg: => String, maxFrames: Int): Unit =
+    devWarning(msg + "\n" + throwableAsString(new Throwable, maxFrames))
 
   /** Prints a stack trace if -Ydebug or equivalent was given, otherwise does nothing. */
   def debugStack(t: Throwable): Unit  = devWarning(throwableAsString(t))