Test for concise error on sinking last statement.

If the last statement in a file used to need sinking, and doing so
threw an exception, we'd spit out an epic low-level backtrace rather
than a nice high-level one. Fix that by making non-EVAL and non-REPL
stuff we compile put the last statement in sink context in the code we
generate rather than having MAIN do the sinking. Also means a trailing
0; in a file now warns.

src/Perl6/Actions.nqp

@@ -294,6 +294,11 @@ class Perl6::Actions is HLL::Actions does STDActions {
             $*POD_PAST,
             statementlist_with_handlers($/)
         );
+        unless $*NEED_RESULT {
+            # Evaluate last statement in sink context, by pushing another
+            # statement after it, unless we need the result.
+            $mainline.push(QAST::WVal.new( :value($*W.find_symbol(['Nil'])) ));
+        }
         fatalize($mainline) if %*PRAGMAS<fatal>;
 
         if %*COMPILING<%?OPTIONS><p> { # also covers the -np case, like Perl 5

src/Perl6/Grammar.nqp

@@ -379,6 +379,9 @@ grammar Perl6::Grammar is HLL::Grammar does STD {
         %*HOW<knowhow> := nqp::knowhow();
         %*HOW<package> := nqp::knowhow();
 
+        # Will we use the result of this? (Yes for EVAL and REPL).
+        my $*NEED_RESULT := nqp::existskey(%*COMPILING<%?OPTIONS>, 'outer_ctx');
+
         # Symbol table and serialization context builder - keeps track of
         # objects that cross the compile-time/run-time boundary that are
         # associated with this compilation unit.

src/main.nqp

@@ -43,15 +43,12 @@ sub MAIN(*@ARGS) {
 sub MAIN(@ARGS) {
 #?endif
     # Enter the compiler.
-    my $result := $comp.command_line(@ARGS, :encoding('utf8'), :transcode('ascii iso-8859-1'));
-    if !nqp::isnull($result) && nqp::isconcrete($result) && nqp::can($result, 'sink') {
-        $result.sink();
-    }
+    $comp.command_line(@ARGS, :encoding('utf8'), :transcode('ascii iso-8859-1'));
 
     # Run any END blocks before exiting.
     my @END := nqp::gethllsym('perl6', '@END_PHASERS');
     while +@END {
-        $result := (@END.shift)();
+        my $result := (@END.shift)();
         nqp::can($result, 'sink') && $result.sink();
         CATCH { $comp.handle-exception($_); }
         CONTROL { $comp.handle-control($_); }