error about & at top level until it is safe

R=ken
OCL=17241
CL=17241

src/cmd/gc/go.h

@@ -468,6 +468,8 @@ EXTERN	int32	thunk;
 
 EXTERN	int	exporting;
 
+EXTERN	int	func;
+
 /*
  *	y.tab.c
  */

src/cmd/gc/go.y

@@ -754,6 +754,8 @@ uexpr:
 	}
 |	'&' uexpr
 	{
+		if($2->op == OCONV && !func)
+			yyerror("& of composite literal at top level");
 		$$ = nod(OADDR, $2, N);
 	}
 |	'+' uexpr
@@ -1179,11 +1181,13 @@ xfndcl:
 	{
 		maxarg = 0;
 		stksize = 0;
+		func++;
 	} fndcl fnbody
 	{
 		$$ = $3;
 		$$->nbody = $4;
 		funcbody($$);
+		func--;
 	}
 
 fndcl: