kconfig: allow only 'config', 'comment', and 'if' inside 'choice'

The code block surrounded by 'if' ... 'endif' is reduced into if_stmt,
which is accepted in the 'choice' context. Therefore, you can write any
statements within a choice block by wrapping 'if y' ... 'end'.

For example, you can create a menu inside a choice, like follows:

---------------->8----------------
  choice
          prompt "choice"

  config A
          bool "A"

  config B
          bool "B"

  if y

  menu "strange menu"

  config C
          bool "C"

  endmenu

  endif

  endchoice
---------------->8----------------

I want to change such a weird structure into a syntax error.

In fact, the USB gadget Kconfig had used nested 'choice' for no good
reason until commit df8df5e4bc ("usb: get rid of 'choice' for
legacy gadget drivers") killed it.

I think the 'source' inside 'choice' is on the fence. It is at least
gramatically sensible as long as the included file contains only
bool/tristate configs. However, it makes the code unreadable, and people
tend to forget the fact that the file is included from the choice
block. Commit 10e5e6c249 ("usb: gadget: move choice ... endchoice to
legacy/Kconfig") got rid of the only usecase.

Going forward, you can only use 'config', 'comment', and 'if' inside
'choice'. This also recursively applies to 'if' blocks inside 'choice'.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/parser.y

@@ -119,20 +119,24 @@ mainmenu_stmt: T_MAINMENU T_WORD_QUOTE T_EOL
 
 stmt_list:
 	  /* empty */
-	| stmt_list common_stmt
+	| stmt_list assignment_stmt
 	| stmt_list choice_stmt
+	| stmt_list comment_stmt
+	| stmt_list config_stmt
+	| stmt_list if_stmt
 	| stmt_list menu_stmt
+	| stmt_list menuconfig_stmt
+	| stmt_list source_stmt
 	| stmt_list T_WORD error T_EOL	{ zconf_error("unknown statement \"%s\"", $2); }
 	| stmt_list error T_EOL		{ zconf_error("invalid statement"); }
 ;
 
-common_stmt:
-	  if_stmt
-	| comment_stmt
-	| config_stmt
-	| menuconfig_stmt
-	| source_stmt
-	| assignment_stmt
+stmt_list_in_choice:
+	  /* empty */
+	| stmt_list_in_choice comment_stmt
+	| stmt_list_in_choice config_stmt
+	| stmt_list_in_choice if_stmt_in_choice
+	| stmt_list_in_choice error T_EOL	{ zconf_error("invalid statement"); }
 ;
 
 /* config/menuconfig entry */
@@ -254,7 +258,7 @@ choice_end: end
 	}
 };
 
-choice_stmt: choice_entry choice_block choice_end
+choice_stmt: choice_entry stmt_list_in_choice choice_end
 ;
 
 choice_option_list:
@@ -305,11 +309,6 @@ default:
 	| T_DEF_BOOL		{ $$ = S_BOOLEAN; }
 	| T_DEF_TRISTATE	{ $$ = S_TRISTATE; }
 
-choice_block:
-	  /* empty */
-	| choice_block common_stmt
-;
-
 /* if entry */
 
 if_entry: T_IF expr T_EOL
@@ -331,6 +330,9 @@ if_end: end
 if_stmt: if_entry stmt_list if_end
 ;
 
+if_stmt_in_choice: if_entry stmt_list_in_choice if_end
+;
+
 /* menu entry */
 
 menu: T_MENU T_WORD_QUOTE T_EOL