warnings: don't complain about m4_foo and b4_foo when from the user

Currently, occurrences of these identifiers in the user's input yield
spurious warnings.

To tell the difference between a legitimate m4_foo from the user, and
a bad m4_foo coming from a non-evaluated macro of a skeleton, escape
the user's identifiers as m4@'_foo.  We already use @' as a special
sequence to be stripped from the skeleton's output.

See <https://lists.gnu.org/r/bug-bison/2021-10/msg00026.html> and
previous commit ("warnings: be less picky about occurrences of m4_/b4_
in the output").

* src/flex-scanner.h (OBSTACK_SGROW): New.
* src/output.c (output_escaped): Escape m4_ and b4_.
* src/scan-code.l: Likewise.
* src/system.h (obstack_escape): Likewise.
And rewrite as a function.
* tests/skeletons.at (Suspicious sequences): Make sure the user can
use m4_foo/b4_foo without spurious warnings.

4 files changed, 76 insertions, 16 deletions

diff --git a/src/flex-scanner.h b/src/flex-scanner.h
index 238ca881..08c0e62c 100644
--- a/src/flex-scanner.h
+++ b/src/flex-scanner.h
@@ -106,6 +106,9 @@ static struct obstack obstack_for_string;
 # define STRING_GROW()                                  \
   obstack_grow (&obstack_for_string, yytext, yyleng)
 
+# define STRING_SGROW(String)                   \
+  obstack_sgrow (&obstack_for_string, String)
+
 # define STRING_FINISH()                                \
   (last_string = obstack_finish0 (&obstack_for_string))
 
diff --git a/src/output.c b/src/output.c
index 4b1bb0d8..07234f2a 100644
--- a/src/output.c
+++ b/src/output.c
@@ -104,20 +104,45 @@ GENERATE_MUSCLE_INSERT_TABLE (muscle_insert_symbol_number_table, symbol_number)
 GENERATE_MUSCLE_INSERT_TABLE (muscle_insert_item_number_table, item_number)
 GENERATE_MUSCLE_INSERT_TABLE (muscle_insert_state_number_table, state_number)
 
+
 /*----------------------------------------------------------------.
 | Print to OUT a representation of CP quoted and escaped for M4.  |
+|                                                                 |
+| Keep sync'ed with obstack_escape.                               |
 `----------------------------------------------------------------*/
 
 static void
 output_escaped (FILE *out, const char *cp)
 {
-  for (; *cp; cp++)
+  for (; *cp; ++cp)
     switch (*cp)
       {
       case '$': fputs ("$][", out); break;
       case '@': fputs ("@@",  out); break;
       case '[': fputs ("@{",  out); break;
       case ']': fputs ("@}",  out); break;
+
+      case 'b':
+        if (STRPREFIX_LIT ("b4_", cp))
+          {
+            fputs ("b4@'_", out);
+            cp += strlen ("b4_") - 1;
+            break;
+          }
+        else
+          goto append;
+
+      case 'm':
+        if (STRPREFIX_LIT ("m4_", cp))
+          {
+            fputs ("m4@'_", out);
+            cp += strlen ("m4_") - 1;
+            break;
+          }
+        else
+          goto append;
+
+      append:
       default:  fputc (*cp,   out); break;
       }
 }
diff --git a/src/scan-code.l b/src/scan-code.l
index fced37d2..e3e765c8 100644
--- a/src/scan-code.l
+++ b/src/scan-code.l
@@ -197,6 +197,9 @@ ref      -?[0-9]+|{id}|"["{id}"]"|"$"
 
 <*>
 {
+  "b4_"   STRING_SGROW ("b4@'_");
+  "m4_"   STRING_SGROW ("m4@'_");
+
   /* Escape M4 quoting characters in C code.  */
   [$@\[\]]    obstack_escape (&obstack_for_string, yytext);
 
diff --git a/src/system.h b/src/system.h
index bbdf9fd0..630ef646 100644
--- a/src/system.h
+++ b/src/system.h
@@ -204,24 +204,53 @@ max_int (int a, int b)
   } while (0)
 
 
-/* Output Str escaped for our postprocessing (i.e., escape M4 special
+/* Output CP escaped for our postprocessing (i.e., escape M4 special
    characters).
 
-   For instance "[foo]" -> "@{foo@}", "$$" -> "$][$][". */
+   For instance "[foo]" -> "@{foo@}", "$$" -> "$][$][".
 
-# define obstack_escape(Obs, Str)                       \
-  do {                                                  \
-    char const *p__;                                    \
-    for (p__ = Str; *p__; p__++)                        \
-      switch (*p__)                                     \
-        {                                               \
-        case '$': obstack_sgrow (Obs, "$]["); break;    \
-        case '@': obstack_sgrow (Obs, "@@" ); break;    \
-        case '[': obstack_sgrow (Obs, "@{" ); break;    \
-        case ']': obstack_sgrow (Obs, "@}" ); break;    \
-        default:  obstack_1grow (Obs, *p__ ); break;    \
-        }                                               \
-  } while (0)
+   All the user's input passed to m4 is processed by this function or
+   by output_escaped, except user actions.  Because of the specific
+   handling of @$, @1, etc., user actions are processed (and escaped)
+   by scan-code.l.
+
+   Keep output_escaped and obstack_escape sync'ed.
+ */
+
+static inline void
+obstack_escape (struct obstack* obs, const char *cp)
+{
+  for (; *cp; ++cp)
+    switch (*cp)
+      {
+      case '$': obstack_sgrow (obs, "$]["); break;
+      case '@': obstack_sgrow (obs, "@@" ); break;
+      case '[': obstack_sgrow (obs, "@{" ); break;
+      case ']': obstack_sgrow (obs, "@}" ); break;
+      case 'b':
+        if (STRPREFIX_LIT ("b4_", cp))
+          {
+            obstack_sgrow (obs, "b4@'_");
+            cp += strlen ("b4_") - 1;
+            break;
+          }
+        else
+          goto append;
+
+      case 'm':
+        if (STRPREFIX_LIT ("m4_", cp))
+          {
+            obstack_sgrow (obs, "m4@'_");
+            cp += strlen ("m4_") - 1;
+            break;
+          }
+        else
+          goto append;
+
+      append:
+      default:  obstack_1grow (obs, *cp); break;
+      }
+}
 
 
 /* Output Str both quoted for M4 (i.e., embed in [[...]]), and escaped