c_str: don't allow symbols.

On 2009-10-02, prior to TXR 014, I made a change to the c_str
function to allow symbolic arguments, so that c_str(sym) could
be used in the code base instead of c_str(symbol_name(sym)).
This was a bad idea, and is allowing numerous functions that
operate on strings to accept symbols also. That behavior is
not documented and not consistently supported. Ironically, I
completely forgot about this and have been consistently using
symbol_name(sym) anyway.  Let us remove this.
Compat support is required because this will break user code
that accidentally depends on this undocumented behavior.

* lib.c (c_str): If the object is type SYM, only return the
symbol_name if compatibility with 231 or older is requested,
otherwise fall through to the error case.

* match.c (dump_var): Fix one case where a symbol is passed
directly to put_string. This fails one of the test cases.
More testing is required to see if any other such cases occur.

* txr.1: New entry in the compatibility notes.

1 files changed, 4 insertions, 2 deletions

diff --git a/lib.c b/lib.c
index f54a71c4..ec17d81b 100644
--- a/lib.c
+++ b/lib.c
@@ -3660,11 +3660,13 @@ const wchar_t *c_str(val obj)
     return litptr(obj);
   case STR:
     return obj->st.str;
-  case SYM:
-    return c_str(symbol_name(obj));
   case LSTR:
     lazy_str_force(obj);
     return c_str(obj->ls.prefix);
+  case SYM:
+    if (opt_compat && opt_compat <= 231)
+      return c_str(symbol_name(obj));
+    /* fallthrough */
   default:
     type_mismatch(lit("~s is not a string"), obj, nao);
   }