maint: add a syntax check to avoid unstyled quoting

* src/paste.c (main): Use our styled wrapper for quotearg_colon().
* cfg.mk (sc_prohibit-quotearg): A new syntax check to avoid
future uses of unstyled quotearg to one of the internal slots,
and thus destined for diagnostic output.

2 files changed, 12 insertions, 2 deletions

diff --git a/cfg.mk b/cfg.mk
index 78f19fa56..d6c4c9f61 100644
--- a/cfg.mk
+++ b/cfg.mk
@@ -218,6 +218,17 @@ sc_error_shell_always_quotes:
 	       exit 1; }  \
 	  || :
 
+# Avoid unstyled quoting to internal slots and thus destined for diagnostics
+# as that can leak unescaped control characters to the output, when using
+# the default "literal" quoting style.
+# Instead use quotef(), or quoteaf() or in edge cases quotearg_n_style_colon().
+# A more general PCRE would be @prohibit='quotearg_.*(?!(style|buffer))'
+sc_prohibit-quotearg:
+	@prohibit='quotearg(_n)?(|_colon|_char|_mem) ' \
+	in_vc_files='\.c$$' \
+	halt='Unstyled diagnostic quoting detected' \
+	  $(_sc_search_regexp)
+
 sc_sun_os_names:
 	@grep -nEi \
 	    'solaris[^[:alnum:]]*2\.(7|8|9|[1-9][0-9])|sunos[^[:alnum:]][6-9]' \
diff --git a/src/paste.c b/src/paste.c
index 761ae8d94..7021a9628 100644
--- a/src/paste.c
+++ b/src/paste.c
@@ -502,10 +502,9 @@ main (int argc, char **argv)
     {
       /* Don't use the quote() quoting style, because that would double the
          number of displayed backslashes, making the diagnostic look bogus.  */
-      set_quoting_style (NULL, c_maybe_quoting_style);
       error (EXIT_FAILURE, 0,
              _("delimiter list ends with an unescaped backslash: %s"),
-             quotearg_colon (delim_arg));
+             quotearg_n_style_colon (0, c_maybe_quoting_style, delim_arg));
     }
 
   if (!serial_merge)