Fix inverted NOCOPY encode/decode parameter (bug#40407)

In {encode,decode}-coding-string, the NOCOPY parameter had the
opposite effect to what was intended and documented.  This 18 year old
bug (introduced in 4031e2bf0a) only affected calls with CODING-SYSTEM
being nil.

* src/coding.c (code_convert_string): Correct use of NOCOPY.
* test/src/coding-tests.el (coding-nocopy-trivial): New test.

1 files changed, 8 insertions, 0 deletions

diff --git a/test/src/coding-tests.el b/test/src/coding-tests.el
index 094a1fad8fa..110ff126964 100644
--- a/test/src/coding-tests.el
+++ b/test/src/coding-tests.el
@@ -375,6 +375,14 @@
                          (with-temp-buffer (insert-file-contents (car file))))))
           (insert (format "%s: %s\n" (car file) result)))))))
 
+(ert-deftest coding-nocopy-trivial ()
+  "Check that the NOCOPY parameter works for the trivial coding system."
+  (let ((s "abc"))
+    (should-not (eq (decode-coding-string s nil nil) s))
+    (should (eq (decode-coding-string s nil t) s))
+    (should-not (eq (encode-coding-string s nil nil) s))
+    (should (eq (encode-coding-string s nil t) s))))
+
 ;; Local Variables:
 ;; byte-compile-warnings: (not obsolete)
 ;; End: