Avoid expensive recoding for ASCII identity cases (bug#40407)

Optimise for the common case of encoding or decoding an ASCII-only
string using an ASCII-compatible coding, for file names in particular.

* src/coding.c (string_ascii_p): New function.
(code_convert_string): Return the input string for ASCII-only inputs
and ASCII-compatible codings.
* test/src/coding-tests.el (coding-nocopy-ascii): New test.

1 files changed, 11 insertions, 0 deletions

diff --git a/test/src/coding-tests.el b/test/src/coding-tests.el
index 110ff126964..93e6709d442 100644
--- a/test/src/coding-tests.el
+++ b/test/src/coding-tests.el
@@ -383,6 +383,17 @@
     (should-not (eq (encode-coding-string s nil nil) s))
     (should (eq (encode-coding-string s nil t) s))))
 
+(ert-deftest coding-nocopy-ascii ()
+  "Check that the NOCOPY parameter works for ASCII-only strings."
+  (let* ((uni (apply #'string (number-sequence 0 127)))
+         (multi (string-to-multibyte uni)))
+    (dolist (s (list uni multi))
+      (dolist (coding '(us-ascii iso-latin-1 utf-8))
+        (should-not (eq (decode-coding-string s coding nil) s))
+        (should-not (eq (encode-coding-string s coding nil) s))
+        (should (eq (decode-coding-string s coding t) s))
+        (should (eq (encode-coding-string s coding t) s))))))
+
 ;; Local Variables:
 ;; byte-compile-warnings: (not obsolete)
 ;; End: