master 98e5639: Fix the tests for 'string-limit'

[Eli Zaretskii]

branch: master
commit 98e5639c3c3caf2424f35e4a9f9c53ff48f43897
Author: Eli Zaretskii <eliz@gnu.org>
Commit: Eli Zaretskii <eliz@gnu.org>

    Fix the tests for 'string-limit'
    
    * test/lisp/emacs-lisp/subr-x-tests.el (subr-string-limit-coding):
    Fix the expected results of string-limit when encoding with
    UTF-16.  Add tests for UTF-8 with BOM.  (Bug#48324)
    
    * lisp/emacs-lisp/subr-x.el (string-limit): Add FIXME comment
    about the current implementation, which is faulty by design.
---
 lisp/emacs-lisp/subr-x.el            | 12 ++++++++++++
 test/lisp/emacs-lisp/subr-x-tests.el |  7 +++++--
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/lisp/emacs-lisp/subr-x.el b/lisp/emacs-lisp/subr-x.el
index 9c8c967..5a8885c 100644
--- a/lisp/emacs-lisp/subr-x.el
+++ b/lisp/emacs-lisp/subr-x.el
@@ -289,6 +289,18 @@ than this function."
       (let ((result nil)
             (result-length 0)
             (index (if end (1- (length string)) 0)))
+        ;; FIXME: This implementation, which uses encode-coding-char
+        ;; to encode the string one character at a time, is in general
+        ;; incorrect: coding-systems that produce prefix or suffix
+        ;; bytes, such as ISO-2022-based or UTF-8/16 with BOM, will
+        ;; produce those bytes for each character, instead of just
+        ;; once for the entire string.  encode-coding-char attempts to
+        ;; remove those extra bytes at least in some situations, but
+        ;; it cannot do that in all cases.  And in any case, producing
+        ;; what is supposed to be a UTF-16 or ISO-2022-CN encoded
+        ;; string which lacks the BOM bytes at the beginning and the
+        ;; charset designation sequences at the head and tail of the
+        ;; result will definitely surprise the callers in some cases.
         (while (let ((encoded (encode-coding-char
                                (aref string index) coding-system)))
                  (and (<= (+ (length encoded) result-length) length)
diff --git a/test/lisp/emacs-lisp/subr-x-tests.el 
b/test/lisp/emacs-lisp/subr-x-tests.el
index 112f3c1..ef04cde 100644
--- a/test/lisp/emacs-lisp/subr-x-tests.el
+++ b/test/lisp/emacs-lisp/subr-x-tests.el
@@ -607,18 +607,21 @@
   (should (equal (string-limit "foó" 4 nil 'utf-8) "fo\303\263"))
   (should (equal (string-limit "foóa" 4 nil 'utf-8) "fo\303\263"))
   (should (equal (string-limit "foóá" 4 nil 'utf-8) "fo\303\263"))
+  (should (equal (string-limit "foóá" 4 nil 'utf-8-with-signature)
+                 "fo\303\263"))
   (should (equal (string-limit "foóa" 4 nil 'iso-8859-1) "fo\363a"))
   (should (equal (string-limit "foóá" 4 nil 'iso-8859-1) "fo\363\341"))
-  (should (equal (string-limit "foóá" 4 nil 'utf-16) "\376\377\000f"))
+  (should (equal (string-limit "foóá" 4 nil 'utf-16) "\000f\000o"))
 
   (should (equal (string-limit "foó" 10 t 'utf-8) "fo\303\263"))
   (should (equal (string-limit "foó" 3 t 'utf-8) "o\303\263"))
   (should (equal (string-limit "foó" 4 t 'utf-8) "fo\303\263"))
   (should (equal (string-limit "foóa" 4 t 'utf-8) "o\303\263a"))
   (should (equal (string-limit "foóá" 4 t 'utf-8) "\303\263\303\241"))
+  (should (equal (string-limit "foóá" 2 t 'utf-8-with-signature) "\303\241"))
   (should (equal (string-limit "foóa" 4 t 'iso-8859-1) "fo\363a"))
   (should (equal (string-limit "foóá" 4 t 'iso-8859-1) "fo\363\341"))
-  (should (equal (string-limit "foóá" 4 t 'utf-16) "\376\377\000\341")))
+  (should (equal (string-limit "foóá" 4 t 'utf-16) "\000\363\000\341")))
 
 (ert-deftest subr-string-lines ()
   (should (equal (string-lines "foo") '("foo")))