Disallow string data resizing (bug#79784)

Only allow string mutation that is certain not to require string data to
be resized and reallocated: writing bytes into a unibyte string, and
changing ASCII to ASCII in a multibyte string.

This ensures that mutation will never transform a unibyte string to
multibyte, that the size of a string in bytes never changes, and that
the byte offsets of characters remain the same.  Most importantly, it
removes a long-standing obstacle to reform of string representation and
allow for future performance improvements.

* src/data.c (Faset): Disallow resizing string mutation.
* src/fns.c (clear_string_char_byte_cache):
* src/alloc.c (resize_string_data):  Remove.
* test/src/data-tests.el (data-aset-string): New test.
* test/lisp/subr-tests.el (subr--subst-char-in-string):
Skip error cases.
* test/src/alloc-tests.el (aset-nbytes-change):
Remove test that is no longer relevant.
* doc/lispref/strings.texi (Modifying Strings):
* doc/lispref/sequences.texi (Array Functions):
* doc/lispref/text.texi (Substitution):  Update manual.
* etc/NEWS: Announce.

1 files changed, 14 insertions, 29 deletions

diff --git a/src/data.c b/src/data.c
index 493a8dd63fc..b8a48203bcf 100644
--- a/src/data.c
+++ b/src/data.c
@@ -2574,7 +2574,10 @@ or a byte-code object.  IDX starts at 0.  */)
 DEFUN ("aset", Faset, Saset, 3, 3, 0,
        doc: /* Store into the element of ARRAY at index IDX the value NEWELT.
 Return NEWELT.  ARRAY may be a vector, a string, a char-table or a
-bool-vector.  IDX starts at 0.  */)
+bool-vector.  IDX starts at 0.
+If ARRAY is a unibyte string, NEWELT must be a single byte (0-255).
+If ARRAY is a multibyte string, NEWELT and the previous character at
+index IDX must both be ASCII (0-127).  */)
   (register Lisp_Object array, Lisp_Object idx, Lisp_Object newelt)
 {
   register EMACS_INT idxval;
@@ -2613,42 +2616,24 @@ bool-vector.  IDX starts at 0.  */)
         args_out_of_range (array, idx);
       CHECK_CHARACTER (newelt);
       int c = XFIXNAT (newelt);
-      ptrdiff_t idxval_byte;
-      int prev_bytes;
-      unsigned char workbuf[MAX_MULTIBYTE_LENGTH], *p0 = workbuf, *p1;
 
       if (STRING_MULTIBYTE (array))
         {
-          idxval_byte = string_char_to_byte (array, idxval);
-          p1 = SDATA (array) + idxval_byte;
-          prev_bytes = BYTES_BY_CHAR_HEAD (*p1);
-        }
-      else if (SINGLE_BYTE_CHAR_P (c))
-        {
-          SSET (array, idxval, c);
-          return newelt;
+          if (c > 0x7f)
+            error ("Attempt to store non-ASCII char into multibyte string");
+          ptrdiff_t idxval_byte = string_char_to_byte (array, idxval);
+          unsigned char *p = SDATA (array) + idxval_byte;
+          if (*p > 0x7f)
+            error ("Attempt to replace non-ASCII char in multibyte string");
+          *p = c;
         }
       else
         {
-          for (ptrdiff_t i = SBYTES (array) - 1; i >= 0; i--)
-            if (!ASCII_CHAR_P (SREF (array, i)))
-              args_out_of_range (array, newelt);
-          /* ARRAY is an ASCII string.  Convert it to a multibyte string.  */
-          STRING_SET_MULTIBYTE (array);
-          idxval_byte = idxval;
-          p1 = SDATA (array) + idxval_byte;
-          prev_bytes = 1;
+          if (c > 0xff)
+            error ("Attempt to store non-byte value into unibyte string");
+          SSET (array, idxval, c);
         }
-
-      int new_bytes = CHAR_STRING (c, p0);
-      if (prev_bytes != new_bytes)
-        p1 = resize_string_data (array, idxval_byte, prev_bytes, new_bytes);
-
-      do
-        *p1++ = *p0++;
-      while (--new_bytes != 0);
     }
-
   return newelt;
 }