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, 0 insertions, 50 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 07ca8474bf3..9ace6f01856 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -1815,56 +1815,6 @@ allocate_string_data (struct Lisp_String *s,
   tally_consing (needed);
 }
 
-/* Reallocate multibyte STRING data when a single character is replaced.
-   The character is at byte offset CIDX_BYTE in the string.
-   The character being replaced is CLEN bytes long,
-   and the character that will replace it is NEW_CLEN bytes long.
-   Return the address where the caller should store the new character.  */
-
-unsigned char *
-resize_string_data (Lisp_Object string, ptrdiff_t cidx_byte,
-                    int clen, int new_clen)
-{
-  eassume (STRING_MULTIBYTE (string));
-  sdata *old_sdata = SDATA_OF_STRING (XSTRING (string));
-  ptrdiff_t nchars = SCHARS (string);
-  ptrdiff_t nbytes = SBYTES (string);
-  ptrdiff_t new_nbytes = nbytes + (new_clen - clen);
-  unsigned char *data = SDATA (string);
-  unsigned char *new_charaddr;
-
-  if (sdata_size (nbytes) == sdata_size (new_nbytes))
-    {
-      /* No need to reallocate, as the size change falls within the
-         alignment slop.  */
-      XSTRING (string)->u.s.size_byte = new_nbytes;
-#ifdef GC_CHECK_STRING_BYTES
-      SDATA_NBYTES (old_sdata) = new_nbytes;
-#endif
-      new_charaddr = data + cidx_byte;
-      memmove (new_charaddr + new_clen, new_charaddr + clen,
-               nbytes - (cidx_byte + (clen - 1)));
-    }
-  else
-    {
-      allocate_string_data (XSTRING (string), nchars, new_nbytes, false, false);
-      unsigned char *new_data = SDATA (string);
-      new_charaddr = new_data + cidx_byte;
-      memcpy (new_charaddr + new_clen, data + cidx_byte + clen,
-              nbytes - (cidx_byte + clen));
-      memcpy (new_data, data, cidx_byte);
-
-      /* Mark old string data as free by setting its string back-pointer
-         to null, and record the size of the data in it.  */
-      SDATA_NBYTES (old_sdata) = nbytes;
-      old_sdata->string = NULL;
-    }
-
-  clear_string_char_byte_cache ();
-
-  return new_charaddr;
-}
-
 
 /* Sweep and compact strings.  */