Re: master 49e243c0c85: Avoid resizing mutation in subst-char-in-string, take two

[Mattias Engdegård]

13 maj 2024 kl. 19.53 skrev Eli Zaretskii <eliz@gnu.org>:
> 
>> +  (if (and (not inplace)
>> +           (if (multibyte-string-p string)
>> +               (> (max fromchar tochar) 127)
>> +             (> tochar 255)))
> 
> Is the above condition correct?  My reading of it is that if INPLACE
> is non-nil, we use aset (which will resize a string) even if TOCHAR
> needs more bytes than FROMCHAR.  Which seems to be in contradiction
> with the goal of the change, as advertised by the log message: "avoid
> resizing mutation".

I agree that it does look a bit odd, but it's intentional. First of all, the 
aim is to insulate non-mutating calls to the function from issues arising from 
mutation in the implementation. If we don't have to mutate and it's faster 
and/or safer not to, then we shouldn't.

Second, the function is documented to change the string in-place if INPLACE is 
non-nil, so in that case we have no choice but to mutate, or we might silently 
break reasonable code.

> why, in the case of a multibyte STRING, does the code look at the
> codepoints of FROMCHAR and TOCHAR and not at the number of bytes they
> take in the internal Emacs representation of the characters?

It's a conservative approximation that is much simpler than computing the size 
of the internal representation. (It's also the condition proposed in bug#70784.)