bug#35062: [PATCH v3 2/3] constify a bit of xterm.c

[Konstantin Kharlamov <hi-angel@yandex.ru>]

В Пт, апр 19, 2019 at 17:31, Paul Eggert <eggert@cs.ucla.edu> 
написал:
> I'd rather not use 'const' on locals. Let me tell you a story as to 
> why.
> 
> I once worked with someone who insisted on using the 'register' 
> keyword on every C local variable that was never taken the address 
> of, on the grounds that this meant the reader could easily see that 
> the variable could never be modified indirectly via a pointer and 
> that this made the code easier to read because you didn't need to 
> worry about aliasing.
> 
> I disagreed then, and still disagree. Saying 'register' nearly all 
> the time clutters up the code, and the cost is not worth the benefit 
> in C. It's pretty easy for a human reader to determine whether a 
> local variable is taken the address of somewher in its function. (If 
> it's hard, then write an Elisp function that will tell you. :-) In 
> hindsight, perhaps C should have been designed so that 'register' was 
> the default for local variables, and that one needed a special word 
> to say "watch out! this variable might have its address taken!"; but 
> the ship has sailed.
> 
> 'const' is like 'register' in this respect. Putting 'const' nearly 
> everywhere clutters C code. It's pretty easy for a human reader to 
> determine whether a local variable is modified.

"const" is only similar to "register" in a way of having an additional 
word. Sure it's easy for a human to find whether variable was modified, 
but this requires to go through the whole function highlighting 
variable usages. Now imagine if visibility scope has lots of variables? 
This is especially relevant for the old C89 style where every variable 
was declared at the beginning of a function.

And btw, current workflow with searching through the body doesn't work 
well in vanilla Emacs. While other editors allow you to highlight 
matches by putting a caret over a symbol, Emacs requires either 
manually making it highlight/unhighlight, or installing a separate 
package highlight-symbol.el which is a bit buggy and wasn't updated 
since 2016. That is to say, having non-modified things declared with 
"const" may help an average Emacs developer.

> (if it's hard, write an Elisp function :-)

But situation in there is similar: declaring a local variable requires 
to use (let…), which means that even if you use variable once in the 
end of a function, the variable will be visible to everything else. And 
it's not like you can can make it immutable either.

>  In hindsight, perhaps C should have been designed so that 'const' 
> was the default for local variables; but the ship has sailed there 
> too.

Yeah… FWIW, there's an "REmacs" effort on rewriting Emacs from C to 
Rust, Rust has everything "const" by default. I'm sure emacs-devel 
would largely disagree on integrating it because Rust isn't stable yet 
(despite being widely deployed) and there's no GCC backend (LLVM only), 
which are reasonable points. Still, I'm hoping at some point it will 
change :)