GNU bug report logs - #31688
26.1.50; Byte compiler confuses two string variables

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Message #14 received at 31688 <at> debbugs.gnu.org (full text, mbox):

From: Drew Adams <drew.adams <at> oracle.com>
To: Noam Postavsky <npostavs <at> gmail.com>, Gemini Lasswell <gazally <at> runbox.com>
Cc: 31688 <at> debbugs.gnu.org
Subject: RE: bug#31688: 26.1.50; Byte compiler confuses two string variables
Date: Sat, 2 Jun 2018 23:03:52 +0000 (UTC)

> I don't think this is a bug, the compiler coalesces equal string
> literals.  `put-text-property' modifies the string destructively, so you
> shouldn't use it on literals, for the same reason you shouldn't use
> destructive operations on quoted list literals.  Another example, not
> dependent on compilation:
> 
>     (defun foo (prop val)
>       (let ((s "xyz"))
>         (put-text-property 0 3 prop val s)
>         s))
> 
>     (foo 'x 1) ;=> #("xyz" 0 3 (x 1))
>     (foo 'y 2) ;=> #("xyz" 0 3 (x 1 y 2))

Yes. See also this, about Common Lisp:
https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node74.html

A snippet of it:

  An additional problem with eq is that the implementation
  is permitted to ``collapse'' constants (or portions thereof)
  appearing in code to be compiled if they are equal. An object
  is considered to be a constant in code to be compiled if it
  is a self-evaluating form or is contained in a quote form.

  This is why (eq "Foo" "Foo") might be true or false; in
  interpreted code it would normally be false, because reading
  in the form (eq "Foo" "Foo") would construct distinct strings
  for the two arguments to eq, but the compiler might choose to
  use the same identical string or two distinct copies as the
  two arguments in the call to eq.

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