bug#64127: 30.0.50; mutate-constant warning with pure function

["Mattias Engdegård" <mattias.engdegard@gmail.com>]

17 juni 2023 kl. 13.32 skrev Basil Contovounesios <contovob@tcd.ie>:

> Given a file foo.el that defines a pure function foo:

> (defun foo (x) (declare (pure t)) (list x))

> and another file bar.el that mutates the result of foo:

> (setcar (foo nil) t)

> byte-compiling bar.el emits a mutate-constant warning, even though the
> result of foo is a fresh list:

No, the `pure` declaration means that the function can be evaluated at compile time which the compiler happily does, yielding a constant list, which your code then attempts to modify.

This is why the function `list` itself is not declared `pure` -- while it does look like a pure function when speaking informally, users relies on it returning a freshly allocated list that can be modified and that makes it non-pure. (If lists were immutable, then `list` would naturally be pure.)

Only the mutate-constant warning is new here; previously, the compiler would have let you make this mistake undisturbed.

Thus either you remove the pure-declaration from your function, or you don't mutate what it returns.