Set custom location for eln-cache

Question

I would like the eln-cache for natively compiled files (I'm using emacs 29, built from the master branch) to be in a custom location. I have this in the opening of my early-init.el file:

;; Dir for eln-cache
(when (boundp 'native-comp-eln-load-path)
  (setcar native-comp-eln-load-path
          (expand-file-name (convert-standard-filename ".local/temp/cache/eln-cache/")
                            user-emacs-directory)))

;; Silence nativecomp warnings popping up
(setq native-comp-async-report-warnings-errors nil)

;; Settings
(setq native-comp-speed 2
      native-comp-deferred-compilation t
      package-native-compile t)

Unfortunately this doesn't seem to work. I will get a few files compiled in the proper place (i.e. in the cache dir) right at the start-up of emacs but a new eln-cache dir is always also created at the root of my .emacs.d and the eln files are all then subsequently created there as well.

I don't understand what the problem is here. Any ideas are welcome.

EDIT: Here's the bug report I filed: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=53891

Answer 1

As it turns out there is not currently an emacs developer-sanctioned way to reliably customize the location of the eln-cache, though this may be coming in emacs 29, at some point in the future. Something like

(when (boundp 'native-comp-eln-load-path)
  (setcar native-comp-eln-load-path
          (expand-file-name (convert-standard-filename ".local/temp/cache/eln-cache/")
                            user-emacs-directory)))

should work on emacs 28 or early branches of 29, but don't expect it to work reliably until a customizable solution has been added.

EDIT: this has already(!) been resolved by the developers. You can now put the following in your early-init file and all will work as expected:

;; Set eln-cache dir
(when (boundp 'native-comp-eln-load-path)
  (startup-redirect-eln-cache (expand-file-name "/path/to/cache/eln-cache/" user-emacs-directory)))

Note the function (startup-redirect-eln-cache).

Answer 2

See normal-top-level in startup.el for where that happens. That same function subsequently calls command-line to process your init files (both early-init-el and init.el), so whatever you do there should be sticking unless there's something else again messing with the value. I tried your code in 28.0.90 and couldn't replicate the problem, though, so whatever that "something else" is seems non-standard to me (at least in this version).

You could use emacs-startup-hook to act after other start-up procedures, or to check what the value is at that point in the proceedings. Failing that, I'd suggest using a variable watcher to try to track down when it's getting modified. See C-hig (elisp)Watching Variables for that.

---

It does indeed seem that native-comp-eln-load-path is used when deciding where to write .eln files. See C-hv comp-el-to-eln-filename for details.

You could also set native-compile-target-directory, although it's not clear to me that this is safe, so it's probably better to manipulate the path list.

Of course you could always just symlink your preferred location to ~/.emacs.d/eln-cache (which is all I did when I'd had a similar requirement).