doc: Mention how to remedy "source file ... newer than compiled error".

* doc/contributing.texi (Running Guix Before It Is Installed): Mention
how to remedy "source file ... newer than compiled" error by running
`make -j`.

Co-authored-by: jgart <jgart@dismail.de>
Co-authored-by: Julien Lepiller <julien@lepiller.eu>
Co-authored-by: Ryan Prior <rprior@protonmail.com>
Co-authored-by: Blake Shaw <blake@nonconstructivism.com>
Signed-off-by: Nicolas Goaziou <mail@nicolasgoaziou.fr>

1 files changed, 18 insertions, 0 deletions

diff --git a/doc/contributing.texi b/doc/contributing.texi
index f258d79bd2..196ac48879 100644
--- a/doc/contributing.texi
+++ b/doc/contributing.texi
@@ -243,6 +243,24 @@ local source tree; it simply updates the @file{~/.config/guix/current}
 symlink (@pxref{Invoking guix pull}).  Run @command{git pull} instead if
 you want to upgrade your local source tree.
 
+Sometimes, especially if you have recently updated your repository,
+running @command{./pre-inst-env} will print a message similar to the
+following example:
+
+@example
+;;; note: source file /home/user/projects/guix/guix/progress.scm
+;;;       newer than compiled /home/user/projects/guix/guix/progress.go
+@end example
+
+This is only a note and you can safely ignore it.  You can get rid of
+the message by running @command{make -j4}.  Until you do, Guile will run
+slightly slower because it will interpret the code instead of using
+prepared Guile object (@file{.go}) files.
+
+You can run @command{make} automatically as you work using
+@command{watchexec} from the @code{watchexec} package.  For example,
+to build again each time you update a package file, you can run
+@samp{watchexec -w gnu/packages make -j4}.
 
 @node The Perfect Setup
 @section The Perfect Setup