bug#38628: Character literals unsupported in source 'snippets'

[Marius Bakke <mbakke@fastmail.com>]

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Guix,

If you try to use a character literal in a source 'snippet', Guix will
complain about an "unsupported input".  I.e. adding the following to the
'hello' package definition:

              (snippet '(begin #\!))

Results in:

guix build: error: /home/marius/guix/gnu/packages/base.scm:73:2: package `hello@2.10' has an invalid input: #\!

This is a fairly recent regression.  I tracked it down to the following commit:

commit 24ab804ce11fe12ff49cd144a3d9c4bfcf55b41c
Author: Ludovic Courtès <ludo@gnu.org>
Date:   Mon Sep 23 22:17:39 2019 +0200

    gexp: Catch and report non-self-quoting gexp inputs.
    
    Previously we would, for example, generate build scripts in the store;
    when trying to run them, we'd get a 'read' error due to the presence
    of #<foo> syntax in there.
    
    * guix/gexp.scm (gexp->sexp)[self-quoting?]: New procedure.
    [reference->sexp]: Check whether the argument in a <gexp-input> box is
    self-quoting.  Raise a '&gexp-input-error' condition if it's not.
    * tests/gexp.scm ("lower-gexp, non-self-quoting input"): New test.

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