Consider this minimal operating-system definition:

(use-modules (gnu))
(use-package-modules gcc)

(operating-system
  (host-name "test")
  (timezone "UTC")
  (bootloader (bootloader-configuration
                (bootloader grub-efi-bootloader)
                (target "/boot/efi")))
  (file-systems (cons*
                  (file-system
                    (device (file-system-label "root"))
                    (mount-point "/")
                    (type "ext4"))
                  %base-file-systems))
  (packages '())
  (services (cons*
              (service special-files-service-type
                       `(("/lib64" ,(directory-union "rustup-libs"
                                      (list
                                        (file-append glibc "/lib")
                                        (file-append #~#$gcc:lib "/lib"))))))
              %base-services)))

I would expect this way of specifying a specific output of a package to
work, but it results in the following error:

ice-9/boot-9.scm:1515:18: object is not an exception of the right type #<&gexp-input-error input: #<gexp #<gexp-input #<package gcc@7.5.0 gnu/packages/gcc.scm:520 7f06c996c960>:lib> 7f06c6b06990>> #<record-type &package-input-error>

This also happens when I omit the directory-union:

              (service special-files-service-type
                       `(("/lib64" ,(file-append #~#$gcc:lib "/lib"))))

What *does* work, is this variant with string-append:

              (service special-files-service-type
                       `(("/lib64" ,#~(string-append #$gcc:lib "/lib"))))

However, using it in the directory-union breaks again:

              (service special-files-service-type
                       `(("/lib64" ,(directory-union "rustup-libs"
                                      (list
                                        (file-append glibc "/lib")
                                        #~(string-append #$gcc:lib "/lib"))))))

ERROR: In procedure opendir:
Wrong type (expecting string): (string-append "/gnu/store/mdxmdhrlkgdik6ay9pzmmy8mjcbibpwb-gcc-7.5.0-lib" "/lib")
builder for `/gnu/store/p5hf7hqxn35fgsb75s5i7326vyzb8jkr-rustup-libs.drv' failed with exit code 1

I have figured out that the following does work:

              (service special-files-service-type
                       `(("/lib64" ,#~(directory-union "rustup-libs"
                                        (list
                                          (string-append #$glibc "/lib")
                                          (string-append #$gcc:lib "/lib"))))))

However, I would expect the other variants to work as well. The
documentation and error messages are lacking in this regard.

Regards,
Jakub Kądziołka