gexp: Add 'ungexp-native' and 'ungexp-native-splicing'.

* guix/gexp.scm (<gexp>)[natives]: New field.
  (write-gexp): Use both 'gexp-references' and
  'gexp-native-references'.
  (gexp->derivation): Use both 'gexp-inputs' and 'gexp-native-inputs',
  and append them.
  (gexp-inputs): Add 'references' parameter and honor it.
  (gexp-native-inputs): New procedure.
  (gexp->sexp)[reference->sexp]: Add 'native?' parameter and honor it.
  Use it, and use 'gexp-native-references'.
  (gexp)[collect-native-escapes]: New procedure.
  [escape->ref]: Handle 'ungexp-native' and 'ungexp-native-splicing'.
  [substitute-ungexp, substitute-ungexp-splicing]: New procedures.
  [substitute-references]: Use them, and handle 'ungexp-native' and
  'ungexp-native-splicing'.
  Adjust generated 'make-gexp' call to provide both normal references
  and native references.
  [read-ungexp]: Support 'ungexp-native' and
  'ungexp-native-splicing'.
  Add reader extension for #+.
* tests/gexp.scm (gexp-native-inputs): New procedure.
  (gexp->sexp*): Add 'target' parameter.
  ("ungexp + ungexp-native",
  "input list + ungexp-native",
  "input list splicing + ungexp-native-splicing",
  "gexp->derivation, ungexp-native",
  "gexp->derivation, ungexp + ungexp-native"): New tests.
  ("sugar"): Add tests for #+ and #+@.
* doc/guix.texi (G-Expressions): Document 'ungexp-native' et al.

doc/guix.texi

@@ -2160,8 +2160,32 @@ substituted to the reference to the @var{coreutils} package in the
 actual build code, and @var{coreutils} is automatically made an input to
 the derivation.  Likewise, @code{#$output} (equivalent to @code{(ungexp
 output)}) is replaced by a string containing the derivation's output
-directory name.  The syntactic form to construct gexps is summarized
-below.
+directory name.
+
+@cindex cross compilation
+In a cross-compilation context, it is useful to distinguish between
+references to the @emph{native} build of a package---that can run on the
+host---versus references to cross builds of a package.  To that end, the
+@code{#+} plays the same role as @code{#$}, but is a reference to a
+native package build:
+
+@example
+(gexp->derivation "vi"
+   #~(begin
+       (mkdir #$output)
+       (system* (string-append #+coreutils "/bin/ln")
+                "-s"
+                (string-append #$emacs "/bin/emacs")
+                (string-append #$output "/bin/vi")))
+   #:target "mips64el-linux")
+@end example
+
+@noindent
+In the example above, the native build of @var{coreutils} is used, so
+that @command{ln} can actually run on the host; but then the
+cross-compiled build of @var{emacs} is referenced.
+
+The syntactic form to construct gexps is summarized below.
 
 @deffn {Scheme Syntax} #~@var{exp}
 @deffnx {Scheme Syntax} (gexp @var{exp})
@@ -2190,6 +2214,13 @@ This is like the form above, but referring explicitly to the
 @var{package-or-derivation} produces multiple outputs (@pxref{Packages
 with Multiple Outputs}).
 
+@item #+@var{obj}
+@itemx #+@var{obj}:output
+@itemx (ungexp-native @var{obj})
+@itemx (ungexp-native @var{obj} @var{output})
+Same as @code{ungexp}, but produces a reference to the @emph{native}
+build of @var{obj} when used in a cross compilation context.
+
 @item #$output[:@var{output}]
 @itemx (ungexp output [@var{output}])
 Insert a reference to derivation output @var{output}, or to the main
@@ -2202,6 +2233,11 @@ This only makes sense for gexps passed to @code{gexp->derivation}.
 Like the above, but splices the contents of @var{lst} inside the
 containing list.
 
+@item #+@@@var{lst}
+@itemx (ungexp-native-splicing @var{lst})
+Like the above, but refers to native builds of the objects listed in
+@var{lst}.
+
 @end table
 
 G-expressions created by @code{gexp} or @code{#~} are run-time objects