build-system/channel: Accept a channel or instance as the source.

* guix/build-system/channel.scm (latest-channel-instances*): New
variable.
(build-channels): New procedure, with code formerly in
'channel-build-system', augmented with clauses for when SOURCE is a
channel instance or a channel.
* doc/guix.texi (Build Systems): Adjust accordingly.

1 files changed, 8 insertions, 4 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 5dab9cf169..306c7b635b 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -9571,10 +9571,14 @@ with @code{build-expression->derivation} (@pxref{Derivations,
 @defvr {Scheme Variable} channel-build-system
 This variable is exported by @code{(guix build-system channel)}.
 
-This build system is meant primarily for internal use.  It requires two
-arguments, @code{#:commit} and @code{#:source}, and builds a Guix
-instance from that channel, in the same way @command{guix time-machine}
-would do it (@pxref{Channels}).
+This build system is meant primarily for internal use.  A package using
+this build system must have a channel specification as its @code{source}
+field (@pxref{Channels}); alternatively, its source can be a directory
+name, in which case an additional @code{#:commit} argument must be
+supplied to specify the commit being built (a hexadecimal string).
+
+The resulting package is a Guix instance of the given channel, similar
+to how @command{guix time-machine} would build it.
 @end defvr
 
 @node Build Phases