guix: import: Report go version for go importer.

* guix/import/go.scm (go-package, go.mod-go-version): New procedures.
(go-module->guix-package): Add the #:go keyword in the generated package
definition if the required go is newer than the default go.
* tests/go.scm (mock-http-get): Use gexps for package arguments.

Change-Id: I8d005740a442330ac307a40a53764c803ceffc4f

guix/import/go.scm

@@ -29,6 +29,7 @@
   #:use-module (guix git)
   #:use-module (guix hash)
   #:use-module (guix i18n)
+  #:use-module ((guix utils) #:select (version>?))
   #:use-module (guix diagnostics)
   #:use-module (guix import utils)
   #:use-module (guix import json)
@@ -93,6 +94,11 @@
 
 ;;; Code:
 
+(define (go-package)
+  "Return the 'go' package.  This is a lazy reference so that we don't
+depend on (gnu packages golang)."
+  (module-ref (resolve-interface '(gnu packages golang)) 'go))
+
 (define http-fetch*
   ;; Like http-fetch, but memoized and returning the body as a string.
   (memoize (lambda args
@@ -378,6 +384,17 @@ DIRECTIVE."
 ;; Prevent inlining of this procedure, which is accessed by unit tests.
 (set! go.mod-requirements go.mod-requirements)
 
+(define (go.mod-go-version go.mod)
+  "Return the minimum version of go required to specified by GO.MOD."
+  (let ((go-version (go.mod-directives go.mod 'go)))
+    (if (null? go-version)
+      ;; If the go directive is missing, go 1.16 is assumed.
+      '(version "1.16")
+      (flatten go-version))))
+
+;; Prevent inlining of this procedure, which is accessed by unit tests.
+(set! go.mod-go-version go.mod-go-version)
+
 (define-record-type <vcs>
   (%make-vcs url-prefix root-regex type)
   vcs?
@@ -610,6 +627,7 @@ When VERSION is unspecified, the latest version available is used."
                     available-versions
                     module-path))
          (content (fetch-go.mod goproxy module-path version*))
+         (min-go-version (second (go.mod-go-version (parse-go.mod content))))
          (dependencies+versions (go.mod-requirements (parse-go.mod content)))
          (dependencies (if pin-versions?
                            dependencies+versions
@@ -634,7 +652,10 @@ When VERSION is unspecified, the latest version available is used."
          ,(vcs->origin vcs-type vcs-repo-url version*))
         (build-system go-build-system)
         (arguments
-         '(#:import-path ,module-path
+         (list ,@(if (version>? min-go-version (package-version (go-package)))
+                     `(#:go ,(string->number min-go-version))
+                     '())
+               #:import-path ,module-path
                ,@(if (string=? module-path-sans-suffix root-module-path)
                      '()
                      `(#:unpack-path ,root-module-path))))

tests/go.scm

@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2021 François Joulaud <francois.joulaud@radiofrance.com>
+;;; Copyright © 2021 François Joulaud <francois.joulaud@radiofrance.com>
-;;; Copyright © 2021 Sarah Morgensen <iskarian@mgsn.dev>
+;;; Copyright © 2021 Sarah Morgensen <iskarian@mgsn.dev>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -387,7 +387,7 @@ require github.com/kr/pretty v0.2.1
           "0sjjj9z1dhilhpc8pq4154czrb79z9cm044jvn75kxcjv6v5l2m5"))))
      (build-system go-build-system)
      (arguments
-      '(#:import-path "github.com/go-check/check"))
+      (list #:import-path "github.com/go-check/check"))
      (propagated-inputs
       `(("go-github-com-kr-pretty" ,go-github-com-kr-pretty)))
      (home-page "https://github.com/go-check/check")