build-system/go: Add pre-built standard library as implicit input.

* gnu/packages/golang.scm (make-go-std): New procedure.
* guix/build-system/go.scm (make-go-std): New procedure.
(lower): Use it.  Add pre-built standard library to inputs.

Signed-off-by: Leo Famulari <leo@famulari.name>

gnu/packages/golang.scm

@@ -831,6 +831,36 @@ in the style of communicating sequential processes (@dfn{CSP}).")
 
 (define-public go go-1.14)
 
+(define-public (make-go-std go)
+  "Return a package which builds the standard library for Go compiler GO."
+  (package
+    (name (string-append (package-name go) "-std"))
+    (version (package-version go))
+    (source #f)
+    (build-system go-build-system)
+    (arguments
+     `(#:import-path "std"
+       #:build-flags `("-pkgdir" "pkg") ; "Install" to build directory.
+       #:allow-go-reference? #t
+       #:substitutable? #f ; Faster to build than download.
+       #:tests? #f ; Already tested in the main Go build.
+       #:go ,go
+       #:phases
+       (modify-phases %standard-phases
+         (delete 'unpack)
+         (replace 'install
+           (lambda* (#:key outputs #:allow-other-keys)
+             (let* ((out (assoc-ref outputs "out"))
+                    (out-cache (string-append out "/var/cache/go/build")))
+               (copy-recursively (getenv "GOCACHE") out-cache)
+               (delete-file (string-append out-cache "/trim.txt"))
+               (delete-file (string-append out-cache "/README")))))
+         (delete 'install-license-files))))
+    (home-page (package-home-page go))
+    (synopsis "Cached standard library build for Go")
+    (description (package-description go))
+    (license (package-license go))))
+
 (define-public go-0xacab-org-leap-shapeshifter
   (let ((commit "0aa6226582efb8e563540ec1d3c5cfcd19200474")
         (revision "12"))

guix/build-system/go.scm

@@ -4,6 +4,7 @@
 ;;; Copyright © 2020 Jakub Kądziołka <kuba@kadziolka.net>
 ;;; Copyright © 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2021 Efraim Flashner <efraim@flashner.co.il>
+;;; Copyright © 2021 Sarah Morgensen <iskarian@mgsn.dev>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -112,6 +113,9 @@ commit hash and its date rather than a proper release tag."
   (let ((go (resolve-interface '(gnu packages golang))))
     (module-ref go 'go)))
 
+(define (make-go-std)
+  (module-ref (resolve-interface '(gnu packages golang)) 'make-go-std))
+
 (define* (lower name
                 #:key source inputs native-inputs outputs system target
                 (go (default-go))
@@ -121,6 +125,14 @@ commit hash and its date rather than a proper release tag."
   (define private-keywords
     '(#:target #:go #:inputs #:native-inputs))
 
+  (define inputs-with-cache
+    ;; XXX: Avoid a circular dependency.  This should be rewritten with
+    ;; 'package-mapping' or similar.
+    (let ((go-std-name (string-append (package-name go) "-std")))
+      (if (string-prefix? go-std-name name)
+          inputs
+          (cons `(,go-std-name ,((make-go-std) go)) inputs))))
+
   (bag
     (name name)
     (system system)
@@ -130,7 +142,7 @@ commit hash and its date rather than a proper release tag."
                         '())
                      ,@`(("go" ,go))
                      ,@native-inputs
-                     ,@(if target '() inputs)
+                     ,@(if target '() inputs-with-cache)
                      ,@(if target
                          ;; Use the standard cross inputs of
                          ;; 'gnu-build-system'.
@@ -138,7 +150,7 @@ commit hash and its date rather than a proper release tag."
                          '())
                      ;; Keep the standard inputs of 'gnu-build-system'.
                      ,@(standard-packages)))
-    (host-inputs (if target inputs '()))
+    (host-inputs (if target inputs-with-cache '()))
 
     ;; The cross-libc is really a target package, but for bootstrapping
     ;; reasons, we can't put it in 'host-inputs'.  Namely, 'cross-gcc' is a