import: crate: Allow imports of a specific version.

* guix/import/crate.scm (crate->guix-package): Add optional 'version'
argument and honor it.
* guix/scripts/import/crate.scm (guix-import-crate): Assume the first
argument is a spec and destructure it with
'package-name->name+version'.  Pass both to 'crate->guix-package'.
* doc/guix.texi (Invoking guix import): Document it.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -8912,7 +8912,17 @@ in Guix.
 @item crate
 @cindex crate
 Import metadata from the crates.io Rust package repository
-@uref{https://crates.io, crates.io}.
+@uref{https://crates.io, crates.io}, as in this example:
+
+@example
+guix import crate blake2-rfc
+@end example
+
+The crate importer also allows you to specify a version string:
+
+@example
+guix import crate constant-time-eq@@0.1.0
+@end example
 
 @item opam
 @cindex OPAM

guix/import/crate.scm

@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2016 David Craven <david@craven.ch>
 ;;; Copyright © 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2019 Martin Becze <mjbecze@riseup.net>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -181,9 +182,11 @@ and LICENSE."
   ;; This regexp matches that.
   (make-regexp "^(.*) OR (.*)$"))
 
-(define (crate->guix-package crate-name)
+(define* (crate->guix-package crate-name #:optional version)
   "Fetch the metadata for CRATE-NAME from crates.io, and return the
-`package' s-expression corresponding to that package, or #f on failure."
+`package' s-expression corresponding to that package, or #f on failure.
+When VERSION is specified, attempt to fetch that version; otherwise fetch the
+latest version of CRATE-NAME."
   (define (string->license string)
     (match (regexp-exec %dual-license-rx string)
       (#f (list (spdx-string->license string)))
@@ -196,12 +199,18 @@ and LICENSE."
   (define crate
     (lookup-crate crate-name))
 
-  (and crate
+  (define version-number
-       (let* ((version        (find (lambda (version)
+    (or version
-                                      (string=? (crate-version-number version)
         (crate-latest-version crate)))
+
+  (define version*
+    (find (lambda (version)
+            (string=? (crate-version-number version)
+                      version-number))
           (crate-versions crate)))
-              (dependencies   (crate-version-dependencies version))
+
+  (and crate version*
+       (let* ((dependencies   (crate-version-dependencies version*))
               (dep-crates     (filter normal-dependency? dependencies))
               (dev-dep-crates (remove normal-dependency? dependencies))
               (cargo-inputs   (sort (map crate-dependency-id dep-crates)
@@ -210,14 +219,14 @@ and LICENSE."
                (sort (map crate-dependency-id dev-dep-crates)
                      string-ci<?)))
          (make-crate-sexp #:name crate-name
-                          #:version (crate-version-number version)
+                          #:version (crate-version-number version*)
                           #:cargo-inputs cargo-inputs
                           #:cargo-development-inputs cargo-development-inputs
                           #:home-page (or (crate-home-page crate)
                                           (crate-repository crate))
                           #:synopsis (crate-description crate)
                           #:description (crate-description crate)
-                          #:license (and=> (crate-version-license version)
+                          #:license (and=> (crate-version-license version*)
                                            string->license)))))
 
 (define (guix-package->crate-name package)

guix/scripts/import/crate.scm

@@ -2,6 +2,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2014 David Thompson <davet@gnu.org>
 ;;; Copyright © 2016 David Craven <david@craven.ch>
+;;; Copyright © 2019 Martin Becze <mjbecze@riseup.net>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -75,6 +76,7 @@ Import and convert the crate.io package for PACKAGE-NAME.\n"))
                   (alist-cons 'argument arg result))
                 %default-options))
 
+
   (let* ((opts (parse-options))
          (args (filter-map (match-lambda
                             (('argument . value)
@@ -82,11 +84,16 @@ Import and convert the crate.io package for PACKAGE-NAME.\n"))
                             (_ #f))
                            (reverse opts))))
     (match args
-      ((package-name)
+      ((spec)
-       (let ((sexp (crate->guix-package package-name)))
+       (define-values (name version)
+         (package-name->name+version spec))
+
+       (let ((sexp (crate->guix-package name version)))
          (unless sexp
            (leave (G_ "failed to download meta-data for package '~a'~%")
-                  package-name))
+                  (if version
+                      (string-append name "@" version)
+                      name)))
          sexp))
       (()
        (leave (G_ "too few arguments~%")))