store: Make 'direct-store-path?' public.

* guix/store.scm (direct-store-path?): New procedure.
* guix/derivations.scm (derivation)[direct-store-path?]: Remove.
* tests/store.scm ("direct-store-path?"): New test.

guix/derivations.scm

@@ -541,15 +541,6 @@ (define* (derivation store name builder args
 When REFERENCES-GRAPHS is true, it must be a list of file name/store path
 pairs.  In that case, the reference graph of each store path is exported in
 the build environment in the corresponding file, in a simple text format."
-  (define direct-store-path?
-    (let ((len (+ 1 (string-length (%store-prefix)))))
-      (lambda (p)
-        ;; Return #t if P is a store path, and not a sub-directory of a
-        ;; store path.  This predicate is needed because files *under* a
-        ;; store path are not valid inputs.
-        (and (store-path? p)
-             (not (string-index (substring p len) #\/))))))
-
   (define (add-output-paths drv)
     ;; Return DRV with an actual store path for each of its output and the
     ;; corresponding environment variable.

guix/store.scm

@@ -85,6 +85,7 @@ (define-module (guix store)
 
             %store-prefix
             store-path?
+            direct-store-path?
             derivation-path?
             store-path-package-name
             store-path-hash-part
@@ -640,6 +641,14 @@ (define (store-path? path)
   ;; `isStorePath' in Nix does something similar.
   (string-prefix? (%store-prefix) path))
 
+(define (direct-store-path? path)
+  "Return #t if PATH is a store path, and not a sub-directory of a store path.
+This predicate is sometimes needed because files *under* a store path are not
+valid inputs."
+  (and (store-path? path)
+       (let ((len (+ 1 (string-length (%store-prefix)))))
+         (not (string-index (substring path len) #\/)))))
+
 (define (derivation-path? path)
   "Return #t if PATH is a derivation path."
   (and (store-path? path) (string-suffix? ".drv" path)))

tests/store.scm

@@ -65,6 +65,15 @@ (define (random-text)
    (string-append (%store-prefix)
                   "/foo/bar/283gqy39v3g9dxjy26rynl0zls82fmcg-guile-2.0.7")))
 
+(test-assert "direct-store-path?"
+  (and (direct-store-path?
+        (string-append (%store-prefix)
+                       "/283gqy39v3g9dxjy26rynl0zls82fmcg-guile-2.0.7"))
+       (not (direct-store-path?
+             (string-append
+              (%store-prefix)
+              "/283gqy39v3g9dxjy26rynl0zls82fmcg-guile-2.0.7/bin/guile")))))
+
 (test-skip (if %store 0 10))
 
 (test-assert "dead-paths"