packages: Add 'package-development-inputs'.

* guix/packages.scm (package-development-inputs): New procedure.
* guix/scripts/environment.scm (package-environment-inputs): Use it.
* tests/packages.scm ("package-development-inputs")
("package-development-inputs, cross-compilation"): New tests.
* doc/guix.texi (package Reference): Document it.

doc/guix.texi

@@ -6848,6 +6848,47 @@ cross-compiling:
 It is an error to refer to @code{this-package} outside a package definition.
 @end deffn
 
+@cindex development inputs, of a package
+@cindex implicit inputs, of a package
+Sometimes you will want to obtain the list of inputs needed to
+@emph{develop} a package---all the inputs that are visible when the
+package is compiled.  This is what the @code{package-development-inputs}
+procedure returns.
+
+@deffn {Scheme Procedure} package-development-inputs @var{package} @
+   [@var{system}] [#:target #f]
+Return the list of inputs required by @var{package} for development
+purposes on @var{system}.  When @var{target} is true, return the inputs
+needed to cross-compile @var{package} from @var{system} to
+@var{triplet}, where @var{triplet} is a triplet such as
+@code{"aarch64-linux-gnu"}.
+
+Note that the result includes both explicit inputs and implicit
+inputs---inputs automatically added by the build system (@pxref{Build
+Systems}).  Let us take the @code{hello} package to illustrate that:
+
+@lisp
+(use-modules (gnu packages base) (guix packages))
+
+hello
+@result{} #<package hello@@2.10 gnu/packages/base.scm:79 7f585d4f6790>
+
+(package-direct-inputs hello)
+@result{} ()
+
+(package-development-inputs hello)
+@result{} (("source" @dots{}) ("tar" #<package tar@@1.32 @dots{}>) @dots{})
+@end lisp
+
+In this example, @code{package-direct-inputs} returns the empty list,
+because @code{hello} has zero explicit dependencies.  Conversely,
+@code{package-development-inputs} includes inputs implicitly added by
+@code{gnu-build-system} that are required to build @code{hello}: tar,
+gzip, GCC, libc, Bash, and more.  To visualize it, @command{guix graph
+hello} would show you explicit inputs, whereas @command{guix graph -t
+bag hello} would include implicit inputs (@pxref{Invoking guix graph}).
+@end deffn
+
 Because packages are regular Scheme objects that capture a complete
 dependency graph and associated build procedures, it is often useful to
 write procedures that take a package and return a modified version

guix/packages.scm

@@ -153,6 +153,7 @@
             bag-transitive-host-inputs
             bag-transitive-build-inputs
             bag-transitive-target-inputs
+            package-development-inputs
             package-closure
 
             default-guile
@@ -1070,6 +1071,15 @@ dependencies are known to build on SYSTEM."
                  (%current-system (bag-system bag)))
     (transitive-inputs (bag-target-inputs bag))))
 
+(define* (package-development-inputs package
+                                     #:optional (system (%current-system))
+                                     #:key target)
+  "Return the list of inputs required by PACKAGE for development purposes on
+SYSTEM.  When TARGET is true, return the inputs needed to cross-compile
+PACKAGE from SYSTEM to TRIPLET, where TRIPLET is a triplet such as
+\"aarch64-linux-gnu\"."
+  (bag-transitive-inputs (package->bag package system target)))
+
 (define* (package-closure packages #:key (system (%current-system)))
   "Return the closure of PACKAGES on SYSTEM--i.e., PACKAGES and the list of
 packages they depend on, recursively."

guix/scripts/environment.scm

@@ -82,7 +82,7 @@ package."
 packages for PACKAGE."
   ;; Remove non-package inputs such as origin records.
   (filter-map input->manifest-entry
-              (bag-transitive-inputs (package->bag package))))
+              (package-development-inputs package)))
 
 (define (show-help)
   (display (G_ "Usage: guix environment [OPTION]... PACKAGE... [-- COMMAND...]

tests/packages.scm

@@ -353,6 +353,20 @@
           (package-transitive-supported-systems d)
           (package-transitive-supported-systems e))))
 
+(test-assert "package-development-inputs"
+  ;; Note: Due to propagated inputs, 'package-development-inputs' returns a
+  ;; couple more inputs, such as 'linux-libre-headers'.
+  (lset<= equal?
+          `(("source" ,(package-source hello)) ,@(standard-packages))
+          (package-development-inputs hello)))
+
+(test-assert "package-development-inputs, cross-compilation"
+  (lset<= equal?
+          `(("source" ,(package-source hello))
+            ,@(standard-cross-packages "mips64el-linux-gnu" 'host)
+            ,@(standard-cross-packages "mips64el-linux-gnu" 'target))
+          (package-development-inputs hello #:target "mips64el-linux-gnu")))
+
 (test-assert "package-closure"
   (let-syntax ((dummy-package/no-implicit
                 (syntax-rules ()