Add link_deps field

In some cases, the linking step requires some dependencies. For example,
passing a version script to the linker. The new `(link_deps)` field
uses the dependency DSL already used in other places.

Closes #852

Signed-off-by: Etienne Millon <etienne@cryptosense.com>

doc/jbuild.rst

@@ -248,6 +248,10 @@ Executables can also be linked as object or shared object files. See
 - ``(link_flags <flags>)`` specifies additional flags to pass to the linker.
   This field supports ``(:include ...)`` forms
 
+- ``(link_deps (<deps-conf list>))`` specifies the dependencies used only by the
+  linker, for example when using a version script. See the `Dependency
+  specification`_ section for more details.
+
 - ``(modules <modules>)`` specifies which modules in the current directory
   Jbuilder should consider when building this executable. Modules not listed
   here will be ignored and cannot be used inside the executable described by

src/exe.ml

@@ -108,6 +108,7 @@ let link_exe
       ~(linkage:Linkage.t)
       ~top_sorted_modules
       ?(link_flags=Build.arr (fun _ -> []))
+      ?(link_deps=Build.arr (fun _ -> []))
       ?(js_of_ocaml=Jbuild.Js_of_ocaml.default)
       cctx
   =
@@ -137,7 +138,9 @@ let link_exe
         artifacts modules ~ext:ctx.ext_obj))
   in
   SC.add_rule sctx
-    (Build.fanout3
+    (link_deps >>^ ignore
+     >>>
+     Build.fanout3
        (register_native_objs_deps modules_and_cm_files >>^ snd)
        (Ocaml_flags.get (CC.flags cctx) mode)
        link_flags
@@ -173,6 +176,7 @@ let build_and_link_many
       ~linkages
       ?already_used
       ?link_flags
+      ?link_deps
       ?(js_of_ocaml=Jbuild.Js_of_ocaml.default)
       cctx
   =
@@ -194,7 +198,8 @@ let build_and_link_many
         ~linkage
         ~top_sorted_modules
         ~js_of_ocaml
-        ?link_flags))
+        ?link_flags
+        ?link_deps))
 
 let build_and_link ~program =
   build_and_link_many ~programs:[program]

src/exe.mli

@@ -1,3 +1,5 @@
+open! Import
+
 (** Compilation and linking of executables *)
 
 module Program : sig
@@ -41,6 +43,7 @@ val build_and_link
   -> linkages:Linkage.t list
   -> ?already_used:Module.Name.Set.t
   -> ?link_flags:(unit, string list) Build.t
+  -> ?link_deps:(unit, Path.t list) Build.t
   -> ?js_of_ocaml:Jbuild.Js_of_ocaml.t
   -> Compilation_context.t
   -> unit
@@ -50,6 +53,7 @@ val build_and_link_many
   -> linkages:Linkage.t list
   -> ?already_used:Module.Name.Set.t
   -> ?link_flags:(unit, string list) Build.t
+  -> ?link_deps:(unit, Path.t list) Build.t
   -> ?js_of_ocaml:Jbuild.Js_of_ocaml.t
   -> Compilation_context.t
   -> unit
@@ -62,6 +66,7 @@ val link_exe
   -> linkage:Linkage.t
   -> top_sorted_modules:(unit, Module.t list) Build.t
   -> ?link_flags:(unit, string list) Build.t
+  -> ?link_deps:(unit, Path.t list) Build.t
   -> ?js_of_ocaml:Jbuild.Js_of_ocaml.t
   -> Compilation_context.t
   -> unit

src/gen_rules.ml

@@ -885,11 +885,16 @@ module Gen(P : Install_rules.Params) = struct
         ~loc:exes.buildable.loc ~modules
     in
 
+    let link_deps =
+      SC.Deps.interpret sctx ~scope ~dir exes.link_deps
+    in
+
     Exe.build_and_link_many cctx
       ~programs
       ~already_used
       ~linkages
       ~link_flags
+      ~link_deps
       ~js_of_ocaml:exes.buildable.js_of_ocaml;
 
     Merlin.make ()

src/jbuild.ml

@@ -875,13 +875,15 @@ module Executables = struct
     { names            : (Loc.t * string) list
     ; link_executables : bool
     ; link_flags       : Ordered_set_lang.Unexpanded.t
+    ; link_deps        : Dep_conf.t list
     ; modes            : Link_mode.Set.t
     ; buildable        : Buildable.t
     }
 
   let common_v1 project names public_names ~multi =
     Buildable.v1 >>= fun buildable ->
     field      "link_executables"   bool ~default:true >>= fun link_executables ->
+    field "link_deps" (list Dep_conf.t) ~default:[]    >>= fun link_deps ->
     field_oslu "link_flags"                            >>= fun link_flags ->
     field "modes" Link_mode.Set.t ~default:Link_mode.Set.default
     >>= fun modes ->
@@ -898,6 +900,7 @@ module Executables = struct
       { names
       ; link_executables
       ; link_flags
+      ; link_deps
       ; modes
       ; buildable
       }

src/jbuild.mli

@@ -259,6 +259,7 @@ module Executables : sig
     { names            : (Loc.t * string) list
     ; link_executables : bool
     ; link_flags       : Ordered_set_lang.Unexpanded.t
+    ; link_deps        : Dep_conf.t list
     ; modes            : Link_mode.Set.t
     ; buildable        : Buildable.t
     }

test/blackbox-tests/dune.inc

@@ -330,6 +330,14 @@
     test-cases/lib-available
     (progn (run ${exe:cram.exe} -test run.t) (diff? run.t run.t.corrected))))))
 
+(alias
+ ((name link-deps)
+  (deps ((package dune) (files_recursively_in test-cases/link-deps)))
+  (action
+   (chdir
+    test-cases/link-deps
+    (progn (run ${exe:cram.exe} -test run.t) (diff? run.t run.t.corrected))))))
+
 (alias
  ((name loop)
   (deps ((package dune) (files_recursively_in test-cases/loop)))
@@ -592,6 +600,7 @@
     (alias installable-dup-private-libs)
     (alias intf-only)
     (alias lib-available)
+    (alias link-deps)
     (alias loop)
     (alias menhir)
     (alias merlin-tests)
@@ -656,6 +665,7 @@
     (alias installable-dup-private-libs)
     (alias intf-only)
     (alias lib-available)
+    (alias link-deps)
     (alias loop)
     (alias merlin-tests)
     (alias meta-gen)

test/blackbox-tests/test-cases/link-deps/dune

@@ -0,0 +1,11 @@
+(alias
+  ((name message)
+   (deps (link_deps.cmo))
+   (action (echo "link\n"))
+   ))
+
+(executable
+  ((name link_deps)
+   (link_deps ((alias message)))
+   )
+  )

test/blackbox-tests/test-cases/link-deps/dune-project

@@ -0,0 +1 @@
+(lang dune 1.0)

test/blackbox-tests/test-cases/link-deps/run.t

@@ -0,0 +1,12 @@
+It is possible to add link-time dependencies.
+
+In particular, these can depend on the result of the compilation (like a .cmo
+file) and be created just before linking.
+
+  $ dune build --display short link_deps.exe
+      ocamldep another.ml.d
+      ocamldep link_deps.ml.d
+        ocamlc .link_deps.eobjs/link_deps.{cmi,cmo,cmt}
+  link
+      ocamlopt .link_deps.eobjs/link_deps.{cmx,o}
+      ocamlopt link_deps.exe