add describe opam-files (ocaml#3577)

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

bin/describe.ml

@@ -1,6 +1,13 @@
 open Stdune
 open Import
 
+(* This command is not yet versioned, but some people are using it in
+   non-released tools. If you change the format of the output, please contact:
+
+   - rotor people for "describe workspace"
+
+   - duniverse people for "describe opam-files" *)
+
 let doc = "Describe the workspace."
 
 let man =
@@ -98,6 +105,31 @@ module Crawl = struct
     Dyn.List (Lib.Set.to_list libs |> List.filter_map ~f:(library sctx))
 end
 
+module Opam_files = struct
+  let get () =
+    let project = Dune.File_tree.root () |> Dune.File_tree.Dir.project in
+    let packages =
+      Dune_project.packages project |> Dune.Package.Name.Map.values
+    in
+    Dyn.List
+      (List.map packages ~f:(fun pkg ->
+           let opam_file = Path.source (Dune.Package.opam_file pkg) in
+           let contents =
+             if not (Dune_project.generate_opam_files project) then
+               Io.read_file opam_file
+             else
+               let template_file = Dune.Opam_create.template_file opam_file in
+               let template =
+                 if Path.exists template_file then
+                   Some (template_file, Io.read_file template_file)
+                 else
+                   None
+               in
+               Dune.Opam_create.generate project pkg ~template
+           in
+           Dyn.Tuple [ String (Path.to_string opam_file); String contents ]))
+end
+
 (* What to describe. To determine what to describe, we convert the positional
    arguments of the command line to a list of atoms and we parse it using the
    regular [Dune_lang.Decoder].
@@ -106,13 +138,15 @@ end
    machinery. This also allow to easily extend this to arbitrary complex phrases
    without hassle. *)
 module What = struct
-  type t = Workspace
+  type t =
+    | Workspace
+    | Opam_files
 
   let default = Workspace
 
   let parse =
     let open Dune_lang.Decoder in
-    sum [ ("workspace", return Workspace) ]
+    sum [ ("workspace", return Workspace); ("opam-files", return Opam_files) ]
 
   let parse ~lang args =
     match args with
@@ -128,6 +162,7 @@ module What = struct
   let describe t setup context =
     match t with
     | Workspace -> Crawl.workspace setup context
+    | Opam_files -> Opam_files.get ()
 end
 
 module Format = struct

src/dune/opam_create.ml

@@ -160,36 +160,46 @@ let opam_fields project (package : Package.t) =
   else
     Opam_file.Create.normalise_field_order fields
 
+let template_file = Path.extend_basename ~suffix:".template"
+
 let opam_template ~opam_path =
-  let opam_template_path =
-    opam_path |> Path.Build.extend_basename ~suffix:".template" |> Path.build
-  in
+  let open Build.O in
+  let opam_template_path = template_file opam_path in
   Build.if_file_exists opam_template_path
-    ~then_:(Build.contents opam_template_path)
-    ~else_:(Build.return "")
+    ~then_:
+      (let+ contents = Build.contents opam_template_path in
+       Some (opam_template_path, contents))
+    ~else_:(Build.return None)
+
+let generate project pkg ~template =
+  let opam_fname = Package.opam_file pkg in
+  let filter_fields =
+    match template with
+    | None -> Fun.id
+    | Some (fname, contents) ->
+      let vars_in_template =
+        Lexbuf.from_string ~fname:(Path.to_string fname) contents
+        |> Opam_file.parse |> Opam_file.existing_variables
+      in
+      List.filter ~f:(fun (v, _) -> not (String.Set.mem vars_in_template v))
+  in
+  let generated_fields =
+    opam_fields project pkg |> filter_fields
+    |> Opam_file.Create.of_bindings ~file:(Path.source opam_fname)
+  in
+  sprintf "# This file is generated by dune, edit dune-project instead\n%s\n%s"
+    (OpamPrinter.opamfile generated_fields)
+    ( match template with
+    | None -> ""
+    | Some (_, s) -> s )
 
 let add_rule sctx ~project ~pkg =
   let open Build.O in
   let build_dir = Super_context.build_dir sctx in
   let opam_path = Path.Build.append_source build_dir (Package.opam_file pkg) in
   let opam_rule =
-    (let+ template = opam_template ~opam_path in
-     let opam_path = Path.build opam_path in
-     let opamfile =
-       Opam_file.parse
-         (Lexbuf.from_string ~fname:(Path.to_string opam_path) template)
-     in
-     let existing_vars_template = Opam_file.existing_variables opamfile in
-     let generated_fields =
-       opam_fields project pkg
-       |> List.filter ~f:(fun (v, _) ->
-              not (String.Set.mem existing_vars_template v))
-       |> Opam_file.Create.of_bindings ~file:opam_path
-     in
-     sprintf
-       "# This file is generated by dune, edit dune-project instead\n%s\n%s"
-       (OpamPrinter.opamfile generated_fields)
-       template)
+    (let+ template = opam_template ~opam_path:(Path.build opam_path) in
+     generate project pkg ~template)
     |> Build.write_file_dyn opam_path
   in
   let dir = Path.Build.append_source build_dir pkg.path in

src/dune/opam_create.mli

@@ -1,3 +1,13 @@
+(** Generate opam files from dune-project metadata *)
+
 open Stdune
 
+(** Given an opam filename, returns the filename of the template file *)
+val template_file : Path.t -> Path.t
+
+(** Generate the contents of an opam file. [template] is the filename and
+    contents of the template file. *)
+val generate :
+  Dune_project.t -> Package.t -> template:(Path.t * string) option -> string
+
 val add_rules : Super_context.t -> dir:Path.Build.t -> unit

test/blackbox-tests/test-cases/describe/run.t

@@ -6,7 +6,10 @@ Setup
 
   $ cat >dune-project <<EOF
   > (lang dune 2.3)
-  > (package (name foo))
+  > (package
+  >  (name foo)
+  >  (synopsis "foo bar baz"))
+  > (generate_opam_files)
   > EOF
 
   $ cat >dune <<EOF
@@ -81,3 +84,30 @@ Test errors
   Usage: dune describe [OPTION]... [STRING]...
   Try `dune describe --help' or `dune --help' for more information.
   [1]
+
+opam file listing
+-----------------
+
+  $ dune describe --lang 0.1 opam-files | sed 's/\\n/\n/g'
+  ((foo.opam
+     "# This file is generated by dune, edit dune-project instead
+  opam-version: \"2.0\"
+  synopsis: \"foo bar baz\"
+  depends: [
+    \"dune\" {>= \"2.3\"}
+  ]
+  build: [
+    [\"dune\" \"subst\"] {pinned}
+    [
+      \"dune\"
+      \"build\"
+      \"-p\"
+      name
+      \"-j\"
+      jobs
+      \"@install\"
+      \"@runtest\" {with-test}
+      \"@doc\" {with-doc}
+    ]
+  ]
+  "))