Use dune-build-info

This uses dune-build-info to compute the version number.

It will be resolved in the following way:

- if (version) is set in (dune-project), it is used. This is what happens
  when using opam pins (through dune subst), or for released versions
  (through dune-release).
- otherwise, a description from [git describe] will be used. Caveat for
  this case: binaries under [_build/] will not have this information, but
  [dune install --prefix _install] will copy a valid binary under
  [_install/bin].

We require at least dune 2.7, since `dune-build-info` is broken on
flambda switches before this version.

See ocaml/dune#3599

dune-project

@@ -1,4 +1,4 @@
-(lang dune 2.2)
+(lang dune 2.7)
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;                                                                        ;
 ;                              OCamlFormat                               ;

lib/Version.ml

@@ -0,0 +1,14 @@
+(**************************************************************************)
+(*                                                                        *)
+(*                              OCamlFormat                               *)
+(*                                                                        *)
+(*            Copyright (c) Facebook, Inc. and its affiliates.            *)
+(*                                                                        *)
+(*      This source code is licensed under the MIT license found in       *)
+(*      the LICENSE file in the root directory of this source tree.       *)
+(*                                                                        *)
+(**************************************************************************)
+
+let version =
+  let open Build_info.V1 in
+  version () |> Option.value_map ~f:Version.to_string ~default:"unknown"

lib/Version.mli

@@ -0,0 +1,22 @@
+(**************************************************************************)
+(*                                                                        *)
+(*                              OCamlFormat                               *)
+(*                                                                        *)
+(*            Copyright (c) Facebook, Inc. and its affiliates.            *)
+(*                                                                        *)
+(*      This source code is licensed under the MIT license found in       *)
+(*      the LICENSE file in the root directory of this source tree.       *)
+(*                                                                        *)
+(**************************************************************************)
+
+val version : string
+(** A version number, or "unknown". This is provided by [dune-build-info],
+    which means that it will be resolved in the following way:
+
+    - if (version) is set in (dune-project), it is used. This is what happens
+      when using opam pins (through dune subst), or for released versions
+      (through dune-release).
+    - otherwise, a description from [git describe] will be used. Caveat for
+      this case: binaries under [_build/] will not have this information, but
+      [dune install --prefix _install] will copy a valid binary under
+      [_install/bin]. *)

lib/dune

@@ -9,13 +9,6 @@
 ;                                                                        ;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-(rule
- (targets Version.ml)
- (deps (universe))
- (action
-  (run ocaml %{dep:../tools/gen_version.mlt} Version.ml))
- (mode fallback))
-
 (ocamllex Literal_lexer)
 
 (library
@@ -24,4 +17,4 @@
   (:standard -open Base -open Import -open Compat))
  ;;INSERT_BISECT_HERE;;
  (libraries format_ import ocaml-migrate-parsetree odoc.model odoc.parser
-   parse_wyc re uuseg uuseg.string token_latest compat))
+   parse_wyc re uuseg uuseg.string token_latest compat dune-build-info))

ocamlformat.opam

@@ -17,7 +17,7 @@ bug-reports: "https://github.com/ocaml-ppx/ocamlformat/issues"
 dev-repo: "git+https://github.com/ocaml-ppx/ocamlformat.git"
 license: "MIT"
 build: [
-  ["ocaml" "tools/gen_version.mlt" "lib/Version.ml" version] {pinned}
+  ["dune" "subst"] {pinned}
   ["dune" "build" "-p" name "-j" jobs]
   ["dune" "runtest" "-p" name "-j" jobs] {with-test}
 ]
@@ -27,8 +27,9 @@ depends: [
   "base" {>= "v0.12.0" & < "v0.15"}
   "base-unix"
   "cmdliner"
-  "dune" {>= "2.2.0"}
+  "dune" {>= "2.7.0"}
   "fix"
+  "dune-build-info"
   "fpath"
   "menhir"
   "ocaml-migrate-parsetree" {>= "1.7.3" & < "2.0.0"}