feature: unused-libs alias

Introduce an alias to detect unused libraries in libraries and
executable stanzas.

This stanza relies on information extracted using ocamlobjinfo to detect
which modules are actually used

Signed-off-by: Rudi Grinberg <[email protected]>

Author: rgrinberg

doc/changes/added/12623.md

@@ -0,0 +1,2 @@
+- Introduce an `unused-libs` stanza to detect unused libraries (#12623, fixes
+  #650, @rgrinberg)

doc/reference/aliases.rst

@@ -82,6 +82,7 @@ Some aliases are defined and managed by Dune itself:
        aliases/runtest
        aliases/fmt
        aliases/lint
+       aliases/unused-libs
 
   .. grid-item::
 

doc/reference/aliases/unused-libs.rst

@@ -0,0 +1,5 @@
+@unused-libs
+============
+
+This alias is used to detect unused entries in the libraries field of
+executables and stanzas.

src/dune_rules/alias0.ml

@@ -26,6 +26,7 @@ let install = standard "install"
 let pkg_install = Alias.Name.of_string "pkg-install"
 let ocaml_index = standard "ocaml-index"
 let runtest = standard "runtest"
+let unused_libs = standard "unused-libs"
 let all = standard "all"
 let default = standard "default"
 let empty = standard "empty"

src/dune_rules/alias0.mli

@@ -15,6 +15,7 @@ val ocaml_index : Name.t
 val install : Name.t
 val pkg_install : Name.t
 val runtest : Name.t
+val unused_libs : Name.t
 val empty : Name.t
 val all : Name.t
 val default : Name.t

src/dune_rules/dep_rules.ml

@@ -36,7 +36,10 @@ let ooi_deps
       let ctx = Super_context.context sctx in
       Context.ocaml ctx
     in
-    Ocamlobjinfo.rules ocaml ~sandbox ~dir ~unit
+    Ocamlobjinfo.rules ocaml ~sandbox ~dir ~units:[ unit ]
+    |> Action_builder.map ~f:(function
+      | [ x ] -> x
+      | [] | _ :: _ -> assert false)
   in
   let add_rule = Super_context.add_rule sctx ~dir in
   let read =

src/dune_rules/exe_rules.ml

@@ -208,6 +208,18 @@ let executables_rules
   in
   let lib_config = ocaml.lib_config in
   let* requires_compile = Compilation_context.requires_compile cctx in
+  let* () =
+    let toolchain = Compilation_context.ocaml cctx in
+    let direct_requires = Lib.Compile.direct_requires compile_info in
+    Unused_libs_rules.gen_rules
+      sctx
+      toolchain
+      exes.buildable.loc
+      ~obj_dir
+      ~modules
+      ~dir
+      ~direct_requires
+  in
   let* () =
     let* dep_graphs =
       (* Building an archive for foreign stubs, we link the corresponding object

src/dune_rules/lib_rules.ml

@@ -621,6 +621,17 @@ let library_rules
     in
     Sub_system.gen_rules
       { super_context = sctx; dir; stanza = lib; scope; source_modules; compile_info }
+  and+ () =
+    let toolchain = Compilation_context.ocaml cctx in
+    let direct_requires = Lib.Compile.direct_requires compile_info in
+    Unused_libs_rules.gen_rules
+      sctx
+      toolchain
+      lib.buildable.loc
+      ~obj_dir
+      ~modules
+      ~dir
+      ~direct_requires
   and+ merlin =
     let+ requires_hidden = Compilation_context.requires_hidden cctx
     and+ parameters = Compilation_context.parameters cctx in

src/dune_rules/ocamlobjinfo.mli

@@ -10,8 +10,19 @@ val rules
   :  Ocaml_toolchain.t
   -> dir:Path.Build.t
   -> sandbox:Sandbox_config.t option
-  -> unit:Path.t
-  -> t Action_builder.t
+  -> units:Path.t list
+  -> t list Action_builder.t
+
+(** Run ocamlobjinfo on an archive to extract module names defined in it *)
+val archive_rules
+  :  Ocaml_toolchain.t
+  -> dir:Path.Build.t
+  -> sandbox:Sandbox_config.t option
+  -> archive:Path.t
+  -> Module_name.Unique.Set.t Action_builder.t
 
 (** For testing only *)
-val parse : string -> t
+val parse : string -> t list
+
+(** Parse archive output to extract module names defined in the archive *)
+val parse_archive : string -> Module_name.Unique.Set.t

src/dune_rules/ocamlobjinfo.mll

@@ -22,23 +22,34 @@ let ws = [' ' '\t']+
 let hash = ['0'-'9' 'a'-'z' '-']+
 let name = ['A'-'Z'] ['A'-'Z' 'a'-'z' '0'-'9' '_']*
 
-rule ocamlobjinfo acc = parse
-  | "Interfaces imported:" newline { intfs acc lexbuf }
-  | "Implementations imported:" newline { impls acc lexbuf }
-  | _ { ocamlobjinfo acc lexbuf }
+rule ocamlobjinfo acc_units acc = parse
+  | "Interfaces imported:" newline { intfs acc_units acc lexbuf }
+  | "Implementations imported:" newline { impls acc_units acc lexbuf }
+  | _ { ocamlobjinfo acc_units acc lexbuf }
+  | eof { acc :: acc_units }
+and intfs acc_units acc = parse
+  | ws hash ws (name as name) newline { intfs acc_units (add_intf acc name) lexbuf }
+  | "Implementations imported:" newline { impls acc_units acc lexbuf }
+  | "File " [^ '\n']+ newline { ocamlobjinfo (acc :: acc_units) empty lexbuf }
+  | _ | eof { acc :: acc_units }
+and impls acc_units acc = parse
+  | ws hash ws (name as name) newline { impls acc_units (add_impl acc name) lexbuf }
+  | "File " [^ '\n']+ newline { ocamlobjinfo (acc :: acc_units) empty lexbuf }
+  | _ | eof { acc :: acc_units }
+
+and archive acc = parse
+  | "Unit name:" ws (name as name) { archive (Module_name.Unique.Set.add acc (Module_name.Unique.of_string name)) lexbuf }
+  | _ { archive acc lexbuf }
   | eof { acc }
-and intfs acc = parse
-  | ws hash ws (name as name) newline { intfs (add_intf acc name) lexbuf }
-  | "Implementations imported:" newline { impls acc lexbuf }
-  | _ | eof { acc }
-and impls acc = parse
-  | ws hash ws (name as name) newline { impls (add_impl acc name) lexbuf }
-  | _ | eof { acc }
 
 {
-let parse s = ocamlobjinfo empty (Lexing.from_string s)
+let parse s = Lexing.from_string s |> ocamlobjinfo [] empty  |> List.rev
+
+let parse_archive s =
+  Lexing.from_string s
+  |> archive Module_name.Unique.Set.empty
 
-let rules (ocaml : Ocaml_toolchain.t) ~dir ~sandbox ~unit =
+let rules (ocaml : Ocaml_toolchain.t) ~dir ~sandbox ~units =
   let no_approx =
     if Ocaml.Version.ooi_supports_no_approx ocaml.version then
       [Command.Args.A "-no-approx"]
@@ -52,7 +63,9 @@ let rules (ocaml : Ocaml_toolchain.t) ~dir ~sandbox ~unit =
       []
   in
   let observing_facts =
-    Dep.Facts.singleton (Dep.file unit) (Dep.Fact.nothing)
+    List.map units ~f:(fun unit ->
+      Dep.Facts.singleton (Dep.file unit) (Dep.Fact.nothing))
+    |> Dep.Facts.union_all
   in
   let open Action_builder.O in
   let* action =
@@ -61,7 +74,7 @@ let rules (ocaml : Ocaml_toolchain.t) ~dir ~sandbox ~unit =
       (List.concat
          [ no_approx
          ; no_code
-         ; [ Dep unit ]
+         ; [ Deps units ]
          ])
   in
   (Dune_engine.Build_system.execute_action_stdout
@@ -73,4 +86,24 @@ let rules (ocaml : Ocaml_toolchain.t) ~dir ~sandbox ~unit =
     }
   |> Action_builder.of_memo)
   >>| parse
+
+let archive_rules (ocaml : Ocaml_toolchain.t) ~dir ~sandbox ~archive =
+  let observing_facts =
+    Dep.Facts.singleton (Dep.file archive) (Dep.Fact.nothing)
+  in
+  let open Action_builder.O in
+  let* action =
+    Command.run' ?sandbox
+      ~dir:(Path.build dir) ocaml.ocamlobjinfo
+      [ Dep archive ]
+  in
+  (Dune_engine.Build_system.execute_action_stdout
+    ~observing_facts
+    { Rule.Anonymous_action.action
+    ; loc = Loc.none
+    ; dir
+    ; alias = None
+    }
+  |> Action_builder.of_memo)
+  >>| parse_archive
 }