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 <me@rgrinberg.com>

feature: allow_unused_libraries

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

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/artifacts_db.ml

@@ -23,6 +23,7 @@ let available_exes ~dir (exes : Executables.t) =
       libs
       (`Exe exes.names)
       exes.buildable.libraries
+      ~allow_unused_libraries:exes.buildable.allow_unused_libraries
       ~pps
       ~dune_version
       ~forbidden_libraries:exes.forbidden_libraries

src/dune_rules/cinaps.ml

@@ -140,6 +140,7 @@ let gen_rules sctx t ~dir ~scope =
       (Scope.libs scope)
       (`Exe names)
       (Lib_dep.Direct (loc, Lib_name.of_string "cinaps.runtime") :: t.libraries)
+      ~allow_unused_libraries:[]
       ~pps:(Preprocess.Per_module.pps t.preprocess)
       ~dune_version
       ~allow_overlaps:false

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/dune_package.ml

@@ -289,6 +289,7 @@ module Lib = struct
            ~plugins
            ~archives
            ~ppx_runtime_deps
+           ~allow_unused_libraries:[]
            ~foreign_archives
            ~native_archives:(Files native_archives)
            ~foreign_dll_files

src/dune_rules/exe_rules.ml

@@ -208,6 +208,20 @@ 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
+    let allow_unused_libraries = Lib.Compile.allow_unused_libraries compile_info in
+    Unused_libs_rules.gen_rules
+      sctx
+      toolchain
+      exes.buildable.loc
+      ~obj_dir
+      ~modules
+      ~dir
+      ~direct_requires
+      ~allow_unused_libraries
+  in
   let* () =
     let* dep_graphs =
       (* Building an archive for foreign stubs, we link the corresponding object
@@ -328,6 +342,7 @@ let compile_info ~scope (exes : Executables.t) =
     (Scope.libs scope)
     (`Exe exes.names)
     exes.buildable.libraries
+    ~allow_unused_libraries:exes.buildable.allow_unused_libraries
     ~pps
     ~dune_version
     ~allow_overlaps:exes.buildable.allow_overlapping_dependencies