feature: add an unused-libs alias (#12623)

* feature: unused-libs alias

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

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

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

Author: rgrinberg

boot/libs.ml

@@ -1,5 +1,5 @@
 open Types
-let external_libraries = [ "unix"; "threads" ]
+let external_libraries = [ "pp"; "unix"; "csexp"; "threads" ]
 
 let local_libraries =
   [ { path = "otherlibs/top-closure"

doc/changes/added/12623.md

@@ -0,0 +1,2 @@
+- Introduce an `unused-libs` alias 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