fix(ctypes): track implicit header dependencies

doc/changes/fixed/16243.md

@@ -0,0 +1,2 @@
+- Rebuild generated Ctypes helpers when headers from Ctypes' implicit include
+  directories change (#16243, @rgrinberg)

src/dune_rules/ctypes/ctypes_rules.ml

@@ -207,23 +207,24 @@ let build_c_program
   in
   let include_args =
     let open Action_builder.O in
-    let+ include_dirs =
-      let* ocaml = Action_builder.of_memo ocaml in
-      let+ ctypes_include_dirs =
-        (* XXX: need glob dependency *)
-        let open Action_builder.O in
-        let+ lib =
-          let ctypes = Lib_name.of_string "ctypes" in
-          Lib.DB.resolve (Scope.libs scope) (Loc.none, ctypes) |> Resolve.Memo.read
-        in
-        Lib_flags.L.include_paths [ lib ] (Ocaml Native) ocaml.lib_config
-        |> Lib_flags.L.include_only
+    let* ocaml = Action_builder.of_memo ocaml in
+    let+ ctypes_include_dirs =
+      let+ lib =
+        let ctypes = Lib_name.of_string "ctypes" in
+        Lib.DB.resolve (Scope.libs scope) (Loc.none, ctypes) |> Resolve.Memo.read
       in
-      let ocaml_where = ocaml.lib_config.stdlib_dir in
-      ocaml_where :: ctypes_include_dirs
+      Lib_flags.L.include_paths [ lib ] (Ocaml Native) ocaml.lib_config
+      |> Lib_flags.L.include_only
+    in
+    let include_arg dir = Command.Args.S [ A "-I"; Path dir ] in
+    let ctypes_include_arg dir =
+      let headers = File_selector.of_glob ~dir (Glob.of_string "*.h") in
+      Command.Args.S
+        [ include_arg dir; Hidden_deps (Dep.Set.singleton (Dep.file_selector headers)) ]
     in
     Command.Args.S
-      (List.map include_dirs ~f:(fun dir -> Command.Args.S [ A "-I"; Path dir ]))
+      (include_arg ocaml.lib_config.stdlib_dir
+       :: List.map ctypes_include_dirs ~f:ctypes_include_arg)
   in
   let extra_deps =
     let source_file_deps =

test/blackbox-tests/test-cases/ctypes/exe-vendored.t/run.t

@@ -36,7 +36,7 @@ With Ctypes 0.3, the final executable link runs outside a sandbox.
   >   ][0]'
   {"sandbox":false,"example_flags":1}
 
-The Ctypes headers found through its implicit include directory are not rule
+The Ctypes headers found through its implicit include directory are rule
 dependencies.
 
   $ dune rules --format=json \
@@ -46,12 +46,14 @@ dependencies.
   >    | select(.deps)
   >    | ruleDepFilePaths
   >    | select(endswith("ctypes_cstubs_internals.h"))
+  >    | {kind: "file", path: basename}
   >    ]
   >    +
   >    [ .[]
   >    | select(.deps)
   >    | ruleDepGlobEntries
   >    | select((.dir | endswith("ctypes"))
   >             and (.predicate | tostring | contains(".h")))
-  >    ]) | length > 0'
-  false
+  >    | {kind: "glob", dir: (.dir | basename), predicate}
+  >    ]) | unique'
+  [{"kind":"glob","dir":"ctypes","predicate":"*.h"}]