Add semgrep rule for Fun.id

.semgrep/fun.yml

@@ -0,0 +1,6 @@
+rules:
+  - id: fun-id
+    pattern: fun $X -> $X
+    message: use Fun.id instead
+    languages: [ocaml]
+    severity: WARNING

src/cdomains/exp.ml

@@ -192,7 +192,7 @@ struct
     let rec separate_fields_index o =
       match o with
       | NoOffset -> None
-      | Index (ie,o) -> Some ((fun x -> x),ie,o)
+      | Index (ie,o) -> Some (Fun.id,ie,o)
       | Field (f,o) ->
         match separate_fields_index o with
         | Some (osf, ie,o) -> Some ((fun o -> Field (f,o)), ie, o)

src/cdomains/mHP.ml

@@ -23,7 +23,7 @@ let pretty () {tid; created; must_joined} =
     else
       Some (Pretty.dprintf "must_joined=%a" ConcDomain.ThreadSet.pretty must_joined)
   in
-  let docs = List.filter_map (fun doc -> doc) [tid_doc; created_doc; must_joined_doc] in
+  let docs = List.filter_map Fun.id [tid_doc; created_doc; must_joined_doc] in
   Pretty.dprintf "{%a}" (Pretty.d_list "; " Pretty.insert) docs
 
 include Printable.SimplePretty (