[dead stores] report on dead stores to values captured by value in a lambda

Summary:
Before D7100561, the frontend translated capture-by-ref and capture-by-value in the same way.
Now we can tell the difference and report bugs in the capture-by-value case.

Reviewed By: jeremydubreil

Differential Revision: D7102214

fbshipit-source-id: e9d3ac7

infer/src/IR/Exp.ml

@@ -206,7 +206,7 @@ let fold_captured ~f exp acc =
         fold_captured_ e1 captured_acc |> fold_captured_ e2
     | Closure {captured_vars} ->
         List.fold captured_vars
-          ~f:(fun acc (_, captured_pvar, _) -> f acc captured_pvar)
+          ~f:(fun acc (captured_exp, _, _) -> f acc captured_exp)
           ~init:captured_acc
     | Const _ | Lvar _ | Var _ | Sizeof _ ->
         captured_acc

infer/src/IR/Exp.mli

@@ -122,8 +122,8 @@ val get_vars : t -> Ident.t list * Pvar.t list
 val program_vars : t -> Pvar.t Sequence.t
 (** all the program variables appearing in the expression *)
 
-val fold_captured : f:('a -> Pvar.t -> 'a) -> t -> 'a -> 'a
-(** Extract the program variables captured by this expression *)
+val fold_captured : f:('a -> t -> 'a) -> t -> 'a -> 'a
+(** Fold over the expressions captured by this expression. *)
 
 val pp_printenv : Pp.env -> (Pp.env -> F.formatter -> Typ.t -> unit) -> F.formatter -> t -> unit
 

infer/src/checkers/liveness.ml

@@ -105,7 +105,16 @@ module CapturedByRefTransferFunctions (CFG : ProcCfg.S) = struct
   let exec_instr astate _ _ instr =
     List.fold (Sil.instr_get_exps instr)
       ~f:(fun acc exp ->
-        Exp.fold_captured ~f:(fun acc pvar -> Domain.add (Var.of_pvar pvar) acc) exp acc )
+        Exp.fold_captured exp
+          ~f:(fun acc exp ->
+            match exp with
+            | Exp.Lvar pvar ->
+                (* captured by reference, add *)
+                Domain.add (Var.of_pvar pvar) acc
+            | _ ->
+                (* captured by value or init-capture, skip *)
+                acc )
+          acc )
       ~init:astate
 end
 

infer/tests/codetoanalyze/cpp/liveness/dead_stores.cpp

@@ -199,9 +199,7 @@ int dead_store_before_capture_by_ref_bad() {
   return lambda();
 }
 
-// frontend can't tell the difference between capture by ref and capture by
-// value yet.
-int FN_capture_by_value_bad() {
+int capture_by_value_bad() {
   int x = 1;
   auto lambda = [=] { return x; };
   x = 2; // this is dead

infer/tests/codetoanalyze/cpp/liveness/issues.exp

@@ -1,4 +1,5 @@
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::FP_assign_array_tricky_ok, 3, DEAD_STORE, [Write of unused value]
+codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::capture_by_value_bad, 3, DEAD_STORE, [Write of unused value]
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::dead_pointer_bad, 2, DEAD_STORE, [Write of unused value]
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::dead_store_before_capture_by_ref_bad, 1, DEAD_STORE, [Write of unused value]
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::dead_struct_no_destructor_bad, 0, DEAD_STORE, [Write of unused value]