[ConfigImpact] Filter known expensive callees when cost is constant

Summary:
This diff filters known expensive callees when cost is constant.

previous:
```
foo() {
  known_expensive_call();
}
```

current:
```
foo() {
  known_expensive_call();
  goo();
}

// cost is constant
goo() {
  known_expensive_call();
  unknown_call();
}
```

When callee's cost is constant and its summary includes known expensive callees, the checker addressed it as a non-constant-cost callee, i.e., it copies all ungated callees from the callee's summary. However, sometimes this full-copying introduces unexpected issues. For example, suppose a callee `goo` is added and `goo`'s cost is constant as above. Since it includes `known_expensive_call`, all ungated callees of its summary is copied to the caller `foo`'s summary:

* `foo`'s ungated callees (before): {`known_expensive_call`}
* `foo`'s ungated callees (after): {`known_expensive_call`, `unknown_call`}

As a result, it would report about `unknown_call` is added. However, this is not what we intended: In the example, `unknown_call` is reported because it is called in the same function with `known_expensive_call`, not because it is expensive.

To fix that issue, this diff filters known expensive callees from `goo`'s summary in that case.

Reviewed By: ezgicicek

Differential Revision: D27852552

fbshipit-source-id: d207eef1c

infer/src/cost/ConfigImpactAnalysis.ml

@@ -116,6 +116,8 @@ module UncheckedCallees = struct
   let pp_without_location f x = UncheckedCallee.pp_without_location_list f (elements x)
 
   let has_known_expensive_callee x = exists (fun {is_known_expensive} -> is_known_expensive) x
+
+  let filter_known_expensive x = filter (fun {is_known_expensive} -> is_known_expensive) x
 end
 
 module UncheckedCalleesCond = struct
@@ -129,6 +131,15 @@ module UncheckedCalleesCond = struct
 
   let replace_location_by_call location fields_map =
     map (UncheckedCallees.replace_location_by_call location) fields_map
+
+
+  let filter_known_expensive fields_map =
+    fold
+      (fun fields callees acc ->
+        let expensive_callees = UncheckedCallees.filter_known_expensive callees in
+        if UncheckedCallees.is_empty expensive_callees then acc
+        else add fields expensive_callees acc )
+      fields_map empty
 end
 
 module Loc = struct
@@ -423,6 +434,15 @@ module Dom = struct
             ; unchecked_callees_cond= callee_summary_cond
             ; has_call_stmt }
           when has_call_stmt ->
+            let callee_summary, callee_summary_cond =
+              if is_cheap_call then
+                (* In this case, the callee is cheap by the heuristics, but its summary includes
+                   some known expensive callees. Thus, it filters the known expensive callees
+                   only. *)
+                ( UncheckedCallees.filter_known_expensive callee_summary
+                , UncheckedCalleesCond.filter_known_expensive callee_summary_cond )
+              else (callee_summary, callee_summary_cond)
+            in
             (* If callee's summary is not leaf, use it. *)
             join_unchecked_callees
               (UncheckedCallees.replace_location_by_call location callee_summary)