Fix SIL to HIL conversion for Exp.Var inside Lfield and Lindex (#1372)

Summary:
When accessing a field or array offset of a pure variable (`Exp.Var`) that does not resolve to an access expression, `HilExp.of_sil` will create an extraneous dereference that causes `HilExp.get_typ` to fail. This pull request wraps variables that are the bases of Lfield or Lindex expressions with AddressOf before they're dereferenced (this is already done for Lvar inside `AccessExpression.of_pvar`) and adds a couple of unit tests that make sure it behaves as expected.

**More details on the bug:**

Given the following code:

```
if (!event_obj->dict)
```

and SIL:

```
n$6=_fun_gdb::ref_ptr<event_object,gdbpy_ref_policy<event_object>>::operator->(&event_obj:gdb::ref_ptr<event_object,gdbpy_ref_policy<event_object>>&) [line 38, column 8];
n$7=*n$6.dict:_object* [line 38, column 8];
PRUNE(!n$7, true); [line 38, column 8];
```

`operator->` has return type `event_object*`, but `n$6.dict` only has access to the type of the struct, `event_object`. `of_sil` [calls](9f98368e49/infer/src/absint/HilExp.ml (L567)) `access_expr_of_lhs_exp` with that type, which [calls](9f98368e49/infer/src/absint/HilExp.ml (L498)) `access_exprs_of_exp` (note that `add_deref` is always true). The Lfield case will then [recurse](9f98368e49/infer/src/absint/HilExp.ml (L469)) to process the Exp.Var, and `AccessExpression.of_id` will return an `AccessPath.base` that is then [dereferenced](9f98368e49/infer/src/absint/HilExp.ml (L440)). When resolving types, `get_typ` will find a non-pointer type wrapped by a `Dereference` and return [None](9f98368e49/infer/src/absint/HilExp.ml (L286)). To fix this, we match what [of_pvar](9f98368e49/infer/src/absint/HilExp.ml (L295)) does and wrap the base in an AddressOf, which is removed by the dereference.

Pull Request resolved: https://github.com/facebook/infer/pull/1372

Reviewed By: ngorogiannis

Differential Revision: D25803049

Pulled By: jvillard

fbshipit-source-id: ceadc8cad

infer/src/absint/HilExp.ml

@@ -435,7 +435,7 @@ and access_exprs_of_exp ~include_array_indexes ~f_resolve_id ~add_deref exp0 typ
           in
           add_accesses access_expr' :: acc
       | None ->
-          let access_expr = AccessExpression.of_id id typ in
+          let access_expr = AccessExpression.address_of_base (AccessExpression.base_of_id id typ) in
           let access_expr' =
             if add_deref then AccessExpression.dereference access_expr else access_expr
           in

infer/src/inferunit.ml

@@ -34,6 +34,7 @@ let () =
         ; DifferentialTests.tests
         ; FileDiffTests.tests
         ; GradleTests.tests
+        ; HilExpTests.tests
         ; IListTests.tests
         ; JavaClassNameTests.tests
         ; JavaProfilerSamplesTest.tests

infer/src/unit/HilExpTests.ml

@@ -0,0 +1,39 @@
+(*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *)
+open! IStd
+open OUnit2
+
+let tests =
+  let struct_typ = Typ.mk_struct (Typ.Name.C.from_string "struct") in
+  let array_typ = Typ.mk_array StdTyp.int in
+  let fieldname = AccessPathTestUtils.make_fieldname "field" in
+  let var_id = Ident.create_normal (Ident.string_to_name "n") 0 in
+  let f_resolve_id _ = None in
+  let test_of_sil =
+    let var_field _ =
+      assert_equal ~printer:(Format.asprintf "%a" HilExp.pp)
+        (HilExp.of_sil ~include_array_indexes:false ~f_resolve_id ~add_deref:true
+           (Exp.Lfield (Exp.Var var_id, fieldname, struct_typ))
+           StdTyp.void)
+        (HilExp.AccessExpression
+           (HilExp.AccessExpression.field_offset
+              (HilExp.AccessExpression.base (Var.of_id var_id, struct_typ))
+              fieldname))
+    in
+    let var_index _ =
+      assert_equal ~printer:(Format.asprintf "%a" HilExp.pp)
+        (HilExp.of_sil ~include_array_indexes:false ~f_resolve_id ~add_deref:true
+           (Exp.Lindex (Exp.Var var_id, Exp.zero))
+           StdTyp.int)
+        (HilExp.AccessExpression
+           (HilExp.AccessExpression.array_offset
+              (HilExp.AccessExpression.base (Var.of_id var_id, array_typ))
+              StdTyp.int None))
+    in
+    "of_sil" >::: ["var_field" >:: var_field; "var_index" >:: var_index]
+  in
+  "hil_exp_suite" >::: [test_of_sil]