[inefficient-keyset-iterator] Add documentation

Reviewed By: ngorogiannis

Differential Revision: D22046100

fbshipit-source-id: 71954ce49

infer/documentation/issues/INEFFICIENT_KEYSET_ITERATOR.md

@@ -0,0 +1,26 @@
+This issue is raised when
+- iterating over a HashMap with `ketSet()` iterator
+- looking up the key each time
+
+Instead, it is more efficient to iterate over the loop with `entrySet` which returns key-vaue pairs and gets rid of the hashMap lookup.
+ For instance, we would raise an issue for the following program:
+
+```java
+void inefficient_loop_bad(HashMap<String, Integer> testMap) {
+ for (String key : testMap.keySet()) {
+   Integer value = testMap.get(key); // extra look-up cost
+   foo(key, value);
+ }
+}
+```
+
+Instead, it is more efficient to have:
+```java
+void efficient_loop_ok(HashMap<String, Integer> testMap) {
+  for (Map.Entry<String, Integer> entry : testMap.entrySet()) {
+    String key = entry.getKey();
+    Integer value = entry.getValue();
+    foo(key, value);
+  }
+}
+```

infer/src/base/IssueType.ml

@@ -633,6 +633,7 @@ let impure_function = register_from_string ~id:"IMPURE_FUNCTION" Error Impurity
 
 let inefficient_keyset_iterator =
   register_from_string ~id:"INEFFICIENT_KEYSET_ITERATOR" Error InefficientKeysetIterator
+    ~user_documentation:[%blob "../../documentation/issues/INEFFICIENT_KEYSET_ITERATOR.md"]
 
 
 let inferbo_alloc_is_big =