Avoid some string copies

Summary: This is a behavior-preserving change.

Reviewed By: wsanville

Differential Revision: D49210587

fbshipit-source-id: 7d1d8ce235feff0d80375635299a51f4db471199

libredex/DexClass.cpp

@@ -110,7 +110,7 @@ std::string get_simple_deobf_name(const T* ref) {
   const auto& full_name = ref->get_deobfuscated_name_or_empty();
   if (full_name.empty()) {
     // This comes up for redex-created methods/fields.
-    return std::string(ref->c_str());
+    return ref->str_copy();
   }
   auto dot_pos = full_name.find('.');
   auto colon_pos = full_name.find(':');

libredex/RedexContext.cpp

@@ -515,7 +515,7 @@ void RedexContext::mutate_field(DexFieldRef* field,
   if (rename_on_collision && s_field_map.find(r) != s_field_map.end()) {
     uint32_t i = 0;
     while (true) {
-      r.name = DexString::make_string(("f$" + std::to_string(i++)).c_str());
+      r.name = DexString::make_string("f$" + std::to_string(i++));
       if (s_field_map.find(r) == s_field_map.end()) {
         break;
       }
@@ -680,7 +680,7 @@ void RedexContext::mutate_method(DexMethodRef* method,
         prefix = r.name->str() + "$";
       }
       do {
-        r.name = DexString::make_string((prefix + std::to_string(i++)).c_str());
+        r.name = DexString::make_string(prefix + std::to_string(i++));
       } while (s_method_map.count(r));
     } else {
       // We are about to change its class. Use a better name to remember its