Move toStdString to FBString.h

Summary: Move `toStdString` to `FBString.h` because it's fbstring-related and remove the dependency between `String.h` and `FBString.h`. This is possible because the only other use of fbstring in `String.h` is removed in previous diff.

Reviewed By: yfeldblum

Differential Revision: D20195591

fbshipit-source-id: b5ff7c918669e063e24bf91155a2496f82d7e975

folly/FBString.h

@@ -2806,6 +2806,23 @@ typedef basic_fbstring<char> fbstring;
 template <class T, class R, class A, class S>
 FOLLY_ASSUME_RELOCATABLE(basic_fbstring<T, R, A, S>);
 
+// Compatibility function, to make sure toStdString(s) can be called
+// to convert a std::string or fbstring variable s into type std::string
+// with very little overhead if s was already std::string
+inline std::string toStdString(const folly::fbstring& s) {
+  return std::string(s.data(), s.size());
+}
+
+inline const std::string& toStdString(const std::string& s) {
+  return s;
+}
+
+// If called with a temporary, the compiler will select this overload instead
+// of the above, so we don't return a (lvalue) reference to a temporary.
+inline std::string&& toStdString(std::string&& s) {
+  return std::move(s);
+}
+
 } // namespace folly
 
 // Hash functions to make fbstring usable with e.g. unordered_map

folly/String.h

@@ -26,31 +26,13 @@
 
 #include <folly/Conv.h>
 #include <folly/ExceptionString.h>
-#include <folly/FBString.h>
 #include <folly/Portability.h>
 #include <folly/Range.h>
 #include <folly/ScopeGuard.h>
 #include <folly/Traits.h>
 
-// Compatibility function, to make sure toStdString(s) can be called
-// to convert a std::string or fbstring variable s into type std::string
-// with very little overhead if s was already std::string
 namespace folly {
 
-inline std::string toStdString(const folly::fbstring& s) {
-  return std::string(s.data(), s.size());
-}
-
-inline const std::string& toStdString(const std::string& s) {
-  return s;
-}
-
-// If called with a temporary, the compiler will select this overload instead
-// of the above, so we don't return a (lvalue) reference to a temporary.
-inline std::string&& toStdString(std::string&& s) {
-  return std::move(s);
-}
-
 /**
  * C-Escape a string, making it suitable for representation as a C string
  * literal.  Appends the result to the output string.