[ADT] Implicitly convert between StringRef and std::string_view when …

…we have C++17

This makes the types almost seamlessly interchangeable in C++17
codebases. Eventually we want to replace StringRef with the standard
type, but that requires C++17 being the default and a huge refactoring
job as StringRef has a lot more functionality.

llvm/include/llvm/ADT/StringRef.h

@@ -18,6 +18,9 @@
 #include <cstring>
 #include <limits>
 #include <string>
+#if __cplusplus > 201402L
+#include <string_view>
+#endif
 #include <type_traits>
 #include <utility>
 
@@ -110,6 +113,12 @@ namespace llvm {
     /*implicit*/ StringRef(const std::string &Str)
       : Data(Str.data()), Length(Str.length()) {}
 
+#if __cplusplus > 201402L
+    /// Construct a string ref from an std::string_view.
+    /*implicit*/ constexpr StringRef(std::string_view Str)
+        : Data(Str.data()), Length(Str.size()) {}
+#endif
+
     static StringRef withNullAsEmpty(const char *data) {
       return StringRef(data ? data : "");
     }
@@ -267,6 +276,12 @@ namespace llvm {
       return str();
     }
 
+#if __cplusplus > 201402L
+    operator std::string_view() const {
+      return std::string_view(data(), size());
+    }
+#endif
+
     /// @}
     /// @name String Predicates
     /// @{

llvm/unittests/ADT/StringRefTest.cpp

@@ -59,6 +59,16 @@ TEST(StringRefTest, Construction) {
   EXPECT_EQ("hello", StringRef("hello"));
   EXPECT_EQ("hello", StringRef("hello world", 5));
   EXPECT_EQ("hello", StringRef(std::string("hello")));
+#if __cplusplus > 201402L
+  EXPECT_EQ("hello", StringRef(std::string_view("hello")));
+#endif
+}
+
+TEST(StringRefTest, Conversion) {
+  EXPECT_EQ("hello", std::string(StringRef("hello")));
+#if __cplusplus > 201402L
+  EXPECT_EQ("hello", std::string_view(StringRef("hello")));
+#endif
 }
 
 TEST(StringRefTest, EmptyInitializerList) {