[SmallString] Add explicit conversion to std::string

With the conversion between StringRef and std::string now being
explicit, converting SmallStrings becomes more tedious. This patch adds
an explicit operator so you can write std::string(Str) instead of
Str.str().str().

Differential revision: https://reviews.llvm.org/D73640

llvm/include/llvm/ADT/SmallString.h

@@ -275,6 +275,8 @@ public:
   /// Implicit conversion to StringRef.
   operator StringRef() const { return str(); }
 
+  explicit operator std::string() const { return str().str(); }
+
   // Extra operators.
   const SmallString &operator=(StringRef RHS) {
     this->clear();

llvm/unittests/ADT/SmallStringTest.cpp

@@ -96,6 +96,20 @@ TEST_F(SmallStringTest, AppendSmallVector) {
   EXPECT_STREQ("abcabc", theString.c_str());
 }
 
+TEST_F(SmallStringTest, StringRefConversion) {
+  StringRef abc = "abc";
+  theString.assign(abc.begin(), abc.end());
+  StringRef theStringRef = theString;
+  EXPECT_EQ("abc", theStringRef);
+}
+
+TEST_F(SmallStringTest, StdStringConversion) {
+  StringRef abc = "abc";
+  theString.assign(abc.begin(), abc.end());
+  std::string theStdString = std::string(theString);
+  EXPECT_EQ("abc", theStdString);
+}
+
 TEST_F(SmallStringTest, Substr) {
   theString = "hello";
   EXPECT_EQ("lo", theString.substr(3));