[SmallString] Add explicit conversion to std::string

[JDevlieghere]

Summary

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().

[MaskRay]

llvm/include/llvm/ADT/SmallString.h

278

The comment is redundant, because the code self explains.

[JDevlieghere]

llvm/include/llvm/ADT/SmallString.h

278

It's the same in StringRef, but I agree!

[merge_guards_bot]

Unit tests: fail. 62303 tests passed, 1 failed and 838 were skipped.

failed: libc++.std/thread/thread_mutex/thread_mutex_requirements/thread_mutex_requirements_mutex/thread_mutex_recursive/try_lock.pass.cpp

clang-tidy: fail. clang-tidy found 0 errors and 4 warnings. 0 of them are added as review comments below (why?).

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

Pre-merge checks is in beta. Report issue. Please join beta or enable it for your project.

[MaskRay]

With the conversion between StringRef and std::string now being explicit, converting SmallStrings becomes more tedious

Explicitly mention 777180a32b61070a10dd330b4f038bf24e916af1 in the description.

[cishida]

This indirection seems like unnecessary work. Could you reimplement this as a direct conversion to std::string?

[JDevlieghere]

In D73640#1847990, @cishida wrote:

This indirection seems like unnecessary work. Could you reimplement this as a direct conversion to std::string?

Good catch, thank you!

[craig.topper]

llvm/include/llvm/ADT/SmallString.h

279

I'm late here, but that should probably be this->data() instead of this->begin(). It does the same thing in this case, but begin() returns an "iterator" while data() returns a pointer. And the constructor that's being called is defined as taking a pointer and size.

[JDevlieghere]

llvm/include/llvm/ADT/SmallString.h

279

Thanks Craig! I've fixed it for the std::string and llvm::StringRef case above.

To github.com:llvm/llvm-project.git
   cfebd777422..a5f479473b2  master -> master

llvm/include/llvm/ADT/SmallString.h

const char* c_str() {
     this->push_back(0);
     this->pop_back();
     return this->data();
   }
 
   /// Implicit conversion to StringRef.
   operator StringRef() const { return str(); }
 
-  explicit operator std::string() const { return str().str(); }
+  explicit operator std::string() const {
+    return std::string(this->begin(), this->size());
+  }
 
   // Extra operators.
   const SmallString &operator=(StringRef RHS) {
     this->clear();
     return *this += RHS;
   }
 
   SmallString &operator+=(StringRef RHS) {