[Support] Replace function with function_ref in writeFileAtomically. NFC

[ilya-biryukov]

Summary

The latter is slightly more efficient and communicates the intent of the
API: writeFileAtomically does not own or copy the callback, it merely
calls it at some point.

[ilya-biryukov]

• Reformat

[jkorous]

Good point! Thanks.

[ilya-biryukov]

Ah, the tests fail to compile because we now have an overload where the only difference is function_ref vs StringRef.
This works fine with std::function, STL probably ensures the conversions from StringRef to std::functtion fail and function_ref will only fail at constructor instantiation time.

I'll keep this open for now, will see if it's feasible to make function_ref similar to std::function in that case.

llvm/include/llvm/Support/FileUtilities.h

 //===- llvm/Support/FileUtilities.h - File System Utilities -----*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file defines a family of utility functions which are useful for doing
 // various things with files.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_FILEUTILITIES_H
 #define LLVM_SUPPORT_FILEUTILITIES_H
 
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 
 namespace llvm {
 
namespace llvm {
   /// writes content of \p Buffer to the file and renames it to \p FinalPath.
   ///
   /// \returns \c AtomicFileWriteError in case of error.
   llvm::Error writeFileAtomically(StringRef TempPathModel, StringRef FinalPath,
                                   StringRef Buffer);
 
   llvm::Error
   writeFileAtomically(StringRef TempPathModel, StringRef FinalPath,
-                      std::function<llvm::Error(llvm::raw_ostream &)> Writer);
+                      llvm::function_ref<llvm::Error(llvm::raw_ostream &)> Writer);
 } // End llvm namespace
 
 #endif

llvm/lib/Support/FileUtilities.cpp

return writeFileAtomically(TempPathModel, FinalPath,
                              [&Buffer](llvm::raw_ostream &OS) {
                                OS.write(Buffer.data(), Buffer.size());
                                return llvm::Error::success();
                              });
 }
 
 llvm::Error llvm::writeFileAtomically(
     StringRef TempPathModel, StringRef FinalPath,
-    std::function<llvm::Error(llvm::raw_ostream &)> Writer) {
+    llvm::function_ref<llvm::Error(llvm::raw_ostream &)> Writer) {
   SmallString<128> GeneratedUniqPath;
   int TempFD;
   if (sys::fs::createUniqueFile(TempPathModel.str(), TempFD,
                                 GeneratedUniqPath)) {
     return llvm::make_error<AtomicFileWriteError>(
         atomic_write_error::failed_to_create_uniq_file);
   }
   llvm::FileRemover RemoveTmpFileOnFail(GeneratedUniqPath);