diff --git a/tools/fuzzing/libfuzzer/FuzzerIO.cpp b/tools/fuzzing/libfuzzer/FuzzerIO.cpp
--- a/tools/fuzzing/libfuzzer/FuzzerIO.cpp
+++ b/tools/fuzzing/libfuzzer/FuzzerIO.cpp
@@ -3,16 +3,17 @@
 // 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
 //
 //===----------------------------------------------------------------------===//
 // IO functions.
 //===----------------------------------------------------------------------===//
 
+#include "mozilla/Unused.h"
 #include "FuzzerDefs.h"
 #include "FuzzerExtFunctions.h"
 #include "FuzzerIO.h"
 #include "FuzzerUtil.h"
 #include <algorithm>
 #include <cstdarg>
 #include <fstream>
 #include <iterator>
@@ -59,17 +60,17 @@ std::string FileToString(const std::stri
 void CopyFileToErr(const std::string &Path) {
   Printf("%s", FileToString(Path).c_str());
 }
 
 void WriteToFile(const Unit &U, const std::string &Path) {
   // Use raw C interface because this function may be called from a sig handler.
   FILE *Out = fopen(Path.c_str(), "w");
   if (!Out) return;
-  fwrite(U.data(), sizeof(U[0]), U.size(), Out);
+  mozilla::Unused << fwrite(U.data(), sizeof(U[0]), U.size(), Out);
   fclose(Out);
 }
 
 void ReadDirToVectorOfUnits(const char *Path, Vector<Unit> *V,
                             long *Epoch, size_t MaxSize, bool ExitOnError) {
   long E = Epoch ? *Epoch : 0;
   Vector<std::string> Files;
   ListFilesInDirRecursive(Path, Epoch, &Files, /*TopDir*/true);
diff --git a/tools/fuzzing/libfuzzer/FuzzerIOPosix.cpp b/tools/fuzzing/libfuzzer/FuzzerIOPosix.cpp
--- a/tools/fuzzing/libfuzzer/FuzzerIOPosix.cpp
+++ b/tools/fuzzing/libfuzzer/FuzzerIOPosix.cpp
@@ -2,16 +2,17 @@
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 // IO functions implementation using Posix API.
 //===----------------------------------------------------------------------===//
+#include "mozilla/Unused.h"
 #include "FuzzerDefs.h"
 #if LIBFUZZER_POSIX || LIBFUZZER_FUCHSIA
 
 #include "FuzzerExtFunctions.h"
 #include "FuzzerIO.h"
 #include <cstdarg>
 #include <cstdio>
 #include <dirent.h>
@@ -154,17 +155,17 @@ bool IsInterestingCoverageFile(const std
   if (FileName.find("/usr/include/") != std::string::npos)
     return false;
   if (FileName == "<null>")
     return false;
   return true;
 }
 
 void RawPrint(const char *Str) {
-  write(2, Str, strlen(Str));
+  mozilla::Unused << write(2, Str, strlen(Str));
 }
 
 void MkDir(const std::string &Path) {
   mkdir(Path.c_str(), 0700);
 }
 
 void RmDir(const std::string &Path) {
   rmdir(Path.c_str());