diff --git a/lldb/include/lldb/Expression/DiagnosticManager.h b/lldb/include/lldb/Expression/DiagnosticManager.h
index 8fe1b4b5d06e..d9024e649b80 100644
--- a/lldb/include/lldb/Expression/DiagnosticManager.h
+++ b/lldb/include/lldb/Expression/DiagnosticManager.h
@@ -13,6 +13,8 @@
 #include "lldb/lldb-defines.h"
 #include "lldb/lldb-types.h"
 
+#include "llvm/ADT/StringRef.h"
+
 #include <string>
 #include <vector>
 
@@ -55,7 +57,7 @@ public:
     }
   }
 
-  Diagnostic(const char *message, DiagnosticSeverity severity,
+  Diagnostic(llvm::StringRef message, DiagnosticSeverity severity,
              DiagnosticOrigin origin, uint32_t compiler_id)
       : m_message(message), m_severity(severity), m_origin(origin),
         m_compiler_id(compiler_id) {}
@@ -72,9 +74,10 @@ public:
 
   uint32_t GetCompilerID() const { return m_compiler_id; }
 
-  const char *GetMessage() const { return m_message.c_str(); }
+  llvm::StringRef GetMessage() const { return m_message; }
 
-  void AppendMessage(const char *message, bool precede_with_newline = true) {
+  void AppendMessage(llvm::StringRef message,
+                     bool precede_with_newline = true) {
     if (precede_with_newline)
       m_message.push_back('\n');
     m_message.append(message);
@@ -114,7 +117,7 @@ public:
     return false;
   }
 
-  void AddDiagnostic(const char *message, DiagnosticSeverity severity,
+  void AddDiagnostic(llvm::StringRef message, DiagnosticSeverity severity,
                      DiagnosticOrigin origin,
                      uint32_t compiler_id = LLDB_INVALID_COMPILER_ID) {
     m_diagnostics.push_back(
@@ -127,11 +130,11 @@ public:
 
   size_t Printf(DiagnosticSeverity severity, const char *format, ...)
       __attribute__((format(printf, 3, 4)));
-  size_t PutCString(DiagnosticSeverity severity, const char *cstr);
+  size_t PutString(DiagnosticSeverity severity, llvm::StringRef str);
 
-  void AppendMessageToDiagnostic(const char *cstr) {
-    if (m_diagnostics.size()) {
-      m_diagnostics.back()->AppendMessage(cstr);
+  void AppendMessageToDiagnostic(llvm::StringRef str) {
+    if (!m_diagnostics.empty()) {
+      m_diagnostics.back()->AppendMessage(str);
     }
   }
 
diff --git a/lldb/source/Expression/DiagnosticManager.cpp b/lldb/source/Expression/DiagnosticManager.cpp
index 37a9df7311bb..ad06600e098a 100644
--- a/lldb/source/Expression/DiagnosticManager.cpp
+++ b/lldb/source/Expression/DiagnosticManager.cpp
@@ -67,15 +67,15 @@ size_t DiagnosticManager::Printf(DiagnosticSeverity severity,
   size_t result = ss.PrintfVarArg(format, args);
   va_end(args);
 
-  AddDiagnostic(ss.GetData(), severity, eDiagnosticOriginLLDB);
+  AddDiagnostic(ss.GetString(), severity, eDiagnosticOriginLLDB);
 
   return result;
 }
 
-size_t DiagnosticManager::PutCString(DiagnosticSeverity severity,
-                                     const char *cstr) {
-  if (!cstr)
+size_t DiagnosticManager::PutString(DiagnosticSeverity severity,
+                                    llvm::StringRef str) {
+  if (str.empty())
     return 0;
-  AddDiagnostic(cstr, severity, eDiagnosticOriginLLDB);
-  return strlen(cstr);
+  AddDiagnostic(str, severity, eDiagnosticOriginLLDB);
+  return str.size();
 }
diff --git a/lldb/source/Expression/FunctionCaller.cpp b/lldb/source/Expression/FunctionCaller.cpp
index 805afda2fba3..f218ccb047a7 100644
--- a/lldb/source/Expression/FunctionCaller.cpp
+++ b/lldb/source/Expression/FunctionCaller.cpp
@@ -1,5 +1,4 @@
-//===-- FunctionCaller.cpp ---------------------------------------*- C++
-//-*-===//
+//===-- FunctionCaller.cpp ---------------------------------------*- C++-*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
@@ -130,9 +129,9 @@ bool FunctionCaller::WriteFunctionArguments(
   // All the information to reconstruct the struct is provided by the
   // StructExtractor.
   if (!m_struct_valid) {
-    diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                  "Argument information was not correctly "
-                                  "parsed, so the function cannot be called.");
+    diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                 "Argument information was not correctly "
+                                 "parsed, so the function cannot be called.");
     return false;
   }
 
@@ -243,7 +242,7 @@ lldb::ThreadPlanSP FunctionCaller::GetThreadPlanToCallFunction(
   // FIXME: Use the errors Stream for better error reporting.
   Thread *thread = exe_ctx.GetThreadPtr();
   if (thread == NULL) {
-    diagnostic_manager.PutCString(
+    diagnostic_manager.PutString(
         eDiagnosticSeverityError,
         "Can't call a function without a valid thread.");
     return NULL;
diff --git a/lldb/source/Expression/LLVMUserExpression.cpp b/lldb/source/Expression/LLVMUserExpression.cpp
index 78207df56b09..379dd98b5cad 100644
--- a/lldb/source/Expression/LLVMUserExpression.cpp
+++ b/lldb/source/Expression/LLVMUserExpression.cpp
@@ -97,7 +97,7 @@ LLVMUserExpression::DoExecute(DiagnosticManager &diagnostic_manager,
       llvm::Function *function = m_execution_unit_sp->GetFunction();
 
       if (!module || !function) {
-        diagnostic_manager.PutCString(
+        diagnostic_manager.PutString(
             eDiagnosticSeverityError,
             "supposed to interpret, but nothing is there");
         return lldb::eExpressionSetupError;
@@ -153,7 +153,7 @@ LLVMUserExpression::DoExecute(DiagnosticManager &diagnostic_manager,
 
       StreamString ss;
       if (!call_plan_sp || !call_plan_sp->ValidatePlan(&ss)) {
-        diagnostic_manager.PutCString(eDiagnosticSeverityError, ss.GetData());
+        diagnostic_manager.PutString(eDiagnosticSeverityError, ss.GetData());
         return lldb::eExpressionSetupError;
       }
 
@@ -198,8 +198,8 @@ LLVMUserExpression::DoExecute(DiagnosticManager &diagnostic_manager,
                                     "Execution was interrupted, reason: %s.",
                                     error_desc);
         else
-          diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                        "Execution was interrupted.");
+          diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                       "Execution was interrupted.");
 
         if ((execution_result == lldb::eExpressionInterrupted &&
              options.DoesUnwindOnError()) ||
@@ -220,7 +220,7 @@ LLVMUserExpression::DoExecute(DiagnosticManager &diagnostic_manager,
 
         return execution_result;
       } else if (execution_result == lldb::eExpressionStoppedForDebug) {
-        diagnostic_manager.PutCString(
+        diagnostic_manager.PutString(
             eDiagnosticSeverityRemark,
             "Execution was halted at the first instruction of the expression "
             "function because \"debug\" was requested.\n"
@@ -243,7 +243,7 @@ LLVMUserExpression::DoExecute(DiagnosticManager &diagnostic_manager,
       return lldb::eExpressionResultUnavailable;
     }
   } else {
-    diagnostic_manager.PutCString(
+    diagnostic_manager.PutString(
         eDiagnosticSeverityError,
         "Expression can't be run, because there is no JIT compiled function");
     return lldb::eExpressionSetupError;
@@ -298,7 +298,7 @@ bool LLVMUserExpression::PrepareToExecuteJITExpression(
   lldb::StackFrameSP frame;
 
   if (!LockAndCheckContext(exe_ctx, target, process, frame)) {
-    diagnostic_manager.PutCString(
+    diagnostic_manager.PutString(
         eDiagnosticSeverityError,
         "The context has changed before we could JIT the expression!");
     return false;
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp
index 1df6246d491c..d91da5f40d94 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp
@@ -624,8 +624,8 @@ unsigned ClangExpressionParser::Parse(DiagnosticManager &diagnostic_manager) {
 
   if (m_pp_callbacks && m_pp_callbacks->hasErrors()) {
     num_errors++;
-    diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                  "while importing modules:");
+    diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                 "while importing modules:");
     diagnostic_manager.AppendMessageToDiagnostic(
         m_pp_callbacks->getErrorString().c_str());
   }
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangFunctionCaller.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangFunctionCaller.cpp
index 87d773867820..b1428bb30058 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangFunctionCaller.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangFunctionCaller.cpp
@@ -203,8 +203,8 @@ ClangFunctionCaller::CompileFunction(lldb::ThreadSP thread_to_use_sp,
 
     num_errors = m_parser->Parse(diagnostic_manager);
   } else {
-    diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                  "no process - unable to inject function");
+    diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                 "no process - unable to inject function");
     num_errors = 1;
   }
 
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.cpp
index 59dd137edd33..5abad71b84a7 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.cpp
@@ -325,21 +325,21 @@ bool ClangUserExpression::Parse(DiagnosticManager &diagnostic_manager,
                 lldb::eLanguageTypeC)) {
       m_result_delegate.RegisterPersistentState(persistent_state);
     } else {
-      diagnostic_manager.PutCString(
+      diagnostic_manager.PutString(
           eDiagnosticSeverityError,
           "couldn't start parsing (no persistent data)");
       return false;
     }
   } else {
-    diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                  "error: couldn't start parsing (no target)");
+    diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                 "error: couldn't start parsing (no target)");
     return false;
   }
 
   ScanContext(exe_ctx, err);
 
   if (!err.Success()) {
-    diagnostic_manager.PutCString(eDiagnosticSeverityWarning, err.AsCString());
+    diagnostic_manager.PutString(eDiagnosticSeverityWarning, err.AsCString());
   }
 
   ////////////////////////////////////
@@ -400,8 +400,8 @@ bool ClangUserExpression::Parse(DiagnosticManager &diagnostic_manager,
 
     if (!source_code->GetText(m_transformed_text, lang_type, m_in_static_method,
                               exe_ctx)) {
-      diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                    "couldn't construct expression body");
+      diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                   "couldn't construct expression body");
       return false;
     }
   }
@@ -416,7 +416,7 @@ bool ClangUserExpression::Parse(DiagnosticManager &diagnostic_manager,
   Target *target = exe_ctx.GetTargetPtr();
 
   if (!target) {
-    diagnostic_manager.PutCString(eDiagnosticSeverityError, "invalid target");
+    diagnostic_manager.PutString(eDiagnosticSeverityError, "invalid target");
     return false;
   }
 
@@ -443,7 +443,7 @@ bool ClangUserExpression::Parse(DiagnosticManager &diagnostic_manager,
   OnExit on_exit([this]() { ResetDeclMap(); });
 
   if (!DeclMap()->WillParse(exe_ctx, m_materializer_ap.get())) {
-    diagnostic_manager.PutCString(
+    diagnostic_manager.PutString(
         eDiagnosticSeverityError,
         "current process state is unsuitable for expression parsing");
 
@@ -508,10 +508,10 @@ bool ClangUserExpression::Parse(DiagnosticManager &diagnostic_manager,
     if (!jit_error.Success()) {
       const char *error_cstr = jit_error.AsCString();
       if (error_cstr && error_cstr[0])
-        diagnostic_manager.PutCString(eDiagnosticSeverityError, error_cstr);
+        diagnostic_manager.PutString(eDiagnosticSeverityError, error_cstr);
       else
-        diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                      "expression can't be interpreted or run");
+        diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                     "expression can't be interpreted or run");
       return false;
     }
   }
@@ -527,8 +527,8 @@ bool ClangUserExpression::Parse(DiagnosticManager &diagnostic_manager,
                                   "couldn't run static initializers: %s\n",
                                   error_cstr);
       else
-        diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                      "couldn't run static initializers\n");
+        diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                     "couldn't run static initializers\n");
       return false;
     }
   }
@@ -597,7 +597,7 @@ bool ClangUserExpression::AddArguments(ExecutionContext &exe_ctx,
     } else if (m_in_objectivec_method) {
       object_name.SetCString("self");
     } else {
-      diagnostic_manager.PutCString(
+      diagnostic_manager.PutString(
           eDiagnosticSeverityError,
           "need object pointer but don't know the language");
       return false;
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangUtilityFunction.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangUtilityFunction.cpp
index d3af620145b2..5a86994037bc 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangUtilityFunction.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangUtilityFunction.cpp
@@ -1,5 +1,4 @@
-//===-- ClangUserExpression.cpp -------------------------------------*- C++
-//-*-===//
+//===-- ClangUserExpression.cpp ----------------------------------*- C++-*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
@@ -63,8 +62,8 @@ ClangUtilityFunction::~ClangUtilityFunction() {}
 bool ClangUtilityFunction::Install(DiagnosticManager &diagnostic_manager,
                                    ExecutionContext &exe_ctx) {
   if (m_jit_start_addr != LLDB_INVALID_ADDRESS) {
-    diagnostic_manager.PutCString(eDiagnosticSeverityWarning,
-                                  "already installed");
+    diagnostic_manager.PutString(eDiagnosticSeverityWarning,
+                                 "already installed");
     return false;
   }
 
@@ -75,14 +74,14 @@ bool ClangUtilityFunction::Install(DiagnosticManager &diagnostic_manager,
   Target *target = exe_ctx.GetTargetPtr();
 
   if (!target) {
-    diagnostic_manager.PutCString(eDiagnosticSeverityError, "invalid target");
+    diagnostic_manager.PutString(eDiagnosticSeverityError, "invalid target");
     return false;
   }
 
   Process *process = exe_ctx.GetProcessPtr();
 
   if (!process) {
-    diagnostic_manager.PutCString(eDiagnosticSeverityError, "invalid process");
+    diagnostic_manager.PutString(eDiagnosticSeverityError, "invalid process");
     return false;
   }
 
@@ -95,7 +94,7 @@ bool ClangUtilityFunction::Install(DiagnosticManager &diagnostic_manager,
   ResetDeclMap(exe_ctx, keep_result_in_memory);
 
   if (!DeclMap()->WillParse(exe_ctx, NULL)) {
-    diagnostic_manager.PutCString(
+    diagnostic_manager.PutString(
         eDiagnosticSeverityError,
         "current process state is unsuitable for expression parsing");
     return false;
@@ -159,8 +158,8 @@ bool ClangUtilityFunction::Install(DiagnosticManager &diagnostic_manager,
     if (error_cstr && error_cstr[0]) {
       diagnostic_manager.Printf(eDiagnosticSeverityError, "%s", error_cstr);
     } else {
-      diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                    "expression can't be interpreted or run");
+      diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                   "expression can't be interpreted or run");
     }
     return false;
   }
diff --git a/lldb/source/Plugins/ExpressionParser/Go/GoUserExpression.cpp b/lldb/source/Plugins/ExpressionParser/Go/GoUserExpression.cpp
index ff20be1d22e5..30cc4aa828ed 100644
--- a/lldb/source/Plugins/ExpressionParser/Go/GoUserExpression.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Go/GoUserExpression.cpp
@@ -216,7 +216,7 @@ bool GoUserExpression::Parse(DiagnosticManager &diagnostic_manager,
     return true;
   const char *error_cstr = m_interpreter->error().AsCString();
   if (error_cstr && error_cstr[0])
-    diagnostic_manager.PutCString(eDiagnosticSeverityError, error_cstr);
+    diagnostic_manager.PutString(eDiagnosticSeverityError, error_cstr);
   else
     diagnostic_manager.Printf(eDiagnosticSeverityError,
                               "expression can't be interpreted or run");
@@ -245,8 +245,8 @@ GoUserExpression::DoExecute(DiagnosticManager &diagnostic_manager,
         log->Printf("== [GoUserExpression::Evaluate] Expression may not run, "
                     "but is not constant ==");
 
-      diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                    "expression needed to run but couldn't");
+      diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                   "expression needed to run but couldn't");
 
       return execution_results;
     }
@@ -260,10 +260,10 @@ GoUserExpression::DoExecute(DiagnosticManager &diagnostic_manager,
   if (!result_val_sp) {
     const char *error_cstr = err.AsCString();
     if (error_cstr && error_cstr[0])
-      diagnostic_manager.PutCString(eDiagnosticSeverityError, error_cstr);
+      diagnostic_manager.PutString(eDiagnosticSeverityError, error_cstr);
     else
-      diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                    "expression can't be interpreted or run");
+      diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                   "expression can't be interpreted or run");
     return lldb::eExpressionDiscarded;
   }
   result.reset(new ExpressionVariable(ExpressionVariable::eKindGo));
diff --git a/lldb/source/Target/Process.cpp b/lldb/source/Target/Process.cpp
index a1f1d951c18a..ffaa19da3539 100644
--- a/lldb/source/Target/Process.cpp
+++ b/lldb/source/Target/Process.cpp
@@ -4815,29 +4815,29 @@ Process::RunThreadPlan(ExecutionContext &exe_ctx,
   std::lock_guard<std::mutex> run_thread_plan_locker(m_run_thread_plan_lock);
 
   if (!thread_plan_sp) {
-    diagnostic_manager.PutCString(
+    diagnostic_manager.PutString(
         eDiagnosticSeverityError,
         "RunThreadPlan called with empty thread plan.");
     return eExpressionSetupError;
   }
 
   if (!thread_plan_sp->ValidatePlan(nullptr)) {
-    diagnostic_manager.PutCString(
+    diagnostic_manager.PutString(
         eDiagnosticSeverityError,
         "RunThreadPlan called with an invalid thread plan.");
     return eExpressionSetupError;
   }
 
   if (exe_ctx.GetProcessPtr() != this) {
-    diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                  "RunThreadPlan called on wrong process.");
+    diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                 "RunThreadPlan called on wrong process.");
     return eExpressionSetupError;
   }
 
   Thread *thread = exe_ctx.GetThreadPtr();
   if (thread == nullptr) {
-    diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                  "RunThreadPlan called with invalid thread.");
+    diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                 "RunThreadPlan called with invalid thread.");
     return eExpressionSetupError;
   }
 
@@ -4864,7 +4864,7 @@ Process::RunThreadPlan(ExecutionContext &exe_ctx,
   thread_plan_sp->SetOkayToDiscard(false);
 
   if (m_private_state.GetValue() != eStateStopped) {
-    diagnostic_manager.PutCString(
+    diagnostic_manager.PutString(
         eDiagnosticSeverityError,
         "RunThreadPlan called while the private state was not stopped.");
     return eExpressionSetupError;
@@ -5028,10 +5028,10 @@ Process::RunThreadPlan(ExecutionContext &exe_ctx,
         uint64_t computed_one_thread_timeout;
         if (option_one_thread_timeout != 0) {
           if (timeout_usec < option_one_thread_timeout) {
-            diagnostic_manager.PutCString(eDiagnosticSeverityError,
-                                          "RunThreadPlan called without one "
-                                          "thread timeout greater than total "
-                                          "timeout");
+            diagnostic_manager.PutString(eDiagnosticSeverityError,
+                                         "RunThreadPlan called without one "
+                                         "thread timeout greater than total "
+                                         "timeout");
             return eExpressionSetupError;
           }
           computed_one_thread_timeout = option_one_thread_timeout;
@@ -5060,7 +5060,7 @@ Process::RunThreadPlan(ExecutionContext &exe_ctx,
 
     Event *other_events = listener_sp->PeekAtNextEvent();
     if (other_events != nullptr) {
-      diagnostic_manager.PutCString(
+      diagnostic_manager.PutString(
           eDiagnosticSeverityError,
           "RunThreadPlan called with pending events on the queue.");
       return eExpressionSetupError;
@@ -5237,9 +5237,8 @@ Process::RunThreadPlan(ExecutionContext &exe_ctx,
             const bool use_run_lock = false;
             Halt(clear_thread_plans, use_run_lock);
             return_value = eExpressionInterrupted;
-            diagnostic_manager.PutCString(
-                eDiagnosticSeverityRemark,
-                "execution halted by user interrupt.");
+            diagnostic_manager.PutString(eDiagnosticSeverityRemark,
+                                         "execution halted by user interrupt.");
             if (log)
               log->Printf("Process::RunThreadPlan(): Got  interrupted by "
                           "eBroadcastBitInterrupted, exiting.");
@@ -5350,7 +5349,7 @@ Process::RunThreadPlan(ExecutionContext &exe_ctx,
               if (stop_state == eStateExited)
                 event_to_broadcast_sp = event_sp;
 
-              diagnostic_manager.PutCString(
+              diagnostic_manager.PutString(
                   eDiagnosticSeverityError,
                   "execution stopped with unexpected state.");
               return_value = eExpressionInterrupted;