[Damangle] convert rustDemangle to use std::string_view

I was doing this API conversion to use std::string_view top-down in
D149104, but this exposed issues in individual demanglers that needed to
get fixed first. There's no issue with the conversion for the Rust
demangler, so convert it first.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D149784

Author: nickdesaulniers

lldb/include/lldb/Utility/ConstString.h

@@ -14,6 +14,7 @@
 #include "llvm/Support/FormatVariadic.h"
 
 #include <cstddef>
+#include <string_view>
 
 namespace lldb_private {
 class Stream;
@@ -182,6 +183,8 @@ class ConstString {
 
   // Implicitly convert \class ConstString instances to \class StringRef.
   operator llvm::StringRef() const { return GetStringRef(); }
+  // Implicitly convert \class ConstString instances to \calss std::string_view.
+  operator std::string_view() const { return std::string_view(m_string, GetLength()); }
 
   /// Get the string value as a C string.
   ///

lldb/source/Core/Mangled.cpp

@@ -25,6 +25,7 @@
 
 #include <mutex>
 #include <string>
+#include <string_view>
 #include <utility>
 
 #include <cstdlib>
@@ -150,7 +151,7 @@ static char *GetItaniumDemangledStr(const char *M) {
   return demangled_cstr;
 }
 
-static char *GetRustV0DemangledStr(const char *M) {
+static char *GetRustV0DemangledStr(std::string_view M) {
   char *demangled_cstr = llvm::rustDemangle(M);
 
   if (Log *log = GetLog(LLDBLog::Demangle)) {
@@ -259,7 +260,7 @@ ConstString Mangled::GetDemangledName() const {
         break;
       }
       case eManglingSchemeRustV0:
-        demangled_name = GetRustV0DemangledStr(mangled_name);
+        demangled_name = GetRustV0DemangledStr(m_mangled);
         break;
       case eManglingSchemeD:
         demangled_name = GetDLangDemangledStr(mangled_name);

llvm/include/llvm/Demangle/Demangle.h

@@ -11,6 +11,7 @@
 
 #include <cstddef>
 #include <string>
+#include <string_view>
 
 namespace llvm {
 /// This is a llvm local version of __cxa_demangle. Other than the name and
@@ -54,7 +55,7 @@ char *microsoftDemangle(const char *mangled_name, size_t *n_read, int *status,
                         MSDemangleFlags Flags = MSDF_None);
 
 // Demangles a Rust v0 mangled symbol.
-char *rustDemangle(const char *MangledName);
+char *rustDemangle(std::string_view MangledName);
 
 // Demangles a D mangled symbol.
 char *dlangDemangle(const char *MangledName);

llvm/lib/Demangle/Demangle.cpp

@@ -51,7 +51,7 @@ bool llvm::nonMicrosoftDemangle(std::string_view MangledName,
   if (isItaniumEncoding(MangledName.data()))
     Demangled = itaniumDemangle(MangledName);
   else if (isRustEncoding(MangledName.data()))
-    Demangled = rustDemangle(MangledName.data());
+    Demangled = rustDemangle(MangledName);
   else if (isDLangEncoding(MangledName.data()))
     Demangled = dlangDemangle(MangledName.data());
 

llvm/lib/Demangle/RustDemangle.cpp

@@ -20,11 +20,13 @@
 #include <cstdint>
 #include <cstring>
 #include <limits>
+#include <string_view>
 
 using namespace llvm;
 
 using llvm::itanium_demangle::OutputBuffer;
 using llvm::itanium_demangle::ScopedOverride;
+using llvm::itanium_demangle::starts_with;
 
 namespace {
 
@@ -146,17 +148,13 @@ class Demangler {
 
 } // namespace
 
-char *llvm::rustDemangle(const char *MangledName) {
-  if (MangledName == nullptr)
-    return nullptr;
-
+char *llvm::rustDemangle(std::string_view MangledName) {
   // Return early if mangled name doesn't look like a Rust symbol.
-  std::string_view Mangled(MangledName);
-  if (!llvm::itanium_demangle::starts_with(Mangled, "_R"))
+  if (MangledName.empty() || !starts_with(MangledName, "_R"))
     return nullptr;
 
   Demangler D;
-  if (!D.demangle(Mangled)) {
+  if (!D.demangle(MangledName)) {
     std::free(D.Output.getBuffer());
     return nullptr;
   }
@@ -196,7 +194,7 @@ bool Demangler::demangle(std::string_view Mangled) {
   RecursionLevel = 0;
   BoundLifetimes = 0;
 
-  if (!llvm::itanium_demangle::starts_with(Mangled, "_R")) {
+  if (!starts_with(Mangled, "_R")) {
     Error = true;
     return false;
   }

llvm/tools/llvm-rust-demangle-fuzzer/llvm-rust-demangle-fuzzer.cpp

@@ -13,7 +13,7 @@
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
   std::string NullTerminatedString((const char *)Data, Size);
-  char *Demangled = llvm::rustDemangle(NullTerminatedString.c_str());
+  char *Demangled = llvm::rustDemangle(NullTerminatedString);
   std::free(Demangled);
   return 0;
 }