[Demangle] Support Rust v0 mangling scheme in llvm::demangle

The llvm::demangle is currently used by llvm-objdump and llvm-readobj,
so this effectively adds support for Rust v0 mangling to those
applications.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D104340

Author: tmiasko

llvm/lib/Demangle/Demangle.cpp

@@ -19,10 +19,17 @@ static bool isItaniumEncoding(const std::string &MangledName) {
   return Pos > 0 && Pos <= 4 && MangledName[Pos] == 'Z';
 }
 
+static bool isRustEncoding(const std::string &MangledName) {
+  return MangledName.size() >= 2 && MangledName[0] == '_' &&
+         MangledName[1] == 'R';
+}
+
 std::string llvm::demangle(const std::string &MangledName) {
   char *Demangled;
   if (isItaniumEncoding(MangledName))
     Demangled = itaniumDemangle(MangledName.c_str(), nullptr, nullptr, nullptr);
+  else if (isRustEncoding(MangledName))
+    Demangled = rustDemangle(MangledName.c_str(), nullptr, nullptr, nullptr);
   else
     Demangled = microsoftDemangle(MangledName.c_str(), nullptr, nullptr,
                                   nullptr, nullptr);

llvm/unittests/Demangle/DemangleTest.cpp

@@ -21,6 +21,7 @@ TEST(Demangle, demangleTest) {
             "invocation function for block in foo(int)");
   EXPECT_EQ(demangle("?foo@@YAXH@Z"), "void __cdecl foo(int)");
   EXPECT_EQ(demangle("foo"), "foo");
+  EXPECT_EQ(demangle("_RNvC3foo3bar"), "foo::bar");
 
   // Regression test for demangling of optional template-args for vendor
   // extended type qualifier (https://bugs.llvm.org/show_bug.cgi?id=48009)