[Demangle][Rust] Parse implementation paths

[dblaikie]

Looks good - please commit it as three separate changes, though - since they're three readily separable features.

llvm/include/llvm/Demangle/RustDemangle.h

class Demangler {
   // Current recursion level.
   size_t RecursionLevel;
 
   // Input string that is being demangled with "_R" prefix removed.
   StringView Input;
   // Position in the input string.
   size_t Position;
 
+  // When true, print methods append the output to the stream.
+  // When false, the output is suppressed.
+  bool Print;
+
   // True if an error occurred.
   bool Error;
-
 public:
   // Demangled output.
   OutputStream Output;
 
   Demangler(size_t MaxRecursionLevel = 500);
 
   bool demangle(StringView MangledName);
 
 private:
   void demanglePath();
+  void demangleImplPath();
   void demangleGenericArg();
   void demangleType();
   void demangleConst();
   void demangleConstInt();
   void demangleConstBool();
   void demangleConstChar();
 
   Identifier parseIdentifier();
   uint64_t parseOptionalBase62Number(char Tag);
   uint64_t parseBase62Number();
   uint64_t parseDecimalNumber();
   uint64_t parseHexNumber(StringView &HexDigits);
 
   void print(char C) {
-    if (Error)
+    if (Error || !Print)
       return;
 
     Output += C;
   }
 
   void print(StringView S) {
-    if (Error)
+    if (Error || !Print)
       return;
 
     Output += S;
   }
 
   void printDecimalNumber(uint64_t N) {
-    if (Error)
+    if (Error || !Print)
       return;
 
     Output << N;
   }
 
   void printBasicType(BasicType);
 
   char look() const {

llvm/lib/Demangle/RustDemangle.cpp

 // Demangles Rust v0 mangled symbol. Returns true when successful, and false
 // otherwise. The demangled symbol is stored in Output field. It is
 // responsibility of the caller to free the memory behind the output stream.
 //
 // <symbol-name> = "_R" <path> [<instantiating-crate>]
 bool Demangler::demangle(StringView Mangled) {
   Position = 0;
   Error = false;
+  Print = true;
   RecursionLevel = 0;
 
   if (!Mangled.consumeFront("_R")) {
     Error = true;
     return false;
   }
   Input = Mangled;
 
void Demangler::demanglePath() {
 
   switch (consume()) {
   case 'C': {
     parseOptionalBase62Number('s');
     Identifier Ident = parseIdentifier();
     print(Ident.Name);
     break;
   }
+  case 'M': {
+    demangleImplPath();
+    print("<");
+    demangleType();
+    print(">");
+    break;
+  }
+  case 'X': {
+    demangleImplPath();
+    print("<");
+    demangleType();
+    print(" as ");
+    demanglePath();
+    print(">");
+    break;
+  }
+  case 'Y': {
+    print("<");
+    demangleType();
+    print(" as ");
+    demanglePath();
+    print(">");
+    break;
+  }
   case 'N': {
     char NS = consume();
     if (!isLower(NS) && !isUpper(NS)) {
       Error = true;
       break;
     }
     demanglePath();
 
void Demangler::demanglePath() {
   }
   default:
     // FIXME parse remaining productions.
     Error = true;
     break;
   }
 }
 
+// <impl-path> = [<disambiguator>] <path>
+// <disambiguator> = "s" <base-62-number>
+void Demangler::demangleImplPath() {
+  SwapAndRestore<bool> SavePrint(Print, false);
+  parseOptionalBase62Number('s');
+  demanglePath();
+}
+
 // <generic-arg> = <lifetime>
 //               | <type>
 //               | "K" <const>
 // <lifetime> = "L" <base-62-number>
 void Demangler::demangleGenericArg() {
   if (consumeIf('K'))
     demangleConst();
   else

llvm/test/Demangle/rust.test

 CHECK: crate::{shim:reify#0}
        _RNSC5crate5reify
 
 ; Unrecognized special namespace
 
 CHECK: crate::{Z:ident#10}
        _RNZC5crates8_5ident
 
+; Inherent impl
+
+CHECK: <_>
+       _RMC5cratep
+
+CHECK: <_>
+       _RMs_C5cratep
+
+; Trait impl
+
+CHECK: <_ as Clone>
+       _RXC5cratepC5Clone
+
+CHECK: <_ as Clone>
+       _RXs_C5cratepC5Clone
+
+; Trait definition
+
+CHECK: <_ as Ord>
+       _RYpC3Ord
+
 ; Generic type arguments
 
 CHECK: generic::<_>
        _RIC7genericpE
 
 CHECK: generic::<_, _>
        _RIC7genericppE