[Demangle][Rust] Parse path backreferences

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D103459

Author: tmiasko

llvm/include/llvm/Demangle/RustDemangle.h

@@ -102,6 +102,21 @@ class Demangler {
   void demangleConstBool();
   void demangleConstChar();
 
+  template <typename Callable> void demangleBackref(Callable Demangler) {
+    uint64_t Backref = parseBase62Number();
+    if (Error || Backref >= Position) {
+      Error = true;
+      return;
+    }
+
+    if (!Print)
+      return;
+
+    SwapAndRestore<size_t> SavePosition(Position, Position);
+    Position = Backref;
+    Demangler();
+  }
+
   Identifier parseIdentifier();
   uint64_t parseOptionalBase62Number(char Tag);
   uint64_t parseBase62Number();

llvm/lib/Demangle/RustDemangle.cpp

@@ -232,8 +232,12 @@ bool Demangler::demanglePath(InType InType, LeaveOpen LeaveOpen) {
       print(">");
     break;
   }
+  case 'B': {
+    bool IsOpen = false;
+    demangleBackref([&] { IsOpen = demanglePath(InType, LeaveOpen); });
+    return IsOpen;
+  }
   default:
-    // FIXME parse remaining productions.
     Error = true;
     break;
   }

llvm/test/Demangle/rust.test

@@ -420,6 +420,22 @@ CHECK: char::<'\u{10ffff}'>
 CHECK: _RIC4charKc1234567_E
        _RIC4charKc1234567_E
 
+; Backreferences
+
+CHECK: backref::<backref::ident>
+       _RIC7backrefNvB0_5identE
+
+; Invalid backreferences
+
+CHECK: _RB_
+       _RB_
+
+CHECK: _RB5_
+       _RB5_
+
+CHECK: _RNvB_1a
+       _RNvB_1a
+
 ; Invalid mangled characters
 
 CHECK: _RNvC2a.1c