Update contrib/restricted/abseil-cpp-tstring to 20240722.0

83a5727000e16bc5a94523a0cf1cce75fa86a191

1 files changed, 925 insertions, 0 deletions

diff --git a/contrib/restricted/abseil-cpp-tstring/y_absl/debugging/internal/demangle_rust.cc b/contrib/restricted/abseil-cpp-tstring/y_absl/debugging/internal/demangle_rust.cc
new file mode 100644
index 0000000000..086e1457f9
--- /dev/null
+++ b/contrib/restricted/abseil-cpp-tstring/y_absl/debugging/internal/demangle_rust.cc
@@ -0,0 +1,925 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "y_absl/debugging/internal/demangle_rust.h"
+
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <limits>
+
+#include "y_absl/base/attributes.h"
+#include "y_absl/base/config.h"
+#include "y_absl/debugging/internal/decode_rust_punycode.h"
+
+namespace y_absl {
+Y_ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+
+namespace {
+
+// Same step limit as the C++ demangler in demangle.cc uses.
+constexpr int kMaxReturns = 1 << 17;
+
+bool IsDigit(char c) { return '0' <= c && c <= '9'; }
+bool IsLower(char c) { return 'a' <= c && c <= 'z'; }
+bool IsUpper(char c) { return 'A' <= c && c <= 'Z'; }
+bool IsAlpha(char c) { return IsLower(c) || IsUpper(c); }
+bool IsIdentifierChar(char c) { return IsAlpha(c) || IsDigit(c) || c == '_'; }
+bool IsLowerHexDigit(char c) { return IsDigit(c) || ('a' <= c && c <= 'f'); }
+
+const char* BasicTypeName(char c) {
+  switch (c) {
+    case 'a': return "i8";
+    case 'b': return "bool";
+    case 'c': return "char";
+    case 'd': return "f64";
+    case 'e': return "str";
+    case 'f': return "f32";
+    case 'h': return "u8";
+    case 'i': return "isize";
+    case 'j': return "usize";
+    case 'l': return "i32";
+    case 'm': return "u32";
+    case 'n': return "i128";
+    case 'o': return "u128";
+    case 'p': return "_";
+    case 's': return "i16";
+    case 't': return "u16";
+    case 'u': return "()";
+    case 'v': return "...";
+    case 'x': return "i64";
+    case 'y': return "u64";
+    case 'z': return "!";
+  }
+  return nullptr;
+}
+
+// Parser for Rust symbol mangling v0, whose grammar is defined here:
+//
+// https://doc.rust-lang.org/rustc/symbol-mangling/v0.html#symbol-grammar-summary
+class RustSymbolParser {
+ public:
+  // Prepares to demangle the given encoding, a Rust symbol name starting with
+  // _R, into the output buffer [out, out_end).  The caller is expected to
+  // continue by calling the new object's Parse function.
+  RustSymbolParser(const char* encoding, char* out, char* const out_end)
+      : encoding_(encoding), out_(out), out_end_(out_end) {
+    if (out_ != out_end_) *out_ = '\0';
+  }
+
+  // Parses the constructor's encoding argument, writing output into the range
+  // [out, out_end).  Returns true on success and false for input whose
+  // structure was not recognized or exceeded implementation limits, such as by
+  // nesting structures too deep.  In either case *this should not be used
+  // again.
+  Y_ABSL_MUST_USE_RESULT bool Parse() && {
+    // Recursively parses the grammar production named by callee, then resumes
+    // execution at the next statement.
+    //
+    // Recursive-descent parsing is a beautifully readable translation of a
+    // grammar, but it risks stack overflow if implemented by naive recursion on
+    // the C++ call stack.  So we simulate recursion by goto and switch instead,
+    // keeping a bounded stack of "return addresses" in the recursion_stack_
+    // member.
+    //
+    // The callee argument is a statement label.  We goto that label after
+    // saving the "return address" on recursion_stack_.  The next continue
+    // statement in the for loop below "returns" from this "call".
+    //
+    // The caller argument names the return point.  Each value of caller must
+    // appear in only one Y_ABSL_DEMANGLER_RECURSE call and be listed in the
+    // definition of enum ReturnAddress.  The switch implements the control
+    // transfer from the end of a "called" subroutine back to the statement
+    // after the "call".
+    //
+    // Note that not all the grammar productions have to be packed into the
+    // switch, but only those which appear in a cycle in the grammar.  Anything
+    // acyclic can be written as ordinary functions and function calls, e.g.,
+    // ParseIdentifier.
+#define Y_ABSL_DEMANGLER_RECURSE(callee, caller) \
+    do { \
+      if (recursion_depth_ == kStackSize) return false; \
+      /* The next continue will switch on this saved value ... */ \
+      recursion_stack_[recursion_depth_++] = caller; \
+      goto callee; \
+      /* ... and will land here, resuming the suspended code. */ \
+      case caller: {} \
+    } while (0)
+
+    // Parse the encoding, counting completed recursive calls to guard against
+    // excessively complex input and infinite-loop bugs.
+    int iter = 0;
+    goto whole_encoding;
+    for (; iter < kMaxReturns && recursion_depth_ > 0; ++iter) {
+      // This switch resumes the code path most recently suspended by
+      // Y_ABSL_DEMANGLER_RECURSE.
+      switch (recursion_stack_[--recursion_depth_]) {
+        //
+        // symbol-name ->
+        // _R decimal-number? path instantiating-crate? vendor-specific-suffix?
+        whole_encoding:
+          if (!Eat('_') || !Eat('R')) return false;
+          // decimal-number? is always empty today, so proceed to path, which
+          // can't start with a decimal digit.
+          Y_ABSL_DEMANGLER_RECURSE(path, kInstantiatingCrate);
+          if (IsAlpha(Peek())) {
+            ++silence_depth_;  // Print nothing more from here on.
+            Y_ABSL_DEMANGLER_RECURSE(path, kVendorSpecificSuffix);
+          }
+          switch (Take()) {
+            case '.': case '$': case '\0': return true;
+          }
+          return false;  // unexpected trailing content
+
+        // path -> crate-root | inherent-impl | trait-impl | trait-definition |
+        //         nested-path | generic-args | backref
+        //
+        // Note that Y_ABSL_DEMANGLER_RECURSE does not work inside a nested switch
+        // (which would hide the generated case label).  Thus we jump out of the
+        // inner switch with gotos before performing any fake recursion.
+        path:
+          switch (Take()) {
+            case 'C': goto crate_root;
+            case 'M': goto inherent_impl;
+            case 'X': goto trait_impl;
+            case 'Y': goto trait_definition;
+            case 'N': goto nested_path;
+            case 'I': goto generic_args;
+            case 'B': goto path_backref;
+            default: return false;
+          }
+
+        // crate-root -> C identifier (C consumed above)
+        crate_root:
+          if (!ParseIdentifier()) return false;
+          continue;
+
+        // inherent-impl -> M impl-path type (M already consumed)
+        inherent_impl:
+          if (!Emit("<")) return false;
+          Y_ABSL_DEMANGLER_RECURSE(impl_path, kInherentImplType);
+          Y_ABSL_DEMANGLER_RECURSE(type, kInherentImplEnding);
+          if (!Emit(">")) return false;
+          continue;
+
+        // trait-impl -> X impl-path type path (X already consumed)
+        trait_impl:
+          if (!Emit("<")) return false;
+          Y_ABSL_DEMANGLER_RECURSE(impl_path, kTraitImplType);
+          Y_ABSL_DEMANGLER_RECURSE(type, kTraitImplInfix);
+          if (!Emit(" as ")) return false;
+          Y_ABSL_DEMANGLER_RECURSE(path, kTraitImplEnding);
+          if (!Emit(">")) return false;
+          continue;
+
+        // impl-path -> disambiguator? path (but never print it!)
+        impl_path:
+          ++silence_depth_;
+          {
+            int ignored_disambiguator;
+            if (!ParseDisambiguator(ignored_disambiguator)) return false;
+          }
+          Y_ABSL_DEMANGLER_RECURSE(path, kImplPathEnding);
+          --silence_depth_;
+          continue;
+
+        // trait-definition -> Y type path (Y already consumed)
+        trait_definition:
+          if (!Emit("<")) return false;
+          Y_ABSL_DEMANGLER_RECURSE(type, kTraitDefinitionInfix);
+          if (!Emit(" as ")) return false;
+          Y_ABSL_DEMANGLER_RECURSE(path, kTraitDefinitionEnding);
+          if (!Emit(">")) return false;
+          continue;
+
+        // nested-path -> N namespace path identifier (N already consumed)
+        // namespace -> lower | upper
+        nested_path:
+          // Uppercase namespaces must be saved on a stack so we can print
+          // ::{closure#0} or ::{shim:vtable#0} or ::{X:name#0} as needed.
+          if (IsUpper(Peek())) {
+            if (!PushNamespace(Take())) return false;
+            Y_ABSL_DEMANGLER_RECURSE(path, kIdentifierInUppercaseNamespace);
+            if (!Emit("::")) return false;
+            if (!ParseIdentifier(PopNamespace())) return false;
+            continue;
+          }
+
+          // Lowercase namespaces, however, are never represented in the output;
+          // they all emit just ::name.
+          if (IsLower(Take())) {
+            Y_ABSL_DEMANGLER_RECURSE(path, kIdentifierInLowercaseNamespace);
+            if (!Emit("::")) return false;
+            if (!ParseIdentifier()) return false;
+            continue;
+          }
+
+          // Neither upper or lower
+          return false;
+
+        // type -> basic-type | array-type | slice-type | tuple-type |
+        //         ref-type | mut-ref-type | const-ptr-type | mut-ptr-type |
+        //         fn-type | dyn-trait-type | path | backref
+        //
+        // We use ifs instead of switch (Take()) because the default case jumps
+        // to path, which will need to see the first character not yet Taken
+        // from the input.  Because we do not use a nested switch here,
+        // Y_ABSL_DEMANGLER_RECURSE works fine in the 'S' case.
+        type:
+          if (IsLower(Peek())) {
+            const char* type_name = BasicTypeName(Take());
+            if (type_name == nullptr || !Emit(type_name)) return false;
+            continue;
+          }
+          if (Eat('A')) {
+            // array-type = A type const
+            if (!Emit("[")) return false;
+            Y_ABSL_DEMANGLER_RECURSE(type, kArraySize);
+            if (!Emit("; ")) return false;
+            Y_ABSL_DEMANGLER_RECURSE(constant, kFinishArray);
+            if (!Emit("]")) return false;
+            continue;
+          }
+          if (Eat('S')) {
+            if (!Emit("[")) return false;
+            Y_ABSL_DEMANGLER_RECURSE(type, kSliceEnding);
+            if (!Emit("]")) return false;
+            continue;
+          }
+          if (Eat('T')) goto tuple_type;
+          if (Eat('R')) {
+            if (!Emit("&")) return false;
+            if (!ParseOptionalLifetime()) return false;
+            goto type;
+          }
+          if (Eat('Q')) {
+            if (!Emit("&mut ")) return false;
+            if (!ParseOptionalLifetime()) return false;
+            goto type;
+          }
+          if (Eat('P')) {
+            if (!Emit("*const ")) return false;
+            goto type;
+          }
+          if (Eat('O')) {
+            if (!Emit("*mut ")) return false;
+            goto type;
+          }
+          if (Eat('F')) goto fn_type;
+          if (Eat('D')) goto dyn_trait_type;
+          if (Eat('B')) goto type_backref;
+          goto path;
+
+        // tuple-type -> T type* E (T already consumed)
+        tuple_type:
+          if (!Emit("(")) return false;
+
+          // The toolchain should call the unit type u instead of TE, but the
+          // grammar and other demanglers also recognize TE, so we do too.
+          if (Eat('E')) {
+            if (!Emit(")")) return false;
+            continue;
+          }
+
+          // A tuple with one element is rendered (type,) instead of (type).
+          Y_ABSL_DEMANGLER_RECURSE(type, kAfterFirstTupleElement);
+          if (Eat('E')) {
+            if (!Emit(",)")) return false;
+            continue;
+          }
+
+          // A tuple with two elements is of course (x, y).
+          if (!Emit(", ")) return false;
+          Y_ABSL_DEMANGLER_RECURSE(type, kAfterSecondTupleElement);
+          if (Eat('E')) {
+            if (!Emit(")")) return false;
+            continue;
+          }
+
+          // And (x, y, z) for three elements.
+          if (!Emit(", ")) return false;
+          Y_ABSL_DEMANGLER_RECURSE(type, kAfterThirdTupleElement);
+          if (Eat('E')) {
+            if (!Emit(")")) return false;
+            continue;
+          }
+
+          // For longer tuples we write (x, y, z, ...), printing none of the
+          // content of the fourth and later types.  Thus we avoid exhausting
+          // output buffers and human readers' patience when some library has a
+          // long tuple as an implementation detail, without having to
+          // completely obfuscate all tuples.
+          if (!Emit(", ...)")) return false;
+          ++silence_depth_;
+          while (!Eat('E')) {
+            Y_ABSL_DEMANGLER_RECURSE(type, kAfterSubsequentTupleElement);
+          }
+          --silence_depth_;
+          continue;
+
+        // fn-type -> F fn-sig (F already consumed)
+        // fn-sig -> binder? U? (K abi)? type* E type
+        // abi -> C | undisambiguated-identifier
+        //
+        // We follow the C++ demangler in suppressing details of function
+        // signatures.  Every function type is rendered "fn...".
+        fn_type:
+          if (!Emit("fn...")) return false;
+          ++silence_depth_;
+          if (!ParseOptionalBinder()) return false;
+          (void)Eat('U');
+          if (Eat('K')) {
+            if (!Eat('C') && !ParseUndisambiguatedIdentifier()) return false;
+          }
+          while (!Eat('E')) {
+            Y_ABSL_DEMANGLER_RECURSE(type, kContinueParameterList);
+          }
+          Y_ABSL_DEMANGLER_RECURSE(type, kFinishFn);
+          --silence_depth_;
+          continue;
+
+        // dyn-trait-type -> D dyn-bounds lifetime (D already consumed)
+        // dyn-bounds -> binder? dyn-trait* E
+        //
+        // The grammar strangely allows an empty trait list, even though the
+        // compiler should never output one.  We follow existing demanglers in
+        // rendering DEL_ as "dyn ".
+        //
+        // Because auto traits lengthen a type name considerably without
+        // providing much value to a search for related source code, it would be
+        // desirable to abbreviate
+        //     dyn main::Trait + std::marker::Copy + std::marker::Send
+        // to
+        //     dyn main::Trait + ...,
+        // eliding the auto traits.  But it is difficult to do so correctly, in
+        // part because there is no guarantee that the mangling will list the
+        // main trait first.  So we just print all the traits in their order of
+        // appearance in the mangled name.
+        dyn_trait_type:
+          if (!Emit("dyn ")) return false;
+          if (!ParseOptionalBinder()) return false;
+          if (!Eat('E')) {
+            Y_ABSL_DEMANGLER_RECURSE(dyn_trait, kBeginAutoTraits);
+            while (!Eat('E')) {
+              if (!Emit(" + ")) return false;
+              Y_ABSL_DEMANGLER_RECURSE(dyn_trait, kContinueAutoTraits);
+            }
+          }
+          if (!ParseRequiredLifetime()) return false;
+          continue;
+
+        // dyn-trait -> path dyn-trait-assoc-binding*
+        // dyn-trait-assoc-binding -> p undisambiguated-identifier type
+        //
+        // We render nonempty binding lists as <>, omitting their contents as
+        // for generic-args.
+        dyn_trait:
+          Y_ABSL_DEMANGLER_RECURSE(path, kContinueDynTrait);
+          if (Peek() == 'p') {
+            if (!Emit("<>")) return false;
+            ++silence_depth_;
+            while (Eat('p')) {
+              if (!ParseUndisambiguatedIdentifier()) return false;
+              Y_ABSL_DEMANGLER_RECURSE(type, kContinueAssocBinding);
+            }
+            --silence_depth_;
+          }
+          continue;
+
+        // const -> type const-data | p | backref
+        //
+        // const is a C++ keyword, so we use the label `constant` instead.
+        constant:
+          if (Eat('B')) goto const_backref;
+          if (Eat('p')) {
+            if (!Emit("_")) return false;
+            continue;
+          }
+
+          // Scan the type without printing it.
+          //
+          // The Rust language restricts the type of a const generic argument
+          // much more than the mangling grammar does.  We do not enforce this.
+          //
+          // We also do not bother printing false, true, 'A', and '\u{abcd}' for
+          // the types bool and char.  Because we do not print generic-args
+          // contents, we expect to print constants only in array sizes, and
+          // those should not be bool or char.
+          ++silence_depth_;
+          Y_ABSL_DEMANGLER_RECURSE(type, kConstData);
+          --silence_depth_;
+
+          // const-data -> n? hex-digit* _
+          //
+          // Although the grammar doesn't say this, existing demanglers expect
+          // that zero is 0, not an empty digit sequence, and no nonzero value
+          // may have leading zero digits.  Also n0_ is accepted and printed as
+          // -0, though a toolchain will probably never write that encoding.
+          if (Eat('n') && !EmitChar('-')) return false;
+          if (!Emit("0x")) return false;
+          if (Eat('0')) {
+            if (!EmitChar('0')) return false;
+            if (!Eat('_')) return false;
+            continue;
+          }
+          while (IsLowerHexDigit(Peek())) {
+            if (!EmitChar(Take())) return false;
+          }
+          if (!Eat('_')) return false;
+          continue;
+
+        // generic-args -> I path generic-arg* E (I already consumed)
+        //
+        // We follow the C++ demangler in omitting all the arguments from the
+        // output, printing only the list opening and closing tokens.
+        generic_args:
+          Y_ABSL_DEMANGLER_RECURSE(path, kBeginGenericArgList);
+          if (!Emit("::<>")) return false;
+          ++silence_depth_;
+          while (!Eat('E')) {
+            Y_ABSL_DEMANGLER_RECURSE(generic_arg, kContinueGenericArgList);
+          }
+          --silence_depth_;
+          continue;
+
+        // generic-arg -> lifetime | type | K const
+        generic_arg:
+          if (Peek() == 'L') {
+            if (!ParseOptionalLifetime()) return false;
+            continue;
+          }
+          if (Eat('K')) goto constant;
+          goto type;
+
+        // backref -> B base-62-number (B already consumed)
+        //
+        // The BeginBackref call parses and range-checks the base-62-number.  We
+        // always do that much.
+        //
+        // The recursive call parses and prints what the backref points at.  We
+        // save CPU and stack by skipping this work if the output would be
+        // suppressed anyway.
+        path_backref:
+          if (!BeginBackref()) return false;
+          if (silence_depth_ == 0) {
+            Y_ABSL_DEMANGLER_RECURSE(path, kPathBackrefEnding);
+          }
+          EndBackref();
+          continue;
+
+        // This represents the same backref production as in path_backref but
+        // parses the target as a type instead of a path.
+        type_backref:
+          if (!BeginBackref()) return false;
+          if (silence_depth_ == 0) {
+            Y_ABSL_DEMANGLER_RECURSE(type, kTypeBackrefEnding);
+          }
+          EndBackref();
+          continue;
+
+        const_backref:
+          if (!BeginBackref()) return false;
+          if (silence_depth_ == 0) {
+            Y_ABSL_DEMANGLER_RECURSE(constant, kConstantBackrefEnding);
+          }
+          EndBackref();
+          continue;
+      }
+    }
+
+    return false;  // hit iteration limit or a bug in our stack handling
+  }
+
+ private:
+  // Enumerates resumption points for Y_ABSL_DEMANGLER_RECURSE calls.
+  enum ReturnAddress : uint8_t {
+    kInstantiatingCrate,
+    kVendorSpecificSuffix,
+    kIdentifierInUppercaseNamespace,
+    kIdentifierInLowercaseNamespace,
+    kInherentImplType,
+    kInherentImplEnding,
+    kTraitImplType,
+    kTraitImplInfix,
+    kTraitImplEnding,
+    kImplPathEnding,
+    kTraitDefinitionInfix,
+    kTraitDefinitionEnding,
+    kArraySize,
+    kFinishArray,
+    kSliceEnding,
+    kAfterFirstTupleElement,
+    kAfterSecondTupleElement,
+    kAfterThirdTupleElement,
+    kAfterSubsequentTupleElement,
+    kContinueParameterList,
+    kFinishFn,
+    kBeginAutoTraits,
+    kContinueAutoTraits,
+    kContinueDynTrait,
+    kContinueAssocBinding,
+    kConstData,
+    kBeginGenericArgList,
+    kContinueGenericArgList,
+    kPathBackrefEnding,
+    kTypeBackrefEnding,
+    kConstantBackrefEnding,
+  };
+
+  // Element counts for the stack arrays.  Larger stack sizes accommodate more
+  // deeply nested names at the cost of a larger footprint on the C++ call
+  // stack.
+  enum {
+    // Maximum recursive calls outstanding at one time.
+    kStackSize = 256,
+
+    // Maximum N<uppercase> nested-paths open at once.  We do not expect
+    // closures inside closures inside closures as much as functions inside
+    // modules inside other modules, so we can use a smaller array here.
+    kNamespaceStackSize = 64,
+
+    // Maximum number of nested backrefs.  We can keep this stack pretty small
+    // because we do not follow backrefs inside generic-args or other contexts
+    // that suppress printing, so deep stacking is unlikely in practice.
+    kPositionStackSize = 16,
+  };
+
+  // Returns the next input character without consuming it.
+  char Peek() const { return encoding_[pos_]; }
+
+  // Consumes and returns the next input character.
+  char Take() { return encoding_[pos_++]; }
+
+  // If the next input character is the given character, consumes it and returns
+  // true; otherwise returns false without consuming a character.
+  Y_ABSL_MUST_USE_RESULT bool Eat(char want) {
+    if (encoding_[pos_] != want) return false;
+    ++pos_;
+    return true;
+  }
+
+  // Provided there is enough remaining output space, appends c to the output,
+  // writing a fresh NUL terminator afterward, and returns true.  Returns false
+  // if the output buffer had less than two bytes free.
+  Y_ABSL_MUST_USE_RESULT bool EmitChar(char c) {
+    if (silence_depth_ > 0) return true;
+    if (out_end_ - out_ < 2) return false;
+    *out_++ = c;
+    *out_ = '\0';
+    return true;
+  }
+
+  // Provided there is enough remaining output space, appends the C string token
+  // to the output, followed by a NUL character, and returns true.  Returns
+  // false if not everything fit into the output buffer.
+  Y_ABSL_MUST_USE_RESULT bool Emit(const char* token) {
+    if (silence_depth_ > 0) return true;
+    const size_t token_length = std::strlen(token);
+    const size_t bytes_to_copy = token_length + 1;  // token and final NUL
+    if (static_cast<size_t>(out_end_ - out_) < bytes_to_copy) return false;
+    std::memcpy(out_, token, bytes_to_copy);
+    out_ += token_length;
+    return true;
+  }
+
+  // Provided there is enough remaining output space, appends the decimal form
+  // of disambiguator (if it's nonnegative) or "?" (if it's negative) to the
+  // output, followed by a NUL character, and returns true.  Returns false if
+  // not everything fit into the output buffer.
+  Y_ABSL_MUST_USE_RESULT bool EmitDisambiguator(int disambiguator) {
+    if (disambiguator < 0) return EmitChar('?');  // parsed but too large
+    if (disambiguator == 0) return EmitChar('0');
+    // Convert disambiguator to decimal text.  Three digits per byte is enough
+    // because 999 > 256.  The bound will remain correct even if future
+    // maintenance changes the type of the disambiguator variable.
+    char digits[3 * sizeof(disambiguator)] = {};
+    size_t leading_digit_index = sizeof(digits) - 1;
+    for (; disambiguator > 0; disambiguator /= 10) {
+      digits[--leading_digit_index] =
+          static_cast<char>('0' + disambiguator % 10);
+    }
+    return Emit(digits + leading_digit_index);
+  }
+
+  // Consumes an optional disambiguator (s123_) from the input.
+  //
+  // On success returns true and fills value with the encoded value if it was
+  // not too big, otherwise with -1.  If the optional disambiguator was omitted,
+  // value is 0.  On parse failure returns false and sets value to -1.
+  Y_ABSL_MUST_USE_RESULT bool ParseDisambiguator(int& value) {
+    value = -1;
+
+    // disambiguator = s base-62-number
+    //
+    // Disambiguators are optional.  An omitted disambiguator is zero.
+    if (!Eat('s')) {
+      value = 0;
+      return true;
+    }
+    int base_62_value = 0;
+    if (!ParseBase62Number(base_62_value)) return false;
+    value = base_62_value < 0 ? -1 : base_62_value + 1;
+    return true;
+  }
+
+  // Consumes a base-62 number like _ or 123_ from the input.
+  //
+  // On success returns true and fills value with the encoded value if it was
+  // not too big, otherwise with -1.  On parse failure returns false and sets
+  // value to -1.
+  Y_ABSL_MUST_USE_RESULT bool ParseBase62Number(int& value) {
+    value = -1;
+
+    // base-62-number = (digit | lower | upper)* _
+    //
+    // An empty base-62 digit sequence means 0.
+    if (Eat('_')) {
+      value = 0;
+      return true;
+    }
+
+    // A nonempty digit sequence denotes its base-62 value plus 1.
+    int encoded_number = 0;
+    bool overflowed = false;
+    while (IsAlpha(Peek()) || IsDigit(Peek())) {
+      const char c = Take();
+      if (encoded_number >= std::numeric_limits<int>::max()/62) {
+        // If we are close to overflowing an int, keep parsing but stop updating
+        // encoded_number and remember to return -1 at the end.  The point is to
+        // avoid undefined behavior while parsing crate-root disambiguators,
+        // which are large in practice but not shown in demangling, while
+        // successfully computing closure and shim disambiguators, which are
+        // typically small and are printed out.
+        overflowed = true;
+      } else {
+        int digit;
+        if (IsDigit(c)) {
+          digit = c - '0';
+        } else if (IsLower(c)) {
+          digit = c - 'a' + 10;
+        } else {
+          digit = c - 'A' + 36;
+        }
+        encoded_number = 62 * encoded_number + digit;
+      }
+    }
+
+    if (!Eat('_')) return false;
+    if (!overflowed) value = encoded_number + 1;
+    return true;
+  }
+
+  // Consumes an identifier from the input, returning true on success.
+  //
+  // A nonzero uppercase_namespace specifies the character after the N in a
+  // nested-identifier, e.g., 'C' for a closure, allowing ParseIdentifier to
+  // write out the name with the conventional decoration for that namespace.
+  Y_ABSL_MUST_USE_RESULT bool ParseIdentifier(char uppercase_namespace = '\0') {
+    // identifier -> disambiguator? undisambiguated-identifier
+    int disambiguator = 0;
+    if (!ParseDisambiguator(disambiguator)) return false;
+
+    return ParseUndisambiguatedIdentifier(uppercase_namespace, disambiguator);
+  }
+
+  // Consumes from the input an identifier with no preceding disambiguator,
+  // returning true on success.
+  //
+  // When ParseIdentifier calls this, it passes the N<namespace> character and
+  // disambiguator value so that "{closure#42}" and similar forms can be
+  // rendered correctly.
+  //
+  // At other appearances of undisambiguated-identifier in the grammar, this
+  // treatment is not applicable, and the call site omits both arguments.
+  Y_ABSL_MUST_USE_RESULT bool ParseUndisambiguatedIdentifier(
+      char uppercase_namespace = '\0', int disambiguator = 0) {
+    // undisambiguated-identifier -> u? decimal-number _? bytes
+    const bool is_punycoded = Eat('u');
+    if (!IsDigit(Peek())) return false;
+    int num_bytes = 0;
+    if (!ParseDecimalNumber(num_bytes)) return false;
+    (void)Eat('_');  // optional separator, needed if a digit follows
+    if (is_punycoded) {
+      DecodeRustPunycodeOptions options;
+      options.punycode_begin = &encoding_[pos_];
+      options.punycode_end = &encoding_[pos_] + num_bytes;
+      options.out_begin = out_;
+      options.out_end = out_end_;
+      out_ = DecodeRustPunycode(options);
+      if (out_ == nullptr) return false;
+      pos_ += static_cast<size_t>(num_bytes);
+    }
+
+    // Emit the beginnings of braced forms like {shim:vtable#0}.
+    if (uppercase_namespace != '\0') {
+      switch (uppercase_namespace) {
+        case 'C':
+          if (!Emit("{closure")) return false;
+          break;
+        case 'S':
+          if (!Emit("{shim")) return false;
+          break;
+        default:
+          if (!EmitChar('{') || !EmitChar(uppercase_namespace)) return false;
+          break;
+      }
+      if (num_bytes > 0 && !Emit(":")) return false;
+    }
+
+    // Emit the name itself.
+    if (!is_punycoded) {
+      for (int i = 0; i < num_bytes; ++i) {
+        const char c = Take();
+        if (!IsIdentifierChar(c) &&
+            // The spec gives toolchains the choice of Punycode or raw UTF-8 for
+            // identifiers containing code points above 0x7f, so accept bytes
+            // with the high bit set.
+            (c & 0x80) == 0) {
+          return false;
+        }
+        if (!EmitChar(c)) return false;
+      }
+    }
+
+    // Emit the endings of braced forms, e.g., "#42}".
+    if (uppercase_namespace != '\0') {
+      if (!EmitChar('#')) return false;
+      if (!EmitDisambiguator(disambiguator)) return false;
+      if (!EmitChar('}')) return false;
+    }
+
+    return true;
+  }
+
+  // Consumes a decimal number like 0 or 123 from the input.  On success returns
+  // true and fills value with the encoded value.  If the encoded value is too
+  // large or otherwise unparsable, returns false and sets value to -1.
+  Y_ABSL_MUST_USE_RESULT bool ParseDecimalNumber(int& value) {
+    value = -1;
+    if (!IsDigit(Peek())) return false;
+    int encoded_number = Take() - '0';
+    if (encoded_number == 0) {
+      // Decimal numbers are never encoded with extra leading zeroes.
+      value = 0;
+      return true;
+    }
+    while (IsDigit(Peek()) &&
+           // avoid overflow
+           encoded_number < std::numeric_limits<int>::max()/10) {
+      encoded_number = 10 * encoded_number + (Take() - '0');
+    }
+    if (IsDigit(Peek())) return false;  // too big
+    value = encoded_number;
+    return true;
+  }
+
+  // Consumes a binder of higher-ranked lifetimes if one is present.  On success
+  // returns true and discards the encoded lifetime count.  On parse failure
+  // returns false.
+  Y_ABSL_MUST_USE_RESULT bool ParseOptionalBinder() {
+    // binder -> G base-62-number
+    if (!Eat('G')) return true;
+    int ignored_binding_count;
+    return ParseBase62Number(ignored_binding_count);
+  }
+
+  // Consumes a lifetime if one is present.
+  //
+  // On success returns true and discards the lifetime index.  We do not print
+  // or even range-check lifetimes because they are a finer detail than other
+  // things we omit from output, such as the entire contents of generic-args.
+  //
+  // On parse failure returns false.
+  Y_ABSL_MUST_USE_RESULT bool ParseOptionalLifetime() {
+    // lifetime -> L base-62-number
+    if (!Eat('L')) return true;
+    int ignored_de_bruijn_index;
+    return ParseBase62Number(ignored_de_bruijn_index);
+  }
+
+  // Consumes a lifetime just like ParseOptionalLifetime, but returns false if
+  // there is no lifetime here.
+  Y_ABSL_MUST_USE_RESULT bool ParseRequiredLifetime() {
+    if (Peek() != 'L') return false;
+    return ParseOptionalLifetime();
+  }
+
+  // Pushes ns onto the namespace stack and returns true if the stack is not
+  // full, else returns false.
+  Y_ABSL_MUST_USE_RESULT bool PushNamespace(char ns) {
+    if (namespace_depth_ == kNamespaceStackSize) return false;
+    namespace_stack_[namespace_depth_++] = ns;
+    return true;
+  }
+
+  // Pops the last pushed namespace.  Requires that the namespace stack is not
+  // empty (namespace_depth_ > 0).
+  char PopNamespace() { return namespace_stack_[--namespace_depth_]; }
+
+  // Pushes position onto the position stack and returns true if the stack is
+  // not full, else returns false.
+  Y_ABSL_MUST_USE_RESULT bool PushPosition(int position) {
+    if (position_depth_ == kPositionStackSize) return false;
+    position_stack_[position_depth_++] = position;
+    return true;
+  }
+
+  // Pops the last pushed input position.  Requires that the position stack is
+  // not empty (position_depth_ > 0).
+  int PopPosition() { return position_stack_[--position_depth_]; }
+
+  // Consumes a base-62-number denoting a backref target, pushes the current
+  // input position on the data stack, and sets the input position to the
+  // beginning of the backref target.  Returns true on success.  Returns false
+  // if parsing failed, the stack is exhausted, or the backref target position
+  // is out of range.
+  Y_ABSL_MUST_USE_RESULT bool BeginBackref() {
+    // backref = B base-62-number (B already consumed)
+    //
+    // Reject backrefs that don't parse, overflow int, or don't point backward.
+    // If the offset looks fine, adjust it to account for the _R prefix.
+    int offset = 0;
+    const int offset_of_this_backref =
+        pos_ - 2 /* _R */ - 1 /* B already consumed */;
+    if (!ParseBase62Number(offset) || offset < 0 ||
+        offset >= offset_of_this_backref) {
+      return false;
+    }
+    offset += 2;
+
+    // Save the old position to restore later.
+    if (!PushPosition(pos_)) return false;
+
+    // Move the input position to the backref target.
+    //
+    // Note that we do not check whether the new position points to the
+    // beginning of a construct matching the context in which the backref
+    // appeared.  We just jump to it and see whether nested parsing succeeds.
+    // We therefore accept various wrong manglings, e.g., a type backref
+    // pointing to an 'l' character inside an identifier, which happens to mean
+    // i32 when parsed as a type mangling.  This saves the complexity and RAM
+    // footprint of remembering which offsets began which kinds of
+    // substructures.  Existing demanglers take similar shortcuts.
+    pos_ = offset;
+    return true;
+  }
+
+  // Cleans up after a backref production by restoring the previous input
+  // position from the data stack.
+  void EndBackref() { pos_ = PopPosition(); }
+
+  // The leftmost recursion_depth_ elements of recursion_stack_ contain the
+  // ReturnAddresses pushed by Y_ABSL_DEMANGLER_RECURSE calls not yet completed.
+  ReturnAddress recursion_stack_[kStackSize] = {};
+  int recursion_depth_ = 0;
+
+  // The leftmost namespace_depth_ elements of namespace_stack_ contain the
+  // uppercase namespace identifiers for open nested-paths, e.g., 'C' for a
+  // closure.
+  char namespace_stack_[kNamespaceStackSize] = {};
+  int namespace_depth_ = 0;
+
+  // The leftmost position_depth_ elements of position_stack_ contain the input
+  // positions to return to after fully printing the targets of backrefs.
+  int position_stack_[kPositionStackSize] = {};
+  int position_depth_ = 0;
+
+  // Anything parsed while silence_depth_ > 0 contributes nothing to the
+  // demangled output.  For constructs omitted from the demangling, such as
+  // impl-path and the contents of generic-args, we will increment
+  // silence_depth_ on the way in and decrement silence_depth_ on the way out.
+  int silence_depth_ = 0;
+
+  // Input: encoding_ points to a Rust mangled symbol, and encoding_[pos_] is
+  // the next input character to be scanned.
+  int pos_ = 0;
+  const char* encoding_ = nullptr;
+
+  // Output: *out_ is where the next output character should be written, and
+  // out_end_ points past the last byte of available space.
+  char* out_ = nullptr;
+  char* out_end_ = nullptr;
+};
+
+}  // namespace
+
+bool DemangleRustSymbolEncoding(const char* mangled, char* out,
+                                size_t out_size) {
+  return RustSymbolParser(mangled, out, out + out_size).Parse();
+}
+
+}  // namespace debugging_internal
+Y_ABSL_NAMESPACE_END
+}  // namespace y_absl