Rewrite LLVM's archive writer in Rust

This allows it to be used by other codegen backends

compiler/rustc_llvm/llvm-wrapper/SymbolWrapper.cpp

@@ -0,0 +1,82 @@
+// Derived from code in LLVM, which is:
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Derived from:
+// * https://github.com/llvm/llvm-project/blob/8ef3e895ad8ab1724e2b87cabad1dacdc7a397a3/llvm/include/llvm/Object/ArchiveWriter.h
+// * https://github.com/llvm/llvm-project/blob/8ef3e895ad8ab1724e2b87cabad1dacdc7a397a3/llvm/lib/Object/ArchiveWriter.cpp
+
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/Object/ObjectFile.h"
+#include "llvm/ADT/Optional.h"
+
+using namespace llvm;
+using namespace llvm::sys;
+using namespace llvm::object;
+
+static bool isArchiveSymbol(const object::BasicSymbolRef &S) {
+  Expected<uint32_t> SymFlagsOrErr = S.getFlags();
+  if (!SymFlagsOrErr)
+    // FIXME: Actually report errors helpfully.
+    report_fatal_error(SymFlagsOrErr.takeError());
+  if (*SymFlagsOrErr & object::SymbolRef::SF_FormatSpecific)
+    return false;
+  if (!(*SymFlagsOrErr & object::SymbolRef::SF_Global))
+    return false;
+  if (*SymFlagsOrErr & object::SymbolRef::SF_Undefined)
+    return false;
+  return true;
+}
+
+typedef void *(*LLVMRustGetSymbolsCallback)(void *, const char *);
+typedef void *(*LLVMRustGetSymbolsErrorCallback)(const char *);
+
+// Note: This is implemented in C++ instead of using the C api from Rust as IRObjectFile doesn't
+// implement getSymbolName, only printSymbolName, which is inaccessible from the C api.
+extern "C" void *LLVMRustGetSymbols(
+  char *BufPtr, size_t BufLen, void *State, LLVMRustGetSymbolsCallback Callback,
+  LLVMRustGetSymbolsErrorCallback ErrorCallback) {
+  std::unique_ptr<MemoryBuffer> Buf =
+    MemoryBuffer::getMemBuffer(StringRef(BufPtr, BufLen), StringRef("LLVMRustGetSymbolsObject"),
+                               false);
+  SmallString<0> SymNameBuf;
+  raw_svector_ostream SymName(SymNameBuf);
+
+  // In the scenario when LLVMContext is populated SymbolicFile will contain a
+  // reference to it, thus SymbolicFile should be destroyed first.
+  LLVMContext Context;
+  std::unique_ptr<object::SymbolicFile> Obj;
+
+  const file_magic Type = identify_magic(Buf->getBuffer());
+  if (Type != file_magic::bitcode) {
+    return ErrorCallback("not bitcode");
+  }
+  auto ObjOrErr = object::SymbolicFile::createSymbolicFile(
+      Buf->getMemBufferRef(), file_magic::bitcode, &Context);
+  if (!ObjOrErr) {
+    Error E = ObjOrErr.takeError();
+    SmallString<0> ErrorBuf;
+    raw_svector_ostream Error(ErrorBuf);
+    Error << E << '\0';
+    return ErrorCallback(Error.str().data());
+  }
+  Obj = std::move(*ObjOrErr);
+
+  for (const object::BasicSymbolRef &S : Obj->symbols()) {
+    if (!isArchiveSymbol(S))
+      continue;
+    if (Error E = S.printName(SymName)) {
+      SmallString<0> ErrorBuf;
+      raw_svector_ostream Error(ErrorBuf);
+      Error << E << '\0';
+      return ErrorCallback(Error.str().data());
+    }
+    SymName << '\0';
+    if (void *E = Callback(State, SymNameBuf.str().data())) {
+      return E;
+    }
+    SymNameBuf.clear();
+  }
+  return 0;
+}