Auto merge of #84171 - ricobbe:raw-dylib-via-llvm, r=petrochenkov

Partial support for raw-dylib linkage

First cut of functionality for issue #58713: add support for `#[link(kind = "raw-dylib")]` on `extern` blocks in lib crates compiled to .rlib files.  Does not yet support `#[link_name]` attributes on functions, or the `#[link_ordinal]` attribute, or `#[link(kind = "raw-dylib")]` on `extern` blocks in bin crates; I intend to publish subsequent PRs to fill those gaps.  It's also not yet clear whether this works for functions in `extern "stdcall"` blocks; I also intend to investigate that shortly and make any necessary changes as a follow-on PR.

This implementation calls out to an LLVM function to construct the actual `.idata` sections as temporary `.lib` files on disk and then links those into the generated .rlib.

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -6,6 +6,7 @@
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/Object/Archive.h"
+#include "llvm/Object/COFFImportFile.h"
 #include "llvm/Object/ObjectFile.h"
 #include "llvm/Bitcode/BitcodeWriterPass.h"
 #include "llvm/Support/Signals.h"
@@ -1722,3 +1723,54 @@ extern "C" LLVMValueRef
 LLVMRustBuildMaxNum(LLVMBuilderRef B, LLVMValueRef LHS, LLVMValueRef RHS) {
     return wrap(unwrap(B)->CreateMaxNum(unwrap(LHS),unwrap(RHS)));
 }
+
+// This struct contains all necessary info about a symbol exported from a DLL.
+// At the moment, it's just the symbol's name, but we use a separate struct to
+// make it easier to add other information like ordinal later.
+struct LLVMRustCOFFShortExport {
+  const char* name;
+};
+
+// Machine must be a COFF machine type, as defined in PE specs.
+extern "C" LLVMRustResult LLVMRustWriteImportLibrary(
+  const char* ImportName,
+  const char* Path,
+  const LLVMRustCOFFShortExport* Exports,
+  size_t NumExports,
+  uint16_t Machine,
+  bool MinGW)
+{
+  std::vector<llvm::object::COFFShortExport> ConvertedExports;
+  ConvertedExports.reserve(NumExports);
+
+  for (size_t i = 0; i < NumExports; ++i) {
+    ConvertedExports.push_back(llvm::object::COFFShortExport{
+      Exports[i].name,  // Name
+      std::string{},    // ExtName
+      std::string{},    // SymbolName
+      std::string{},    // AliasTarget
+      0,                // Ordinal
+      false,            // Noname
+      false,            // Data
+      false,            // Private
+      false             // Constant
+    });
+  }
+
+  auto Error = llvm::object::writeImportLibrary(
+    ImportName,
+    Path,
+    ConvertedExports,
+    static_cast<llvm::COFF::MachineTypes>(Machine),
+    MinGW);
+  if (Error) {
+    std::string errorString;
+    llvm::raw_string_ostream stream(errorString);
+    stream << Error;
+    stream.flush();
+    LLVMRustSetLastError(errorString.c_str());
+    return LLVMRustResult::Failure;
+  } else {
+    return LLVMRustResult::Success;
+  }
+}