[WIP] Maybe this works on Windows?

Author: bjorn3

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2083,6 +2083,8 @@ unsafe extern "C" {
         ValueLen: size_t,
     );
 
+    pub(crate) fn LLVMRustAddLinkerOptions(M: &Module, Value: *const c_char, ValueLen: size_t);
+
     pub(crate) fn LLVMRustDIBuilderCreateCompileUnit<'a>(
         Builder: &DIBuilder<'a>,
         Lang: c_uint,

compiler/rustc_codegen_llvm/src/llvm/mod.rs

@@ -441,6 +441,12 @@ pub(crate) fn add_module_flag_str(
     }
 }
 
+pub(crate) fn add_module_linker_option(module: &Module, value: &str) {
+    unsafe {
+        LLVMRustAddLinkerOptions(module, value.as_c_char_ptr(), value.len());
+    }
+}
+
 pub(crate) fn set_dllimport_storage_class<'ll>(v: &'ll Value) {
     unsafe {
         LLVMSetDLLStorageClass(v, DLLStorageClass::DllImport);

compiler/rustc_codegen_llvm/src/mono_item.rs

@@ -83,33 +83,56 @@ impl<'tcx> PreDefineCodegenMethods<'tcx> for CodegenCx<'_, 'tcx> {
 
         let symbol_name = self.tcx.symbol_name(instance);
         if symbol_name.name.contains("__rdl_alloc") {
-            self.weak_alias(lldecl, &mangle_internal_symbol(self.tcx, "__rust_alloc"));
+            self.weak_alias(
+                lldecl,
+                symbol_name.name,
+                &mangle_internal_symbol(self.tcx, "__rust_alloc"),
+            );
         }
         if symbol_name.name.contains("__rdl_dealloc") {
-            self.weak_alias(lldecl, &mangle_internal_symbol(self.tcx, "__rust_dealloc"));
+            self.weak_alias(
+                lldecl,
+                symbol_name.name,
+                &mangle_internal_symbol(self.tcx, "__rust_dealloc"),
+            );
         }
         if symbol_name.name.contains("__rdl_realloc") {
-            self.weak_alias(lldecl, &mangle_internal_symbol(self.tcx, "__rust_realloc"));
+            self.weak_alias(
+                lldecl,
+                symbol_name.name,
+                &mangle_internal_symbol(self.tcx, "__rust_realloc"),
+            );
         }
         if symbol_name.name.contains("__rdl_alloc_zeroed") {
-            self.weak_alias(lldecl, &mangle_internal_symbol(self.tcx, "__rust_alloc_zeroed"));
+            self.weak_alias(
+                lldecl,
+                symbol_name.name,
+                &mangle_internal_symbol(self.tcx, "__rust_alloc_zeroed"),
+            );
         }
 
         self.instances.borrow_mut().insert(instance, lldecl);
     }
 
-    fn weak_alias(&self, aliasee: Self::Function, name: &str) {
-        let ty = self.get_type_of_global(aliasee);
-        let alias = llvm::add_alias(
-            self.llmod,
-            ty,
-            AddressSpace::DATA,
-            aliasee,
-            &CString::new(name).unwrap(),
-        );
-
-        llvm::set_linkage(alias, llvm::Linkage::WeakAnyLinkage);
-        self.add_compiler_used_global(alias);
+    fn weak_alias(&self, aliasee: Self::Function, aliasee_name: &str, name: &str) {
+        if self.tcx.sess.target.is_like_msvc {
+            llvm::add_module_linker_option(
+                self.llmod,
+                &format!("/alternatename:{name}={aliasee_name}"),
+            );
+        } else {
+            let ty = self.get_type_of_global(aliasee);
+            let alias = llvm::add_alias(
+                self.llmod,
+                ty,
+                AddressSpace::DATA,
+                aliasee,
+                &CString::new(name).unwrap(),
+            );
+
+            llvm::set_linkage(alias, llvm::Linkage::WeakAnyLinkage);
+            self.add_compiler_used_global(alias);
+        }
     }
 }
 

compiler/rustc_codegen_ssa/src/traits/declare.rs

@@ -20,5 +20,5 @@ pub trait PreDefineCodegenMethods<'tcx>: BackendTypes {
         symbol_name: &str,
     );
 
-    fn weak_alias(&self, aliasee: Self::Function, name: &str);
+    fn weak_alias(&self, aliasee: Self::Function, aliasee_name: &str, name: &str);
 }

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -906,6 +906,15 @@ extern "C" void LLVMRustAddModuleFlagString(
       MDString::get(unwrap(M)->getContext(), StringRef(Value, ValueLen)));
 }
 
+extern "C" void LLVMRustAddLinkerOptions(LLVMModuleRef M, const char *Value,
+                                         size_t ValueLen) {
+
+  auto *Opts = unwrap(M)->getOrInsertNamedMetadata("llvm.linker.options");
+  Opts->addOperand(MDNode::get(
+      unwrap(M)->getContext(),
+      {MDString::get(unwrap(M)->getContext(), StringRef(Value, ValueLen))}));
+}
+
 extern "C" LLVMValueRef LLVMRustGetLastInstruction(LLVMBasicBlockRef BB) {
   auto Point = unwrap(BB)->rbegin();
   if (Point != unwrap(BB)->rend())