turn this into a codegen test so that we actually ensure it behaves correctly

Author: RalfJung

tests/codegen/abi-win64-zst.rs

@@ -0,0 +1,33 @@
+//@ revisions: linux
+//@[linux] compile-flags: --target x86_64-unknown-linux-gnu
+//@[linux] needs-llvm-components: x86
+
+//@ revisions: windows-gnu
+//@[windows-gnu] compile-flags: --target x86_64-pc-windows-gnu
+//@[windows-gnu] needs-llvm-components: x86
+
+//@ revisions: windows-msvc
+//@[windows-msvc] compile-flags: --target x86_64-pc-windows-msvc
+//@[windows-msvc] needs-llvm-components: x86
+
+#![feature(no_core, lang_items, rustc_attrs)]
+#![no_core]
+#![crate_type = "lib"]
+
+#[lang = "sized"]
+trait Sized {}
+
+// Make sure the argument is always ignored when explicitly requesting "win64" ABI.
+
+// CHECK: define win64cc void @pass_zst_win64()
+#[no_mangle]
+extern "win64" fn pass_zst_win64(_: ()) {}
+
+// For `extern "C"` functions, ZST are *not* ignored on windows-gnu.
+// That's likely an accident on their side but the ABI is what it is.
+
+// linux: define void @pass_zst_c()
+// windows-msvc: define void @pass_zst_c()
+// windows-gnu: define void @pass_zst_c(ptr {{.*}})
+#[no_mangle]
+extern "C" fn pass_zst_c(_: ()) {}