vectorcall should also ignore ZST

Author: RalfJung

compiler/rustc_target/src/callconv/x86_win64.rs

@@ -42,12 +42,12 @@ pub(crate) fn compute_abi_info<Ty>(cx: &impl HasTargetSpec, fn_abi: &mut FnAbi<'
     }
     for arg in fn_abi.args.iter_mut() {
         if arg.is_ignore() {
-            // x86_64-pc-windows-gnu doesn't ignore ZSTs -- except in `extern "win64"`
-            // functions which are supposed to match MSVC.
+            // x86_64-pc-windows-gnu doesn't ignore ZSTs for their native "C" ABI.
+            // However, other special ABIs ("win64", "fastcall", etc) should match MSVC behavior.
             if cx.target_spec().os == "windows"
                 && cx.target_spec().env == "gnu"
                 && arg.layout.is_zst()
-                && fn_abi.conv != Conv::X86_64Win64
+                && fn_abi.conv == Conv::C
             {
                 arg.make_indirect_from_ignore();
             }

tests/codegen/abi-win64-zst.rs

@@ -10,7 +10,7 @@
 //@[windows-msvc] compile-flags: --target x86_64-pc-windows-msvc
 //@[windows-msvc] needs-llvm-components: x86
 
-#![feature(no_core, lang_items, rustc_attrs)]
+#![feature(no_core, lang_items, rustc_attrs, abi_vectorcall)]
 #![no_core]
 #![crate_type = "lib"]
 
@@ -27,6 +27,10 @@ extern "win64" fn pass_zst_win64(_: ()) {}
 #[no_mangle]
 extern "sysv64" fn pass_zst_sysv64(_: ()) {}
 
+// CHECK: define x86_vectorcallcc void @pass_zst_vectorcall()
+#[no_mangle]
+extern "vectorcall" fn pass_zst_vectorcall(_: ()) {}
+
 // 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.