fix handling of ZST in win64 ABI on windows-gnu targets

Author: RalfJung

compiler/rustc_abi/src/extern_abi/mod.rs

@@ -192,6 +192,10 @@ pub fn is_enabled(
     s
 }
 
+/// Returns whether the ABI is stable to use.
+///
+/// Note that there is a separate check determining whether the ABI is even supported
+/// on the current target; see `fn is_abi_supported` in `rustc_target::spec`.
 pub fn is_stable(name: &str) -> Result<(), AbiDisabled> {
     match name {
         // Stable

compiler/rustc_target/src/callconv/x86_win64.rs

@@ -1,5 +1,6 @@
 use rustc_abi::{BackendRepr, Float, Primitive};
 
+use super::Conv;
 use crate::abi::call::{ArgAbi, FnAbi, Reg};
 use crate::spec::HasTargetSpec;
 
@@ -41,10 +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.
+            // x86_64-pc-windows-gnu doesn't ignore ZSTs -- except in `extern "win64"`
+            // functions which are supposed to match MSVC.
             if cx.target_spec().os == "windows"
                 && cx.target_spec().env == "gnu"
                 && arg.layout.is_zst()
+                && fn_abi.conv != Conv::X86_64Win64
             {
                 arg.make_indirect_from_ignore();
             }

tests/ui/abi/win64-zst.rs

@@ -20,5 +20,7 @@
 #[lang = "sized"]
 trait Sized {}
 
+// Should use `mode: Ignore` on *all* targets. This ABI is explicitly
+// meant to be the same across the MSVC and GNU targets.
 #[rustc_abi(debug)]
 extern "win64" fn pass_zst(_: ()) {} //~ ERROR: fn_abi

tests/ui/abi/win64-zst.x86_64-windows-gnu.stderr

@@ -24,18 +24,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                            unadjusted_abi_align: $SOME_ALIGN,
                        },
                    },
-                   mode: Indirect {
-                       attrs: ArgAttributes {
-                           regular: NoAlias | NoCapture | NonNull | NoUndef,
-                           arg_ext: None,
-                           pointee_size: Size(0 bytes),
-                           pointee_align: Some(
-                               Align(1 bytes),
-                           ),
-                       },
-                       meta_attrs: None,
-                       on_stack: false,
-                   },
+                   mode: Ignore,
                },
            ],
            ret: ArgAbi {