assert that #[rustc_pass_indirectly_in_non_rustic_abis] is respected

compiler/rustc_target/src/callconv/mod.rs

@@ -703,13 +703,6 @@ impl<'a, Ty> FnAbi<'a, Ty> {
             "bpf" => bpf::compute_abi_info(cx, self),
             arch => panic!("no lowering implemented for {arch}"),
         }
-        // Double check that any argument types annotated with the
-        // `#[rustc_pass_indirectly_in_non_rustic_abis]` attribute are passed indirectly.
-        for arg in &self.args {
-            if arg.layout.pass_indirectly_in_non_rustic_abis(cx) {
-                assert!(matches!(arg.mode, PassMode::Indirect { on_stack: false, .. }));
-            }
-        }
     }
 
     pub fn adjust_for_rust_abi<C>(&mut self, cx: &C)

compiler/rustc_ty_utils/src/abi.rs

@@ -1,3 +1,4 @@
+use std::assert_matches::assert_matches;
 use std::iter;
 
 use rustc_abi::Primitive::Pointer;
@@ -388,6 +389,12 @@ fn fn_abi_sanity_check<'tcx>(
             if let PassMode::Indirect { on_stack, .. } = arg.mode {
                 assert!(!on_stack, "rust abi shouldn't use on_stack");
             }
+        } else if arg.layout.pass_indirectly_in_non_rustic_abis(cx) {
+            assert_matches!(
+                arg.mode,
+                PassMode::Indirect { on_stack: false, .. },
+                "the {spec_abi} ABI does not implement `#[rustc_pass_indirectly_in_non_rustic_abis]`"
+            );
         }
 
         match &arg.mode {

library/core/src/ffi/va_list.rs

@@ -299,3 +299,15 @@ impl<'f> Drop for VaListImpl<'f> {
         // This works for now, since `va_end` is a no-op on all current LLVM targets.
     }
 }
+
+// Checks (via an assert in `compiler/rustc_ty_utils/src/abi.rs`) that the C ABI for the current
+// target correctly implements `rustc_pass_indirectly_in_non_rustic_abis`.
+const _: () = {
+    #[repr(C)]
+    #[rustc_pass_indirectly_in_non_rustic_abis]
+    struct Type(usize);
+
+    const extern "C" fn c(_: Type) {}
+
+    c(Type(0))
+};