fix: NonNull<()> doesn't lower to void *

src/bindgen/ir/ty.rs

@@ -526,8 +526,16 @@ impl Type {
             _ => return None,
         };
 
-        if path.generics().is_empty() {
-            return None;
+        // Special-case: when a ZST like `()` is used as the generic parameter of `NonNull`, it
+        // evaporates during parsing and `NonNull` ends up with zero generics. In that case, treat
+        // it as `void*` so aliases like `type Id = NonNull<()>;` lower correctly.
+        if path.generics().is_empty() && path.name() == "NonNull" {
+            return Some(Type::Ptr {
+                ty: Box::new(Type::Primitive(PrimitiveType::Void)),
+                is_const: false,
+                is_nullable: false,
+                is_ref: false,
+            });
         }
 
         if path.generics().len() != 1 {

tests/expectations-symbols/nonnull_unit.c.sym

@@ -0,0 +1,4 @@
+{
+takes_id;
+takes_unit_ptr;
+};

tests/expectations/nonnull_unit.c

@@ -0,0 +1,10 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef void *MyId;
+
+void takes_id(MyId id);
+
+void takes_unit_ptr(void *id);

tests/expectations/nonnull_unit.compat.c

@@ -0,0 +1,18 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef void *MyId;
+
+#ifdef __cplusplus
+extern "C" {
+#endif // __cplusplus
+
+void takes_id(MyId id);
+
+void takes_unit_ptr(void *id);
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif  // __cplusplus

tests/expectations/nonnull_unit.cpp

@@ -0,0 +1,15 @@
+#include <cstdarg>
+#include <cstdint>
+#include <cstdlib>
+#include <ostream>
+#include <new>
+
+using MyId = void*;
+
+extern "C" {
+
+void takes_id(MyId id);
+
+void takes_unit_ptr(void *id);
+
+}  // extern "C"

tests/expectations/nonnull_unit.pyx

@@ -0,0 +1,13 @@
+from libc.stdint cimport int8_t, int16_t, int32_t, int64_t, intptr_t
+from libc.stdint cimport uint8_t, uint16_t, uint32_t, uint64_t, uintptr_t
+cdef extern from *:
+  ctypedef bint bool
+  ctypedef struct va_list
+
+cdef extern from *:
+
+  ctypedef void *MyId;
+
+  void takes_id(MyId id);
+
+  void takes_unit_ptr(void *id);

tests/rust/nonnull_unit.rs

@@ -0,0 +1,16 @@
+use core::ptr::NonNull;
+
+// Reproducer: generic argument is a ZST `()` which evaporates in parsing.
+pub type MyId = NonNull<()>;
+
+#[no_mangle]
+pub extern "C" fn takes_id(id: MyId) {
+    let _ = id;
+}
+
+#[no_mangle]
+pub extern "C" fn takes_unit_ptr(id: *mut ()) {
+    let _ = id;
+}
+
+