Allow zero sized unions to be defined

A union may be defined in C that contains only zero sized types. We
should in turn generate a rust union in this case.

bindgen-tests/tests/expectations/tests/struct_with_anon_union_of_array_members.rs

diff --git a/bindgen-tests/tests/expectations/tests/struct_with_anon_union_of_array_members.rs b/bindgen-tests/tests/expectations/tests/struct_with_anon_union_of_array_members.rs
new file mode 100644
index 0000000..7190160
--- /dev/null
+++ b/bindgen-tests/tests/expectations/tests/struct_with_anon_union_of_array_members.rs
@@ -0,0 +1,148 @@
+#![allow(dead_code, non_snake_case, non_camel_case_types, non_upper_case_globals)]
+#[repr(C)]
+#[derive(Default)]
+pub struct __IncompleteArrayField<T>(::std::marker::PhantomData<T>, [T; 0]);
+impl<T> __IncompleteArrayField<T> {
+    #[inline]
+    pub const fn new() -> Self {
+        __IncompleteArrayField(::std::marker::PhantomData, [])
+    }
+    #[inline]
+    pub fn as_ptr(&self) -> *const T {
+        self as *const _ as *const T
+    }
+    #[inline]
+    pub fn as_mut_ptr(&mut self) -> *mut T {
+        self as *mut _ as *mut T
+    }
+    #[inline]
+    pub unsafe fn as_slice(&self, len: usize) -> &[T] {
+        ::std::slice::from_raw_parts(self.as_ptr(), len)
+    }
+    #[inline]
+    pub unsafe fn as_mut_slice(&mut self, len: usize) -> &mut [T] {
+        ::std::slice::from_raw_parts_mut(self.as_mut_ptr(), len)
+    }
+}
+impl<T> ::std::fmt::Debug for __IncompleteArrayField<T> {
+    fn fmt(&self, fmt: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+        fmt.write_str("__IncompleteArrayField")
+    }
+}
+#[repr(C)]
+pub struct bpf_array {
+    pub __bindgen_anon_1: bpf_array__bindgen_ty_1,
+}
+#[repr(C)]
+pub union bpf_array__bindgen_ty_1 {
+    pub __bindgen_anon_1: ::std::mem::ManuallyDrop<
+        bpf_array__bindgen_ty_1__bindgen_ty_1,
+    >,
+    pub __bindgen_anon_2: ::std::mem::ManuallyDrop<
+        bpf_array__bindgen_ty_1__bindgen_ty_2,
+    >,
+}
+#[repr(C)]
+#[derive(Debug, Default)]
+pub struct bpf_array__bindgen_ty_1__bindgen_ty_1 {
+    pub __empty_value1: bpf_array__bindgen_ty_1__bindgen_ty_1__bindgen_ty_1,
+    pub value1: __IncompleteArrayField<::std::os::raw::c_char>,
+}
+#[repr(C)]
+#[derive(Debug, Default, Copy, Clone)]
+pub struct bpf_array__bindgen_ty_1__bindgen_ty_1__bindgen_ty_1 {}
+#[allow(clippy::unnecessary_operation, clippy::identity_op)]
+const _: () = {
+    [
+        "Size of bpf_array__bindgen_ty_1__bindgen_ty_1__bindgen_ty_1",
+    ][::std::mem::size_of::<bpf_array__bindgen_ty_1__bindgen_ty_1__bindgen_ty_1>()
+        - 0usize];
+    [
+        "Alignment of bpf_array__bindgen_ty_1__bindgen_ty_1__bindgen_ty_1",
+    ][::std::mem::align_of::<bpf_array__bindgen_ty_1__bindgen_ty_1__bindgen_ty_1>()
+        - 1usize];
+};
+#[allow(clippy::unnecessary_operation, clippy::identity_op)]
+const _: () = {
+    [
+        "Size of bpf_array__bindgen_ty_1__bindgen_ty_1",
+    ][::std::mem::size_of::<bpf_array__bindgen_ty_1__bindgen_ty_1>() - 0usize];
+    [
+        "Alignment of bpf_array__bindgen_ty_1__bindgen_ty_1",
+    ][::std::mem::align_of::<bpf_array__bindgen_ty_1__bindgen_ty_1>() - 1usize];
+    [
+        "Offset of field: bpf_array__bindgen_ty_1__bindgen_ty_1::__empty_value1",
+    ][::std::mem::offset_of!(bpf_array__bindgen_ty_1__bindgen_ty_1, __empty_value1)
+        - 0usize];
+    [
+        "Offset of field: bpf_array__bindgen_ty_1__bindgen_ty_1::value1",
+    ][::std::mem::offset_of!(bpf_array__bindgen_ty_1__bindgen_ty_1, value1) - 0usize];
+};
+#[repr(C)]
+#[derive(Debug, Default)]
+pub struct bpf_array__bindgen_ty_1__bindgen_ty_2 {
+    pub __empty_value2: bpf_array__bindgen_ty_1__bindgen_ty_2__bindgen_ty_1,
+    pub value2: __IncompleteArrayField<::std::os::raw::c_char>,
+}
+#[repr(C)]
+#[derive(Debug, Default, Copy, Clone)]
+pub struct bpf_array__bindgen_ty_1__bindgen_ty_2__bindgen_ty_1 {}
+#[allow(clippy::unnecessary_operation, clippy::identity_op)]
+const _: () = {
+    [
+        "Size of bpf_array__bindgen_ty_1__bindgen_ty_2__bindgen_ty_1",
+    ][::std::mem::size_of::<bpf_array__bindgen_ty_1__bindgen_ty_2__bindgen_ty_1>()
+        - 0usize];
+    [
+        "Alignment of bpf_array__bindgen_ty_1__bindgen_ty_2__bindgen_ty_1",
+    ][::std::mem::align_of::<bpf_array__bindgen_ty_1__bindgen_ty_2__bindgen_ty_1>()
+        - 1usize];
+};
+#[allow(clippy::unnecessary_operation, clippy::identity_op)]
+const _: () = {
+    [
+        "Size of bpf_array__bindgen_ty_1__bindgen_ty_2",
+    ][::std::mem::size_of::<bpf_array__bindgen_ty_1__bindgen_ty_2>() - 0usize];
+    [
+        "Alignment of bpf_array__bindgen_ty_1__bindgen_ty_2",
+    ][::std::mem::align_of::<bpf_array__bindgen_ty_1__bindgen_ty_2>() - 1usize];
+    [
+        "Offset of field: bpf_array__bindgen_ty_1__bindgen_ty_2::__empty_value2",
+    ][::std::mem::offset_of!(bpf_array__bindgen_ty_1__bindgen_ty_2, __empty_value2)
+        - 0usize];
+    [
+        "Offset of field: bpf_array__bindgen_ty_1__bindgen_ty_2::value2",
+    ][::std::mem::offset_of!(bpf_array__bindgen_ty_1__bindgen_ty_2, value2) - 0usize];
+};
+#[allow(clippy::unnecessary_operation, clippy::identity_op)]
+const _: () = {
+    [
+        "Size of bpf_array__bindgen_ty_1",
+    ][::std::mem::size_of::<bpf_array__bindgen_ty_1>() - 0usize];
+    [
+        "Alignment of bpf_array__bindgen_ty_1",
+    ][::std::mem::align_of::<bpf_array__bindgen_ty_1>() - 1usize];
+};
+impl Default for bpf_array__bindgen_ty_1 {
+    fn default() -> Self {
+        let mut s = ::std::mem::MaybeUninit::<Self>::uninit();
+        unsafe {
+            ::std::ptr::write_bytes(s.as_mut_ptr(), 0, 1);
+            s.assume_init()
+        }
+    }
+}
+#[allow(clippy::unnecessary_operation, clippy::identity_op)]
+const _: () = {
+    ["Size of bpf_array"][::std::mem::size_of::<bpf_array>() - 0usize];
+    ["Alignment of bpf_array"][::std::mem::align_of::<bpf_array>() - 1usize];
+};
+impl Default for bpf_array {
+    fn default() -> Self {
+        let mut s = ::std::mem::MaybeUninit::<Self>::uninit();
+        unsafe {
+            ::std::ptr::write_bytes(s.as_mut_ptr(), 0, 1);
+            s.assume_init()
+        }
+    }
+}

bindgen-tests/tests/headers/struct_with_anon_union_of_array_members.h

diff --git a/bindgen-tests/tests/headers/struct_with_anon_union_of_array_members.h b/bindgen-tests/tests/headers/struct_with_anon_union_of_array_members.h
new file mode 100644
index 0000000..c4d6d8f
--- /dev/null
+++ b/bindgen-tests/tests/headers/struct_with_anon_union_of_array_members.h
@@ -0,0 +1,13 @@
+// bindgen-flags: --default-non-copy-union-style manually_drop
+struct bpf_array {
+  union {
+    struct {
+      struct {} __empty_value1;
+      char value1[];
+    };
+    struct {
+      struct {} __empty_value2;
+      char value2[];
+    };
+  };
+};

bindgen/ir/comp.rs

diff --git a/bindgen/ir/comp.rs b/bindgen/ir/comp.rs
index edf9937..92e69a2 100644
--- a/bindgen/ir/comp.rs
+++ b/bindgen/ir/comp.rs
@@ -1801,7 +1801,9 @@
             return (false, false);
         }
 
-        if layout.is_some_and(|l| l.size == 0) {
+        if layout.is_some_and(|l| l.size == 0) &&
+            union_style != NonCopyUnionStyle::ManuallyDrop
+        {
             return (false, false);
         }