add command-line option for disabling untagged unions

One more thing that we can configure from the command line.

3 files changed, 97 insertions, 0 deletions

diff --git a/src/options.rs b/src/options.rs
index b630bb4b..c2de1e5a 100644
--- a/src/options.rs
+++ b/src/options.rs
@@ -267,6 +267,11 @@ where
                      names like \"bar\" instead of \"foo_bar\" for a nested \
                      definition \"struct foo { struct bar { } b; };\"."
                 ),
+            Arg::with_name("disable-untagged-union")
+                .long("disable-untagged-union")
+                .help(
+                    "Disable support for native Rust unions.",
+                ),
             Arg::with_name("ignore-functions")
                 .long("ignore-functions")
                 .help(
@@ -667,6 +672,10 @@ where
         builder = builder.disable_nested_struct_naming();
     }
 
+    if matches.is_present("disable-untagged-union") {
+        builder = builder.disable_untagged_union();
+    }
+
     if matches.is_present("ignore-functions") {
         builder = builder.ignore_functions();
     }
diff --git a/tests/expectations/tests/disable-untagged-union.rs b/tests/expectations/tests/disable-untagged-union.rs
new file mode 100644
index 00000000..9661ef79
--- /dev/null
+++ b/tests/expectations/tests/disable-untagged-union.rs
@@ -0,0 +1,82 @@
+/* automatically generated by rust-bindgen */
+
+#![allow(
+    dead_code,
+    non_snake_case,
+    non_camel_case_types,
+    non_upper_case_globals
+)]
+
+#[repr(C)]
+pub struct __BindgenUnionField<T>(::std::marker::PhantomData<T>);
+impl<T> __BindgenUnionField<T> {
+    #[inline]
+    pub const fn new() -> Self {
+        __BindgenUnionField(::std::marker::PhantomData)
+    }
+    #[inline]
+    pub unsafe fn as_ref(&self) -> &T {
+        ::std::mem::transmute(self)
+    }
+    #[inline]
+    pub unsafe fn as_mut(&mut self) -> &mut T {
+        ::std::mem::transmute(self)
+    }
+}
+impl<T> ::std::default::Default for __BindgenUnionField<T> {
+    #[inline]
+    fn default() -> Self {
+        Self::new()
+    }
+}
+impl<T> ::std::clone::Clone for __BindgenUnionField<T> {
+    #[inline]
+    fn clone(&self) -> Self {
+        Self::new()
+    }
+}
+impl<T> ::std::marker::Copy for __BindgenUnionField<T> {}
+impl<T> ::std::fmt::Debug for __BindgenUnionField<T> {
+    fn fmt(&self, fmt: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+        fmt.write_str("__BindgenUnionField")
+    }
+}
+impl<T> ::std::hash::Hash for __BindgenUnionField<T> {
+    fn hash<H: ::std::hash::Hasher>(&self, _state: &mut H) {}
+}
+impl<T> ::std::cmp::PartialEq for __BindgenUnionField<T> {
+    fn eq(&self, _other: &__BindgenUnionField<T>) -> bool {
+        true
+    }
+}
+impl<T> ::std::cmp::Eq for __BindgenUnionField<T> {}
+#[repr(C)]
+#[derive(Debug, Default, Copy, Clone)]
+pub struct Foo {
+    pub bar: __BindgenUnionField<::std::os::raw::c_int>,
+    pub baz: __BindgenUnionField<::std::os::raw::c_uint>,
+    pub bindgen_union_field: u32,
+}
+#[test]
+fn bindgen_test_layout_Foo() {
+    assert_eq!(
+        ::std::mem::size_of::<Foo>(),
+        4usize,
+        concat!("Size of: ", stringify!(Foo))
+    );
+    assert_eq!(
+        ::std::mem::align_of::<Foo>(),
+        4usize,
+        concat!("Alignment of ", stringify!(Foo))
+    );
+    assert_eq!(
+        unsafe { &(*(::std::ptr::null::<Foo>())).bar as *const _ as usize },
+        0usize,
+        concat!("Offset of field: ", stringify!(Foo), "::", stringify!(bar))
+    );
+    assert_eq!(
+        unsafe { &(*(::std::ptr::null::<Foo>())).baz as *const _ as usize },
+        0usize,
+        concat!("Offset of field: ", stringify!(Foo), "::", stringify!(baz))
+    );
+}
diff --git a/tests/headers/disable-untagged-union.hpp b/tests/headers/disable-untagged-union.hpp
new file mode 100644
index 00000000..44623181
--- /dev/null
+++ b/tests/headers/disable-untagged-union.hpp
@@ -0,0 +1,6 @@
+// bindgen-flags: --disable-untagged-union
+
+union Foo {
+  int bar;
+  unsigned int baz;
+};