don't emit #[repr(align(0))], and reduced test case

src/ir/comp.rs

@@ -1072,6 +1072,12 @@ impl CompInfo {
             return None;
         }
 
+        // By definition, we don't have the right layout information here if
+        // we're a forward declaration.
+        if self.is_forward_declaration() {
+            return None;
+        }
+
         let mut max_size = 0;
         let mut max_align = 0;
         for field in self.fields() {

tests/expectations/tests/forward_declared_complex_types_1_0.rs

@@ -1,6 +1,11 @@
 /* automatically generated by rust-bindgen */
 
-#![allow(dead_code, non_snake_case, non_camel_case_types, non_upper_case_globals)]
+#![allow(
+    dead_code,
+    non_snake_case,
+    non_camel_case_types,
+    non_upper_case_globals
+)]
 
 #[repr(C)]
 #[derive(Debug, Default, Copy)]
@@ -73,7 +78,7 @@ extern "C" {
     pub fn baz_struct(f: *mut Foo);
 }
 #[repr(C)]
-#[derive(Copy)]
+#[derive(Debug, Copy)]
 pub struct Union {
     _unused: [u8; 0],
 }

tests/expectations/tests/forward_declared_opaque.rs

@@ -0,0 +1,19 @@
+/* automatically generated by rust-bindgen */
+
+#![allow(
+    dead_code,
+    non_snake_case,
+    non_camel_case_types,
+    non_upper_case_globals
+)]
+
+#[repr(C)]
+#[derive(Copy, Clone)]
+pub struct a {
+    _unused: [u8; 0],
+}
+#[repr(C)]
+#[derive(Debug, Default, Copy, Clone)]
+pub struct b {
+    _unused: [u8; 0],
+}

tests/headers/forward_declared_opaque.h

@@ -0,0 +1,4 @@
+// bindgen-flags: --opaque-type "*"
+
+union a;
+struct b;