Inconsistent behaviour of `derive_default` for opaque types

[wmmc88]

There seems to be inconsistent behavior for derive_default based on whether the type is detected to be opaque by bindgen or explicitly marked as opaque by bindgen.

For a C definition of struct Foo; for an opaque struct, bindgen will generate the following with derive_default(true):

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct Foo {
    _unused: [u8; 0],
}

When Foo is also explicitly marked opaque with .opaque_type("Foo"), it generates the following:

#[repr(C)]
#[derive(Debug, Default, Copy, Clone)]
pub struct Foo {
    _unused: [u8; 0],
}

I don't see why explicitly marking the type as opaque should change whether or not Default is derived. I also think that it makes even less sense that Default gets derived for a type that is supposed to be Opaque, since opaque types should never be directly accessed and should be handled only via pointers.

Other Information
rust-bindgen 0.71.1
clang version 19.1.7
Target: x86_64-pc-windows-msvc

Full Code Sample
build.rs:

use std::env;
use std::path::PathBuf;

fn main() {
    tracing_subscriber::fmt::init();

    let builder = bindgen::Builder::default()
        .header_contents(
            "input.h",
            r#"
struct Foo;

struct BAR__{int unused;};
typedef struct BAR__ *BAR;
"#,
        )
        .parse_callbacks(Box::new(bindgen::CargoCallbacks::new()))
        .derive_default(true);

    let out_path = PathBuf::from(env::var("OUT_DIR").unwrap());

    builder
        .clone()
        .generate()
        .expect("Unable to generate bindings")
        .write_to_file((out_path).join("not-explicit-opaque.rs"))
        .expect("Couldn't write bindings!");

    builder
        .opaque_type("Foo")
        .opaque_type("BAR__") // Only BAR__ should be marked as opaque. The convenience type BAR should remain as a pointer to the opaque BAR__. 
        .generate()
        .expect("Unable to generate bindings")
        .write_to_file((out_path).join("explicit-opaque.rs"))
        .expect("Couldn't write bindings!");
}

Output:
not-explicit-opaque.rs:

/* automatically generated by rust-bindgen 0.71.1 */

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct Foo {
    _unused: [u8; 0],
}
#[repr(C)]
#[derive(Debug, Default, Copy, Clone)]
pub struct BAR__ {
    pub unused: ::std::os::raw::c_int,
}
#[allow(clippy::unnecessary_operation, clippy::identity_op)]
const _: () = {
    ["Size of BAR__"][::std::mem::size_of::<BAR__>() - 4usize];
    ["Alignment of BAR__"][::std::mem::align_of::<BAR__>() - 4usize];
    ["Offset of field: BAR__::unused"][::std::mem::offset_of!(BAR__, unused) - 0usize];
};
pub type BAR = *mut BAR__;

explicit-opaque.rs:

/* automatically generated by rust-bindgen 0.71.1 */

#[repr(C)]
#[derive(Debug, Default, Copy, Clone)]
pub struct Foo {
    _unused: [u8; 0],
}
#[repr(C)]
#[repr(align(4))]
#[derive(Debug, Default, Copy, Clone)]
pub struct BAR__ {
    pub _bindgen_opaque_blob: u32,
}
#[allow(clippy::unnecessary_operation, clippy::identity_op)]
const _: () = {
    ["Size of BAR__"][::std::mem::size_of::<BAR__>() - 4usize];
    ["Alignment of BAR__"][::std::mem::align_of::<BAR__>() - 4usize];
};
pub type BAR = *mut BAR__;

Output Diff (left is no explicit opaque. right is explicit opaque):

[jschwe]

Opaque types shouldn't derive any of Copy, Clone or default. See also: #1656.

( I have some patches related to opaque types on my fork of bindgen, but didn't have the time yet to clean the ones related to opaque types up enough to open a PR)