Generic namespaced pointers are dropped

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
62/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
cpp, rust
Domain
compilers, devtools

Research direction

Use the minimal example in src/bindgen_entry.h and run the shown bindgen command with C++ parsing enabled. Compare the namespaced pointer alias output with the unnamespaced case, then trace the handling of generic parameters and namespaced template specializations. Done means f is generated with Quux<baz_Foo> and the behavior has a regression test.

Written by the indexing model from the issue text.

Description

In my test program, generic parameters are dropped, leading to incorrect code.

with src/bindgen_entry.h:

namespace baz {
class Foo;
}

template <typename T>
using Quux=T*;

void f(Quux<baz::Foo> bar);

$ bindgen src/bindgen_entry.h -- -x c++ 
/* automatically generated by rust-bindgen 0.73.2 */

#[repr(C)]
#[derive(Debug)]
pub struct baz_Foo {
    _unused: [u8; 0],
}
pub type Quux<T> = *mut T;
unsafe extern "C" {
    #[link_name = "\u{1}__Z1fPN3baz3FooE"]
    pub fn f(bar: Quux);
}

I would expect to see pub fn f(bar: Quux<baz_Foo>).

If Quux is defined without the pointer, Quux gets ignored entirely (which is likely correct, or at least will compile):

namespace baz {
class Foo;
}

template <typename T>
using Quux=T;

void f(Quux<baz::Foo> bar);

$ bindgen src/bindgen_entry.h -- -x c++ 
/* automatically generated by rust-bindgen 0.73.2 */

#[repr(C)]
#[derive(Debug)]
pub struct baz_Foo {
    _unused: [u8; 0],
}
pub type Quux<T> = T;
unsafe extern "C" {
    #[link_name = "\u{1}__Z1fN3baz3FooE"]
    pub fn f(bar: baz_Foo);
}

Note, though, that if I drop the namespace in the C++ code, it works as expected, even with the pointer:

class Foo;

template <typename T>
using Quux=T*;

void f(Quux<Foo> bar);

$ bindgen src/bindgen_entry.h -- -x c++ 
/* automatically generated by rust-bindgen 0.73.2 */

#[repr(C)]
#[derive(Debug)]
pub struct Foo {
    _unused: [u8; 0],
}
pub type Quux<T> = *mut T;
#[allow(clippy::unnecessary_operation, clippy::identity_op)]
const _: () = {
    ["Size of template specialization: Quux_open0_Foo_close0"]
        [::std::mem::size_of::<Quux<Foo>>() - 8usize];
    ["Align of template specialization: Quux_open0_Foo_close0"]
        [::std::mem::align_of::<Quux<Foo>>() - 8usize];
};
unsafe extern "C" {
    #[link_name = "\u{1}__Z1fP3Foo"]
    pub fn f(bar: Quux<Foo>);
}

None of --enable-cxx-namespaces, --disable-name-namespacing, nor --conservative-inline-namespaces correct this behavior.

I believe this is related to, but not quite the same as, https://github.com/rust-lang/rust-bindgen/issues/3373

Dominant language
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