Non-type template parameters and failing layout tests

[fitzgen]

Input C/C++ Header

template <int, typename> struct a {};
enum { b };
namespace JS {
template <typename c> using d = a<b, c>;
template <typename c> class e { d<c> f; };
class AutoIdVector : e<int> {};
}

Bindgen Invokation

$ bindgen input.hpp -- -std=c++14

Actual Results

/* automatically generated by rust-bindgen */

pub const b: _bindgen_ty_1 = _bindgen_ty_1::b;
#[repr(u32)]
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum _bindgen_ty_1 { b = 0, }
pub type JS_d = ();
#[repr(C)]
pub struct JS_e {
    pub f: JS_d,
}
#[repr(C)]
pub struct JS_AutoIdVector {
    pub _base: JS_e,
}
#[test]
fn bindgen_test_layout_JS_AutoIdVector() {
    assert_eq!(::std::mem::size_of::<JS_AutoIdVector>() , 1usize , concat ! (
               "Size of: " , stringify ! ( JS_AutoIdVector ) ));
    assert_eq! (::std::mem::align_of::<JS_AutoIdVector>() , 1usize , concat !
                ( "Alignment of " , stringify ! ( JS_AutoIdVector ) ));
}
#[test]
fn __bindgen_test_layout_JS_e_instantiation_16() {
    assert_eq!(::std::mem::size_of::<JS_e>() , 1usize , concat ! (
               "Size of template specialization: " , stringify ! ( JS_e ) ));
    assert_eq!(::std::mem::align_of::<JS_e>() , 1usize , concat ! (
               "Alignment of template specialization: " , stringify ! ( JS_e )
               ));
}

Compiling with --test and running the generated layout tests results in these failures:

running 2 tests
test root::JS::bindgen_test_layout_AutoIdVector ... FAILED
test root::__bindgen_test_layout_e_instantiation_16 ... FAILED

failures:

---- root::JS::bindgen_test_layout_AutoIdVector stdout ----
	thread 'root::JS::bindgen_test_layout_AutoIdVector' panicked at 'assertion failed: `(left == right)` (left: `0`, right: `1`): Size of: AutoIdVector', ./js.rs:22
note: Run with `RUST_BACKTRACE=1` for a backtrace.

---- root::__bindgen_test_layout_e_instantiation_16 stdout ----
	thread 'root::__bindgen_test_layout_e_instantiation_16' panicked at 'assertion failed: `(left == right)` (left: `0`, right: `1`): Size of template specialization: root :: JS :: e', ./js.rs:31


failures:
    root::JS::bindgen_test_layout_AutoIdVector
    root::__bindgen_test_layout_e_instantiation_16

test result: FAILED. 0 passed; 2 failed; 0 ignored; 0 measured

Expected Results

Although we generally can't support non-type template parameters and instantiations of templates with such parameters, we shouldn't be generating code + layout tests that fail. We need to do a better job of detecting this and emitting an opaque blob instead, or even completely skipping this stuff in codegen (which we do some of the time but not always it seems).

[fitzgen]

Ok so this issue seems to involve our choice of () as the default representation for when a template definition has non-type parameters.