Struct containing a pointer to multi-dimensional array that has one zero sized dimension.

Hello,
I'm working on generating some valid C headers as property tests for bindgen, issue #970.
While filling in the generation logic I'm starting to come across code that makes
the csmith-fuzzing/predicate.py script cough. I don't know how many of these there
might be (or how many might be false alarms like #1151) so I'm open to just keeping a
public running list somewhere if raising issues as I encounter them isn't the preference.
I'm also open to sharing some of the generated
bonkers preliminary code
that bindgen is handling happily.

This also seems related to issue #684 which the property tests have run into as well.
This one only fails the csmith-fuzzing/predicate.py script though,
rustc seems to compile without errors so I thought I'd bring it up.

Sorry if the code looks insane, it's generated! 😰

Here's the result of rustup show:

Default host: x86_64-unknown-linux-gnu

installed toolchains
--------------------

stable-x86_64-unknown-linux-gnu
nightly-x86_64-unknown-linux-gnu (default)

active toolchain
----------------

nightly-x86_64-unknown-linux-gnu (default)
rustc 1.23.0-nightly (ff0f5de3b 2017-11-14)

Input C/C++ Header

struct {
    char *decl_4_0[0][4]; // fails
} struct_2_0;

// struct {
//     char decl_4_0[0][4]; // succeeds
// } struct_2_0;

// struct {
//     char *decl_4_0[1][4]; // succeeds
// } struct_2_0;

Bindgen Invocation

I've been using the csmith-fuzzing/predicate.py script to test so the failing invocation would look like this assuming that definition is in a header named prop_test.h

$ ./csmith-fuzzing/predicate.py prop_test.h

Actual Results

Script output:

Running: ['cargo', 'run', '--manifest-path', '/home/username/src/rust-bindgen/Cargo.toml', '--', 'prop_test.h', '-o', '/tmp/bindings-7_gwph30.rs', '--with-derive-partialeq', '--with-derive-eq', '--with-derive-partialord', '--with-derive-ord', '--with-derive-hash', '--with-derive-default']
Running: ['rustc', '--crate-type', 'lib', '--test', '-o', '/tmp/layout-tests-br4ipbog', '/tmp/bindings-7_gwph30.rs']
Running: ['/tmp/layout-tests-br4ipbog']
Error: running the compiled bindings' layout tests failed
+
+running 1 test
+test bindgen_test_layout__bindgen_ty_1 ... FAILED
+
+failures:
+
+---- bindgen_test_layout__bindgen_ty_1 stdout ----
+	thread 'bindgen_test_layout__bindgen_ty_1' panicked at 'assertion failed: `(left == right)`
+  left: `1`,
+ right: `8`: Alignment of _bindgen_ty_1', /tmp/bindings-7_gwph30.rs:52:4
+stack backtrace:
+   0:     0x55f6f4d49f63 - std::sys::imp::backtrace::tracing::imp::unwind_backtrace::h58b4c7cfb400b18c
+                               at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
+   1:     0x55f6f4d463ec - std::sys_common::backtrace::_print::h091e19687805d26e
+                               at /checkout/src/libstd/sys_common/backtrace.rs:68
+   2:     0x55f6f4d4c3b1 - std::panicking::default_hook::{{closure}}::h839e2a2e96c19d98
+                               at /checkout/src/libstd/sys_common/backtrace.rs:57
+                               at /checkout/src/libstd/panicking.rs:381
+   3:     0x55f6f4d4c0ac - std::panicking::default_hook::h892c050099854db5
+                               at /checkout/src/libstd/panicking.rs:391
+   4:     0x55f6f4d4c884 - std::panicking::rust_panic_with_hook::h09c5f7a0435ac463
+                               at /checkout/src/libstd/panicking.rs:577
+   5:     0x55f6f4d4c6bc - std::panicking::begin_panic::h321753ef82401d62
+                               at /checkout/src/libstd/panicking.rs:538
+   6:     0x55f6f4d4c639 - std::panicking::begin_panic_fmt::h24fe390b9054223d
+                               at /checkout/src/libstd/panicking.rs:522
+   7:     0x55f6f4d0839c - bindings_7_gwph30::bindgen_test_layout__bindgen_ty_1::hbdde4efabb33c38f
+   8:     0x55f6f4d17f61 - <F as test::FnBox<T>>::call_box::hc30672f430bdb600
+                               at /checkout/src/libtest/lib.rs:1491
+                               at /checkout/src/libcore/ops/function.rs:223
+                               at /checkout/src/libtest/lib.rs:142
+   9:     0x55f6f4d54ebe - __rust_maybe_catch_panic
+                               at /checkout/src/libpanic_unwind/lib.rs:99
+  10:     0x55f6f4d093b0 - std::sys_common::backtrace::__rust_begin_short_backtrace::h904f706bf2295342
+                               at /checkout/src/libstd/panicking.rs:459
+                               at /checkout/src/libstd/panic.rs:365
+                               at /checkout/src/libtest/lib.rs:1430
+                               at /checkout/src/libstd/sys_common/backtrace.rs:133
+  11:     0x55f6f4d0a052 - std::panicking::try::do_call::hba965e573c3ab44f
+                               at /checkout/src/libstd/thread/mod.rs:406
+                               at /checkout/src/libstd/panic.rs:300
+                               at /checkout/src/libstd/panicking.rs:480
+  12:     0x55f6f4d54ebe - __rust_maybe_catch_panic
+                               at /checkout/src/libpanic_unwind/lib.rs:99
+  13:     0x55f6f4d11942 - <F as alloc::boxed::FnBox<A>>::call_box::h3f8cad64dddfc3c1
+                               at /checkout/src/libstd/panicking.rs:459
+                               at /checkout/src/libstd/panic.rs:365
+                               at /checkout/src/libstd/thread/mod.rs:405
+                               at /checkout/src/liballoc/boxed.rs:762
+  14:     0x55f6f4d4b78b - std::sys::imp::thread::Thread::new::thread_start::he0774ede68aa21ab
+                               at /checkout/src/liballoc/boxed.rs:772
+                               at /checkout/src/libstd/sys_common/thread.rs:23
+                               at /checkout/src/libstd/sys/unix/thread.rs:90
+  15:     0x7f5de07de6b9 - start_thread
+  16:     0x7f5de02fe3dc - clone
+  17:                0x0 - <unknown>
+
+
+failures:
+    bindgen_test_layout__bindgen_ty_1
+
+test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
+

rustc output when given the generated code:

warning: found zero-sized type composed only of phantom-data in a foreign-function.
  --> err.rs:60:32
   |
60 |     pub static mut struct_2_0: _bindgen_ty_1;
   |                                ^^^^^^^^^^^^^
   |
   = note: #[warn(improper_ctypes)] on by default

Generated code:

/* automatically generated by rust-bindgen */

#[repr(C)]
#[derive(Default)]
pub struct __IncompleteArrayField<T>(::std::marker::PhantomData<T>);
impl<T> __IncompleteArrayField<T> {
    #[inline]
    pub fn new() -> Self {
        __IncompleteArrayField(::std::marker::PhantomData)
    }
    #[inline]
    pub unsafe fn as_ptr(&self) -> *const T {
        ::std::mem::transmute(self)
    }
    #[inline]
    pub unsafe fn as_mut_ptr(&mut self) -> *mut T {
        ::std::mem::transmute(self)
    }
    #[inline]
    pub unsafe fn as_slice(&self, len: usize) -> &[T] {
        ::std::slice::from_raw_parts(self.as_ptr(), len)
    }
    #[inline]
    pub unsafe fn as_mut_slice(&mut self, len: usize) -> &mut [T] {
        ::std::slice::from_raw_parts_mut(self.as_mut_ptr(), len)
    }
}
impl<T> ::std::fmt::Debug for __IncompleteArrayField<T> {
    fn fmt(&self, fmt: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
        fmt.write_str("__IncompleteArrayField")
    }
}
impl<T> ::std::clone::Clone for __IncompleteArrayField<T> {
    #[inline]
    fn clone(&self) -> Self {
        Self::new()
    }
}
impl<T> ::std::marker::Copy for __IncompleteArrayField<T> {}
#[repr(C)]
#[derive(Debug)]
pub struct _bindgen_ty_1 {
    pub decl_4_0: __IncompleteArrayField<[*mut ::std::os::raw::c_char; 4usize]>,
}
#[test]
fn bindgen_test_layout__bindgen_ty_1() {
    assert_eq!(
        ::std::mem::size_of::<_bindgen_ty_1>(),
        0usize,
        concat!("Size of: ", stringify!(_bindgen_ty_1))
    );
    assert_eq!(
        ::std::mem::align_of::<_bindgen_ty_1>(),
        8usize,
        concat!("Alignment of ", stringify!(_bindgen_ty_1))
    );
}
extern "C" {
    #[link_name = "\u{1}_Z10struct_2_0"]
    pub static mut struct_2_0: _bindgen_ty_1;
}

Expected Results

🙃 I'm not quite fluent enough in bindgen output to answer this one.

Start with the reduced C++ header and run csmith-fuzzing/predicate.py prop_test.h to reproduce the generated layout-test failure. Inspect the generated Rust binding and the alignment assertion, then establish the expected handling for a pointer array with a zero-sized dimension; done means the predicate test reflects correct layout behavior without this failure.