ice: 'PassMode::Direct' for aggregates only allowed on wasm targets

[matthiaskrgr]

Code

#![feature(unsized_tuple_coercion)]
#![feature(unsized_fn_params)]

fn bad() -> extern "C" fn(([u8],)) {
    todo!()
}

fn main() {
    let f = bad();
    let slice: Box<([u8],)> = Box::new(([1; 8],));
    f(*slice);
}

Meta

rustc --version --verbose:

rustc 1.74.0-nightly (8142a319e 2023-09-13)
binary: rustc
commit-hash: 8142a319ed5c1d1f96e5a1881a6546e463b77c8f
commit-date: 2023-09-13
host: x86_64-unknown-linux-gnu
release: 1.74.0-nightly
LLVM version: 17.0.0

Error output

<output>

Backtrace

warning: `extern` fn uses type `([u8],)`, which is not FFI-safe
 --> b.rs:4:13
  |
4 | fn bad() -> extern "C" fn(([u8],)) {
  |             ^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
  |
  = help: consider using a struct instead
  = note: tuples have unspecified layout
  = note: `#[warn(improper_ctypes_definitions)]` on by default

thread 'rustc' panicked at compiler/rustc_codegen_llvm/src/abi.rs:360:25:
`PassMode::Direct` for aggregates only allowed on wasm targets
Problematic type: TyAndLayout {
    ty: ([u8],),
    layout: Layout {
        size: Size(0 bytes),
        align: AbiAndPrefAlign {
            abi: Align(1 bytes),
            pref: Align(8 bytes),
        },
        abi: Aggregate {
            sized: false,
        },
        fields: Arbitrary {
            offsets: [
                Size(0 bytes),
            ],
            memory_index: [
                0,
            ],
        },
        largest_niche: None,
        variants: Single {
            index: 0,
        },
        max_repr_align: None,
        unadjusted_abi_align: Align(1 bytes),
    },
}
stack backtrace:
   0:     0x7fbb7af62efc - std::backtrace_rs::backtrace::libunwind::trace::h7d696a3fe83931ea
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7fbb7af62efc - std::backtrace_rs::backtrace::trace_unsynchronized::h32ed3b706756d880
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7fbb7af62efc - std::sys_common::backtrace::_print_fmt::h234b1d6e2050b1c7
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x7fbb7af62efc - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hdba83e95158d24b7
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7fbb7afc915c - core::fmt::rt::Argument::fmt::h56eddd6c4c48048d
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/core/src/fmt/rt.rs:138:9
   5:     0x7fbb7afc915c - core::fmt::write::heb62e4111c27e516
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/core/src/fmt/mod.rs:1094:21
   6:     0x7fbb7af5593e - std::io::Write::write_fmt::hbe56a2148663bb0e
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/io/mod.rs:1714:15
   7:     0x7fbb7af62ce4 - std::sys_common::backtrace::_print::h17c88990d1e7ffc8
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7fbb7af62ce4 - std::sys_common::backtrace::print::h18bfa712e4237a54
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7fbb7af65dda - std::panicking::panic_hook_with_disk_dump::{{closure}}::h29435e023d2fef53
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/panicking.rs:280:22
  10:     0x7fbb7af65ad5 - std::panicking::panic_hook_with_disk_dump::h747314e0eb3083d0
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/panicking.rs:314:9
  11:     0x7fbb7e144229 - <rustc_driver_impl[6872457824a876c8]::install_ice_hook::{closure#0} as core[468698755226bf15]::ops::function::FnOnce<(&core[468698755226bf15]::panic::panic_info::PanicInfo,)>>::call_once::{shim:vtable#0}
  12:     0x7fbb7af66693 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h28a1f6f4d204892e
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/alloc/src/boxed.rs:2021:9
  13:     0x7fbb7af66693 - std::panicking::rust_panic_with_hook::hcbc867ec0eddd0c3
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/panicking.rs:757:13
  14:     0x7fbb7af66411 - std::panicking::begin_panic_handler::{{closure}}::h3c9eb8c1fbf3ea52
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/panicking.rs:631:13
  15:     0x7fbb7af63426 - std::sys_common::backtrace::__rust_end_short_backtrace::hd2324a4c2985e4f3
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/sys_common/backtrace.rs:170:18
  16:     0x7fbb7af66152 - rust_begin_unwind
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/panicking.rs:619:5
  17:     0x7fbb7afc5505 - core::panicking::panic_fmt::h583d94a3776bd1d2
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/core/src/panicking.rs:72:14
  18:     0x7fbb7cd895bd - <rustc_codegen_ssa[33c9f48d12f8e4aa]::mir::block::TerminatorCodegenHelper>::do_call::<rustc_codegen_llvm[863ddb282f34b5ae]::builder::Builder>
  19:     0x7fbb7cd2d4a4 - rustc_codegen_ssa[33c9f48d12f8e4aa]::mir::codegen_mir::<rustc_codegen_llvm[863ddb282f34b5ae]::builder::Builder>
  20:     0x7fbb7d7985a1 - rustc_codegen_llvm[863ddb282f34b5ae]::base::compile_codegen_unit::module_codegen
  21:     0x7fbb7d7963f5 - rustc_codegen_llvm[863ddb282f34b5ae]::base::compile_codegen_unit
  22:     0x7fbb7d831567 - rustc_codegen_ssa[33c9f48d12f8e4aa]::base::codegen_crate::<rustc_codegen_llvm[863ddb282f34b5ae]::LlvmCodegenBackend>
  23:     0x7fbb7d830f9f - <rustc_codegen_llvm[863ddb282f34b5ae]::LlvmCodegenBackend as rustc_codegen_ssa[33c9f48d12f8e4aa]::traits::backend::CodegenBackend>::codegen_crate
  24:     0x7fbb7d67f702 - <rustc_session[bf85c9c3ca58fcd0]::session::Session>::time::<alloc[5760c3642cffe391]::boxed::Box<dyn core[468698755226bf15]::any::Any>, rustc_interface[89ffa3a3953ed9b2]::passes::start_codegen::{closure#0}>
  25:     0x7fbb7d67f25b - rustc_interface[89ffa3a3953ed9b2]::passes::start_codegen
  26:     0x7fbb7d679baa - <rustc_middle[a5aa27c2964d564a]::ty::context::GlobalCtxt>::enter::<<rustc_interface[89ffa3a3953ed9b2]::queries::Queries>::ongoing_codegen::{closure#0}, core[468698755226bf15]::result::Result<alloc[5760c3642cffe391]::boxed::Box<dyn core[468698755226bf15]::any::Any>, rustc_span[3e6de59a27d5529]::ErrorGuaranteed>>
  27:     0x7fbb7d678fb2 - <rustc_interface[89ffa3a3953ed9b2]::interface::Compiler>::enter::<rustc_driver_impl[6872457824a876c8]::run_compiler::{closure#1}::{closure#2}, core[468698755226bf15]::result::Result<core[468698755226bf15]::option::Option<rustc_interface[89ffa3a3953ed9b2]::queries::Linker>, rustc_span[3e6de59a27d5529]::ErrorGuaranteed>>
  28:     0x7fbb7d6721a4 - std[1d99e526d3fccd95]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[89ffa3a3953ed9b2]::util::run_in_thread_with_globals<rustc_interface[89ffa3a3953ed9b2]::interface::run_compiler<core[468698755226bf15]::result::Result<(), rustc_span[3e6de59a27d5529]::ErrorGuaranteed>, rustc_driver_impl[6872457824a876c8]::run_compiler::{closure#1}>::{closure#0}, core[468698755226bf15]::result::Result<(), rustc_span[3e6de59a27d5529]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[468698755226bf15]::result::Result<(), rustc_span[3e6de59a27d5529]::ErrorGuaranteed>>
  29:     0x7fbb7d6718fe - <<std[1d99e526d3fccd95]::thread::Builder>::spawn_unchecked_<rustc_interface[89ffa3a3953ed9b2]::util::run_in_thread_with_globals<rustc_interface[89ffa3a3953ed9b2]::interface::run_compiler<core[468698755226bf15]::result::Result<(), rustc_span[3e6de59a27d5529]::ErrorGuaranteed>, rustc_driver_impl[6872457824a876c8]::run_compiler::{closure#1}>::{closure#0}, core[468698755226bf15]::result::Result<(), rustc_span[3e6de59a27d5529]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[468698755226bf15]::result::Result<(), rustc_span[3e6de59a27d5529]::ErrorGuaranteed>>::{closure#1} as core[468698755226bf15]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  30:     0x7fbb7af71075 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::ha4104662ef3c5bbd
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/alloc/src/boxed.rs:2007:9
  31:     0x7fbb7af71075 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hb8d3ac4a3d377b23
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/alloc/src/boxed.rs:2007:9
  32:     0x7fbb7af71075 - std::sys::unix::thread::Thread::new::thread_start::h1cee3a3217d3e553
                               at /rustc/8142a319ed5c1d1f96e5a1881a6546e463b77c8f/library/std/src/sys/unix/thread.rs:108:17
  33:     0x7fbb7aa8c9eb - <unknown>
  34:     0x7fbb7ab10dfc - <unknown>
  35:                0x0 - <unknown>

error: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please attach the file at `/tmp/lol/icemaker_reduced/rustc-ice-2023-09-14T15:15:58.855984188Z-3701302.txt` to your bug report

query stack during panic:
end of query stack
warning: 1 warning emitted

[bjorn3]

cc @RalfJung

[RalfJung]

And that's on x86-64? Fun fun fun... (My PR triggered the ICE but a Direct ABI for the unsized slice makes no sense at all so the bug was probably pre-existing.) How did you get that testcase?^^

[matthiaskrgr]

fuzzing :D https://github.com/matthiaskrgr/icemaker/

[RalfJung]

For the Rust ABI we have logic that makes all unsized arguments indirect:

rust/compiler/rustc_ty_utils/src/abi.rs

Lines 527 to 528 in ae9465f

	if arg.layout.is_unsized() || size > Pointer(AddressSpace::DATA).size(cx) {
	arg.make_indirect();

For other ABIs however, currently every single target ABI adjustment has to re-implement that.

Honestly I think we should just forbid unsized argument on non-Rust ABIs.

[RalfJung]

Honestly I think we should just forbid unsized argument on non-Rust ABIs.

Oh unfortunately this will be more annoying than I thought. I thought without the feature gate we would reject fn types with unsized arguments, but we don't, this builds on stable:

type T = extern "C" fn(str);
fn foo(_x: T) {}

So we can't just adjust fn WF-checking to reject unsized arguments on non-Rust ABIs, that would be a breaking change. Instead we'll need two checks, one for the caller side (that fixes the reproducer above) and one for the callee side that is required to fix this one

#![feature(unsized_tuple_coercion)]
#![feature(unsized_fn_params)]

pub extern "C" fn foo(_x: str) {}

[matthiaskrgr]

another one: (already fixed with the pr above)

#![crate_type = "lib"]
#![feature(c_unwind, unsized_fn_params)]

#[no_mangle]
pub extern "C" fn rust_item_that_cannot_unwind() {
}

#[no_mangle]
pub extern "system" fn rust_item_that_can_unwind(f: dyn FnOnce()) {
}