ICE with unsizing an extern type

[DutchGhost]

Code

#![feature(extern_types)]
#![feature(unsized_locals)]

extern { 
    type Device;
}

unsafe fn make_device() -> Box<Device> {
    Box::from_raw(0 as *mut _)
}

fn main() {
    let d: Device = unsafe { *make_device() };
}

Meta

rustc --version --verbose:

rustc 1.50.0-nightly (1c389ffef 2020-11-24)

Error output

thread 'rustc' panicked at 'assertion failed: `(left == right)`
  left: `false`,
 right: `true`', compiler/rustc_codegen_llvm/src/builder.rs:445:9

Backtrace

stack backtrace:
   0: rust_begin_unwind
             at /rustc/1c389ffeff814726dec325f0f2b0c99107df2673/library/std/src/panicking.rs:493:5
   1: core::panicking::panic_fmt
             at /rustc/1c389ffeff814726dec325f0f2b0c99107df2673/library/core/src/panicking.rs:92:14
   2: <rustc_codegen_llvm::builder::Builder as rustc_codegen_ssa::traits::builder::BuilderMethods>::load_operand
   3: rustc_codegen_ssa::mir::operand::<impl rustc_codegen_ssa::mir::FunctionCx<Bx>>::codegen_consume
   4: rustc_codegen_ssa::mir::operand::<impl rustc_codegen_ssa::mir::FunctionCx<Bx>>::codegen_operand
   5: rustc_codegen_ssa::mir::block::<impl rustc_codegen_ssa::mir::FunctionCx<Bx>>::codegen_block
   6: rustc_codegen_ssa::mir::codegen_mir
   7: rustc_codegen_ssa::base::codegen_instance
   8: <rustc_middle::mir::mono::MonoItem as rustc_codegen_ssa::mono_item::MonoItemExt>::define
   9: rustc_codegen_llvm::base::compile_codegen_unit::module_codegen
  10: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  11: rustc_codegen_llvm::base::compile_codegen_unit
  12: rustc_codegen_ssa::base::codegen_crate
  13: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  14: rustc_interface::queries::Queries::ongoing_codegen
  15: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  16: rustc_span::with_source_map
  17: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: internal compiler error: unexpected panic

[SNCPlay42]

It seems clear to me that this shouldn't compile: there's no way to determine, even at run time, how many bytes of storage d needs (short of looking at the external code to see the size of Device, or calling some function in the compiled extern code to ask what the size of Device is, neither of which can be done in general). So we just can't support an unsized local of an extern type.

But this presents a pretty big problem: unsized locals of ?Sized generic parameter types. This program can't be correct (and indeed it ICEs the same way as the original code), but which part of it is in error?

#![feature(extern_types)]
#![feature(unsized_locals)]

extern {
    type Foo;
}

fn mk_foo() -> Box<Foo> {
    unimplemented!()
}

fn unsized_local_generic_parameter<T: ?Sized>(x: Box<T>) {
    // `unsized_locals` allows you to create locals of generic `?Sized`
    // parameter types
    let _x = *x;
}

fn main() {
    // But `extern_types` allows you to create pointers to extern types, and
    // pass them to functions expecting a pointer to a `?Sized` parameter type
    unsized_local_generic_parameter(mk_foo());
}

It seems to me that these features can't coexist as they are currently - extern_types breaks unsized_locals's fundamental assumption that at runtime we can determine the size of any type, even ?Sized ones.

I think this needs discussion from the lang team.

[Enselic]

Triage: Still reproduces in Playground on current nightly, now at compiler/rustc_codegen_llvm/src/builder.rs:491:9.

[matthiaskrgr]

thread 'rustc' panicked at compiler/rustc_codegen_llvm/src/builder.rs:493:17:
unsized locals must not be `extern` types
stack backtrace:
   0:     0x7f5a38b8c7e6 - std::backtrace_rs::backtrace::libunwind::trace::h92488e4bb2264071
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/../../backtrace/src/backtrace/libunwind.rs:104:5
   1:     0x7f5a38b8c7e6 - std::backtrace_rs::backtrace::trace_unsynchronized::h97e02be157039c96
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f5a38b8c7e6 - std::sys_common::backtrace::_print_fmt::h89a2b01cb3c33339
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/sys_common/backtrace.rs:68:5
   3:     0x7f5a38b8c7e6 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::ha79772b33774e7af
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f5a38bdeed0 - core::fmt::rt::Argument::fmt::h6d2228239beb9437
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/core/src/fmt/rt.rs:142:9
   5:     0x7f5a38bdeed0 - core::fmt::write::h9bcb8c2e898ce4b2
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/core/src/fmt/mod.rs:1120:17
   6:     0x7f5a38b801df - std::io::Write::write_fmt::he7ba80727a5f8d75
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/io/mod.rs:1810:15
   7:     0x7f5a38b8c5c4 - std::sys_common::backtrace::_print::h9bd2ce6833902b42
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f5a38b8c5c4 - std::sys_common::backtrace::print::h09be6e715f2e0cc6
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f5a38b8f357 - std::panicking::default_hook::{{closure}}::h81ac1ebc9ea0357f
  10:     0x7f5a38b8f0b9 - std::panicking::default_hook::hfc7d2f2254fbbaf6
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/panicking.rs:292:9
  11:     0x7f5a3b94e03c - std[e13db6861558cd66]::panicking::update_hook::<alloc[74e4b4cff019550b]::boxed::Box<rustc_driver_impl[f3f951fd75793e59]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x7f5a38b8faa6 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h864b8f962a42f35c
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/alloc/src/boxed.rs:2030:9
  13:     0x7f5a38b8faa6 - std::panicking::rust_panic_with_hook::he5915ec9da668475
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/panicking.rs:783:13
  14:     0x7f5a38b8f7b9 - std::panicking::begin_panic_handler::{{closure}}::h8912c4af58ae3d2d
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/panicking.rs:649:13
  15:     0x7f5a38b8cce6 - std::sys_common::backtrace::__rust_end_short_backtrace::hfbc35781d5527627
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/sys_common/backtrace.rs:171:18
  16:     0x7f5a38b8f550 - rust_begin_unwind
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/panicking.rs:645:5
  17:     0x7f5a38bdb5d5 - core::panicking::panic_fmt::hc3a8fce14bfb5d21
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/core/src/panicking.rs:72:14
  18:     0x7f5a3ce77ab6 - <rustc_codegen_ssa[ab39db10c7744d49]::mir::FunctionCx<rustc_codegen_llvm[5233fca0b810d365]::builder::Builder>>::codegen_operand
  19:     0x7f5a3d48fb17 - <rustc_codegen_ssa[ab39db10c7744d49]::mir::FunctionCx<rustc_codegen_llvm[5233fca0b810d365]::builder::Builder>>::codegen_block
  20:     0x7f5a3d789783 - rustc_codegen_ssa[ab39db10c7744d49]::mir::codegen_mir::<rustc_codegen_llvm[5233fca0b810d365]::builder::Builder>
  21:     0x7f5a3d776f52 - rustc_codegen_llvm[5233fca0b810d365]::base::compile_codegen_unit::module_codegen
  22:     0x7f5a3db89cab - <rustc_codegen_llvm[5233fca0b810d365]::LlvmCodegenBackend as rustc_codegen_ssa[ab39db10c7744d49]::traits::backend::ExtraBackendMethods>::compile_codegen_unit
  23:     0x7f5a3dba85f9 - rustc_codegen_ssa[ab39db10c7744d49]::base::codegen_crate::<rustc_codegen_llvm[5233fca0b810d365]::LlvmCodegenBackend>
  24:     0x7f5a3dba7d35 - <rustc_codegen_llvm[5233fca0b810d365]::LlvmCodegenBackend as rustc_codegen_ssa[ab39db10c7744d49]::traits::backend::CodegenBackend>::codegen_crate
  25:     0x7f5a3dba60f0 - rustc_interface[1dfaeeba2be2315d]::passes::start_codegen
  26:     0x7f5a3dba5826 - <rustc_interface[1dfaeeba2be2315d]::queries::Queries>::codegen_and_build_linker
  27:     0x7f5a3d9169de - rustc_interface[1dfaeeba2be2315d]::interface::run_compiler::<core[6f83431ac1278f74]::result::Result<(), rustc_span[b49aba93e0aaec5a]::ErrorGuaranteed>, rustc_driver_impl[f3f951fd75793e59]::run_compiler::{closure#0}>::{closure#0}
  28:     0x7f5a3dbd04c6 - std[e13db6861558cd66]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[1dfaeeba2be2315d]::util::run_in_thread_with_globals<rustc_interface[1dfaeeba2be2315d]::util::run_in_thread_pool_with_globals<rustc_interface[1dfaeeba2be2315d]::interface::run_compiler<core[6f83431ac1278f74]::result::Result<(), rustc_span[b49aba93e0aaec5a]::ErrorGuaranteed>, rustc_driver_impl[f3f951fd75793e59]::run_compiler::{closure#0}>::{closure#0}, core[6f83431ac1278f74]::result::Result<(), rustc_span[b49aba93e0aaec5a]::ErrorGuaranteed>>::{closure#0}, core[6f83431ac1278f74]::result::Result<(), rustc_span[b49aba93e0aaec5a]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[6f83431ac1278f74]::result::Result<(), rustc_span[b49aba93e0aaec5a]::ErrorGuaranteed>>
  29:     0x7f5a3dbd02f3 - <<std[e13db6861558cd66]::thread::Builder>::spawn_unchecked_<rustc_interface[1dfaeeba2be2315d]::util::run_in_thread_with_globals<rustc_interface[1dfaeeba2be2315d]::util::run_in_thread_pool_with_globals<rustc_interface[1dfaeeba2be2315d]::interface::run_compiler<core[6f83431ac1278f74]::result::Result<(), rustc_span[b49aba93e0aaec5a]::ErrorGuaranteed>, rustc_driver_impl[f3f951fd75793e59]::run_compiler::{closure#0}>::{closure#0}, core[6f83431ac1278f74]::result::Result<(), rustc_span[b49aba93e0aaec5a]::ErrorGuaranteed>>::{closure#0}, core[6f83431ac1278f74]::result::Result<(), rustc_span[b49aba93e0aaec5a]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[6f83431ac1278f74]::result::Result<(), rustc_span[b49aba93e0aaec5a]::ErrorGuaranteed>>::{closure#1} as core[6f83431ac1278f74]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  30:     0x7f5a38b999d5 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hb53ed9031b4e247a
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/alloc/src/boxed.rs:2016:9
  31:     0x7f5a38b999d5 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hfb9a49e9b795584d
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/alloc/src/boxed.rs:2016:9
  32:     0x7f5a38b999d5 - std::sys::unix::thread::Thread::new::thread_start::h4db5c6ec45610220
                               at /rustc/ca663b06c5492ac2dde5e53cd11579fa8e4d68bd/library/std/src/sys/unix/thread.rs:108:17
  33:     0x7f5a389859eb - <unknown>
  34:     0x7f5a38a097cc - <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/ices/rustc-ice-2024-01-09T22_37_00-1727225.txt` to your bug report

query stack during panic:
end of query stack
warning: 2 warnings emitted