rustc should not suggest `#![feature(core_intrinsics)]`

[kaya3]

Rust beginner here. I was trying something out on the Playground, turns out I didn't understand how to use wrapping_add correctly; but following a sequence of suggested fixes from the compiler I arrived at the below, which gives two internal compiler errors. I asked on Discord and was told it is worth reporting it here, even though this code is incorrect and uses an unstable library feature.

Code

#![feature(core_intrinsics)]
use std::intrinsics::wrapping_add;

#[derive(Clone, Copy)]
struct WrapInt8 {
    value: u8
}

impl std::ops::Add for WrapInt8 {
    type Output = WrapInt8;
    fn add(self, other: WrapInt8) -> WrapInt8 {
        wrapping_add(self, other)
    }
}

fn main() {
    let p = WrapInt8 { value: 123 };
    let q = WrapInt8 { value: 234 };
    println!("{}", (p + q).value);
}

Meta

This occurred on the Playground, using the current nightly build (1.63.0-nightly, 2022-05-27 ebbcbfc).

Error output

   Compiling playground v0.0.1 (/playground)
error: internal compiler error: no errors encountered even though `delay_span_bug` issued

error: internal compiler error: broken MIR in Item(WithOptConstParam { did: DefId(0:8 ~ playground[3faf]::{impl#0}::add), const_param_did: None }) (end of phase transition to Optimized) at bb0[2]:
                                Cannot perform arithmetic on type WrapInt8
  --> src/main.rs:12:9
   |
12 |         wrapping_add(self, other)
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: delayed at compiler/rustc_const_eval/src/transform/validate.rs:127:36

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1368:13
stack backtrace:
   0:     0x7f216626ffcd - std::backtrace_rs::backtrace::libunwind::trace::h9dd319c001cd5d5b
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7f216626ffcd - std::backtrace_rs::backtrace::trace_unsynchronized::h5cdb9413f7b7f703
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f216626ffcd - std::sys_common::backtrace::_print_fmt::h62e01fe71d2f0609
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/std/src/sys_common/backtrace.rs:66:5
   3:     0x7f216626ffcd - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hb568664ce7d28696
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/std/src/sys_common/backtrace.rs:45:22
   4:     0x7f21662cbe5c - core::fmt::write::hff09181fc8f150c2
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/core/src/fmt/mod.rs:1196:17
   5:     0x7f21662616c1 - std::io::Write::write_fmt::h77801493f17dbb5a
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/std/src/io/mod.rs:1654:15
   6:     0x7f2166272cb5 - std::sys_common::backtrace::_print::hace3cc1f256ece45
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/std/src/sys_common/backtrace.rs:48:5
   7:     0x7f2166272cb5 - std::sys_common::backtrace::print::hc1b81d3f2867b83b
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/std/src/sys_common/backtrace.rs:35:9
   8:     0x7f2166272cb5 - std::panicking::default_hook::{{closure}}::he802b1e81f85775c
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/std/src/panicking.rs:295:22
   9:     0x7f21662729d6 - std::panicking::default_hook::h57b0d510bf2c9fe7
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/std/src/panicking.rs:314:9
  10:     0x7f2166a3c211 - rustc_driver[8a2d58f6f4bf431a]::DEFAULT_HOOK::{closure#0}::{closure#0}
  11:     0x7f216627338a - std::panicking::rust_panic_with_hook::hdd373bc6c22fa56f
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/std/src/panicking.rs:702:17
  12:     0x7f2167bdb181 - std[aa967257b9f8f73b]::panicking::begin_panic::<rustc_errors[1ed68885dba1d6d1]::ExplicitBug>::{closure#0}
  13:     0x7f2167bd9e06 - std[aa967257b9f8f73b]::sys_common::backtrace::__rust_end_short_backtrace::<std[aa967257b9f8f73b]::panicking::begin_panic<rustc_errors[1ed68885dba1d6d1]::ExplicitBug>::{closure#0}, !>
  14:     0x7f2167bedd56 - std[aa967257b9f8f73b]::panicking::begin_panic::<rustc_errors[1ed68885dba1d6d1]::ExplicitBug>
  15:     0x7f2167bf1ec6 - std[aa967257b9f8f73b]::panic::panic_any::<rustc_errors[1ed68885dba1d6d1]::ExplicitBug>
  16:     0x7f21694638e1 - <rustc_errors[1ed68885dba1d6d1]::HandlerInner as core[8467f2214112d7a4]::ops::drop::Drop>::drop
  17:     0x7f2168bcf7a8 - core[8467f2214112d7a4]::ptr::drop_in_place::<rustc_session[f5a33ef05da8b8fa]::parse::ParseSess>
  18:     0x7f2168bd1ac3 - <alloc[e5cd0633e65a6c1f]::rc::Rc<rustc_session[f5a33ef05da8b8fa]::session::Session> as core[8467f2214112d7a4]::ops::drop::Drop>::drop
  19:     0x7f2168bce36d - core[8467f2214112d7a4]::ptr::drop_in_place::<rustc_interface[b93c8180971df832]::interface::Compiler>
  20:     0x7f2168bcdd94 - rustc_span[4bbade9624b8160a]::with_source_map::<core[8467f2214112d7a4]::result::Result<(), rustc_errors[1ed68885dba1d6d1]::ErrorGuaranteed>, rustc_interface[b93c8180971df832]::interface::create_compiler_and_run<core[8467f2214112d7a4]::result::Result<(), rustc_errors[1ed68885dba1d6d1]::ErrorGuaranteed>, rustc_driver[8a2d58f6f4bf431a]::run_compiler::{closure#1}>::{closure#1}>
  21:     0x7f2168bba454 - rustc_interface[b93c8180971df832]::interface::create_compiler_and_run::<core[8467f2214112d7a4]::result::Result<(), rustc_errors[1ed68885dba1d6d1]::ErrorGuaranteed>, rustc_driver[8a2d58f6f4bf431a]::run_compiler::{closure#1}>
  22:     0x7f2168ba4792 - <scoped_tls[6dc72d687799c0be]::ScopedKey<rustc_span[4bbade9624b8160a]::SessionGlobals>>::set::<rustc_interface[b93c8180971df832]::interface::run_compiler<core[8467f2214112d7a4]::result::Result<(), rustc_errors[1ed68885dba1d6d1]::ErrorGuaranteed>, rustc_driver[8a2d58f6f4bf431a]::run_compiler::{closure#1}>::{closure#0}, core[8467f2214112d7a4]::result::Result<(), rustc_errors[1ed68885dba1d6d1]::ErrorGuaranteed>>
  23:     0x7f2168bbbb3f - std[aa967257b9f8f73b]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[b93c8180971df832]::util::run_in_thread_pool_with_globals<rustc_interface[b93c8180971df832]::interface::run_compiler<core[8467f2214112d7a4]::result::Result<(), rustc_errors[1ed68885dba1d6d1]::ErrorGuaranteed>, rustc_driver[8a2d58f6f4bf431a]::run_compiler::{closure#1}>::{closure#0}, core[8467f2214112d7a4]::result::Result<(), rustc_errors[1ed68885dba1d6d1]::ErrorGuaranteed>>::{closure#0}, core[8467f2214112d7a4]::result::Result<(), rustc_errors[1ed68885dba1d6d1]::ErrorGuaranteed>>
  24:     0x7f2168bbbc79 - <<std[aa967257b9f8f73b]::thread::Builder>::spawn_unchecked_<rustc_interface[b93c8180971df832]::util::run_in_thread_pool_with_globals<rustc_interface[b93c8180971df832]::interface::run_compiler<core[8467f2214112d7a4]::result::Result<(), rustc_errors[1ed68885dba1d6d1]::ErrorGuaranteed>, rustc_driver[8a2d58f6f4bf431a]::run_compiler::{closure#1}>::{closure#0}, core[8467f2214112d7a4]::result::Result<(), rustc_errors[1ed68885dba1d6d1]::ErrorGuaranteed>>::{closure#0}, core[8467f2214112d7a4]::result::Result<(), rustc_errors[1ed68885dba1d6d1]::ErrorGuaranteed>>::{closure#1} as core[8467f2214112d7a4]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  25:     0x7f216627d2a3 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hf3233b13e94151c3
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/alloc/src/boxed.rs:1872:9
  26:     0x7f216627d2a3 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h4e8ef0a8db850d9e
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/alloc/src/boxed.rs:1872:9
  27:     0x7f216627d2a3 - std::sys::unix::thread::Thread::new::thread_start::haa7458da1d6aafbd
                               at /rustc/ebbcbfc236ced21d5e6a92269edb704692ff26b8/library/std/src/sys/unix/thread.rs:108:17
  28:     0x7f21661a3609 - start_thread
  29:     0x7f21660c6133 - clone
  30:                0x0 - <unknown>

note: 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: rustc 1.63.0-nightly (ebbcbfc23 2022-05-27) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
error: could not compile `playground`

[compiler-errors]

searched nightlies: from nightly-2022-03-01 to nightly-2022-05-28
regressed nightly: nightly-2022-04-13
searched commit range: 90ca447...52ca603
regressed commit: 2a83fbc

bisected with cargo-bisect-rustc v0.6.2

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start=2022-03-01 --end=2022-05-28 --regress=ice 

Makes sense to fail in #95320, cc @JakobDegen. Maybe this should be a hard error instead of a delayed bug?

[JakobDegen]

Thanks for the report!

Before that PR, this:

#![feature(core_intrinsics)]

pub fn add(a: bool, b: bool) -> bool {
    std::intrinsics::wrapping_add(a, b)
}

just happily compiled, and this:

#![feature(core_intrinsics)]
use std::intrinsics::wrapping_add;

#[derive(Clone, Copy)]
struct WrapInt8 {
    value: u8,
    value2: u16,
}

impl std::ops::Add for WrapInt8 {
    type Output = WrapInt8;
    fn add(self, other: WrapInt8) -> WrapInt8 {
        wrapping_add(self, other)
    }
}

fn main() {
    let p = WrapInt8 { value: 1, value2: 2 };
    let q = WrapInt8 { value: 3, value2: 4 };
    println!("{}", (p + q).value);
}

already ICEd, so I don't think this is a regression. I'm tempted to say that we shouldn't bother trying to fix this, and that #![feature(core_intrinsics)] is not "supported" in the sense that this bug wants it to be. I'm not sure what the procedure is for making a decision on this though.

Edit: Maybe the actual bug here is that we recommend #![feature(core_intrinsics)] in diagnostics?

[kaya3]

Maybe the actual bug here is that we recommend #![feature(core_intrinsics)] in diagnostics?

I would agree with this. It probably makes sense to exclude std::intrinsics and core::intrinsics from suggested fixes, too.

[scottmcm]

I agree that ICEing for this is actually pretty expected -- intrinsics are super-internal, and ICEing for misuse is basically fine.

👍 to seeing if we can make sure not to suggest core_intrinsics the feature, or really anything with the extern "rust-intrinsic" ABI called via the intrinsics module. (Sadly there are some stable things with that ABI.)

Maybe for the things that are stable we can have them emit warnings/errors pointing to the stable versions? Since this example really wants u8::wrapping_add, not the intrinsic. (Well, really it wants num::Wrapping<u8>, but we're probably not able to be that smart about it.)

[RalfJung]

We might also want to change this message if the crate uses the core_intrinsic feature:

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

ICEs are not always bugs when intrinsics are used directly.