ICE: broken mir with no-core and 1 => 1 match arm

[matthiaskrgr]

Code

#![feature(lang_items, no_core)]
#![no_std]
#![no_core]

#[lang = "sized"]
pub trait Sized {}

#[lang = "copy"]
trait Copy {}

fn bla(argc: isize)  {
    match argc {
        1 => 1, // removing this arm fixes the ICE
        _ => 1,
    };
}

Meta

rustc --version --verbose:

rustc 1.66.0-nightly (ce7f0f1aa 2022-09-28)
binary: rustc
commit-hash: ce7f0f1aa0f02c45cad0749e63f3086234b1f422
commit-date: 2022-09-28
host: x86_64-unknown-linux-gnu
release: 1.66.0-nightly
LLVM version: 15.0.0

Error output

rustc 92495.rs --crate-type lib

<output>

Backtrace

warning: function `bla` is never used
  --> 92495.rs:11:4
   |
11 | fn bla(argc: isize)  {
   |    ^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: 1 warning emitted

error: internal compiler error: no errors encountered even though `delay_span_bug` issued

error: internal compiler error: broken MIR in DefId(0:3 ~ 92495[a75f]::bla) (NoSolution): could not prove Binder(TraitPredicate(<isize as Copy>, polarity:Positive), [])
  |
  = note: delayed at compiler/rustc_borrowck/src/type_check/canonical.rs:156:13

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1530:13
stack backtrace:
   0:     0x7f97106c3170 - std::backtrace_rs::backtrace::libunwind::trace::hac9b0f7f9661e748
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   1:     0x7f97106c3170 - std::backtrace_rs::backtrace::trace_unsynchronized::hc44795987fc42d34
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f97106c3170 - std::sys_common::backtrace::_print_fmt::h11541e17c3fc86a7
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/std/src/sys_common/backtrace.rs:66:5
   3:     0x7f97106c3170 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hc17ff1666cc882db
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/std/src/sys_common/backtrace.rs:45:22
   4:     0x7f971071de2e - core::fmt::write::h55fcab10ae826150
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/core/src/fmt/mod.rs:1209:17
   5:     0x7f97106b3db5 - std::io::Write::write_fmt::hf963d6d18d14f181
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/std/src/io/mod.rs:1679:15
   6:     0x7f97106c5ef3 - std::sys_common::backtrace::_print::h994ce2c0e5b405c8
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/std/src/sys_common/backtrace.rs:48:5
   7:     0x7f97106c5ef3 - std::sys_common::backtrace::print::hcc56026dcd969e04
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/std/src/sys_common/backtrace.rs:35:9
   8:     0x7f97106c5ef3 - std::panicking::default_hook::{{closure}}::h88f2c5d855241086
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/std/src/panicking.rs:267:22
   9:     0x7f97106c5bdf - std::panicking::default_hook::h8c95d81babfc3452
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/std/src/panicking.rs:286:9
  10:     0x7f9712efb801 - rustc_driver[b4131074c7377c18]::DEFAULT_HOOK::{closure#0}::{closure#0}
  11:     0x7f97106c672b - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h67c5cbe4132c274b
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/alloc/src/boxed.rs:1952:9
  12:     0x7f97106c672b - std::panicking::rust_panic_with_hook::h60207dae86acc93c
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/std/src/panicking.rs:673:13
  13:     0x7f9714014701 - std[1bac5bf8d99344e6]::panicking::begin_panic::<rustc_errors[6dd3b64aee5d90d7]::ExplicitBug>::{closure#0}
  14:     0x7f9714013026 - std[1bac5bf8d99344e6]::sys_common::backtrace::__rust_end_short_backtrace::<std[1bac5bf8d99344e6]::panicking::begin_panic<rustc_errors[6dd3b64aee5d90d7]::ExplicitBug>::{closure#0}, !>
  15:     0x7f971400cfb6 - std[1bac5bf8d99344e6]::panicking::begin_panic::<rustc_errors[6dd3b64aee5d90d7]::ExplicitBug>
  16:     0x7f971400f7d6 - std[1bac5bf8d99344e6]::panic::panic_any::<rustc_errors[6dd3b64aee5d90d7]::ExplicitBug>
  17:     0x7f9712a681c8 - <rustc_errors[6dd3b64aee5d90d7]::HandlerInner as core[de81bf0691074480]::ops::drop::Drop>::drop
  18:     0x7f9711d1e468 - core[de81bf0691074480]::ptr::drop_in_place::<rustc_session[d3255eaf8224a584]::parse::ParseSess>
  19:     0x7f9711cdc3b8 - <alloc[c8bb264001a6b8ff]::rc::Rc<rustc_session[d3255eaf8224a584]::session::Session> as core[de81bf0691074480]::ops::drop::Drop>::drop
  20:     0x7f9711cd908d - core[de81bf0691074480]::ptr::drop_in_place::<rustc_interface[f0f2c906f605917e]::interface::Compiler>
  21:     0x7f9711cd6734 - rustc_interface[f0f2c906f605917e]::interface::create_compiler_and_run::<core[de81bf0691074480]::result::Result<(), rustc_errors[6dd3b64aee5d90d7]::ErrorGuaranteed>, rustc_driver[b4131074c7377c18]::run_compiler::{closure#1}>
  22:     0x7f9711cd4c41 - <scoped_tls[77ed516bf348563d]::ScopedKey<rustc_span[a9093c78adc8a30e]::SessionGlobals>>::set::<rustc_interface[f0f2c906f605917e]::interface::run_compiler<core[de81bf0691074480]::result::Result<(), rustc_errors[6dd3b64aee5d90d7]::ErrorGuaranteed>, rustc_driver[b4131074c7377c18]::run_compiler::{closure#1}>::{closure#0}, core[de81bf0691074480]::result::Result<(), rustc_errors[6dd3b64aee5d90d7]::ErrorGuaranteed>>
  23:     0x7f9711cd492f - std[1bac5bf8d99344e6]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[f0f2c906f605917e]::util::run_in_thread_pool_with_globals<rustc_interface[f0f2c906f605917e]::interface::run_compiler<core[de81bf0691074480]::result::Result<(), rustc_errors[6dd3b64aee5d90d7]::ErrorGuaranteed>, rustc_driver[b4131074c7377c18]::run_compiler::{closure#1}>::{closure#0}, core[de81bf0691074480]::result::Result<(), rustc_errors[6dd3b64aee5d90d7]::ErrorGuaranteed>>::{closure#0}, core[de81bf0691074480]::result::Result<(), rustc_errors[6dd3b64aee5d90d7]::ErrorGuaranteed>>
  24:     0x7f9712d5d600 - <<std[1bac5bf8d99344e6]::thread::Builder>::spawn_unchecked_<rustc_interface[f0f2c906f605917e]::util::run_in_thread_pool_with_globals<rustc_interface[f0f2c906f605917e]::interface::run_compiler<core[de81bf0691074480]::result::Result<(), rustc_errors[6dd3b64aee5d90d7]::ErrorGuaranteed>, rustc_driver[b4131074c7377c18]::run_compiler::{closure#1}>::{closure#0}, core[de81bf0691074480]::result::Result<(), rustc_errors[6dd3b64aee5d90d7]::ErrorGuaranteed>>::{closure#0}, core[de81bf0691074480]::result::Result<(), rustc_errors[6dd3b64aee5d90d7]::ErrorGuaranteed>>::{closure#1} as core[de81bf0691074480]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  25:     0x7f97106d0393 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hd5e3cab38cec9739
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/alloc/src/boxed.rs:1938:9
  26:     0x7f97106d0393 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h970a034a5216df96
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/alloc/src/boxed.rs:1938:9
  27:     0x7f97106d0393 - std::sys::unix::thread::Thread::new::thread_start::h19f111f22a37ea01
                               at /rustc/ce7f0f1aa0f02c45cad0749e63f3086234b1f422/library/std/src/sys/unix/thread.rs:108:17
  28:     0x7f971042878d - <unknown>
  29:     0x7f97104a98e4 - 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 1.66.0-nightly (ce7f0f1aa 2022-09-28) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib

query stack during panic:
end of query stack

[lcnr]

I feel like we shouldn't really work on fixing this. Parts of the compiler short-cut trivially Copy builtin-types without going through the trait system. Failing with an ICE when missing these impls feels acceptable

[matthiaskrgr]

ah, so we try to optimize away 1 => 1 which assume isize/usize is copy but it isn't here in no_core/no_std which causes the ICE?

[lcnr]

not sure where exactly the ICE happens but pretty much: we compile while assuming that isize: Copy holds without checking it in the trait system

at some point we actually check that in the trait system, see that it doesn't hold, realize that we have a broken assumption and ICE

[compiler-errors]

Yeah, I think we've gone back and forth on whether or not ICEs with an ill-defined #![no_core] program are worth the effort, code review, and resulting compiler code complexity of fixing. Perhaps it's worthwhile revisiting this as a policy rather than making a specific decision every time, especially since it's very likely that #![no_core] will never be stabilized?

[JohnTitor]

Fixed on the latest nightly, marking as E-needs-test
@rustbot labels: +E-needs-test

[Noratrieb]

wontfix,we do not want a test: rust-lang/compiler-team#620