ICE: transmute: `not implemented: Cycle`

[matthiaskrgr]

auto-reduced (treereduce-rust):

mod assert {
    use std::mem::{Assume, BikeshedIntrinsicFrom};
    pub struct Context;

    pub fn is_maybe_transmutable<Src, Dst>()
    where
        Dst: BikeshedIntrinsicFrom<Src, Context>,
    {
    }
}

fn should_pad_explicitly_packed_field() {
    #[repr(C)]
    struct ExplicitlyPadded(ExplicitlyPadded);

    assert::is_maybe_transmutable::<ExplicitlyPadded, ()>();
}

original:

mod assert {
    use std::mem::{Assume, BikeshedIntrinsicFrom};
    pub struct Context;

    pub fn is_maybe_transmutable<Src, Dst>()
    where
        Dst: BikeshedIntrinsicFrom<Src, Context>,
    {
    }
}

fn should_pad_explicitly_packed_field() {
    enum V0u8 {
        V = 0,
    }

    pub union Uninit {
        a: (),
        b: V0u8,
    }

    #[repr(C)]
    struct ExplicitlyPadded(ExplicitlyPadded);

    assert::is_maybe_transmutable::<ExplicitlyPadded, ()>();
}

Version information

rustc 1.75.0-nightly (11cd1f002 2023-11-01)
binary: rustc
commit-hash: 11cd1f00268061dff447bc0df3d679b35d460875
commit-date: 2023-11-01
host: x86_64-unknown-linux-gnu
release: 1.75.0-nightly
LLVM version: 17.0.4

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output

warning: unused import: `Assume`
 --> /tmp/icemaker_global_tempdir.fRJN4fR6sfYc/rustc_testrunner_tmpdir_reporting.9qgozuo6451M/mvce.rs:2:20
  |
2 |     use std::mem::{Assume, BikeshedIntrinsicFrom};
  |                    ^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.fRJN4fR6sfYc/rustc_testrunner_tmpdir_reporting.9qgozuo6451M/mvce.rs:17:2
   |
17 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.fRJN4fR6sfYc/rustc_testrunner_tmpdir_reporting.9qgozuo6451M/mvce.rs`

error[E0658]: use of unstable library feature 'transmutability'
 --> /tmp/icemaker_global_tempdir.fRJN4fR6sfYc/rustc_testrunner_tmpdir_reporting.9qgozuo6451M/mvce.rs:2:20
  |
2 |     use std::mem::{Assume, BikeshedIntrinsicFrom};
  |                    ^^^^^^
  |
  = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
  = help: add `#![feature(transmutability)]` to the crate attributes to enable

error[E0658]: use of unstable library feature 'transmutability'
 --> /tmp/icemaker_global_tempdir.fRJN4fR6sfYc/rustc_testrunner_tmpdir_reporting.9qgozuo6451M/mvce.rs:2:28
  |
2 |     use std::mem::{Assume, BikeshedIntrinsicFrom};
  |                            ^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
  = help: add `#![feature(transmutability)]` to the crate attributes to enable

error[E0658]: use of unstable library feature 'transmutability'
 --> /tmp/icemaker_global_tempdir.fRJN4fR6sfYc/rustc_testrunner_tmpdir_reporting.9qgozuo6451M/mvce.rs:7:14
  |
7 |         Dst: BikeshedIntrinsicFrom<Src, Context>,
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
  = help: add `#![feature(transmutability)]` to the crate attributes to enable

error[E0072]: recursive type `ExplicitlyPadded` has infinite size
  --> /tmp/icemaker_global_tempdir.fRJN4fR6sfYc/rustc_testrunner_tmpdir_reporting.9qgozuo6451M/mvce.rs:14:5
   |
14 |     struct ExplicitlyPadded(ExplicitlyPadded);
   |     ^^^^^^^^^^^^^^^^^^^^^^^ ---------------- recursive without indirection
   |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
   |
14 |     struct ExplicitlyPadded(Box<ExplicitlyPadded>);
   |                             ++++                +

error[E0391]: cycle detected when computing layout of `should_pad_explicitly_packed_field::ExplicitlyPadded`
  |
  = note: ...which immediately requires computing layout of `should_pad_explicitly_packed_field::ExplicitlyPadded` again
  = note: cycle used when evaluating trait selection obligation `(): core::mem::transmutability::BikeshedIntrinsicFrom<should_pad_explicitly_packed_field::ExplicitlyPadded, assert::Context, core::mem::transmutability::Assume { alignment: false, lifetimes: false, safety: false, validity: false }>`
  = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

thread 'rustc' panicked at compiler/rustc_transmute/src/layout/tree.rs:202:24:
not implemented: Cycle
stack backtrace:
   0:     0x7f44f4f69cfc - std::backtrace_rs::backtrace::libunwind::trace::hf337e9a1b2eefec7
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/../../backtrace/src/backtrace/libunwind.rs:104:5
   1:     0x7f44f4f69cfc - std::backtrace_rs::backtrace::trace_unsynchronized::hd403e7c33ea47db5
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f44f4f69cfc - std::sys_common::backtrace::_print_fmt::hb82fff7b01334105
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x7f44f4f69cfc - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hbef0c62239df8317
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f44f4fcbef0 - core::fmt::rt::Argument::fmt::h3a98781498209cfb
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/core/src/fmt/rt.rs:142:9
   5:     0x7f44f4fcbef0 - core::fmt::write::h99e91fba6b08deab
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/core/src/fmt/mod.rs:1117:17
   6:     0x7f44f4f5dc1f - std::io::Write::write_fmt::he13b638451da3657
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/io/mod.rs:1763:15
   7:     0x7f44f4f69ae4 - std::sys_common::backtrace::_print::hbde38cb8fe94ee2d
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f44f4f69ae4 - std::sys_common::backtrace::print::h65eb3e10b81811f1
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f44f4f6c777 - std::panicking::default_hook::{{closure}}::hd25239f22521c22b
  10:     0x7f44f4f6c4df - std::panicking::default_hook::hff58cd128fac3a73
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/panicking.rs:292:9
  11:     0x7f44f7ad9c30 - std[114aeea37183646d]::panicking::update_hook::<alloc[3407882df4067ddc]::boxed::Box<rustc_driver_impl[4a240edf19cfe501]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x7f44f4f6ceb8 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::hd4e46d91a21e0a2f
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/alloc/src/boxed.rs:2021:9
  13:     0x7f44f4f6ceb8 - std::panicking::rust_panic_with_hook::hc42c4990ce4accb0
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/panicking.rs:783:13
  14:     0x7f44f4f6cc0e - std::panicking::begin_panic_handler::{{closure}}::h259e895efabde9cf
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/panicking.rs:657:13
  15:     0x7f44f4f6a1c6 - std::sys_common::backtrace::__rust_end_short_backtrace::hdc0b762c5f2d3290
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/sys_common/backtrace.rs:170:18
  16:     0x7f44f4f6c972 - rust_begin_unwind
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/panicking.rs:645:5
  17:     0x7f44f4fc8615 - core::panicking::panic_fmt::hf2677c1e3f814967
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/core/src/panicking.rs:72:14
  18:     0x7f44f85187be - <rustc_transmute[2c16d89708a25746]::layout::tree::rustc::Err as core[e1aaa68dbbc63f77]::convert::From<&rustc_middle[f8698e1d8382d76c]::ty::layout::LayoutError>>::from
  19:     0x7f44f8519216 - <rustc_transmute[2c16d89708a25746]::layout::tree::Tree<rustc_transmute[2c16d89708a25746]::layout::rustc::Def, rustc_transmute[2c16d89708a25746]::layout::rustc::Ref>>::from_ty
  20:     0x7f44f851c4f7 - <rustc_transmute[2c16d89708a25746]::rustc::TransmuteTypeEnv>::is_transmutable
  21:     0x7f44f9627b16 - <rustc_trait_selection[2f80e02d5b7ffcda]::traits::select::SelectionContext>::confirm_candidate
  22:     0x7f44f960bd56 - <rustc_trait_selection[2f80e02d5b7ffcda]::traits::select::SelectionContext>::evaluate_candidate
  23:     0x7f44f93c8d6b - <rustc_trait_selection[2f80e02d5b7ffcda]::traits::select::SelectionContext>::evaluate_trait_predicate_recursively
  24:     0x7f44f93c231c - <rustc_trait_selection[2f80e02d5b7ffcda]::traits::select::SelectionContext>::evaluation_probe::<<rustc_trait_selection[2f80e02d5b7ffcda]::traits::select::SelectionContext>::evaluate_root_obligation::{closure#0}>
  25:     0x7f44f8e7a3c6 - rustc_traits[ca87abc2a558096c]::evaluate_obligation::evaluate_obligation
  26:     0x7f44f8e79e20 - rustc_query_impl[92b020c0f41871dc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[92b020c0f41871dc]::query_impl::evaluate_obligation::dynamic_query::{closure#2}::{closure#0}, rustc_middle[f8698e1d8382d76c]::query::erase::Erased<[u8; 2usize]>>
  27:     0x7f44f8e797f8 - rustc_query_system[2501f8730129aa9c]::query::plumbing::try_execute_query::<rustc_query_impl[92b020c0f41871dc]::DynamicConfig<rustc_query_system[2501f8730129aa9c]::query::caches::DefaultCache<rustc_type_ir[84234e71f859f58e]::canonical::Canonical<rustc_middle[f8698e1d8382d76c]::ty::context::TyCtxt, rustc_middle[f8698e1d8382d76c]::ty::ParamEnvAnd<rustc_middle[f8698e1d8382d76c]::ty::Predicate>>, rustc_middle[f8698e1d8382d76c]::query::erase::Erased<[u8; 2usize]>>, false, false, false>, rustc_query_impl[92b020c0f41871dc]::plumbing::QueryCtxt, false>
  28:     0x7f44f8e794df - rustc_query_impl[92b020c0f41871dc]::query_impl::evaluate_obligation::get_query_non_incr::__rust_end_short_backtrace
  29:     0x7f44f5f9e184 - <rustc_trait_selection[2f80e02d5b7ffcda]::traits::fulfill::FulfillProcessor as rustc_data_structures[22f3e173f4dc4ddb]::obligation_forest::ObligationProcessor>::process_obligation
  30:     0x7f44f8c11055 - <rustc_data_structures[22f3e173f4dc4ddb]::obligation_forest::ObligationForest<rustc_trait_selection[2f80e02d5b7ffcda]::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection[2f80e02d5b7ffcda]::traits::fulfill::FulfillProcessor>
  31:     0x7f44f95d5fe1 - <rustc_hir_typeck[4d3bbc2cd4689c0f]::fn_ctxt::FnCtxt>::check_argument_types
  32:     0x7f44f95ce6da - <rustc_hir_typeck[4d3bbc2cd4689c0f]::fn_ctxt::FnCtxt>::check_call
  33:     0x7f44f96a8794 - <rustc_hir_typeck[4d3bbc2cd4689c0f]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  34:     0x7f44f90f49d9 - <rustc_hir_typeck[4d3bbc2cd4689c0f]::fn_ctxt::FnCtxt>::check_block_with_expected
  35:     0x7f44f96a88e9 - <rustc_hir_typeck[4d3bbc2cd4689c0f]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  36:     0x7f44f941b119 - rustc_hir_typeck[4d3bbc2cd4689c0f]::check::check_fn
  37:     0x7f44f8f29c0a - rustc_hir_typeck[4d3bbc2cd4689c0f]::typeck
  38:     0x7f44f8f2907d - rustc_query_impl[92b020c0f41871dc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[92b020c0f41871dc]::query_impl::typeck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[f8698e1d8382d76c]::query::erase::Erased<[u8; 8usize]>>
  39:     0x7f44f8d1006c - rustc_query_system[2501f8730129aa9c]::query::plumbing::try_execute_query::<rustc_query_impl[92b020c0f41871dc]::DynamicConfig<rustc_query_system[2501f8730129aa9c]::query::caches::VecCache<rustc_span[9b73632204a5702]::def_id::LocalDefId, rustc_middle[f8698e1d8382d76c]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[92b020c0f41871dc]::plumbing::QueryCtxt, false>
  40:     0x7f44f8d0fd50 - rustc_query_impl[92b020c0f41871dc]::query_impl::typeck::get_query_non_incr::__rust_end_short_backtrace
  41:     0x7f44f90c28d9 - rustc_hir_analysis[48c9c6619aead8a8]::check_crate
  42:     0x7f44f91b7116 - rustc_interface[65dc9bd9189cd136]::passes::analysis
  43:     0x7f44f91b6b35 - rustc_query_impl[92b020c0f41871dc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[92b020c0f41871dc]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[f8698e1d8382d76c]::query::erase::Erased<[u8; 1usize]>>
  44:     0x7f44f9708ba6 - rustc_query_system[2501f8730129aa9c]::query::plumbing::try_execute_query::<rustc_query_impl[92b020c0f41871dc]::DynamicConfig<rustc_query_system[2501f8730129aa9c]::query::caches::SingleCache<rustc_middle[f8698e1d8382d76c]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[92b020c0f41871dc]::plumbing::QueryCtxt, false>
  45:     0x7f44f97089d5 - rustc_query_impl[92b020c0f41871dc]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  46:     0x7f44f982ba86 - rustc_interface[65dc9bd9189cd136]::interface::run_compiler::<core[e1aaa68dbbc63f77]::result::Result<(), rustc_span[9b73632204a5702]::ErrorGuaranteed>, rustc_driver_impl[4a240edf19cfe501]::run_compiler::{closure#1}>::{closure#0}
  47:     0x7f44f9827e35 - std[114aeea37183646d]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[65dc9bd9189cd136]::util::run_in_thread_with_globals<rustc_interface[65dc9bd9189cd136]::interface::run_compiler<core[e1aaa68dbbc63f77]::result::Result<(), rustc_span[9b73632204a5702]::ErrorGuaranteed>, rustc_driver_impl[4a240edf19cfe501]::run_compiler::{closure#1}>::{closure#0}, core[e1aaa68dbbc63f77]::result::Result<(), rustc_span[9b73632204a5702]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[e1aaa68dbbc63f77]::result::Result<(), rustc_span[9b73632204a5702]::ErrorGuaranteed>>
  48:     0x7f44f9827c8d - <<std[114aeea37183646d]::thread::Builder>::spawn_unchecked_<rustc_interface[65dc9bd9189cd136]::util::run_in_thread_with_globals<rustc_interface[65dc9bd9189cd136]::interface::run_compiler<core[e1aaa68dbbc63f77]::result::Result<(), rustc_span[9b73632204a5702]::ErrorGuaranteed>, rustc_driver_impl[4a240edf19cfe501]::run_compiler::{closure#1}>::{closure#0}, core[e1aaa68dbbc63f77]::result::Result<(), rustc_span[9b73632204a5702]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[e1aaa68dbbc63f77]::result::Result<(), rustc_span[9b73632204a5702]::ErrorGuaranteed>>::{closure#1} as core[e1aaa68dbbc63f77]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  49:     0x7f44f4f77c25 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h05e145a7807cc5b2
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/alloc/src/boxed.rs:2007:9
  50:     0x7f44f4f77c25 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h9003b8eee91ef1fa
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/alloc/src/boxed.rs:2007:9
  51:     0x7f44f4f77c25 - std::sys::unix::thread::Thread::new::thread_start::h277cb4377eb8d1f1
                               at /rustc/11cd1f00268061dff447bc0df3d679b35d460875/library/std/src/sys/unix/thread.rs:108:17
  52:     0x7f44f4d419eb - <unknown>
  53:     0x7f44f4dc57cc - <unknown>
  54:                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: rustc 1.75.0-nightly (11cd1f002 2023-11-01) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [evaluate_obligation] evaluating trait selection obligation `(): core::mem::transmutability::BikeshedIntrinsicFrom<should_pad_explicitly_packed_field::ExplicitlyPadded, assert::Context, core::mem::transmutability::Assume { alignment: false, lifetimes: false, safety: false, validity: false }>`
#1 [typeck] type-checking `should_pad_explicitly_packed_field`
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 6 previous errors; 1 warning emitted

Some errors have detailed explanations: E0072, E0391, E0601, E0658.
For more information about an error, try `rustc --explain E0072`.

[matthiaskrgr]

a56bd2b / #116849 ping @oli-obk 😁