internal compiler error: unexpected panic

[bwinton]

Apologies for the poor title, but I'm not sure what to put there… 🙂

Code

https://gist.github.com/0f61ed0d930363d807431f16cb055b5a

Meta

rustc 1.58.0-nightly (65c55bf93 2021-11-23)
binary: rustc
commit-hash: 65c55bf931a55e6b1e5ed14ad8623814a7386424
commit-date: 2021-11-23
host: aarch64-apple-darwin
release: 1.58.0-nightly
LLVM version: 13.0.0

Error output

thread 'rustc' panicked at 'index out of bounds: the len is 2 but the index is 2', compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs:450:53
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

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 1.58.0-nightly (65c55bf93 2021-11-23) running on aarch64-apple-darwin

note: compiler flags: -C embed-bitcode=no -C split-debuginfo=unpacked -C debuginfo=2 -C incremental --crate-type bin

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

query stack during panic:
#0 [mir_borrowck] borrow-checking `q24::solve`
#1 [analysis] running analysis passes on this crate
end of query stack
error: could not compile `adventofcode`

Backtrace

   Compiling adventofcode v0.1.0 (/Users/bwinton/Programming/advent-of-code)
thread 'rustc' panicked at 'index out of bounds: the len is 2 but the index is 2', compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs:450:53
stack backtrace:
   0: _rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic_bounds_check
   3: <rustc_borrowck::MirBorrowckCtxt>::report_mutability_error
   4: <rustc_borrowck::MirBorrowckCtxt>::access_place
   5: <rustc_borrowck::MirBorrowckCtxt as rustc_mir_dataflow::framework::visitor::ResultsVisitor>::visit_statement_before_primary_effect
   6: <rustc_mir_dataflow::framework::direction::Forward as rustc_mir_dataflow::framework::direction::Direction>::visit_results_in_block::<rustc_borrowck::dataflow::BorrowckAnalyses<rustc_index::bit_set::BitSet<rustc_borrowck::dataflow::BorrowIndex>, rustc_index::bit_set::BitSet<rustc_mir_dataflow::move_paths::MovePathIndex>, rustc_index::bit_set::BitSet<rustc_mir_dataflow::move_paths::InitIndex>>, rustc_borrowck::dataflow::BorrowckAnalyses<rustc_mir_dataflow::framework::engine::Results<rustc_borrowck::dataflow::Borrows>, rustc_mir_dataflow::framework::engine::Results<rustc_mir_dataflow::impls::MaybeUninitializedPlaces>, rustc_mir_dataflow::framework::engine::Results<rustc_mir_dataflow::impls::EverInitializedPlaces>>, rustc_borrowck::MirBorrowckCtxt>
   7: rustc_mir_dataflow::framework::visitor::visit_results::<rustc_borrowck::dataflow::BorrowckAnalyses<rustc_index::bit_set::BitSet<rustc_borrowck::dataflow::BorrowIndex>, rustc_index::bit_set::BitSet<rustc_mir_dataflow::move_paths::MovePathIndex>, rustc_index::bit_set::BitSet<rustc_mir_dataflow::move_paths::InitIndex>>, rustc_borrowck::dataflow::BorrowckAnalyses<rustc_mir_dataflow::framework::engine::Results<rustc_borrowck::dataflow::Borrows>, rustc_mir_dataflow::framework::engine::Results<rustc_mir_dataflow::impls::MaybeUninitializedPlaces>, rustc_mir_dataflow::framework::engine::Results<rustc_mir_dataflow::impls::EverInitializedPlaces>>, core::iter::adapters::map::Map<rustc_middle::mir::traversal::ReversePostorder, rustc_borrowck::do_mir_borrowck::{closure#2}>, rustc_borrowck::MirBorrowckCtxt>
   8: rustc_borrowck::do_mir_borrowck
   9: <rustc_infer::infer::InferCtxtBuilder>::enter::<rustc_middle::mir::query::BorrowCheckResult, rustc_borrowck::mir_borrowck::{closure#0}>
  10: rustc_borrowck::mir_borrowck
  11: <rustc_borrowck::provide::{closure#0} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, rustc_span::def_id::LocalDefId)>>::call_once
  12: <rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::with_task::<rustc_middle::ty::context::TyCtxt, rustc_span::def_id::LocalDefId, &rustc_middle::mir::query::BorrowCheckResult>
  13: rustc_data_structures::stack::ensure_sufficient_stack::<(&rustc_middle::mir::query::BorrowCheckResult, rustc_query_system::dep_graph::graph::DepNodeIndex), rustc_query_system::query::plumbing::execute_job<rustc_query_impl::plumbing::QueryCtxt, rustc_span::def_id::LocalDefId, &rustc_middle::mir::query::BorrowCheckResult>::{closure#3}>
  14: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::LocalDefId, &rustc_middle::mir::query::BorrowCheckResult>>
  15: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::mir_borrowck, rustc_query_impl::plumbing::QueryCtxt>
  16: <rustc_session::session::Session>::time::<(), rustc_interface::passes::analysis::{closure#2}>
  17: rustc_interface::passes::analysis
  18: <rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::with_task::<rustc_middle::ty::context::TyCtxt, (), core::result::Result<(), rustc_errors::ErrorReported>>
  19: rustc_data_structures::stack::ensure_sufficient_stack::<(core::result::Result<(), rustc_errors::ErrorReported>, rustc_query_system::dep_graph::graph::DepNodeIndex), rustc_query_system::query::plumbing::execute_job<rustc_query_impl::plumbing::QueryCtxt, (), core::result::Result<(), rustc_errors::ErrorReported>>::{closure#3}>
  20: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<(), core::result::Result<(), rustc_errors::ErrorReported>>>
  21: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::analysis, rustc_query_impl::plumbing::QueryCtxt>
  22: <rustc_interface::passes::QueryContext>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}::{closure#3}, core::result::Result<(), rustc_errors::ErrorReported>>
  23: <rustc_interface::interface::Compiler>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_errors::ErrorReported>>
  24: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorReported>, rustc_interface::interface::create_compiler_and_run<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>::{closure#1}>
  25: rustc_interface::interface::create_compiler_and_run::<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>
  26: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorReported>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_errors::ErrorReported>>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: internal compiler error: unexpected panic

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 1.58.0-nightly (65c55bf93 2021-11-23) running on aarch64-apple-darwin

note: compiler flags: -C embed-bitcode=no -C split-debuginfo=unpacked -C debuginfo=2 -C incremental --crate-type bin

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

query stack during panic:
#0 [mir_borrowck] borrow-checking `q24::solve`
#1 [analysis] running analysis passes on this crate
end of query stack
error: could not compile `adventofcode`
warning: build failed, waiting for other jobs to finish...
error: build failed```

</p>
</details>

[BGR360]

The code you provided in the gist isn't enough to attempt a repro on my end, I'm missing a few things:

error: couldn't read src/data/q24.data: No such file or directory (os error 2)
  --> src/main.rs:14:22
   |
14 | static INPUT: &str = include_str!("data/q24.data");
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this error originates in the macro `include_str` (in Nightly builds, run with -Z macro-backtrace for more info)

error: cannot find macro `q_impl` in this scope
   --> src/main.rs:300:1
    |
300 | q_impl!("24");
    | ^^^^^^

Is there by any chance a git repo that I can clone and a revision that I can checkout that has the code in the same state it was in when you encountered the ICE?

[bwinton]

I'm not pushing the code yet, but I took some time to trim down the sample, and have published a new revision at https://gist.github.com/bwinton/0f61ed0d930363d807431f16cb055b5a that runs on its own and still has the error. If I comment out the level.sort();, it all works correctly (I mean, it gives me a bunch of dead_code warnings, but it compiles cleanly)…

[BGR360]

Amazing! That is immensely helpful.

I was able to reproduce, here's the backtrace with RUST_BACKTRACE=full:

Backtrace

thread 'rustc' panicked at 'index out of bounds: the len is 2 but the index is 2', compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs:450:53
stack backtrace:
   0:        0x1012819c8 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h61a7d3e4c5d884f5
   1:        0x1012ca980 - core::fmt::write::h15b38023853d48c8
   2:        0x1012738a0 - std::io::Write::write_fmt::h8d01a6f9467a53cf
   3:        0x1012847cc - std::panicking::default_hook::{{closure}}::h202969861cea2a40
   4:        0x1012843ac - std::panicking::default_hook::h6075a10f4c58af64
   5:        0x10849ba3c - rustc_driver[35b3b9d305ec1a86]::DEFAULT_HOOK::{closure#0}::{closure#0}
   6:        0x101284fa0 - std::panicking::rust_panic_with_hook::h0d2ad4d26e65e971
   7:        0x101284ac0 - std::panicking::begin_panic_handler::{{closure}}::h81487f6587d82c75
   8:        0x101281e94 - std::sys_common::backtrace::__rust_end_short_backtrace::hc543c9309bc89e91
   9:        0x101284a28 - _rust_begin_unwind
  10:        0x1012fa170 - core::panicking::panic_fmt::h61e5a76b978edc22
  11:        0x1012fa144 - core::panicking::panic_bounds_check::hbcdb5a8d91c8ce55
  12:        0x10ad8e170 - <rustc_borrowck[6143a6a723f63a46]::MirBorrowckCtxt>::report_mutability_error
  13:        0x10ad95914 - <rustc_borrowck[6143a6a723f63a46]::MirBorrowckCtxt>::access_place
  14:        0x10ad94244 - <rustc_borrowck[6143a6a723f63a46]::MirBorrowckCtxt as rustc_mir_dataflow[2505fbb23d1ca289]::framework::visitor::ResultsVisitor>::visit_statement_before_primary_effect
  15:        0x10ad2044c - <rustc_mir_dataflow[2505fbb23d1ca289]::framework::direction::Forward as rustc_mir_dataflow[2505fbb23d1ca289]::framework::direction::Direction>::visit_results_in_block::<rustc_borrowck[6143a6a723f63a46]::dataflow::BorrowckAnalyses<rustc_index[64c13988d092f625]::bit_set::BitSet<rustc_borrowck[6143a6a723f63a46]::dataflow::BorrowIndex>, rustc_index[64c13988d092f625]::bit_set::BitSet<rustc_mir_dataflow[2505fbb23d1ca289]::move_paths::MovePathIndex>, rustc_index[64c13988d092f625]::bit_set::BitSet<rustc_mir_dataflow[2505fbb23d1ca289]::move_paths::InitIndex>>, rustc_borrowck[6143a6a723f63a46]::dataflow::BorrowckAnalyses<rustc_mir_dataflow[2505fbb23d1ca289]::framework::engine::Results<rustc_borrowck[6143a6a723f63a46]::dataflow::Borrows>, rustc_mir_dataflow[2505fbb23d1ca289]::framework::engine::Results<rustc_mir_dataflow[2505fbb23d1ca289]::impls::MaybeUninitializedPlaces>, rustc_mir_dataflow[2505fbb23d1ca289]::framework::engine::Results<rustc_mir_dataflow[2505fbb23d1ca289]::impls::EverInitializedPlaces>>, rustc_borrowck[6143a6a723f63a46]::MirBorrowckCtxt>
  16:        0x10acae320 - rustc_mir_dataflow[2505fbb23d1ca289]::framework::visitor::visit_results::<rustc_borrowck[6143a6a723f63a46]::dataflow::BorrowckAnalyses<rustc_index[64c13988d092f625]::bit_set::BitSet<rustc_borrowck[6143a6a723f63a46]::dataflow::BorrowIndex>, rustc_index[64c13988d092f625]::bit_set::BitSet<rustc_mir_dataflow[2505fbb23d1ca289]::move_paths::MovePathIndex>, rustc_index[64c13988d092f625]::bit_set::BitSet<rustc_mir_dataflow[2505fbb23d1ca289]::move_paths::InitIndex>>, rustc_borrowck[6143a6a723f63a46]::dataflow::BorrowckAnalyses<rustc_mir_dataflow[2505fbb23d1ca289]::framework::engine::Results<rustc_borrowck[6143a6a723f63a46]::dataflow::Borrows>, rustc_mir_dataflow[2505fbb23d1ca289]::framework::engine::Results<rustc_mir_dataflow[2505fbb23d1ca289]::impls::MaybeUninitializedPlaces>, rustc_mir_dataflow[2505fbb23d1ca289]::framework::engine::Results<rustc_mir_dataflow[2505fbb23d1ca289]::impls::EverInitializedPlaces>>, core[489ab2277f19021d]::iter::adapters::map::Map<rustc_middle[29c0f47ba1981f7e]::mir::traversal::ReversePostorder, rustc_borrowck[6143a6a723f63a46]::do_mir_borrowck::{closure#2}>, rustc_borrowck[6143a6a723f63a46]::MirBorrowckCtxt>
  17:        0x10ad9ab04 - rustc_borrowck[6143a6a723f63a46]::do_mir_borrowck
  18:        0x10ad0cc44 - <rustc_infer[233d2e4639a00b1a]::infer::InferCtxtBuilder>::enter::<rustc_middle[29c0f47ba1981f7e]::mir::query::BorrowCheckResult, rustc_borrowck[6143a6a723f63a46]::mir_borrowck::{closure#0}>
  19:        0x10ad939c0 - rustc_borrowck[6143a6a723f63a46]::mir_borrowck
  20:        0x10ad75d14 - <rustc_borrowck[6143a6a723f63a46]::provide::{closure#0} as core[489ab2277f19021d]::ops::function::FnOnce<(rustc_middle[29c0f47ba1981f7e]::ty::context::TyCtxt, rustc_span[6eae83f828617b72]::def_id::LocalDefId)>>::call_once
  21:        0x10b118a10 - rustc_query_system[2ed05f00384aea15]::query::plumbing::try_execute_query::<rustc_query_impl[1e8f6af660f906f5]::plumbing::QueryCtxt, rustc_query_system[2ed05f00384aea15]::query::caches::DefaultCache<rustc_span[6eae83f828617b72]::def_id::LocalDefId, &rustc_middle[29c0f47ba1981f7e]::mir::query::BorrowCheckResult>>
  22:        0x10b1a40c4 - rustc_query_system[2ed05f00384aea15]::query::plumbing::get_query::<rustc_query_impl[1e8f6af660f906f5]::queries::mir_borrowck, rustc_query_impl[1e8f6af660f906f5]::plumbing::QueryCtxt>
  23:        0x10863f7e0 - <rustc_session[5f6c487c9c714a47]::session::Session>::time::<(), rustc_interface[3e6b3c7def9e9d82]::passes::analysis::{closure#2}>
  24:        0x1085acbcc - rustc_interface[3e6b3c7def9e9d82]::passes::analysis
  25:        0x10b148548 - rustc_query_system[2ed05f00384aea15]::query::plumbing::try_execute_query::<rustc_query_impl[1e8f6af660f906f5]::plumbing::QueryCtxt, rustc_query_system[2ed05f00384aea15]::query::caches::DefaultCache<(), core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>>>
  26:        0x10b1dac04 - rustc_query_system[2ed05f00384aea15]::query::plumbing::get_query::<rustc_query_impl[1e8f6af660f906f5]::queries::analysis, rustc_query_impl[1e8f6af660f906f5]::plumbing::QueryCtxt>
  27:        0x1084fad6c - <rustc_interface[3e6b3c7def9e9d82]::passes::QueryContext>::enter::<rustc_driver[35b3b9d305ec1a86]::run_compiler::{closure#1}::{closure#2}::{closure#3}, core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>>
  28:        0x1084df010 - <rustc_interface[3e6b3c7def9e9d82]::interface::Compiler>::enter::<rustc_driver[35b3b9d305ec1a86]::run_compiler::{closure#1}::{closure#2}, core[489ab2277f19021d]::result::Result<core[489ab2277f19021d]::option::Option<rustc_interface[3e6b3c7def9e9d82]::queries::Linker>, rustc_errors[1a053f9350564121]::ErrorReported>>
  29:        0x1084a4608 - rustc_span[6eae83f828617b72]::with_source_map::<core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>, rustc_interface[3e6b3c7def9e9d82]::interface::create_compiler_and_run<core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>, rustc_driver[35b3b9d305ec1a86]::run_compiler::{closure#1}>::{closure#1}>
  30:        0x1084df898 - rustc_interface[3e6b3c7def9e9d82]::interface::create_compiler_and_run::<core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>, rustc_driver[35b3b9d305ec1a86]::run_compiler::{closure#1}>
  31:        0x1084affc0 - <scoped_tls[34b11748aea080ce]::ScopedKey<rustc_span[6eae83f828617b72]::SessionGlobals>>::set::<rustc_interface[3e6b3c7def9e9d82]::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface[3e6b3c7def9e9d82]::interface::run_compiler<core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>, rustc_driver[35b3b9d305ec1a86]::run_compiler::{closure#1}>::{closure#0}, core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>>::{closure#0}::{closure#0}, core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>>
  32:        0x1084ae02c - std[edbc36b44a871839]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[3e6b3c7def9e9d82]::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface[3e6b3c7def9e9d82]::interface::run_compiler<core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>, rustc_driver[35b3b9d305ec1a86]::run_compiler::{closure#1}>::{closure#0}, core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>>::{closure#0}, core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>>
  33:        0x108500048 - <<std[edbc36b44a871839]::thread::Builder>::spawn_unchecked<rustc_interface[3e6b3c7def9e9d82]::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface[3e6b3c7def9e9d82]::interface::run_compiler<core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>, rustc_driver[35b3b9d305ec1a86]::run_compiler::{closure#1}>::{closure#0}, core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>>::{closure#0}, core[489ab2277f19021d]::result::Result<(), rustc_errors[1a053f9350564121]::ErrorReported>>::{closure#1} as core[489ab2277f19021d]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  34:        0x10128e318 - std::sys::unix::thread::Thread::new::thread_start::h02ceb9b16148ae76
  35:        0x1bf9754ec - _pthread_from_mach_thread_np

error: internal compiler error: unexpected panic

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 1.58.0-nightly (936f2600b 2021-11-22) running on aarch64-apple-darwin

query stack during panic:
#0 [mir_borrowck] borrow-checking `solve`
#1 [analysis] running analysis passes on this crate
end of query stack

This issue does not occur on stable 1.57 nor on beta-1.58:

ben@Bens-MBP issue-92265 % rustc +stable src/error.rs
warning: variable does not need to be mutable
 --> src/error.rs:7:9
  |
7 |     let mut level = levels[i].get(&z).unwrap();
  |         ----^^^^^
  |         |
  |         help: remove this `mut`
  |
  = note: `#[warn(unused_mut)]` on by default

error[E0596]: cannot borrow `*level` as mutable, as it is behind a `&` reference
 --> src/error.rs:8:5
  |
7 |     let mut level = levels[i].get(&z).unwrap();
  |         --------- help: consider changing this to be a mutable reference: `&mut Vec<(i128, i128)>`
8 |     level.sort();
  |     ^^^^^^^^^^^^ `level` is a `&` reference, so the data it refers to cannot be borrowed as mutable

error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0596`.
ben@Bens-MBP issue-92265 % rustc +beta src/error.rs
warning: variable does not need to be mutable
 --> src/error.rs:7:9
  |
7 |     let mut level = levels[i].get(&z).unwrap();
  |         ----^^^^^
  |         |
  |         help: remove this `mut`
  |
  = note: `#[warn(unused_mut)]` on by default

error[E0596]: cannot borrow `*level` as mutable, as it is behind a `&` reference
 --> src/error.rs:8:5
  |
7 |     let mut level = levels[i].get(&z).unwrap();
  |         --------- help: consider changing this to be a mutable reference: `&mut Vec<(i128, i128)>`
8 |     level.sort();
  |     ^^^^^^^^^^^^ `level` is a `&` reference, so the data it refers to cannot be borrowed as mutable

error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0596`.

I'm going to try to bisect to find which revision broke things.

@rustbot label +regression-from-stable-to-nightly

[BGR360]

Oh wait, I'm sorry, the beta that I have installed is more recent than your nightly:

ben@Bens-MBP rust % rustc +beta --version -v
rustc 1.58.0-beta.2 (0e07bcb68 2021-12-04)
binary: rustc
commit-hash: 0e07bcb68b82b54c0c4ec6fe076e9d75b02109cf
commit-date: 2021-12-04
host: aarch64-apple-darwin
release: 1.58.0-beta.2
LLVM version: 13.0.0

So it appears this has been fixed. And indeed, doing a quick issue search for mutability_errors.rs, I found: #92172 which links to #91212.

Use rustup to get a more recent nightly and that should fix you.

@rustbot label -regression-from-stable-to-nightly -I-prioritize

[bwinton]

That's great to hear!
I might wait until tomorrow, when the Advent of Code ends, just so that I don't have to spend time fixing anything that might have changed since I last upgraded… 😉

[BGR360]

Cool. Mind closing the issue?