borrow-checking panic when mutating unwrapped immutable reference #92015

thread 'rustc' panicked at 'index out of bounds: the len is 7 but the index is 7', compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs:450:53
stack backtrace:
   0: rust_begin_unwind
             at /rustc/65c55bf931a55e6b1e5ed14ad8623814a7386424/library/std/src/panicking.rs:498:5
   1: core::panicking::panic_fmt
             at /rustc/65c55bf931a55e6b1e5ed14ad8623814a7386424/library/core/src/panicking.rs:107:14
   2: core::panicking::panic_bounds_check
             at /rustc/65c55bf931a55e6b1e5ed14ad8623814a7386424/library/core/src/panicking.rs:75:5
   3: <core::option::Option<&rustc_middle::mir::Location>>::map::<rustc_span::span_encoding::Span, <rustc_borrowck::MirBorrowckCtxt>::report_mutability_error::{closure#2}>
   4: <rustc_borrowck::MirBorrowckCtxt>::report_mutability_error
   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_borrowck::do_mir_borrowck
   8: <rustc_infer::infer::InferCtxtBuilder>::enter::<rustc_middle::mir::query::BorrowCheckResult, rustc_borrowck::mir_borrowck::{closure#0}>
   9: rustc_borrowck::mir_borrowck
  10: <rustc_borrowck::provide::{closure#0} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, rustc_span::def_id::LocalDefId)>>::call_once
  11: <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>
  12: 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}>
  13: 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>>
  14: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_borrowck
  15: <rustc_session::session::Session>::time::<(), rustc_interface::passes::analysis::{closure#2}>
  16: rustc_interface::passes::analysis
  17: <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>>
  18: 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}>
  19: 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>>>
  20: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::analysis, rustc_query_impl::plumbing::QueryCtxt>
  21: <rustc_interface::passes::QueryContext>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}::{closure#3}, core::result::Result<(), rustc_errors::ErrorReported>>
  22: <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>>
  23: 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}>
  24: rustc_interface::interface::create_compiler_and_run::<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>
  25: <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.