error[E0596]: cannot borrow `a` as mutable, as it is not declared as mutable --> /tmp/icemaker_global_tempdir.6eZNMIGu1a2g/rustc_testrunner_tmpdir_reporting.MPJwrcNMKUbA/mvce.rs:10:31 | 5 | let a: HashMap<String, Box<dyn Debug>> = HashMap::new(); | - help: consider changing this to be mutable: `mut a` ... 10 | _ => Box::new(|x| a.get_mut(x).unwrap() as &Box<dyn Debug>) | ^ cannot borrow as mutable error: captured variable cannot escape `FnMut` closure body --> /tmp/icemaker_global_tempdir.6eZNMIGu1a2g/rustc_testrunner_tmpdir_reporting.MPJwrcNMKUbA/mvce.rs:10:31 | 5 | let a: HashMap<String, Box<dyn Debug>> = HashMap::new(); | - variable defined here ... 10 | _ => Box::new(|x| a.get_mut(x).unwrap() as &Box<dyn Debug>) | - -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | | | | | returns a reference to a captured variable which escapes the closure body | | variable captured here | inferred to be a `FnMut` closure | = note: `FnMut` closures only have access to their captured variables while they are executing... = note: ...therefore, they cannot allow references to captured variables to escape thread 'rustc' panicked at compiler/rustc_borrowck/src/dataflow.rs:432:13: assertion `left == right` failed: the loans out of scope must be the same as the borrows out of scope left: {bb13[0]: [bw1, bw2, bw3], bb7[0]: [bw1], bb10[0]: [bw2, bw3]} right: {bb14[0]: [bw0], bb11[0]: [bw0], bb13[0]: [bw1, bw2, bw3], bb7[0]: [bw1], bb10[0]: [bw2, bw3]} stack backtrace: 0: 0x7fbefbbcb17c - std::backtrace_rs::backtrace::libunwind::trace::hcc0166ed719b5803 at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5 1: 0x7fbefbbcb17c - std::backtrace_rs::backtrace::trace_unsynchronized::hbbfdb0cdfd5a3694 at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5 2: 0x7fbefbbcb17c - std::sys_common::backtrace::_print_fmt::h2acdf0dd6d88fb6d at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/sys_common/backtrace.rs:67:5 3: 0x7fbefbbcb17c - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h8c769e1ecfb97334 at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/sys_common/backtrace.rs:44:22 4: 0x7fbefbc2d400 - core::fmt::rt::Argument::fmt::h0b648738cd9c3905 at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/core/src/fmt/rt.rs:142:9 5: 0x7fbefbc2d400 - core::fmt::write::h8a5a209f873b8707 at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/core/src/fmt/mod.rs:1117:17 6: 0x7fbefbbbf00f - std::io::Write::write_fmt::h78561a5ec2d97aeb at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/io/mod.rs:1762:15 7: 0x7fbefbbcaf64 - std::sys_common::backtrace::_print::ha6123e36392a6105 at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/sys_common/backtrace.rs:47:5 8: 0x7fbefbbcaf64 - std::sys_common::backtrace::print::h1b26d911e86a441a at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/sys_common/backtrace.rs:34:9 9: 0x7fbefbbcdbf7 - std::panicking::default_hook::{{closure}}::h0eac92245a13cc26 10: 0x7fbefbbcd95f - std::panicking::default_hook::ha6311ff3ada643b9 at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/panicking.rs:292:9 11: 0x7fbef8cc47b0 - std[59eea237a8c90763]::panicking::update_hook::<alloc[63ea5590ce573fa8]::boxed::Box<rustc_driver_impl[4dab64b5eaf3dea0]::install_ice_hook::{closure#0}>>::{closure#0} 12: 0x7fbefbbce338 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h6c50cfa831996edb at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/alloc/src/boxed.rs:2021:9 13: 0x7fbefbbce338 - std::panicking::rust_panic_with_hook::h57526d95e3d6ee7d at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/panicking.rs:735:13 14: 0x7fbefbbce08e - std::panicking::begin_panic_handler::{{closure}}::h0e4be4e1cec2f135 at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/panicking.rs:609:13 15: 0x7fbefbbcb646 - std::sys_common::backtrace::__rust_end_short_backtrace::h5b8d993df457b165 at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/sys_common/backtrace.rs:170:18 16: 0x7fbefbbcddf2 - rust_begin_unwind at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/panicking.rs:597:5 17: 0x7fbefbc29b25 - core::panicking::panic_fmt::h6845268c474f8c17 at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/core/src/panicking.rs:72:14 18: 0x7fbefbc2a181 - core::panicking::assert_failed_inner::h7a8a298c50edfefb 19: 0x7fbef8a36cf7 - core[7a9d9f52bc7b45fb]::panicking::assert_failed::<indexmap[f4a693edbf7a6e33]::map::IndexMap<rustc_middle[ecb43faa60cb20bb]::mir::Location, alloc[63ea5590ce573fa8]::vec::Vec<rustc_borrowck[d33a3d5eefa9b602]::dataflow::BorrowIndex>, core[7a9d9f52bc7b45fb]::hash::BuildHasherDefault<rustc_hash[a030f08e88c01fdb]::FxHasher>>, indexmap[f4a693edbf7a6e33]::map::IndexMap<rustc_middle[ecb43faa60cb20bb]::mir::Location, alloc[63ea5590ce573fa8]::vec::Vec<rustc_borrowck[d33a3d5eefa9b602]::dataflow::BorrowIndex>, core[7a9d9f52bc7b45fb]::hash::BuildHasherDefault<rustc_hash[a030f08e88c01fdb]::FxHasher>>> 20: 0x7fbefab9c599 - rustc_borrowck[d33a3d5eefa9b602]::do_mir_borrowck 21: 0x7fbefab7ce39 - rustc_borrowck[d33a3d5eefa9b602]::mir_borrowck 22: 0x7fbefab7c9d9 - rustc_query_impl[cc19e181f2c3ddb7]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[cc19e181f2c3ddb7]::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[ecb43faa60cb20bb]::query::erase::Erased<[u8; 8usize]>> 23: 0x7fbef9f4702c - rustc_query_system[1e36878fdb69a02b]::query::plumbing::try_execute_query::<rustc_query_impl[cc19e181f2c3ddb7]::DynamicConfig<rustc_query_system[1e36878fdb69a02b]::query::caches::VecCache<rustc_span[a9e0b1e4bb2a439]::def_id::LocalDefId, rustc_middle[ecb43faa60cb20bb]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[cc19e181f2c3ddb7]::plumbing::QueryCtxt, false> 24: 0x7fbef9f46c50 - rustc_query_impl[cc19e181f2c3ddb7]::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace 25: 0x7fbefa3fecec - rustc_interface[a6146946eb64d03a]::passes::analysis 26: 0x7fbefa3fe621 - rustc_query_impl[cc19e181f2c3ddb7]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[cc19e181f2c3ddb7]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[ecb43faa60cb20bb]::query::erase::Erased<[u8; 1usize]>> 27: 0x7fbefa922c26 - rustc_query_system[1e36878fdb69a02b]::query::plumbing::try_execute_query::<rustc_query_impl[cc19e181f2c3ddb7]::DynamicConfig<rustc_query_system[1e36878fdb69a02b]::query::caches::SingleCache<rustc_middle[ecb43faa60cb20bb]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[cc19e181f2c3ddb7]::plumbing::QueryCtxt, false> 28: 0x7fbefa922a55 - rustc_query_impl[cc19e181f2c3ddb7]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace 29: 0x7fbefab0300d - std[59eea237a8c90763]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[a6146946eb64d03a]::util::run_in_thread_with_globals<rustc_interface[a6146946eb64d03a]::interface::run_compiler<core[7a9d9f52bc7b45fb]::result::Result<(), rustc_span[a9e0b1e4bb2a439]::ErrorGuaranteed>, rustc_driver_impl[4dab64b5eaf3dea0]::run_compiler::{closure#1}>::{closure#0}, core[7a9d9f52bc7b45fb]::result::Result<(), rustc_span[a9e0b1e4bb2a439]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[7a9d9f52bc7b45fb]::result::Result<(), rustc_span[a9e0b1e4bb2a439]::ErrorGuaranteed>> 30: 0x7fbefab022f3 - <<std[59eea237a8c90763]::thread::Builder>::spawn_unchecked_<rustc_interface[a6146946eb64d03a]::util::run_in_thread_with_globals<rustc_interface[a6146946eb64d03a]::interface::run_compiler<core[7a9d9f52bc7b45fb]::result::Result<(), rustc_span[a9e0b1e4bb2a439]::ErrorGuaranteed>, rustc_driver_impl[4dab64b5eaf3dea0]::run_compiler::{closure#1}>::{closure#0}, core[7a9d9f52bc7b45fb]::result::Result<(), rustc_span[a9e0b1e4bb2a439]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[7a9d9f52bc7b45fb]::result::Result<(), rustc_span[a9e0b1e4bb2a439]::ErrorGuaranteed>>::{closure#1} as core[7a9d9f52bc7b45fb]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0} 31: 0x7fbefbbd9105 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h1e5011952b28f75a at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/alloc/src/boxed.rs:2007:9 32: 0x7fbefbbd9105 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::ha8fd064af386c0c4 at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/alloc/src/boxed.rs:2007:9 33: 0x7fbefbbd9105 - std::sys::unix::thread::Thread::new::thread_start::h8b6a97c3db8ef5a6 at /rustc/151256bd4b577f92922c0fbdf94b12d69cfb08d3/library/std/src/sys/unix/thread.rs:108:17 34: 0x7fbef60759eb - <unknown> 35: 0x7fbef60f97cc - <unknown> 36: 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 (151256bd4 2023-10-24) running on x86_64-unknown-linux-gnu note: compiler flags: -Z polonius=next -Z dump-mir-dir=dir query stack during panic: #0 [mir_borrowck] borrow-checking `main` #1 [analysis] running analysis passes on this crate end of query stack error: aborting due to 2 previous errors For more information about this error, try `rustc --explain E0596`.