ice: the loans out of scope must be the same as the borrows out of scope

[matthiaskrgr]

File: /tmp/im/2/code.rs

snippet:

use std::collections::HashMap;
use std::fmt::Debug;

fn main() {
    let a: HashMap<String, Box<dyn Debug>> = HashMap::new();
    let b = "hello";
    println!(
        "{:?}",
        (match &b[0..=0] {
            _ => Box::new(|x| a.get_mut(x).unwrap() as &Box<dyn Debug>)
                as Box<dyn FnMut(&str) -> &Box<dyn Debug>>,
        })("world")
    );
}

Version information

rustc 1.75.0-nightly (151256bd4 2023-10-24)
binary: rustc
commit-hash: 151256bd4b577f92922c0fbdf94b12d69cfb08d3
commit-date: 2023-10-24
host: x86_64-unknown-linux-gnu
release: 1.75.0-nightly
LLVM version: 17.0.3

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zpolonius=next

Program output

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`.

[matthiaskrgr]

cc @lqd

removing the println macro from around the match avoids the ice 🤔

[lqd]

Fun :)

[lqd]

A more minimal version would be:

fn main() {
    let a = ();
    let b = |_| &a;
    bad(&b);
}

fn bad<F: Fn(&()) -> &()>(_: F) {}