[ICE]: `expected region 'static to be of kind ReVar`

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
42/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
rust
Domain
compilers

Research direction

Start with the minimized Rust example in the issue and the backtrace, which points to GatherBorrows::gather_reborrows during MIR borrow checking. Reproduce the ICE with the reported nightly compiler, then trace how reborrow regions are handled in the borrow checker. Done when the example reports a compiler diagnostic instead of panicking and a regression test covers it.

Written by the indexing model from the issue text.

Description

C-bug F-reborrow I-ICE T-compiler
Code
#![feature(reborrow)]
use std::marker::Reborrow;

struct CustomMarker<'a> {
    x: &'a str,
}
struct CustomMarkerTwo<'a> {
    x: CustomMarker<'static>,
    y: &'a u64,
}

impl<'a> Reborrow for CustomMarker<'a> {}
impl<'a> Reborrow for CustomMarkerTwo<'a> {}

fn reborrow(_: CustomMarkerTwo) {
    let a;
    let a = CustomMarkerTwo { x: a, y: &0 };
    reborrow(a)
}

fn main() {}
Meta

rustc --version --verbose:

rustc 1.102.0-nightly (1a4d65fa9 2026-11-08)
binary: rustc
commit-hash: 1a4d65fa9d3ff3990d766c52e741c7027ffa4fc6
commit-date: 2026-11-08
host: x86_64-unknown-linux-gnu
release: 1.102.0-nightly
LLVM version: 23.1.5
Error output
`--edition` is unspecified, defaulting to `2015` while the latest is `2024`; it must be one of: <2015|2018|2021|2024|future>

warning: function cannot return without recursing
  --> code.rs:15:1
   |
15 | fn reborrow(_: CustomMarkerTwo) {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
...
18 |     reborrow(a)
   |     ----------- recursive call site
   |
   = help: a `loop` may express intention better if this is on purpose
   = note: `#[warn(unconditional_recursion)]` on by default
Backtrace 

thread 'rustc' (894146) panicked at /rustc-dev/1a4d65fa9d3ff3990d766c52e741c7027ffa4fc6/compiler/rustc_type_ir/src/sty/mod.rs:192:18:
expected region 'static to be of kind ReVar
stack backtrace:
   0:     0x7fd0d7fb66b6 - <<std[1c3b28271551a578]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[7deae0c0451d2349]::fmt::Display>::fmt
   1:     0x7fd0d860cd0f - core[7deae0c0451d2349]::fmt::write
   2:     0x7fd0d7fcb26c - <std[1c3b28271551a578]::sys::stdio::unix::Stderr as core[7deae0c0451d2349]::io::write::Write>::write_fmt
   3:     0x7fd0d7f8779a - std[1c3b28271551a578]::panicking::default_hook::{closure#0}
   4:     0x7fd0d7faa013 - std[1c3b28271551a578]::panicking::default_hook
   5:     0x7fd0d6d4e279 - std[1c3b28271551a578]::panicking::update_hook::<alloc[93e18355c711b731]::boxed::Box<rustc_driver_impl[b729d44308fea6fe]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fd0d7faa4b2 - std[1c3b28271551a578]::panicking::panic_with_hook
   7:     0x7fd0d7f87852 - std[1c3b28271551a578]::panicking::panic_handler::{closure#0}
   8:     0x7fd0d7f7def9 - std[1c3b28271551a578]::sys::backtrace::__rust_end_short_backtrace::<std[1c3b28271551a578]::panicking::panic_handler::{closure#0}, !>
   9:     0x7fd0d7f895ad - __rustc[98f11d4ffb4a54fb]::rust_begin_unwind
  10:     0x7fd0d481c7dc - core[7deae0c0451d2349]::panicking::panic_fmt
  11:     0x7fd0d69fc447 - <rustc_borrowck[b1a3a3dc4772d542]::borrow_set::GatherBorrows>::gather_reborrows
  12:     0x7fd0d69fc405 - <rustc_borrowck[b1a3a3dc4772d542]::borrow_set::GatherBorrows>::gather_reborrows
  13:     0x7fd0d5070238 - <rustc_borrowck[b1a3a3dc4772d542]::borrow_set::BorrowSet>::build
  14:     0x7fd0d994097d - rustc_borrowck[b1a3a3dc4772d542]::borrowck_collect_region_constraints
  15:     0x7fd0d994f478 - <rustc_borrowck[b1a3a3dc4772d542]::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
  16:     0x7fd0d8a5b9d5 - rustc_borrowck[b1a3a3dc4772d542]::mir_borrowck
  17:     0x7fd0d8a5b6e3 - rustc_query_impl[a2c45f6ddb9da276]::query_vtables::mir_borrowck::invoke_provider_fn::__rust_begin_short_backtrace
  18:     0x7fd0d8a5a10e - rustc_query_impl[a2c45f6ddb9da276]::execution::try_execute_query::<rustc_data_structures[3465cae9aa008a31]::vec_cache::VecCache<rustc_span[2a58d878c64e1a2c]::def_id::LocalDefId, rustc_middle[57c98a37ae0235db]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[57c98a37ae0235db]::dep_graph::graph::DepNodeIndex>, false>
  19:     0x7fd0d8a59c67 - rustc_query_impl[a2c45f6ddb9da276]::query_vtables::mir_borrowck::execute_query_non_incr::__rust_end_short_backtrace
  20:     0x7fd0d8a57d23 - <rustc_middle[57c98a37ae0235db]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[9638b089b1d97f8b]::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  21:     0x7fd0d8a57078 - rustc_interface[9638b089b1d97f8b]::passes::analysis
  22:     0x7fd0d9788147 - rustc_query_impl[a2c45f6ddb9da276]::execution::try_execute_query::<rustc_middle[57c98a37ae0235db]::query::caches::SingleCache<rustc_middle[57c98a37ae0235db]::query::erase::ErasedData<[u8; 0usize]>>, false>
  23:     0x7fd0d9787f2b - rustc_query_impl[a2c45f6ddb9da276]::query_vtables::analysis::execute_query_non_incr::__rust_end_short_backtrace
  24:     0x7fd0d983f567 - rustc_interface[9638b089b1d97f8b]::interface::run_compiler::<(), rustc_driver_impl[b729d44308fea6fe]::compiler_entrypoint::{closure#0}>::{closure#2}
  25:     0x7fd0d98c9ec1 - std[1c3b28271551a578]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[9638b089b1d97f8b]::util::run_in_thread_with_globals<rustc_interface[9638b089b1d97f8b]::util::run_in_thread_pool_with_globals<rustc_interface[9638b089b1d97f8b]::interface::run_compiler<(), rustc_driver_impl[b729d44308fea6fe]::compiler_entrypoint::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  26:     0x7fd0d98c9c6d - <std[1c3b28271551a578]::thread::lifecycle::spawn_unchecked<rustc_interface[9638b089b1d97f8b]::util::run_in_thread_with_globals<rustc_interface[9638b089b1d97f8b]::util::run_in_thread_pool_with_globals<rustc_interface[9638b089b1d97f8b]::interface::run_compiler<(), rustc_driver_impl[b729d44308fea6fe]::compiler_entrypoint::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[7deae0c0451d2349]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  27:     0x7fd0d98c4c2d - <std[1c3b28271551a578]::sys::thread::unix::Thread>::new::thread_start
  28:     0x7fd0d2898042 - <unknown>
  29:     0x7fd0d292072c - <unknown>
  30:                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: please make sure that you have updated to the latest nightly

note: please attach the file at `/home/matthias/vcs/github/CRED/rustc-ice-2026-11-08T18_38_21-894144.txt` to your bug report

note: rustc 1.102.0-nightly (1a4d65fa9 2026-11-08) running on x86_64-unknown-linux-gnu

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

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