rust-lang / rust-lang/rust

[ICE]: expected region '{region error} to be of kind ReVar

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C-bug F-reborrow I-ICE needs-triage T-compiler WG-trait-system-refactor
Dominant language
Rust
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Description

auto-reduced (treereduce-rust):

//@compile-flags: -Znext-solver=globally
#![feature(reborrow)]
use std::marker::{CoerceShared, Reborrow};

struct CustomMut<'a, T>(&'a mut T);

impl<'a, T> CoerceShared for CustomMut<'reborrow, T> {}

struct CustomRef<'a, T>(&'CustomMut T);

fn method(_a: CustomRef<'_, ()>) {}

fn main() {
    let a = CustomMut(&mut ());
    method(a);
}

original:

//@ run-pass

#![feature(reborrow)]
use std::marker::{CoerceShared, Reborrow};

#[allow(unused)]
struct CustomMut<'a, T>(&'a mut T);
impl<'a, T> Reborrow for CustomMut<'a, T> {
    fn clone(&self) -> Self {
        Self(self.0)
    }
}
impl<'a, T> CoerceShared<CustomRef<'a, T>> for CustomMut<'reborrow, T> {}

struct CustomRef<'a, T>(&'CustomMut T);

impl<'a, T> Clone for CoerceShared<CustomRef<'a, T>> {
    fn clone(&self) -> Self {
        Self(self.0)
    }
}
impl<'a, T> Copy for CustomRef<'a, T> {}

fn method(_a: CustomRef<'_, ()>) {}

fn main() {
    let a = CustomMut(&mut ());
    method(a);
}

Version information

rustc 1.97.0-nightly (3e353d735 2026-05-08)
binary: rustc
commit-hash: 3e353d7353b0cd6fa4eefd5c882457cc2708184c
commit-date: 2026-05-08
host: x86_64-unknown-linux-gnu
release: 1.97.0-nightly
LLVM version: 22.1.4

Possibly related line of code:
https://github.com/rust-lang/rust/blob/3e353d7353b0cd6fa4eefd5c882457cc2708184c/compiler/rustc_middle/src/ty/region.rs#L323-L335

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Znext-solver=globally

Program output 

error[E0261]: use of undeclared lifetime name `'reborrow`
 --> /tmp/icemaker_global_tempdir.dtYE6wFp6BMl/rustc_testrunner_tmpdir_reporting.kFLBoocGaF3W/mvce.rs:6:40
  |
6 | impl<'a, T> CoerceShared for CustomMut<'reborrow, T> {}
  |                                        ^^^^^^^^^ undeclared lifetime
  |
help: consider introducing lifetime `'reborrow` here
  |
6 | impl<'reborrow, 'a, T> CoerceShared for CustomMut<'reborrow, T> {}
  |      ++++++++++

error[E0261]: use of undeclared lifetime name `'CustomMut`
 --> /tmp/icemaker_global_tempdir.dtYE6wFp6BMl/rustc_testrunner_tmpdir_reporting.kFLBoocGaF3W/mvce.rs:8:26
  |
8 | struct CustomRef<'a, T>(&'CustomMut T);
  |                          ^^^^^^^^^^ undeclared lifetime
  |
help: consider introducing lifetime `'CustomMut` here
  |
8 | struct CustomRef<'CustomMut, 'a, T>(&'CustomMut T);
  |                  +++++++++++

warning: unused import: `Reborrow`
 --> /tmp/icemaker_global_tempdir.dtYE6wFp6BMl/rustc_testrunner_tmpdir_reporting.kFLBoocGaF3W/mvce.rs:2:33
  |
2 | use std::marker::{CoerceShared, Reborrow};
  |                                 ^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default

error[E0107]: missing generics for trait `CoerceShared`
    --> /tmp/icemaker_global_tempdir.dtYE6wFp6BMl/rustc_testrunner_tmpdir_reporting.kFLBoocGaF3W/mvce.rs:6:13
     |
   6 | impl<'a, T> CoerceShared for CustomMut<'reborrow, T> {}
     |             ^^^^^^^^^^^^ expected 1 generic argument
     |
note: trait defined here, with 1 generic parameter: `Target`
    --> /home/matthias/.rustup/toolchains/master/lib/rustlib/src/rust/library/core/src/marker.rs:1363:11
     |
1363 | pub trait CoerceShared<Target: Copy>: Reborrow {
     |           ^^^^^^^^^^^^ ------
help: add missing generic argument
     |
   6 | impl<'a, T> CoerceShared<Target> for CustomMut<'reborrow, T> {}
     |                         ++++++++

error[E0377]: the trait `CoerceShared` may only be implemented for a coercion between structures
 --> /tmp/icemaker_global_tempdir.dtYE6wFp6BMl/rustc_testrunner_tmpdir_reporting.kFLBoocGaF3W/mvce.rs:6:1
  |
6 | impl<'a, T> CoerceShared for CustomMut<'reborrow, T> {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: internal compiler error: /rustc-dev/3e353d7353b0cd6fa4eefd5c882457cc2708184c/compiler/rustc_middle/src/ty/region.rs:329:18: expected region '{region error} to be of kind ReVar


thread 'rustc' (4171729) panicked at /rustc-dev/3e353d7353b0cd6fa4eefd5c882457cc2708184c/compiler/rustc_middle/src/ty/region.rs:329:18:
Box<dyn Any>
stack backtrace:
   0:     0x7f7d48474239 - <<std[db65628f2330fd11]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[b3c617e8375f9bd0]::fmt::Display>::fmt
   1:     0x7f7d48a1b788 - core[b3c617e8375f9bd0]::fmt::write
   2:     0x7f7d4848ac66 - <std[db65628f2330fd11]::sys::stdio::unix::Stderr as std[db65628f2330fd11]::io::Write>::write_fmt
   3:     0x7f7d4844a2de - std[db65628f2330fd11]::panicking::default_hook::{closure#0}
   4:     0x7f7d484678f3 - std[db65628f2330fd11]::panicking::default_hook
   5:     0x7f7d473544d1 - std[db65628f2330fd11]::panicking::update_hook::<alloc[b10a1c065189447]::boxed::Box<rustc_driver_impl[1f444ccaa450aabc]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f7d48467bd2 - std[db65628f2330fd11]::panicking::panic_with_hook
   7:     0x7f7d47381f51 - std[db65628f2330fd11]::panicking::begin_panic::<rustc_errors[4e25407aa9dfcb98]::ExplicitBug>::{closure#0}
   8:     0x7f7d4737af46 - std[db65628f2330fd11]::sys::backtrace::__rust_end_short_backtrace::<std[db65628f2330fd11]::panicking::begin_panic<rustc_errors[4e25407aa9dfcb98]::ExplicitBug>::{closure#0}, !>
   9:     0x7f7d4737acab - std[db65628f2330fd11]::panicking::begin_panic::<rustc_errors[4e25407aa9dfcb98]::ExplicitBug>
  10:     0x7f7d4738d431 - <rustc_errors[4e25407aa9dfcb98]::diagnostic::BugAbort as rustc_errors[4e25407aa9dfcb98]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7f7d4799cf69 - rustc_middle[4a2cff3451d2bad5]::util::bug::opt_span_bug_fmt::<rustc_span[749aae40fdac821d]::span_encoding::Span>::{closure#0}
  12:     0x7f7d4799d0d2 - rustc_middle[4a2cff3451d2bad5]::ty::context::tls::with_opt::<rustc_middle[4a2cff3451d2bad5]::util::bug::opt_span_bug_fmt<rustc_span[749aae40fdac821d]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  13:     0x7f7d4798b82b - rustc_middle[4a2cff3451d2bad5]::ty::context::tls::with_context_opt::<rustc_middle[4a2cff3451d2bad5]::ty::context::tls::with_opt<rustc_middle[4a2cff3451d2bad5]::util::bug::opt_span_bug_fmt<rustc_span[749aae40fdac821d]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  14:     0x7f7d45331134 - rustc_middle[4a2cff3451d2bad5]::util::bug::bug_fmt
  15:     0x7f7d4a9450e9 - <rustc_middle[4a2cff3451d2bad5]::ty::region::Region>::as_var.cold
  16:     0x7f7d498c22f6 - <rustc_borrowck[543291a75b63dc2a]::type_check::TypeChecker as rustc_middle[4a2cff3451d2bad5]::mir::visit::Visitor>::visit_body
  17:     0x7f7d4987286c - rustc_borrowck[543291a75b63dc2a]::type_check::type_check
  18:     0x7f7d494d7176 - <rustc_borrowck[543291a75b63dc2a]::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
  19:     0x7f7d494d2b1b - rustc_borrowck[543291a75b63dc2a]::mir_borrowck
  20:     0x7f7d494d28e9 - rustc_query_impl[3b9ecabc2cca1a8f]::query_impl::mir_borrowck::invoke_provider_fn::__rust_begin_short_backtrace
  21:     0x7f7d48b6d6a6 - rustc_query_impl[3b9ecabc2cca1a8f]::execution::try_execute_query::<rustc_data_structures[4a3bb46d707d6722]::vec_cache::VecCache<rustc_span[749aae40fdac821d]::def_id::LocalDefId, rustc_middle[4a2cff3451d2bad5]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[4a2cff3451d2bad5]::dep_graph::graph::DepNodeIndex>, false>
  22:     0x7f7d48b6fdc3 - rustc_query_impl[3b9ecabc2cca1a8f]::query_impl::mir_borrowck::execute_query_non_incr::__rust_end_short_backtrace
  23:     0x7f7d48b6ff94 - <rustc_middle[4a2cff3451d2bad5]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[232b63f46aa35c0e]::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  24:     0x7f7d48b6f276 - rustc_interface[232b63f46aa35c0e]::passes::analysis
  25:     0x7f7d49b822c9 - rustc_query_impl[3b9ecabc2cca1a8f]::execution::try_execute_query::<rustc_middle[4a2cff3451d2bad5]::query::caches::SingleCache<rustc_middle[4a2cff3451d2bad5]::query::erase::ErasedData<[u8; 0usize]>>, false>
  26:     0x7f7d49b81f33 - rustc_query_impl[3b9ecabc2cca1a8f]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  27:     0x7f7d49bc765b - rustc_interface[232b63f46aa35c0e]::interface::run_compiler::<(), rustc_driver_impl[1f444ccaa450aabc]::run_compiler::{closure#0}>::{closure#1}
  28:     0x7f7d49b75c3e - std[db65628f2330fd11]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[232b63f46aa35c0e]::util::run_in_thread_with_globals<rustc_interface[232b63f46aa35c0e]::util::run_in_thread_pool_with_globals<rustc_interface[232b63f46aa35c0e]::interface::run_compiler<(), rustc_driver_impl[1f444ccaa450aabc]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  29:     0x7f7d49b7632d - <std[db65628f2330fd11]::thread::lifecycle::spawn_unchecked<rustc_interface[232b63f46aa35c0e]::util::run_in_thread_with_globals<rustc_interface[232b63f46aa35c0e]::util::run_in_thread_pool_with_globals<rustc_interface[232b63f46aa35c0e]::interface::run_compiler<(), rustc_driver_impl[1f444ccaa450aabc]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[b3c617e8375f9bd0]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  30:     0x7f7d49b7712c - <std[db65628f2330fd11]::sys::thread::unix::Thread>::new::thread_start
  31:     0x7f7d436a597a - <unknown>
  32:     0x7f7d437292bc - <unknown>
  33:                0x0 - <unknown>

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: rustc 1.97.0-nightly (3e353d735 2026-05-08) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z next-solver=globally -Z dump-mir-dir=dir

query stack during panic:
#0 [mir_borrowck] borrow-checking `main`
#1 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 5 previous errors; 1 warning emitted

Some errors have detailed explanations: E0107, E0261, E0377.
For more information about an error, try `rustc --explain E0107`.

@rustbot label +F-reborrow +WG-trait-system-refactor

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  4. Open a pull request that references the issue number.

Research direction

Start with the reduced Rust program in a.rs and reproduce it using rustc -Znext-solver=globally. Read compiler/rustc_middle/src/ty/region.rs around lines 323-335, then follow the reported mir_borrowck query and rustc_borrowck type-checking path. Done means the case produces a normal compiler diagnostic without an internal compiler error.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Quiet
Clarity
Mostly clear
Newbie friendliness
35/100

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