rust-lang / rust-lang/rust

[ICE]: `Cannot implement Reborrow on a type containing a &'static mut T field`

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C-bug F-reborrow I-ICE needs-triage T-compiler
Dominant language
Rust
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Description

auto-reduced (treereduce-rust):

#![feature(reborrow)]


use std::marker::{CoerceShared, Reborrow};

struct ReorderMut<'a> {
    a: &'static mut u8,
    b: &'a mut u16,
}




struct ReorderRef<'a> {
    ,
    ,
}

impl<'a> CoerceShared<> for ReorderMut<'a> {}
//~^ ERROR

fn read(value: ReorderRef<'_>) -> (u16, u8) {
    
}

fn main() {
    let mut a = 1;
    let mut b = 2;
    let wrapped = ReorderMut { a: &mut a, b: &mut b };

    assert_eq!(read(wrapped), (2, 1));
}

original:

#![feature(reborrow)]
#![allow(dead_code)]

use std::marker::{CoerceShared, Reborrow};

struct ReorderMut<'a> {
    a: &'static mut u8,
    b: &'a mut u16,
}

impl<'a> Reborrow for ReorderMut<'a> {}

#[derive(Clone, Copy)]
struct ReorderRef<'a> {
    b: &'a u16,
    a: &'a u8,
}

impl<'a> CoerceShared<ReorderRef<'a>> for ReorderMut<'a> {}
//~^ ERROR

fn read(value: ReorderRef<'_>) -> (u16, u8) {
    (*value.b, *value.a)
}

fn main() {
    let mut a = 1;
    let mut b = 2;
    let wrapped = ReorderMut { a: &mut a, b: &mut b };

    assert_eq!(read(wrapped), (2, 1));
}

Version information

rustc 1.100.0-nightly (a4330234a 2026-09-01)
binary: rustc
commit-hash: a4330234a776684c36428d001721d0320d24dd77
commit-date: 2026-09-01
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.0

Possibly related line of code:
https://github.com/rust-lang/rust/blob/a4330234a776684c36428d001721d0320d24dd77/compiler/rustc_borrowck/src/borrow_set.rs#L304-L316

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output 

error: expected identifier, found `,`
  --> /tmp/icemaker_global_tempdir.oxCtheU3rtFf/rustc_testrunner_tmpdir_reporting.qRBWn5QyOL8e/mvce.rs:15:5
   |
14 | struct ReorderRef<'a> {
   |        ---------- while parsing this struct
15 |     ,
   |     ^ expected identifier

warning: unused import: `Reborrow`
 --> /tmp/icemaker_global_tempdir.oxCtheU3rtFf/rustc_testrunner_tmpdir_reporting.qRBWn5QyOL8e/mvce.rs:4:33
  |
4 | use std::marker::{CoerceShared, Reborrow};
  |                                 ^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default

error[E0107]: trait takes 1 generic argument but 0 generic arguments were supplied
    --> /tmp/icemaker_global_tempdir.oxCtheU3rtFf/rustc_testrunner_tmpdir_reporting.qRBWn5QyOL8e/mvce.rs:19:10
     |
  19 | impl<'a> CoerceShared<> for ReorderMut<'a> {}
     |          ^^^^^^^^^^^^ 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:1369:11
     |
1369 | pub trait CoerceShared<Target: Copy>: Reborrow {
     |           ^^^^^^^^^^^^ ------
help: add missing generic argument
     |
  19 | impl<'a> CoerceShared<Target> for ReorderMut<'a> {}
     |                       ++++++

error[E0377]: the trait `CoerceShared` may only be implemented for a coercion between structures
  --> /tmp/icemaker_global_tempdir.oxCtheU3rtFf/rustc_testrunner_tmpdir_reporting.qRBWn5QyOL8e/mvce.rs:19:1
   |
19 | impl<'a> CoerceShared<> for ReorderMut<'a> {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
  --> /tmp/icemaker_global_tempdir.oxCtheU3rtFf/rustc_testrunner_tmpdir_reporting.qRBWn5QyOL8e/mvce.rs:22:35
   |
22 | fn read(value: ReorderRef<'_>) -> (u16, u8) {
   |    ----                           ^^^^^^^^^ expected `(u16, u8)`, found `()`
   |    |
   |    implicitly returns `()` as its body has no tail or `return` expression
   |
   = note:  expected tuple `(u16, u8)`
           found unit type `()`

error: internal compiler error: /rustc-dev/a4330234a776684c36428d001721d0320d24dd77/compiler/rustc_borrowck/src/borrow_set.rs:310:25: Cannot implement Reborrow on a type containing a &'static mut T field


thread 'rustc' (2938508) panicked at /rustc-dev/a4330234a776684c36428d001721d0320d24dd77/compiler/rustc_borrowck/src/borrow_set.rs:310:25:
Box<dyn Any>
stack backtrace:
   0:     0x7fbbfd287576 - <<std[30cbc90d73eb2f7]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[37d23c544a63de81]::fmt::Display>::fmt
   1:     0x7fbbfd81138f - core[37d23c544a63de81]::fmt::write
   2:     0x7fbbfd29c64c - <std[30cbc90d73eb2f7]::sys::stdio::unix::Stderr as core[37d23c544a63de81]::io::write::Write>::write_fmt
   3:     0x7fbbfd259e4a - std[30cbc90d73eb2f7]::panicking::default_hook::{closure#0}
   4:     0x7fbbfd27b8a3 - std[30cbc90d73eb2f7]::panicking::default_hook
   5:     0x7fbbfc1f1d8f - std[30cbc90d73eb2f7]::panicking::update_hook::<alloc[1ae459ecdd033a44]::boxed::Box<rustc_driver_impl[42633c3e3ce72fef]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fbbfd27bd42 - std[30cbc90d73eb2f7]::panicking::panic_with_hook
   7:     0x7fbbfc220324 - std[30cbc90d73eb2f7]::panicking::begin_panic::<rustc_errors[9274a714df388628]::ExplicitBug>::{closure#0}
   8:     0x7fbbfc2190d6 - std[30cbc90d73eb2f7]::sys::backtrace::__rust_end_short_backtrace::<std[30cbc90d73eb2f7]::panicking::begin_panic<rustc_errors[9274a714df388628]::ExplicitBug>::{closure#0}, !>
   9:     0x7fbbfc218e0b - std[30cbc90d73eb2f7]::panicking::begin_panic::<rustc_errors[9274a714df388628]::ExplicitBug>
  10:     0x7fbbfc22bd71 - <rustc_errors[9274a714df388628]::diagnostic::BugAbort as rustc_errors[9274a714df388628]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7fbbfc7b8d59 - rustc_middle[a86b7b2986aebea0]::util::bug::opt_span_bug_fmt::<rustc_span[b1cb78db3bb7e75b]::span_encoding::Span>::{closure#0}
  12:     0x7fbbfc7b9152 - rustc_middle[a86b7b2986aebea0]::ty::context::tls::with_opt::<rustc_middle[a86b7b2986aebea0]::util::bug::opt_span_bug_fmt<rustc_span[b1cb78db3bb7e75b]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  13:     0x7fbbfc7a3f70 - rustc_middle[a86b7b2986aebea0]::ty::context::tls::with_context_opt::<rustc_middle[a86b7b2986aebea0]::ty::context::tls::with_opt<rustc_middle[a86b7b2986aebea0]::util::bug::opt_span_bug_fmt<rustc_span[b1cb78db3bb7e75b]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  14:     0x7fbbf941e6f4 - rustc_middle[a86b7b2986aebea0]::util::bug::bug_fmt
  15:     0x7fbbfbeff308 - <rustc_borrowck[1cacfe858ddd23c2]::borrow_set::GatherBorrows>::gather_reborrows
  16:     0x7fbbf9d36c94 - <rustc_borrowck[1cacfe858ddd23c2]::borrow_set::BorrowSet>::build
  17:     0x7fbbfea5522b - rustc_borrowck[1cacfe858ddd23c2]::borrowck_collect_region_constraints
  18:     0x7fbbfea665f2 - <rustc_borrowck[1cacfe858ddd23c2]::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
  19:     0x7fbbfea65515 - rustc_borrowck[1cacfe858ddd23c2]::mir_borrowck
  20:     0x7fbbfea65219 - rustc_query_impl[97c88ea65335a4b0]::query_vtables::mir_borrowck::invoke_provider_fn::__rust_begin_short_backtrace
  21:     0x7fbbfe57eaf5 - rustc_query_impl[97c88ea65335a4b0]::execution::try_execute_query::<rustc_data_structures[2f5d70850e22934]::vec_cache::VecCache<rustc_span[b1cb78db3bb7e75b]::def_id::LocalDefId, rustc_middle[a86b7b2986aebea0]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[a86b7b2986aebea0]::dep_graph::graph::DepNodeIndex>, false>
  22:     0x7fbbfe57e667 - rustc_query_impl[97c88ea65335a4b0]::query_vtables::mir_borrowck::execute_query_non_incr::__rust_end_short_backtrace
  23:     0x7fbbfe581223 - <rustc_middle[a86b7b2986aebea0]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[a3585332d3a3ff1e]::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  24:     0x7fbbfe580580 - rustc_interface[a3585332d3a3ff1e]::passes::analysis
  25:     0x7fbbfea051ca - rustc_query_impl[97c88ea65335a4b0]::execution::try_execute_query::<rustc_middle[a86b7b2986aebea0]::query::caches::SingleCache<rustc_middle[a86b7b2986aebea0]::query::erase::ErasedData<[u8; 0usize]>>, false>
  26:     0x7fbbfea04f6b - rustc_query_impl[97c88ea65335a4b0]::query_vtables::analysis::execute_query_non_incr::__rust_end_short_backtrace
  27:     0x7fbbfeb5a659 - rustc_interface[a3585332d3a3ff1e]::interface::run_compiler::<(), rustc_driver_impl[42633c3e3ce72fef]::run_compiler::{closure#0}>::{closure#2}
  28:     0x7fbbfebad7c2 - std[30cbc90d73eb2f7]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[a3585332d3a3ff1e]::util::run_in_thread_with_globals<rustc_interface[a3585332d3a3ff1e]::util::run_in_thread_pool_with_globals<rustc_interface[a3585332d3a3ff1e]::interface::run_compiler<(), rustc_driver_impl[42633c3e3ce72fef]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  29:     0x7fbbfebad56d - <std[30cbc90d73eb2f7]::thread::lifecycle::spawn_unchecked<rustc_interface[a3585332d3a3ff1e]::util::run_in_thread_with_globals<rustc_interface[a3585332d3a3ff1e]::util::run_in_thread_pool_with_globals<rustc_interface[a3585332d3a3ff1e]::interface::run_compiler<(), rustc_driver_impl[42633c3e3ce72fef]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[37d23c544a63de81]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  30:     0x7fbbfebb5ec7 - <std[30cbc90d73eb2f7]::sys::thread::unix::Thread>::new::thread_start
  31:     0x7fbbf7e980a2 - <unknown>
  32:     0x7fbbf7f2080c - <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 1.100.0-nightly (a4330234a 2026-09-01) running on x86_64-unknown-linux-gnu

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, E0308, E0377.
For more information about an error, try `rustc --explain E0107`.

@rustbot label +F-reborrow

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the reduced and original reproducers in a.rs, then inspect compiler/rustc_borrowck/src/borrow_set.rs around lines 304-316 and the gather_reborrows call shown in the backtrace. Run the reproducer with the specified nightly; done means the valid Reborrow/CoerceShared case no longer triggers an internal compiler error and still reports the intended diagnostics.

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
Active
Clarity
Mostly clear
Newbie friendliness
52/100

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