rust-lang / rust-lang/rust

[ICE]: called `Result::unwrap()` on an `Err` value: NoSolution

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

this one triggers a different unwrap than #156309
auto-reduced (treereduce-rust):

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

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

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

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

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

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

impl<'a, T> Clone for 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_borrowck/src/type_check/mod.rs#L2505-L2517

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

Program output 

error[E0106]: missing lifetime specifier
 --> /tmp/icemaker_global_tempdir.5uK8My7zlvE6/rustc_testrunner_tmpdir_reporting.tX0I7qYXjCUc/mvce.rs:8:29
  |
8 | struct CustomRef<'a, T>(&'a CustomMut);
  |                             ^^^^^^^^^ expected named lifetime parameter
  |
help: consider using the `'a` lifetime
  |
8 | struct CustomRef<'a, T>(&'a CustomMut<'a>);
  |                                      ++++

warning: unused import: `Reborrow`
 --> /tmp/icemaker_global_tempdir.5uK8My7zlvE6/rustc_testrunner_tmpdir_reporting.tX0I7qYXjCUc/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 struct `CustomMut`
 --> /tmp/icemaker_global_tempdir.5uK8My7zlvE6/rustc_testrunner_tmpdir_reporting.tX0I7qYXjCUc/mvce.rs:8:29
  |
8 | struct CustomRef<'a, T>(&'a CustomMut);
  |                             ^^^^^^^^^ expected 1 generic argument
  |
note: struct defined here, with 1 generic parameter: `T`
 --> /tmp/icemaker_global_tempdir.5uK8My7zlvE6/rustc_testrunner_tmpdir_reporting.tX0I7qYXjCUc/mvce.rs:4:8
  |
4 | struct CustomMut<'a, T>(&'a mut T);
  |        ^^^^^^^^^     -
help: add missing generic argument
  |
8 | struct CustomRef<'a, T>(&'a CustomMut<T>);
  |                                      +++


thread 'rustc' (194646) panicked at /rustc-dev/3e353d7353b0cd6fa4eefd5c882457cc2708184c/compiler/rustc_borrowck/src/type_check/mod.rs:2511:22:
called `Result::unwrap()` on an `Err` value: NoSolution
stack backtrace:
   0:     0x7fc759874239 - <<std[db65628f2330fd11]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[b3c617e8375f9bd0]::fmt::Display>::fmt
   1:     0x7fc759e1b788 - core[b3c617e8375f9bd0]::fmt::write
   2:     0x7fc75988ac66 - <std[db65628f2330fd11]::sys::stdio::unix::Stderr as std[db65628f2330fd11]::io::Write>::write_fmt
   3:     0x7fc75984a2de - std[db65628f2330fd11]::panicking::default_hook::{closure#0}
   4:     0x7fc7598678f3 - std[db65628f2330fd11]::panicking::default_hook
   5:     0x7fc7587544d1 - std[db65628f2330fd11]::panicking::update_hook::<alloc[b10a1c065189447]::boxed::Box<rustc_driver_impl[1f444ccaa450aabc]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fc759867bd2 - std[db65628f2330fd11]::panicking::panic_with_hook
   7:     0x7fc75984a3a2 - std[db65628f2330fd11]::panicking::panic_handler::{closure#0}
   8:     0x7fc7598410b9 - std[db65628f2330fd11]::sys::backtrace::__rust_end_short_backtrace::<std[db65628f2330fd11]::panicking::panic_handler::{closure#0}, !>
   9:     0x7fc75984be0d - __rustc[87b1f669d989e0ab]::rust_begin_unwind
  10:     0x7fc75692c50c - core[b3c617e8375f9bd0]::panicking::panic_fmt
  11:     0x7fc755f99682 - core[b3c617e8375f9bd0]::result::unwrap_failed
  12:     0x7fc75acc23c7 - <rustc_borrowck[543291a75b63dc2a]::type_check::TypeChecker as rustc_middle[4a2cff3451d2bad5]::mir::visit::Visitor>::visit_body
  13:     0x7fc75ac7286c - rustc_borrowck[543291a75b63dc2a]::type_check::type_check
  14:     0x7fc75a8d7176 - <rustc_borrowck[543291a75b63dc2a]::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
  15:     0x7fc75a8d2b1b - rustc_borrowck[543291a75b63dc2a]::mir_borrowck
  16:     0x7fc75a8d28e9 - rustc_query_impl[3b9ecabc2cca1a8f]::query_impl::mir_borrowck::invoke_provider_fn::__rust_begin_short_backtrace
  17:     0x7fc759f6d6a6 - 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>
  18:     0x7fc759f6fdc3 - rustc_query_impl[3b9ecabc2cca1a8f]::query_impl::mir_borrowck::execute_query_non_incr::__rust_end_short_backtrace
  19:     0x7fc759f6ff94 - <rustc_middle[4a2cff3451d2bad5]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[232b63f46aa35c0e]::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  20:     0x7fc759f6f276 - rustc_interface[232b63f46aa35c0e]::passes::analysis
  21:     0x7fc75af822c9 - rustc_query_impl[3b9ecabc2cca1a8f]::execution::try_execute_query::<rustc_middle[4a2cff3451d2bad5]::query::caches::SingleCache<rustc_middle[4a2cff3451d2bad5]::query::erase::ErasedData<[u8; 0usize]>>, false>
  22:     0x7fc75af81f33 - rustc_query_impl[3b9ecabc2cca1a8f]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  23:     0x7fc75afc765b - rustc_interface[232b63f46aa35c0e]::interface::run_compiler::<(), rustc_driver_impl[1f444ccaa450aabc]::run_compiler::{closure#0}>::{closure#1}
  24:     0x7fc75af75c3e - 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}, ()>
  25:     0x7fc75af7632d - <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}
  26:     0x7fc75af7712c - <std[db65628f2330fd11]::sys::thread::unix::Thread>::new::thread_start
  27:     0x7fc754aa097a - <unknown>
  28:     0x7fc754b242bc - <unknown>
  29:                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: rustc 1.97.0-nightly (3e353d735 2026-05-08) 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 2 previous errors; 1 warning emitted

Some errors have detailed explanations: E0106, E0107.
For more information about an error, try `rustc --explain E0106`.

@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

Reproduce the reduced a.rs case with the reported nightly compiler, then inspect compiler/rustc_borrowck/src/type_check/mod.rs around line 2511 and the mir_borrowck query for main. Done means this input no longer causes rustc to panic with the NoSolution unwrap while reporting its regular compilation errors.

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
48/100

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