rust-lang/rust

ICE: `unreachable` in compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

Opened this issue · 2 comments

cushionbadak commented

Code

struct C;

struct S<T>(Option<T>);

trait Tr {
    type Out;
}

impl<T> Clone for S<C>
where
    S<T>: Tr<Out = T>,
{
    fn clone(&self) -> Self {
        *self
    }
}

fn main() {}

A mutant of tests/ui/associated-types/issue-25700-1.rs

Meta

rustc --version --verbose:

rustc 1.93.0-nightly (c90bcb957 2025-11-06)
binary: rustc
commit-hash: c90bcb9571b7aab0d8beaa2ce8a998ffaf079d38
commit-date: 2025-11-06
host: x86_64-apple-darwin
release: 1.93.0-nightly
LLVM version: 21.1.3

Error output

Command: rustc

error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
 --> 08.rs:9:6
  |
9 | impl<T> Clone for S<C>
  |      ^ unconstrained type parameter


thread 'rustc' (6161155) panicked at compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs:1353:27:
internal error: entered unreachable code
Backtrace 

thread 'rustc' (6161155) panicked at compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs:1353:27:
internal error: entered unreachable code
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic
   3: <rustc_borrowck::MirBorrowckCtxt>::suggest_cloning
   4: <rustc_borrowck::MirBorrowckCtxt>::report
   5: <rustc_borrowck::MirBorrowckCtxt>::report_move_errors
   6: rustc_borrowck::borrowck_check_region_constraints
   7: <rustc_borrowck::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
   8: rustc_borrowck::mir_borrowck
      [... omitted 1 frame ...]
   9: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#1}::{closure#0}>::{closure#0}
  10: rustc_interface::passes::analysis
      [... omitted 1 frame ...]
  11: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>
  12: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 `/Users/jb/Library/CloudStorage/Dropbox/Desk/2503_rustc_ice/20251106 ice 보고용/rustc-ice-2025-11-07T05_57_22-5613.txt` to your bug report

query stack during panic:
#0 [mir_borrowck] borrow-checking `<impl at 08.rs:9:1: 11:23>::clone`
#1 [analysis] running analysis passes on crate `08`
end of query stack
error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0207`.

Notes

  • ICE location: compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs Line-1353

    rust/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

    Lines 1346 to 1358 in c90bcb9

    let types: Vec<_> = types.into_iter().collect();
    let msg = match &types[..] {
    [only] => format!("`{only}`"),
    [head @ .., last] => format!(
    "{} and `{last}`",
    head.iter().map(|t| format!("`{t}`")).collect::<Vec<_>>().join(", ")
    ),
    [] => unreachable!(),
    };
    err.span_note(
    span,
    format!("if {msg} implemented `Clone`, you could clone the value"),
    );

  • Duplication Check

cyrgani commented

bit smaller:

struct C;

struct S<T>(T);

trait Tr {}

impl<T> Clone for S<C>
where
    S<T>: Tr,
{
    fn clone(&self) -> Self {
        *self
    }
}

fn main() {}
  • 👍1
cushionbadak commented

searched nightlies: from nightly-2025-01-01 to nightly-2025-11-07
regressed nightly: nightly-2025-07-27
searched commit range: 430d6ed...ce5fdd7
regressed commit: 051d0e8

bisected with cargo-bisect-rustc v0.6.9

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --end=2025-11-07 --regress ice --preserve --script rustc -- 148631.rs
Regression in https://github.com/rust-lang-ci/rust/commit/802b638406d6cbfe6b6e1d7791ac355b03ba6897
The PR introducing the regression in this rollup is #144201: Mention type that could be `Clone` but isn't in more cases