ICE: `None` in `rustc_mir_build/src/builder/matches/mod.rs`

[cushionbadak]

Code

#![feature(never_patterns)]
#![allow(incomplete_features)]
enum Void {}

enum Either {
    One(X),
    Two(X),
}

struct X;

fn move_into_fnmut() {
    let x = Either::One(X);
    || {
        let Either::Two(!) = x;
    };
}

fn main() {}

A mutant of glacier2's fixed/109523.rs

Meta

rustc --version --verbose:

rustc 1.89.0-nightly (283db70ac 2025-05-25)
binary: rustc
commit-hash: 283db70ace62a0ae704a624e43b68c2ee44b87a6
commit-date: 2025-05-25
host: aarch64-apple-darwin
release: 1.89.0-nightly
LLVM version: 20.1.5

Error output

command: rustc --edition=2021

error: mismatched types
  --> never_1.rs:15:25
   |
15 |         let Either::Two(!) = x;
   |                         ^ a never pattern must be used on an uninhabited type
   |
   = note: the matched value is of type `X`


thread 'rustc' panicked at compiler/rustc_mir_build/src/builder/matches/mod.rs:2110:44:
called `Option::unwrap()` on a `None` value

Backtrace

thread 'rustc' panicked at compiler/rustc_mir_build/src/builder/matches/mod.rs:2110:44:
called `Option::unwrap()` on a `None` value
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic
   3: core::option::unwrap_failed
   4: <rustc_mir_build::builder::Builder>::match_candidates_inner
   5: <rustc_mir_build::builder::Builder>::lower_match_tree
   6: <rustc_mir_build::builder::Builder>::place_into_pattern
   7: <rustc_mir_build::builder::Builder>::ast_block_stmts
   8: <rustc_mir_build::builder::Builder>::expr_into_dest
   9: <rustc_mir_build::builder::Builder>::as_temp_inner
  10: <rustc_mir_build::builder::Builder>::expr_into_dest
  11: <rustc_mir_build::builder::Builder>::expr_into_dest::{closure#0}
  12: <rustc_mir_build::builder::Builder>::expr_into_dest
  13: rustc_mir_build::builder::construct_fn
  14: rustc_mir_build::builder::build_mir
  15: rustc_mir_transform::mir_built
      [... omitted 1 frame ...]
  16: <rustc_mir_build::check_unsafety::UnsafetyVisitor>::visit_inner_body
  17: <rustc_mir_build::check_unsafety::UnsafetyVisitor as rustc_middle::thir::visit::Visitor>::visit_expr
  18: <rustc_mir_build::check_unsafety::UnsafetyVisitor as rustc_middle::thir::visit::Visitor>::visit_expr
  19: <rustc_mir_build::check_unsafety::UnsafetyVisitor as rustc_middle::thir::visit::Visitor>::visit_block
  20: <rustc_mir_build::check_unsafety::UnsafetyVisitor as rustc_middle::thir::visit::Visitor>::visit_expr
  21: <rustc_mir_build::check_unsafety::UnsafetyVisitor as rustc_middle::thir::visit::Visitor>::visit_expr
  22: rustc_mir_build::check_unsafety::check_unsafety
      [... omitted 1 frame ...]
  23: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  24: rustc_interface::passes::run_required_analyses
  25: rustc_interface::passes::analysis
      [... omitted 1 frame ...]
  26: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>
  27: 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/jisukbyun/workspace/250203 scratch/rustc-ice-2025-05-26T13_36_19-63737.txt` to your bug report

query stack during panic:
#0 [mir_built] building MIR for `move_into_fnmut::{closure#0}`
#1 [check_unsafety] unsafety-checking `move_into_fnmut`
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 1 previous error

Bisects to

cargo-bisect-rustc 0.6.9

command: cargo-bisect-rustc --start=2023-05-01 --end=2024-05-01 --regress ice --preserve --script rustc -- never_1.rs --edition=2021

********************************************************************************
Regression in nightly-2024-01-21
********************************************************************************

fetching https://static.rust-lang.org/dist/2024-01-20/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2024-01-20: 40 B / 40 B [=======================================================] 100.00 % 779.89 KB/s converted 2024-01-20 to 88189a71e4e4376eea82ac61db6a539612eb200a
fetching https://static.rust-lang.org/dist/2024-01-21/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2024-01-21: 40 B / 40 B [=======================================================] 100.00 % 761.38 KB/s converted 2024-01-21 to 4cb17b4e78e0540e49d2da884cc621a6bf6f47fa
looking for regression commit between 2024-01-20 and 2024-01-21
fetching (via remote github) commits from max(88189a71e4e4376eea82ac61db6a539612eb200a, 2024-01-18) to 4cb17b4e78e0540e49d2da884cc621a6bf6f47fa
ending github query because we found starting sha: 88189a71e4e4376eea82ac61db6a539612eb200a
get_commits_between returning commits, len: 11
  commit[0] 2024-01-19: Auto merge of #120123 - lcnr:sadboi-compat, r=jackh726
  commit[1] 2024-01-20: Auto merge of #116672 - maurer:128-align, r=nikic
  commit[2] 2024-01-20: Auto merge of #120136 - matthiaskrgr:rollup-3zzb0z9, r=matthiaskrgr
  commit[3] 2024-01-20: Auto merge of #119821 - oli-obk:reveal_all_const_evals, r=lcnr
  commit[4] 2024-01-20: Auto merge of #120157 - matthiaskrgr:rollup-f0p3wkk, r=matthiaskrgr
  commit[5] 2024-01-20: Auto merge of #120003 - Mark-Simulacrum:opt-promoted, r=davidtwco
  commit[6] 2024-01-20: Auto merge of #116185 - Zoxc:rem-one-thread, r=cjgillot
  commit[7] 2024-01-20: Auto merge of #108359 - Zoxc:side-effects-tweak, r=cjgillot
  commit[8] 2024-01-20: Auto merge of #117756 - a1phyr:hashmap_fold, r=the8472
  commit[9] 2024-01-20: Auto merge of #120170 - GuillaumeGomez:rollup-edqdf30, r=GuillaumeGomez
  commit[10] 2024-01-20: Auto merge of #111803 - scottmcm:simple-swap-alternative, r=Mark-Simulacrum
ERROR: no CI builds available between 88189a71e4e4376eea82ac61db6a539612eb200a and 4cb17b4e78e0540e49d2da884cc621a6bf6f47fa within last 167 days

I guess it bisects to PR-120136, which includes PR-120009.

Notes

• ICE location: compiler/rustc_mir_build/src/builder/matches/mod.rs Line-2110
  

  rust/compiler/rustc_mir_build/src/builder/matches/mod.rs

  Lines 2105 to 2114 in 283db70

  	fn pick_test(&mut self, candidates: &[&mut Candidate<'tcx>]) -> (Place<'tcx>, Test<'tcx>) {
  	// Extract the match-pair from the highest priority candidate
  	let match_pair = &candidates[0].match_pairs[0];
  	let test = self.pick_test_for_match_pair(match_pair);
  	// Unwrap is ok after simplification.
  	let match_place = match_pair.place.unwrap();
  	debug!(?test, ?match_pair);
  	(match_place, test)
  	}

• internal compiler error: two identical projections #137467 has the same ICE location and query stack but does not use the never_patterns feature. I'm not sure if this report is a duplicate of that.

• In the case of never_patterns, None in rustc_mir_build/src/builder/matches/mod.rs #136852 also shows the same ICE location.

@rustbot label +F-never_patterns

[matthiaskrgr]

Yeah this looks like a duplicate of #137467 (comment)

in general, if you take a pattern that is already know to crash the compiler, then add another feature (making the crash depend on that presence of this feature) and file that as a different ice as "the other one did not need the feature", I don't think this is very helpful 😅

[cushionbadak]

Got it. I’ll be more careful not to submit cases like this in the future.

I posted this one because the ICE in issue-137467 happens with an or pattern on the LHS of a let binding, while this report involves the never_patterns feature, so I thought they might be different in terms of the actual cause.