ICE: upvar: assertion failed: `1 == 2` `-Wrust-2021-incompatible-closure-captures`

[matthiaskrgr]

auto-reduced (treereduce-rust):

//@compile-flags: -Wrust-2021-incompatible-closure-captures
enum Either {
    One(XorShiftRng),
    Two(X),
}

struct X;

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

original:

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

struct X;

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

fn main() { }

Version information

rustc 1.88.0-nightly (1f76d219c 2025-04-17)
binary: rustc
commit-hash: 1f76d219c906f0112bb1872f33aa977164c53fa6
commit-date: 2025-04-17
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2

Possibly related line of code:

rust/compiler/rustc_hir_typeck/src/upvar.rs

Lines 1610 to 1622 in 1f76d21

	ty::Ref(..) => unreachable!(),
	ty::RawPtr(..) => unreachable!(),
	ty::Adt(def, args) => {
	// Multi-variant enums are captured in entirety,
	// which would've been handled in the case of single empty slice in `captured_by_move_projs`.
	assert_eq!(def.variants().len(), 1);
	// Only Field projections can be applied to a non-box Adt.
	assert!(
	captured_by_move_projs.iter().all(|projs| matches!(
	projs.first().unwrap().kind,
	ProjectionKind::Field(..)

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Wrust-2021-incompatible-closure-captures

Program output

error[E0412]: cannot find type `XorShiftRng` in this scope
 --> /tmp/icemaker_global_tempdir.mjs3HhQOcIiO/rustc_testrunner_tmpdir_reporting.JVHxPnPMCvzx/mvce.rs:2:9
  |
2 |     One(XorShiftRng),
  |         ^^^^^^^^^^^ not found in this scope

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.mjs3HhQOcIiO/rustc_testrunner_tmpdir_reporting.JVHxPnPMCvzx/mvce.rs:13:2
   |
13 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.mjs3HhQOcIiO/rustc_testrunner_tmpdir_reporting.JVHxPnPMCvzx/mvce.rs`


thread 'rustc' panicked at compiler/rustc_hir_typeck/src/upvar.rs:1616:17:
assertion `left == right` failed
  left: 2
 right: 1
stack backtrace:
   0:     0x74c9c96c4bf3 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h72d2f767860f01c8
   1:     0x74c9c9e05b07 - core::fmt::write::ha0040902920efb26
   2:     0x74c9cb297351 - std::io::Write::write_fmt::h92d425739cf7f0f8
   3:     0x74c9c96c4a52 - std::sys::backtrace::BacktraceLock::print::h60480fc7af7284f9
   4:     0x74c9c96c83ca - std::panicking::default_hook::{{closure}}::h0210b2339a694596
   5:     0x74c9c96c7f4f - std::panicking::default_hook::h26e462c1b992a8d3
   6:     0x74c9c8745753 - std[5360fa45f3745eb7]::panicking::update_hook::<alloc[3e73228144efd5c3]::boxed::Box<rustc_driver_impl[9f43b67c13dcf142]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x74c9c96c8c43 - std::panicking::rust_panic_with_hook::h9046e71ecea68928
   8:     0x74c9c96c893a - std::panicking::begin_panic_handler::{{closure}}::hb8579fcd3517556a
   9:     0x74c9c96c50c9 - std::sys::backtrace::__rust_end_short_backtrace::h7c2200761e8b2677
  10:     0x74c9c96c85fd - __rustc[5b4c3b5a2bd13629]::rust_begin_unwind
  11:     0x74c9c60863c0 - core::panicking::panic_fmt::h39077f0ed79e3d52
  12:     0x74c9c80c3eca - core::panicking::assert_failed_inner::hd76d8c96d70011e6
  13:     0x74c9c80c3df9 - core::panicking::assert_failed::he4657e61c1bba572
  14:     0x74c9c8af5f9a - <rustc_hir_typeck[19dd2fb0ef174692]::fn_ctxt::FnCtxt>::has_significant_drop_outside_of_captures
  15:     0x74c9c8af1fc4 - <rustc_hir_typeck[19dd2fb0ef174692]::fn_ctxt::FnCtxt>::perform_2229_migration_analysis
  16:     0x74c9ca6e0807 - <rustc_hir_typeck[19dd2fb0ef174692]::fn_ctxt::FnCtxt>::analyze_closure
  17:     0x74c9ca6b0052 - <rustc_hir_typeck[19dd2fb0ef174692]::upvar::InferBorrowKindVisitor as rustc_hir[5929cbe382679e99]::intravisit::Visitor>::visit_expr
  18:     0x74c9ca6aff58 - <rustc_hir_typeck[19dd2fb0ef174692]::upvar::InferBorrowKindVisitor as rustc_hir[5929cbe382679e99]::intravisit::Visitor>::visit_block
  19:     0x74c9ca6b332a - rustc_hir_typeck[19dd2fb0ef174692]::typeck_with_inspect::{closure#0}
  20:     0x74c9ca6b19b6 - rustc_query_impl[5554cc5a49e13358]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[5554cc5a49e13358]::query_impl::typeck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[be3fd842b5d62089]::query::erase::Erased<[u8; 8usize]>>
  21:     0x74c9ca071e6c - rustc_query_system[6a51e1997cb665e8]::query::plumbing::try_execute_query::<rustc_query_impl[5554cc5a49e13358]::DynamicConfig<rustc_data_structures[c6be5df131b5adc5]::vec_cache::VecCache<rustc_span[34967910c7e58b20]::def_id::LocalDefId, rustc_middle[be3fd842b5d62089]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[6a51e1997cb665e8]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[5554cc5a49e13358]::plumbing::QueryCtxt, false>
  22:     0x74c9ca0718cb - rustc_query_impl[5554cc5a49e13358]::query_impl::typeck::get_query_non_incr::__rust_end_short_backtrace
  23:     0x74c9ca070348 - rustc_hir_analysis[4ccd349711354d63]::check_crate
  24:     0x74c9ca1fc074 - rustc_interface[ea4bb220888d6441]::passes::run_required_analyses
  25:     0x74c9cacc229e - rustc_interface[ea4bb220888d6441]::passes::analysis
  26:     0x74c9cacc2273 - rustc_query_impl[5554cc5a49e13358]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[5554cc5a49e13358]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[be3fd842b5d62089]::query::erase::Erased<[u8; 0usize]>>
  27:     0x74c9cacc7d3d - rustc_query_system[6a51e1997cb665e8]::query::plumbing::try_execute_query::<rustc_query_impl[5554cc5a49e13358]::DynamicConfig<rustc_query_system[6a51e1997cb665e8]::query::caches::SingleCache<rustc_middle[be3fd842b5d62089]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[5554cc5a49e13358]::plumbing::QueryCtxt, false>
  28:     0x74c9cacc7a38 - rustc_query_impl[5554cc5a49e13358]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  29:     0x74c9caf4e0fe - rustc_interface[ea4bb220888d6441]::passes::create_and_enter_global_ctxt::<core[6a8fd511579d4332]::option::Option<rustc_interface[ea4bb220888d6441]::queries::Linker>, rustc_driver_impl[9f43b67c13dcf142]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  30:     0x74c9caf3c484 - rustc_interface[ea4bb220888d6441]::interface::run_compiler::<(), rustc_driver_impl[9f43b67c13dcf142]::run_compiler::{closure#0}>::{closure#1}
  31:     0x74c9caf2cab4 - std[5360fa45f3745eb7]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[ea4bb220888d6441]::util::run_in_thread_with_globals<rustc_interface[ea4bb220888d6441]::util::run_in_thread_pool_with_globals<rustc_interface[ea4bb220888d6441]::interface::run_compiler<(), rustc_driver_impl[9f43b67c13dcf142]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  32:     0x74c9caf2cef4 - <<std[5360fa45f3745eb7]::thread::Builder>::spawn_unchecked_<rustc_interface[ea4bb220888d6441]::util::run_in_thread_with_globals<rustc_interface[ea4bb220888d6441]::util::run_in_thread_pool_with_globals<rustc_interface[ea4bb220888d6441]::interface::run_compiler<(), rustc_driver_impl[9f43b67c13dcf142]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[6a8fd511579d4332]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  33:     0x74c9caf2e2eb - std::sys::pal::unix::thread::Thread::new::thread_start::h7e071d267c3a6526
  34:     0x74c9c4ca370a - <unknown>
  35:     0x74c9c4d27aac - <unknown>
  36:                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 1.88.0-nightly (1f76d219c 2025-04-17) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z dump-mir-dir=dir

query stack during panic:
#0 [typeck] type-checking `move_into_fnmut`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 2 previous errors

Some errors have detailed explanations: E0412, E0601.
For more information about an error, try `rustc --explain E0412`.

[matthiaskrgr]

smaller:

enum b {
    c(d),
    e(f),
}
struct f;
fn g() {
    let h;
    || b::e(a) = h;
}