ICE `error performing operation: fully_perform`

[RedRam567]

ICE when const {} initializing mutable references in [expr; N] array initializers where N is >= 2. Does not happen when setting every element explicitly [expr, expr]. Interestingly you cannot use const { &mut[] } directly and must use a function call to get around it. I will make another issue for that.

Related issue: "Compile error when using &mut in explicity const context directly" #140126

Code

// error[E0764]: mutable references are not allowed in the final value of constants
// pub const NO_COMPILE: [&mut [f32]; 2] = [const { &mut[] }; 2];
const OK: [&mut [()]; 2] = [empty_mut(), empty_mut()];
const ICE: [&mut [()]; 2] = [const { empty_mut() }; 2];

// Any kind of fn call gets around E0764.
const fn empty_mut() -> &'static mut [()] {
    &mut []
}

Meta

Occurs in version >= 1.83.0. Older versions fail to compile normally due to mutable references in const.

rustc --version --verbose:

rustc 1.88.0-nightly (b8c54d635 2025-04-20)
binary: rustc
commit-hash: b8c54d6358926028ac2fab1ec2b8665c70edb1c0
commit-date: 2025-04-20
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2

Error output

   Compiling rust_playground2 v0.1.0 (/tmp/rust_playground2)
note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: error performing operation: fully_perform
 --> src/lib.rs:1:34
  |
1 | pub const ICE: [&mut [f32]; 2] = [const { empty_mut() }; 2];
  |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: delayed at /rustc/b8c54d6358926028ac2fab1ec2b8665c70edb1c0/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs:87:25 - disabled backtrace
 --> src/lib.rs:1:34
  |
1 | pub const ICE: [&mut [f32]; 2] = [const { empty_mut() }; 2];
  |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^

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 `/tmp/rust_playground2/rustc-ice-2025-04-21T16_27_11-21059.txt` to your bug report

note: compiler flags: --crate-type lib -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED]

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
error: could not compile `rust_playground2` (lib)

Caused by:
  process didn't exit successfully: `/home/redram/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --crate-name rust_playground2 --edition=2024 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=1f5395c8c0077482 -C extra-filename=-be321adee2c2c0a3 --out-dir /mnt/extra/cargo_target/debug/deps -C incremental=/mnt/extra/cargo_target/debug/incremental -L dependency=/mnt/extra/cargo_target/debug/deps` (exit status: 101)

Backtrace

delayed bug: error performing operation: fully_perform
   0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
   1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
   2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
   3: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, alloc::string::String>
   4: <rustc_borrowck::type_check::TypeChecker>::prove_trait_ref
   5: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_rvalue
   6: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_body
   7: rustc_borrowck::do_mir_borrowck
   8: rustc_borrowck::mir_borrowck
   9: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
  10: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 8]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, true>
  11: rustc_query_impl::query_impl::mir_borrowck::get_query_incr::__rust_end_short_backtrace
  12: rustc_mir_transform::mir_drops_elaborated_and_const_checked
  13: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::mir_drops_elaborated_and_const_checked::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
  14: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 8]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, true>
  15: rustc_query_impl::query_impl::mir_drops_elaborated_and_const_checked::get_query_incr::__rust_end_short_backtrace
  16: rustc_mir_transform::mir_for_ctfe
  17: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::mir_for_ctfe::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
  18: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::Erased<[u8; 8]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, true>
  19: rustc_query_impl::query_impl::mir_for_ctfe::get_query_incr::__rust_end_short_backtrace
  20: <rustc_const_eval::interpret::eval_context::InterpCx<rustc_const_eval::const_eval::machine::CompileTimeMachine>>::load_mir
  21: rustc_const_eval::const_eval::eval_queries::eval_to_allocation_raw_provider
  22: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::eval_to_allocation_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 24]>>
  23: <rustc_query_impl::query_impl::eval_to_allocation_raw::dynamic_query::{closure#2} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, rustc_middle::ty::PseudoCanonicalInput<rustc_middle::mir::interpret::GlobalId>)>>::call_once
  24: <rustc_query_system::query::plumbing::execute_job_incr<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::PseudoCanonicalInput<rustc_middle::mir::interpret::GlobalId>, rustc_middle::query::erase::Erased<[u8; 24]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt>::{closure#2}::{closure#2} as core::ops::function::FnOnce<((rustc_query_impl::plumbing::QueryCtxt, rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::PseudoCanonicalInput<rustc_middle::mir::interpret::GlobalId>, rustc_middle::query::erase::Erased<[u8; 24]>>, false, false, false>), rustc_middle::ty::PseudoCanonicalInput<rustc_middle::mir::interpret::GlobalId>)>>::call_once
  25: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::PseudoCanonicalInput<rustc_middle::mir::interpret::GlobalId>, rustc_middle::query::erase::Erased<[u8; 24]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, true>
  26: rustc_query_impl::query_impl::eval_to_allocation_raw::get_query_incr::__rust_end_short_backtrace
  27: rustc_const_eval::const_eval::eval_queries::eval_to_const_value_raw_provider
  28: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::eval_to_const_value_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 24]>>
  29: <rustc_query_impl::query_impl::eval_to_const_value_raw::dynamic_query::{closure#2} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, rustc_middle::ty::PseudoCanonicalInput<rustc_middle::mir::interpret::GlobalId>)>>::call_once
  30: <rustc_query_system::query::plumbing::execute_job_incr<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::PseudoCanonicalInput<rustc_middle::mir::interpret::GlobalId>, rustc_middle::query::erase::Erased<[u8; 24]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt>::{closure#2}::{closure#2} as core::ops::function::FnOnce<((rustc_query_impl::plumbing::QueryCtxt, rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::PseudoCanonicalInput<rustc_middle::mir::interpret::GlobalId>, rustc_middle::query::erase::Erased<[u8; 24]>>, false, false, false>), rustc_middle::ty::PseudoCanonicalInput<rustc_middle::mir::interpret::GlobalId>)>>::call_once
  31: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::PseudoCanonicalInput<rustc_middle::mir::interpret::GlobalId>, rustc_middle::query::erase::Erased<[u8; 24]>>, false, true, false>, rustc_query_impl::plumbing::QueryCtxt, true>
  32: rustc_query_impl::query_impl::eval_to_const_value_raw::get_query_incr::__rust_end_short_backtrace
  33: rustc_hir_analysis::check_crate
  34: rustc_interface::passes::run_required_analyses
  35: rustc_interface::passes::analysis
  36: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 0]>>
  37: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, true>
  38: rustc_query_impl::query_impl::analysis::get_query_incr::__rust_end_short_backtrace
  39: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  40: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
  41: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  42: <<std::thread::Builder>::spawn_unchecked_<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  43: std::sys::pal::unix::thread::Thread::new::thread_start
  44: <unknown>
  45: <unknown>



rustc version: 1.88.0-nightly (b8c54d635 2025-04-20)
platform: x86_64-unknown-linux-gnu

[compiler-errors]

// MIR for `ICE` after built

const ICE: [&mut [()]; 2] = {
    let mut _0: [&mut [()]; 2];
    let mut _1: &mut [()];
    let mut _2: &mut [()];

    bb0: {
        StorageLive(_1);
        StorageLive(_2);
        _2 = const ICE::{constant#1};
        _1 = &mut (*_2);
        _0 = [move _1; 2];
        StorageDead(_2);
        StorageDead(_1);
        return;
    }
}

We're reborrowing the &mut [()] argument at _1 = &mut (*_2), b/c we always reborrow unsized mut refs in coercion I think.

This could be fixed, but we still have other operations that still cause ICEs like real unsize coercions:

trait Trait {}

impl Trait for [(); 0] {}

const ICE: [&mut dyn Trait; 2] = [const { empty_mut() }; 2];

const fn empty_mut() -> &'static mut [(); 0] {
    &mut []
}

Which also leads to an ICE:

// MIR for `ICE` after built

const ICE: [&mut dyn Trait; 2] = {
    let mut _0: [&mut dyn Trait; 2];
    let mut _1: &mut dyn Trait;
    let mut _2: &mut dyn Trait;
    let mut _3: &mut dyn Trait;

    bb0: {
        StorageLive(_1);
        StorageLive(_2);
        StorageLive(_3);
        _3 = const ICE::{constant#1};
        _2 = &mut (*_3);
        _1 = move _2 as &mut dyn Trait (PointerCoercion(Unsize, Implicit));
        StorageDead(_2);
        _0 = [move _1; 2];
        StorageDead(_3);
        StorageDead(_1);
        return;
    }
}

[RedRam567]

This also ICEs when using std::slice::from_raw_parts_mut instead.

const ICE: [&mut [()]; 2] = [const { empty_mut() }; 2];

const fn empty_mut() -> &'static mut [()] {
    unsafe {
        std::slice::from_raw_parts_mut(std::ptr::dangling_mut(), 0)
    }
}

[theemathas]

A variant that doesn't involve unsized coercions:

const ICE: [&mut [(); 0]; 2] = [const { empty_mut() }; 2];

const fn empty_mut() -> &'static mut [(); 0] {
    &mut []
}

[theemathas]

A variant without a function call:

const ICE: [&mut [(); 0]; 2] = [const {
    let x = &mut [];
    x
}; 2];

[moxian]

@rustbot label: +A-const-eval +S-has-mcve -needs-triage