ICE : `not yet implemented: Iterator is not yet const`

[trait-Cong]

I tried this code:

//@compile-flags: -Znext-solver=globally
trait KeySchema {
    const KEYS: & [& str];
    
}
const fn construct_successor_array<T: KeySchema, >() -> [& str; compute_successor_size::<T>()] {
    let tail_keys = T::KEYS;
    for (i, ) in tail_keys.iter().enumerate() {
    }
}
fn main(){}

Meta

rustc --version --verbose:

rustc 1.95.0-nightly (6a979b3e3 2026-02-26)
binary: rustc
commit-hash: 6a979b3e32522049d0acb4a47f7ae44b7c8abfd5
commit-date: 2026-02-26
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 22.1.0

Backtrace

thread 'rustc' panicked at /rustc-dev/6a979b3e32522049d0acb4a47f7ae44b7c8abfd5/compiler/rustc_next_trait_solver/src/solve/effect_goals.rs:358:9:
not yet implemented: Iterator is not yet const
stack backtrace:
   0:     0x7f9071dba08b - <std[a42f3b87e642314d]::backtrace::Backtrace>::create
   1:     0x7f9071db9fd5 - <std[a42f3b87e642314d]::backtrace::Backtrace>::force_capture
   2:     0x7f9070db33b4 - std[a42f3b87e642314d]::panicking::update_hook::<alloc[bf50147f3a277199]::boxed::Box<rustc_driver_impl[b2dff6da787942eb]::install_ice_hook::{closure#1}>>::{closure#0}
   3:     0x7f9071dcc602 - std[a42f3b87e642314d]::panicking::panic_with_hook
   4:     0x7f9071daf08a - std[a42f3b87e642314d]::panicking::panic_handler::{closure#0}
   5:     0x7f9071da35a9 - std[a42f3b87e642314d]::sys::backtrace::__rust_end_short_backtrace::<std[a42f3b87e642314d]::panicking::panic_handler::{closure#0}, !>
   6:     0x7f9071db0abd - __rustc[12c998373637a003]::rust_begin_unwind
   7:     0x7f906eb7df9c - core[fe7c17ffc1da903f]::panicking::panic_fmt
   8:     0x7f90739f6889 - <rustc_next_trait_solver[8d952b094deec32a]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate, rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>::assemble_and_evaluate_candidates::<rustc_type_ir[379519b7c7821119]::predicate::HostEffectPredicate<rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>
   9:     0x7f90739f5897 - <rustc_next_trait_solver[8d952b094deec32a]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate, rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>::compute_host_effect_goal
  10:     0x7f9073389534 - <rustc_next_trait_solver[8d952b094deec32a]::solve::search_graph::SearchGraphDelegate<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate> as rustc_type_ir[379519b7c7821119]::search_graph::Delegate>::compute_goal::{closure#0}
  11:     0x7f907338776b - <rustc_type_ir[379519b7c7821119]::search_graph::SearchGraph<rustc_next_trait_solver[8d952b094deec32a]::solve::search_graph::SearchGraphDelegate<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate>, rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>::evaluate_goal::{closure#0}::{closure#2}
  12:     0x7f9072e771b9 - <rustc_next_trait_solver[8d952b094deec32a]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate, rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>::evaluate_goal_raw
  13:     0x7f907306ae06 - <rustc_next_trait_solver[8d952b094deec32a]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate, rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>::try_evaluate_added_goals
  14:     0x7f9072e855f1 - <rustc_next_trait_solver[8d952b094deec32a]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate, rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>::evaluate_added_goals_and_make_canonical_response
  15:     0x7f90739ee25d - <rustc_next_trait_solver[8d952b094deec32a]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate, rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>::assemble_impl_candidates::<rustc_type_ir[379519b7c7821119]::predicate::HostEffectPredicate<rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>::{closure#0}
  16:     0x7f90739f7653 - <rustc_next_trait_solver[8d952b094deec32a]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate, rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>::assemble_and_evaluate_candidates::<rustc_type_ir[379519b7c7821119]::predicate::HostEffectPredicate<rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>
  17:     0x7f90739f5897 - <rustc_next_trait_solver[8d952b094deec32a]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate, rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>::compute_host_effect_goal
  18:     0x7f9073389534 - <rustc_next_trait_solver[8d952b094deec32a]::solve::search_graph::SearchGraphDelegate<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate> as rustc_type_ir[379519b7c7821119]::search_graph::Delegate>::compute_goal::{closure#0}
  19:     0x7f907338776b - <rustc_type_ir[379519b7c7821119]::search_graph::SearchGraph<rustc_next_trait_solver[8d952b094deec32a]::solve::search_graph::SearchGraphDelegate<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate>, rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>::evaluate_goal::{closure#0}::{closure#2}
  20:     0x7f9072e771b9 - <rustc_next_trait_solver[8d952b094deec32a]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d548f1357b902480]::solve::delegate::SolverDelegate, rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>>::evaluate_goal_raw
  21:     0x7f90730659e5 - <rustc_trait_selection[d548f1357b902480]::solve::fulfill::FulfillmentCtxt<rustc_trait_selection[d548f1357b902480]::traits::FulfillmentError> as rustc_infer[7061af908568ad8f]::traits::engine::TraitEngine<rustc_trait_selection[d548f1357b902480]::traits::FulfillmentError>>::try_evaluate_obligations
  22:     0x7f9072820655 - <rustc_hir_typeck[c30244052031a4ef]::fn_ctxt::FnCtxt>::check_expr_call
  23:     0x7f9072803f50 - <rustc_hir_typeck[c30244052031a4ef]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  24:     0x7f90727dd4e5 - <rustc_hir_typeck[c30244052031a4ef]::fn_ctxt::FnCtxt>::check_expr_match
  25:     0x7f907280533b - <rustc_hir_typeck[c30244052031a4ef]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  26:     0x7f9072805915 - <rustc_hir_typeck[c30244052031a4ef]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  27:     0x7f90727fe170 - <rustc_hir_typeck[c30244052031a4ef]::fn_ctxt::FnCtxt>::check_expr_block
  28:     0x7f9072803ed7 - <rustc_hir_typeck[c30244052031a4ef]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  29:     0x7f90727d2c7c - rustc_hir_typeck[c30244052031a4ef]::check::check_fn
  30:     0x7f907322fe34 - rustc_hir_typeck[c30244052031a4ef]::typeck_with_inspect::{closure#0}
  31:     0x7f907322e8b2 - rustc_query_impl[d8d8891b690f31a]::query_impl::typeck::invoke_provider_fn::__rust_begin_short_backtrace
  32:     0x7f907259698a - rustc_query_impl[d8d8891b690f31a]::execution::try_execute_query::<rustc_data_structures[8d05224b8dc85cc0]::vec_cache::VecCache<rustc_span[6ddfab8b4d347fa5]::def_id::LocalDefId, rustc_middle[11b273479f5b7c30]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[11b273479f5b7c30]::dep_graph::graph::DepNodeIndex>, false>
  33:     0x7f90725963c3 - rustc_query_impl[d8d8891b690f31a]::query_impl::typeck::execute_query_non_incr::__rust_end_short_backtrace
  34:     0x7f90729ec948 - <rustc_middle[11b273479f5b7c30]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis[eea337f167820684]::check_crate::{closure#2}>::{closure#0}
  35:     0x7f90729ec369 - rustc_hir_analysis[eea337f167820684]::check_crate
  36:     0x7f9072593393 - rustc_interface[454e58b8a783baef]::passes::analysis
  37:     0x7f90736673da - rustc_query_impl[d8d8891b690f31a]::execution::try_execute_query::<rustc_middle[11b273479f5b7c30]::query::caches::SingleCache<rustc_middle[11b273479f5b7c30]::query::erase::ErasedData<[u8; 0usize]>>, false>
  38:     0x7f90736671a9 - rustc_query_impl[d8d8891b690f31a]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  39:     0x7f90736830ad - rustc_interface[454e58b8a783baef]::interface::run_compiler::<(), rustc_driver_impl[b2dff6da787942eb]::run_compiler::{closure#0}>::{closure#1}
  40:     0x7f90736544fe - std[a42f3b87e642314d]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[454e58b8a783baef]::util::run_in_thread_with_globals<rustc_interface[454e58b8a783baef]::util::run_in_thread_pool_with_globals<rustc_interface[454e58b8a783baef]::interface::run_compiler<(), rustc_driver_impl[b2dff6da787942eb]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  41:     0x7f9073654b60 - <std[a42f3b87e642314d]::thread::lifecycle::spawn_unchecked<rustc_interface[454e58b8a783baef]::util::run_in_thread_with_globals<rustc_interface[454e58b8a783baef]::util::run_in_thread_pool_with_globals<rustc_interface[454e58b8a783baef]::interface::run_compiler<(), rustc_driver_impl[b2dff6da787942eb]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[fe7c17ffc1da903f]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  42:     0x7f90736559ec - <std[a42f3b87e642314d]::sys::thread::unix::Thread>::new::thread_start
  43:     0x7f906d1de609 - start_thread
                               at /build/glibc-B3wQXB/glibc-2.31/nptl/pthread_create.c:477:8
  44:     0x7f906d103353 - clone
                               at /build/glibc-B3wQXB/glibc-2.31/misc/../sysdeps/unix/sysv/linux/x86_64/clone.S:95:0
  45:                0x0 - <unknown>


rustc version: 1.95.0-nightly (6a979b3e3 2026-02-26)
platform: x86_64-unknown-linux-gnu

query stack during panic:
#0 [typeck] type-checking `construct_successor_array`
#1 [analysis] running analysis passes on crate `bug`
end of query stack

@rustbot label +I-ICE +WG-trait-system-refactor

[matthiaskrgr]

duplicate of #151901

[RodBurman]

I tried building the example code using:

 % cargo -vV             
cargo 1.96.0 (30a34c682 2026-05-25)
release: 1.96.0
commit-hash: 30a34c6821b57de0aaec83a901aca39f88f6778c
commit-date: 2026-05-25
host: aarch64-apple-darwin
libgit2: 1.9.2 (sys:0.20.4 vendored)
libcurl: 8.7.1 (sys:0.4.87+curl-8.19.0 system ssl:(SecureTransport) LibreSSL/3.3.6)
ssl: OpenSSL 3.5.4 30 Sep 2025
os: Mac OS 26.5.1 [64-bit]

Which produced:

% cargo build
   Compiling iss_153195 v0.1.0 (/Users/rod/code/rust/triage/2026/06/iss_153195)
error: generic parameters may not be used in const operations
 --> src/main.rs:6:90
  |
6 | const fn construct_successor_array<T: KeySchema, >() -> [& str; compute_successor_size::<T>()] {
  |                                                                                          ^ cannot perform const operation using `T`
  |
  = note: type parameters may not be used in const expressions

error[E0106]: missing lifetime specifier
 --> src/main.rs:6:58
  |
6 | const fn construct_successor_array<T: KeySchema, >() -> [& str; compute_successor_size::<T>()] {
  |                                                          ^ expected named lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
  |
6 | const fn construct_successor_array<T: KeySchema, >() -> [&'static  str; compute_successor_size::<T>()] {
  |                                                           +++++++
help: instead, you are more likely to want to return an owned value
  |
6 - const fn construct_successor_array<T: KeySchema, >() -> [& str; compute_successor_size::<T>()] {
6 + const fn construct_successor_array<T: KeySchema, >() -> [String; compute_successor_size::<T>()] {
  |

error[E0425]: cannot find function `compute_successor_size` in this scope
 --> src/main.rs:6:65
  |
6 | const fn construct_successor_array<T: KeySchema, >() -> [& str; compute_successor_size::<T>()] {
  |                                                                 ^^^^^^^^^^^^^^^^^^^^^^ not found in this scope

error[E0308]: mismatched types
 --> src/main.rs:8:9
  |
8 |     for (i, ) in tail_keys.iter().enumerate() {
  |         ^^^^^    ---------------------------- this is an iterator with items of type `(usize, &&str)`
  |         |
  |         expected a tuple with 2 elements, found one with 1 element
  |
  = note: expected tuple `(usize, &&str)`
             found tuple `(_,)`

Some errors have detailed explanations: E0106, E0308, E0425.
For more information about an error, try `rustc --explain E0106`.
error: could not compile `iss_153195` (bin "iss_153195") due to 4 previous errors

So the ICE has disappeared, though the code till does not compile.
Similarly I tried the code from the duplicate issue #151901:

//@compile-flags: -Znext-solver=globally
const { for _ in 1.. {} }

Which produced:

error[E0658]: const block items are experimental
 --> lib.rs:2:1
  |
2 | const { for _ in 1.. {} }
  | ^^^^^
  |
  = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information

error[E0015]: cannot use `for` loop on `std::ops::RangeFrom<i32>` in constants
 --> lib.rs:2:18
  |
2 | const { for _ in 1.. {} }
  |                  ^^^
  |
  = note: calls in constants are limited to constant functions, tuple structs and tuple variants

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0015, E0658.
For more information about an error, try `rustc --explain E0015`.

So again the compiler did not crash but the code had errors.

[lcnr]

the issue has been fixed. Given the underlying bug, I don't think we need to add a test here. Iterator is const now i'd assume🤣