Const generic default ICE: The type checker should prevent reading from a never-written local

[DutchGhost]

Const generic defaults are being stabilized, #90207. But I found an issue that occured earlier in the length expressions of arrays, see #72115

Code

struct X<const N: usize = {
    let s: &str; s.len()
}>;

Meta

rustc --version --verbose:

rustc 1.60.0-nightly (4e8fb743c 2022-02-03)

Error output

error: internal compiler error: compiler/rustc_const_eval/src/interpret/eval_context.rs:202:17: The type checker should prevent reading from a never-written local

Backtrace

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1160:9
stack backtrace:
   0:     0x7fd976ea7ebc - std::backtrace_rs::backtrace::libunwind::trace::hddd5a7e459ce4327
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7fd976ea7ebc - std::backtrace_rs::backtrace::trace_unsynchronized::h69db3e09cfd8bf11
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7fd976ea7ebc - std::sys_common::backtrace::_print_fmt::h83b461b46804fdab
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/sys_common/backtrace.rs:66:5
   3:     0x7fd976ea7ebc - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h2f6e5cd8872f3d74
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/sys_common/backtrace.rs:45:22
   4:     0x7fd976f092dc - core::fmt::write::hd64a00f3787c90bd
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/core/src/fmt/mod.rs:1190:17
   5:     0x7fd976e97383 - std::io::Write::write_fmt::h6347402c88d40325
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/io/mod.rs:1653:15
   6:     0x7fd976eac181 - std::sys_common::backtrace::_print::h989064c3e0b39e3c
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/sys_common/backtrace.rs:48:5
   7:     0x7fd976eac181 - std::sys_common::backtrace::print::h4af8c55ce731a045
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/sys_common/backtrace.rs:35:9
   8:     0x7fd976eac181 - std::panicking::default_hook::{{closure}}::h41ef6f6a9fac6b53
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/panicking.rs:295:22
   9:     0x7fd976eabe3f - std::panicking::default_hook::hbaf11e031e0e4a5f
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/panicking.rs:314:9
  10:     0x7fd977689541 - rustc_driver[7b59b981d4c23d31]::DEFAULT_HOOK::{closure#0}::{closure#0}
  11:     0x7fd976eaca5b - std::panicking::rust_panic_with_hook::h4788f0bd625b1005
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/panicking.rs:702:17
  12:     0x7fd97874d281 - std[daefdbcf0dca51d0]::panicking::begin_panic::<rustc_errors[b77837aed2b0d3e7]::ExplicitBug>::{closure#0}
  13:     0x7fd97874d1f6 - std[daefdbcf0dca51d0]::sys_common::backtrace::__rust_end_short_backtrace::<std[daefdbcf0dca51d0]::panicking::begin_panic<rustc_errors[b77837aed2b0d3e7]::ExplicitBug>::{closure#0}, !>
  14:     0x7fd978751a2f - std[daefdbcf0dca51d0]::panicking::begin_panic::<rustc_errors[b77837aed2b0d3e7]::ExplicitBug>
  15:     0x7fd97875e0cd - std[daefdbcf0dca51d0]::panic::panic_any::<rustc_errors[b77837aed2b0d3e7]::ExplicitBug>
  16:     0x7fd97875d37d - <rustc_errors[b77837aed2b0d3e7]::HandlerInner>::bug
  17:     0x7fd97875ced0 - <rustc_errors[b77837aed2b0d3e7]::Handler>::bug
  18:     0x7fd9785e36d9 - rustc_middle[e664fb30aec2b69e]::ty::context::tls::with_opt::<rustc_middle[e664fb30aec2b69e]::util::bug::opt_span_bug_fmt<rustc_span[26d1c5a8904e1196]::span_encoding::Span>::{closure#0}, ()>
  19:     0x7fd9785e4830 - rustc_middle[e664fb30aec2b69e]::util::bug::opt_span_bug_fmt::<rustc_span[26d1c5a8904e1196]::span_encoding::Span>
  20:     0x7fd9785e47a6 - rustc_middle[e664fb30aec2b69e]::util::bug::bug_fmt
  21:     0x7fd978e7fcf6 - <rustc_const_eval[55e19033940159c7]::interpret::eval_context::InterpCx<rustc_const_eval[55e19033940159c7]::const_eval::machine::CompileTimeInterpreter>>::run
  22:     0x7fd978ebed71 - rustc_const_eval[55e19033940159c7]::const_eval::eval_queries::eval_to_allocation_raw_provider
  23:     0x7fd978f40a9c - rustc_query_system[59fab566a51c2c24]::query::plumbing::try_execute_query::<rustc_query_impl[ae50953882263c5d]::plumbing::QueryCtxt, rustc_query_system[59fab566a51c2c24]::query::caches::DefaultCache<rustc_middle[e664fb30aec2b69e]::ty::ParamEnvAnd<rustc_middle[e664fb30aec2b69e]::mir::interpret::GlobalId>, core[90c82cfbe27f5d33]::result::Result<rustc_middle[e664fb30aec2b69e]::mir::interpret::value::ConstAlloc, rustc_middle[e664fb30aec2b69e]::mir::interpret::error::ErrorHandled>>>
  24:     0x7fd978fb1ebf - <rustc_query_impl[ae50953882263c5d]::Queries as rustc_middle[e664fb30aec2b69e]::ty::query::QueryEngine>::eval_to_allocation_raw
  25:     0x7fd978ebd9b2 - rustc_const_eval[55e19033940159c7]::const_eval::eval_queries::eval_to_const_value_raw_provider
  26:     0x7fd978f4194b - rustc_query_system[59fab566a51c2c24]::query::plumbing::try_execute_query::<rustc_query_impl[ae50953882263c5d]::plumbing::QueryCtxt, rustc_query_system[59fab566a51c2c24]::query::caches::DefaultCache<rustc_middle[e664fb30aec2b69e]::ty::ParamEnvAnd<rustc_middle[e664fb30aec2b69e]::mir::interpret::GlobalId>, core[90c82cfbe27f5d33]::result::Result<rustc_middle[e664fb30aec2b69e]::mir::interpret::value::ConstValue, rustc_middle[e664fb30aec2b69e]::mir::interpret::error::ErrorHandled>>>
  27:     0x7fd978fb207b - <rustc_query_impl[ae50953882263c5d]::Queries as rustc_middle[e664fb30aec2b69e]::ty::query::QueryEngine>::eval_to_const_value_raw
  28:     0x7fd9793b9b4a - <rustc_middle[e664fb30aec2b69e]::ty::context::TyCtxt>::const_eval_global_id
  29:     0x7fd979d3fa49 - <rustc_middle[e664fb30aec2b69e]::ty::context::TyCtxt>::const_eval_resolve
  30:     0x7fd979ce2391 - <rustc_infer[835d7697e80d8145]::infer::InferCtxt>::const_eval_resolve
  31:     0x7fd979cb6659 - rustc_trait_selection[99ac7681cfb15a86]::traits::const_evaluatable::is_const_evaluatable
  32:     0x7fd9792a1679 - <rustc_trait_selection[99ac7681cfb15a86]::traits::fulfill::FulfillProcessor>::progress_changed_obligations
  33:     0x7fd9792aa834 - <rustc_data_structures[9901de6203b925f4]::obligation_forest::ObligationForest<rustc_trait_selection[99ac7681cfb15a86]::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection[99ac7681cfb15a86]::traits::fulfill::FulfillProcessor, rustc_data_structures[9901de6203b925f4]::obligation_forest::Outcome<rustc_trait_selection[99ac7681cfb15a86]::traits::fulfill::PendingPredicateObligation, rustc_infer[835d7697e80d8145]::traits::FulfillmentErrorCode>>
  34:     0x7fd97929d198 - <rustc_trait_selection[99ac7681cfb15a86]::traits::fulfill::FulfillmentContext as rustc_infer[835d7697e80d8145]::traits::engine::TraitEngine>::select_all_or_error
  35:     0x7fd978a5eda5 - <rustc_typeck[95242508bb98a5fb]::check::fn_ctxt::FnCtxt>::select_all_obligations_or_error
  36:     0x7fd979796ba7 - <rustc_infer[835d7697e80d8145]::infer::InferCtxtBuilder>::enter::<(), <rustc_typeck[95242508bb98a5fb]::check::inherited::InheritedBuilder>::enter<<rustc_typeck[95242508bb98a5fb]::check::wfcheck::CheckWfFcxBuilder>::with_fcx<rustc_typeck[95242508bb98a5fb]::check::wfcheck::check_type_defn<rustc_typeck[95242508bb98a5fb]::check::wfcheck::check_item_well_formed::{closure#2}>::{closure#0}>::{closure#0}, ()>::{closure#0}>
  37:     0x7fd978b2caae - rustc_typeck[95242508bb98a5fb]::check::wfcheck::check_item_well_formed
  38:     0x7fd978f55d9e - rustc_query_system[59fab566a51c2c24]::query::plumbing::try_execute_query::<rustc_query_impl[ae50953882263c5d]::plumbing::QueryCtxt, rustc_query_system[59fab566a51c2c24]::query::caches::DefaultCache<rustc_span[26d1c5a8904e1196]::def_id::LocalDefId, ()>>
  39:     0x7fd978fb7ee5 - <rustc_query_impl[ae50953882263c5d]::Queries as rustc_middle[e664fb30aec2b69e]::ty::query::QueryEngine>::check_item_well_formed
  40:     0x7fd978af6b74 - <rustc_middle[e664fb30aec2b69e]::hir::map::Map>::par_visit_all_item_likes::<rustc_typeck[95242508bb98a5fb]::check::wfcheck::CheckTypeWellFormedVisitor>
  41:     0x7fd97978b21c - <rustc_session[bcbf86d149cb5e94]::session::Session>::track_errors::<rustc_typeck[95242508bb98a5fb]::check_crate::{closure#5}, ()>
  42:     0x7fd9797707cb - rustc_typeck[95242508bb98a5fb]::check_crate
  43:     0x7fd979523287 - rustc_interface[be440f50dcda62f9]::passes::analysis
  44:     0x7fd9799bb729 - rustc_query_system[59fab566a51c2c24]::query::plumbing::try_execute_query::<rustc_query_impl[ae50953882263c5d]::plumbing::QueryCtxt, rustc_query_system[59fab566a51c2c24]::query::caches::DefaultCache<(), core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>>>
  45:     0x7fd9799f98a5 - rustc_query_system[59fab566a51c2c24]::query::plumbing::get_query::<rustc_query_impl[ae50953882263c5d]::queries::analysis, rustc_query_impl[ae50953882263c5d]::plumbing::QueryCtxt>
  46:     0x7fd9794f980d - <rustc_interface[be440f50dcda62f9]::passes::QueryContext>::enter::<rustc_driver[7b59b981d4c23d31]::run_compiler::{closure#1}::{closure#2}::{closure#3}, core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>>
  47:     0x7fd9794f781a - <rustc_interface[be440f50dcda62f9]::interface::Compiler>::enter::<rustc_driver[7b59b981d4c23d31]::run_compiler::{closure#1}::{closure#2}, core[90c82cfbe27f5d33]::result::Result<core[90c82cfbe27f5d33]::option::Option<rustc_interface[be440f50dcda62f9]::queries::Linker>, rustc_errors[b77837aed2b0d3e7]::ErrorReported>>
  48:     0x7fd9794fa876 - rustc_span[26d1c5a8904e1196]::with_source_map::<core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>, rustc_interface[be440f50dcda62f9]::interface::create_compiler_and_run<core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>, rustc_driver[7b59b981d4c23d31]::run_compiler::{closure#1}>::{closure#1}>
  49:     0x7fd9794f71fe - rustc_interface[be440f50dcda62f9]::interface::create_compiler_and_run::<core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>, rustc_driver[7b59b981d4c23d31]::run_compiler::{closure#1}>
  50:     0x7fd9794dd302 - std[daefdbcf0dca51d0]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[be440f50dcda62f9]::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface[be440f50dcda62f9]::interface::run_compiler<core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>, rustc_driver[7b59b981d4c23d31]::run_compiler::{closure#1}>::{closure#0}, core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>>::{closure#0}, core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>>
  51:     0x7fd9794daa19 - <<std[daefdbcf0dca51d0]::thread::Builder>::spawn_unchecked_<rustc_interface[be440f50dcda62f9]::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface[be440f50dcda62f9]::interface::run_compiler<core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>, rustc_driver[7b59b981d4c23d31]::run_compiler::{closure#1}>::{closure#0}, core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>>::{closure#0}, core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>>::{closure#1} as core[90c82cfbe27f5d33]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  52:     0x7fd976eb8803 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hed49a6c6e767ba83
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/alloc/src/boxed.rs:1854:9
  53:     0x7fd976eb8803 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h14238373d01dc9bc
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/alloc/src/boxed.rs:1854:9
  54:     0x7fd976eb8803 - std::sys::unix::thread::Thread::new::thread_start::hc6dbf73cd24f68cc
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/sys/unix/thread.rs:108:17
  55:     0x7fd976dec609 - start_thread
  56:     0x7fd976d06293 - clone
  57:                0x0 - <unknown>

[compiler-errors]

so this is because we're trying to evaluate an const expression that doesn't typeck. specifically, we do emit a typeck error when checking the const generic, but still proceed with wfcheck on the containing function signature. this wfcheck requires us to evaluate the const generic, which causes the ICE.

@BoxyUwU, this seems like it could be fixed if we attempted something like #86325 again. do you think I could base a new PR off of the work you did in that one, or do you think the fallout issues you were dealing with were too messy to be worth it?

alternatively, we could just delay a bunch of bugs in CTFE instead of ICEing immediately, since we'll have emitted an error anyways when typecking the default const. seems a bit more of a bandaid solution than properly handling the bubbling up evaluation errors in the queries themselves.

alternatively, I could move the const generic default-is-evaluable check from wf, and move it to rustc_lint where we currently check that const items are themselves evaluable. checking that const generic defaults are evaluable are kinda along the same vein as checking that const generic defaults are evaluable...

cc: @oli-obk, you might have opinions about this as well

[BoxyUwU]

I too thought this might be fixed by #86325 when I saw this issue 😅 feel free to base a PR off that work though I can't really speak to how "right" the solution in that PR is as i never properly looked into why it was failing.

[compiler-errors]

cool, thanks for the response! i'll try to hack something this weekend perhaps 👀

[oli-obk]

Yea, delay span bugs will just push the problems into more and more obscure situations. We should totally try to find out what was wrong with the last attempt at fixing it.

[compiler-errors]

imma claim this

@rustbot claim