Recursive fn with varying const generic parameter causes ICE.

[goddessfreya]

#![feature(const_generics)]

fn mk_array<T, const TO: usize>() -> [T; TO] {
    panic!();
}

fn uperms<const DICE_NUM: usize>() -> Vec<[u8; DICE_NUM]> {
    if DICE_NUM <= 1 {
        return vec![mk_array::<u8, {DICE_NUM}>()];
    } else {
        let pperms = uperms::<{ DICE_NUM - 1 }>();
        panic!();
    }
}

fn main() {
    let perms1 = uperms::<{ 1 }>();
}

Play link

Backtrace:

   Compiling farklemaster v0.1.0 (/home/gentz/Documents/gfx/farklemaster)
warning: the feature `const_generics` is incomplete and may cause the compiler to crash
 --> src/main.rs:1:12
  |
1 | #![feature(const_generics)]
  |            ^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default

error: internal compiler error: src/librustc/ty/subst.rs:597: const parameter `DICE_NUM/#0` (Const { ty: usize, val: Param(DICE_NUM/#0) }/0) out of range when substituting substs=[]

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:578:9
stack backtrace:
   0:     0x7fbf7a370902 - backtrace::backtrace::libunwind::trace::h4ce370bcd1ee6179
                               at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.35/src/backtrace/libunwind.rs:88
   1:     0x7fbf7a370902 - backtrace::backtrace::trace_unsynchronized::ha1849ec6dc042df0
                               at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.35/src/backtrace/mod.rs:66
   2:     0x7fbf7a370902 - std::sys_common::backtrace::_print::hb67777a898188c39
                               at src/libstd/sys_common/backtrace.rs:47
   3:     0x7fbf7a370902 - std::sys_common::backtrace::print::hadbba928c72abbae
                               at src/libstd/sys_common/backtrace.rs:36
   4:     0x7fbf7a370902 - std::panicking::default_hook::{{closure}}::h277bed7d7e46968e
                               at src/libstd/panicking.rs:200
   5:     0x7fbf7a3705e6 - std::panicking::default_hook::hc04dc2fea0cc3aac
                               at src/libstd/panicking.rs:214
   6:     0x7fbf7c24da31 - rustc::util::common::panic_hook::hadd0b8f85b4067ef
   7:     0x7fbf7a37113c - std::panicking::rust_panic_with_hook::h7ed0ac6f9e15991f
                               at src/libstd/panicking.rs:481
   8:     0x7fbf7c1dc53d - std::panicking::begin_panic::h1c40c95922eba485
   9:     0x7fbf7bd1e9ad - rustc_errors::Handler::span_bug::hac0d34a0564a9731
  10:     0x7fbf7befe51e - rustc::util::bug::opt_span_bug_fmt::{{closure}}::h139ee24d6fa9531e
  11:     0x7fbf7bef8ed3 - rustc::ty::context::tls::with_opt::{{closure}}::h4c0650730b3d87b0
  12:     0x7fbf7bef8e43 - rustc::ty::context::tls::with_context_opt::h0b052a3db2fd7393
  13:     0x7fbf7bef8e87 - rustc::ty::context::tls::with_opt::h0247b58216cb47e8
  14:     0x7fbf7befe428 - rustc::util::bug::opt_span_bug_fmt::h3dc854d51555accb
  15:     0x7fbf7befe3da - rustc::util::bug::span_bug_fmt::h6a2695b88c32fe61
  16:     0x7fbf7bd086fb - <rustc::ty::subst::SubstFolder as rustc::ty::fold::TypeFolder>::fold_const::h7e3469995479b33a
  17:     0x7fbf7b2cee00 - rustc::traits::codegen::<impl rustc::ty::context::TyCtxt>::subst_and_normalize_erasing_regions::he1889485709b37ce
  18:     0x7fbf7b44b907 - rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_operand::hb2ea8eb42ae9cf9e
  19:     0x7fbf7b45758b - rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::run::h2c4cd78b015ece1f
  20:     0x7fbf7b23711a - rustc_mir::const_eval::const_eval_raw_provider::hf3ff85bb694929ee
  21:     0x7fbf7b2ed073 - rustc::ty::query::__query_compute::const_eval_raw::h5ba229962fe4016d
  22:     0x7fbf7b24515a - rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval_raw>::compute::h565e5bce23bd5cf5
  23:     0x7fbf7b3a2f6a - rustc::dep_graph::graph::DepGraph::with_task_impl::ha30d8a9044e56636
  24:     0x7fbf7b26bbd5 - rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query::h37ea358b8ec9d7c6
  25:     0x7fbf7b235c6e - rustc_mir::const_eval::const_eval_provider::h792d7cb0f1f28534
  26:     0x7fbf7b2ec463 - rustc::ty::query::__query_compute::const_eval::hefa01c0bd68294d6
  27:     0x7fbf7b244f6a - rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval>::compute::h6bb6e558896acff8
  28:     0x7fbf7b3a444d - rustc::dep_graph::graph::DepGraph::with_task_impl::hac6e9df0a3e6f199
  29:     0x7fbf7b267350 - rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query::h3025814bdb98c9f5
  30:     0x7fbf7b235bfc - rustc_mir::const_eval::const_eval_provider::h792d7cb0f1f28534
  31:     0x7fbf7bd1693a - rustc::ty::query::__query_compute::const_eval::hb4aa5926d8dc5c9d
  32:     0x7fbf7c10f44a - rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval>::compute::h6bb6e558896acff8
  33:     0x7fbf7bc68f39 - rustc::dep_graph::graph::DepGraph::with_task_impl::h69bc137031276732
  34:     0x7fbf7bf7f3a7 - rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query::h36bdc4fed882ea78
  35:     0x7fbf7c16adee - rustc::ty::sty::Const::eval::hb401cdbfd953e0ba
  36:     0x7fbf7bd3401d - <smallvec::SmallVec<A> as core::iter::traits::collect::FromIterator<<A as smallvec::Array>::Item>>::from_iter::he9e2ab7fd3307967
  37:     0x7fbf7c15773b - rustc::ty::fold::TypeFoldable::fold_with::haa043f760b7edf82
  38:     0x7fbf7c14daee - <rustc::traits::project::AssocTypeNormalizer as rustc::ty::fold::TypeFolder>::fold_ty::h86a6eb69d7cae1d0
  39:     0x7fbf7b0113e4 - rustc::traits::project::normalize::hcd8ad6288aa2ebc9
  40:     0x7fbf7ae452cd - rustc_typeck::check::FnCtxt::instantiate_value_path::hedaf5ff2b8b867b2
  41:     0x7fbf7ae0bf30 - rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind::hf72747da8aab128d
  42:     0x7fbf7ae08ee5 - rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs::hd4290fcb743dc65a
  43:     0x7fbf7ae24603 - rustc_typeck::check::callee::<impl rustc_typeck::check::FnCtxt>::check_call::h543b73b94215b502
  44:     0x7fbf7ae0991e - rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind::hf72747da8aab128d
  45:     0x7fbf7ae08ee5 - rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs::hd4290fcb743dc65a
  46:     0x7fbf7ae40dd1 - rustc_typeck::check::FnCtxt::check_decl_initializer::h0df3aa7cb5f326b2
  47:     0x7fbf7ae40e85 - rustc_typeck::check::FnCtxt::check_decl_local::h16dc6a267bb855a1
  48:     0x7fbf7ae40f9b - rustc_typeck::check::FnCtxt::check_stmt::hb8d30f7fa9c2c1f7
  49:     0x7fbf7ae4142b - rustc_typeck::check::FnCtxt::check_block_with_expected::h272b260cd492369d
  50:     0x7fbf7ae08ee5 - rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs::hd4290fcb743dc65a
  51:     0x7fbf7adf1a7f - rustc_typeck::check::_match::<impl rustc_typeck::check::FnCtxt>::check_match::hd3b0418651de0d88
  52:     0x7fbf7ae09d40 - rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind::hf72747da8aab128d
  53:     0x7fbf7ae08ee5 - rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs::hd4290fcb743dc65a
  54:     0x7fbf7ae4146b - rustc_typeck::check::FnCtxt::check_block_with_expected::h272b260cd492369d
  55:     0x7fbf7ae08ee5 - rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs::hd4290fcb743dc65a
  56:     0x7fbf7ae135fb - rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_return_expr::h9a52a562d86b5052
  57:     0x7fbf7ae2ed25 - rustc_typeck::check::check_fn::hcb029ffa950299ec
  58:     0x7fbf7af9c709 - rustc::ty::context::GlobalCtxt::enter_local::hcc1e02c358261193
  59:     0x7fbf7ae2dcb8 - rustc_typeck::check::typeck_tables_of::h6f0eb1f8c7b9f5e9
  60:     0x7fbf7ae7ddca - rustc::ty::query::__query_compute::typeck_tables_of::h62d65d4026b99e27
  61:     0x7fbf7af10fcb - rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute::hca69de331eee9759
  62:     0x7fbf7aeb7d02 - rustc::dep_graph::graph::DepGraph::with_task_impl::ha7be3be332b95d17
  63:     0x7fbf7af20931 - rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query::h10565f1243b7ad5c
  64:     0x7fbf7af0dd7f - rustc::ty::<impl rustc::ty::context::TyCtxt>::par_body_owners::h3a830552646698d8
  65:     0x7fbf7ae2d72d - rustc_typeck::check::typeck_item_bodies::h7b6a9b66bd0d82f5
  66:     0x7fbf7ae7e215 - rustc::ty::query::__query_compute::typeck_item_bodies::hd0a39d56e65efaff
  67:     0x7fbf7aeb9004 - rustc::dep_graph::graph::DepGraph::with_task_impl::hb953ebfe5f7d123d
  68:     0x7fbf7af2c6dd - rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query::h25e7e246720c4835
  69:     0x7fbf7ae82d5e - rustc::util::common::time::h0ca0fd77a3a38e7f
  70:     0x7fbf7b058a9f - rustc_typeck::check_crate::h6efa337673fd0bf9
  71:     0x7fbf7a9573da - rustc_interface::passes::analysis::h5ca1dcf5c3fa00a4
  72:     0x7fbf7a862481 - rustc::ty::query::__query_compute::analysis::hb2ca05cf73df93a7
  73:     0x7fbf7a845759 - rustc::dep_graph::graph::DepGraph::with_task_impl::hada062624bcc756b
  74:     0x7fbf7a859c5e - rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query::hfa3b77f4283d964e
  75:     0x7fbf7a8767aa - rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}::h2cccf239a8fcfaff
  76:     0x7fbf7a9d3b0a - rustc_interface::passes::create_global_ctxt::{{closure}}::h411ea04f079efa05
  77:     0x7fbf7a87926e - rustc_interface::interface::run_compiler_in_existing_thread_pool::h495ac89e699120f3
  78:     0x7fbf7a888522 - std::thread::local::LocalKey<T>::with::h2b6e98dea32965c7
  79:     0x7fbf7a89c21e - scoped_tls::ScopedKey<T>::set::h5242c74f962b8d66
  80:     0x7fbf7a8d1482 - syntax::with_globals::h36bb832d23aa0930
  81:     0x7fbf7a83f940 - std::sys_common::backtrace::__rust_begin_short_backtrace::h5ac2bb0fb1bde18d
  82:     0x7fbf7a3817fa - __rust_maybe_catch_panic
                               at src/libpanic_unwind/lib.rs:80
  83:     0x7fbf7a864619 - core::ops::function::FnOnce::call_once{{vtable.shim}}::h999a457b750ab865
  84:     0x7fbf7a3543ff - <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once::h8937f8752182f20d
                               at /rustc/4295eea903a9e1014ee30f82930f5ec08d888077/src/liballoc/boxed.rs:922
  85:     0x7fbf7a3804a0 - <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once::h2cc60d9264e9fe0a
                               at /rustc/4295eea903a9e1014ee30f82930f5ec08d888077/src/liballoc/boxed.rs:922
  86:     0x7fbf7a3804a0 - std::sys_common::thread::start_thread::h7aa97315f70bdaea
                               at src/libstd/sys_common/thread.rs:13
  87:     0x7fbf7a3804a0 - std::sys::unix::thread::Thread::new::thread_start::h15d5599c4b4a4cef
                               at src/libstd/sys/unix/thread.rs:79
  88:     0x7fbf7a28957f - start_thread
  89:     0x7fbf7a19c0e3 - __clone
  90:                0x0 - <unknown>
query stack during panic:
#0 [const_eval_raw] const-evaluating `uperms::{{constant}}#1`
#1 [const_eval] const-evaluating + checking `uperms::{{constant}}#1`
#2 [const_eval] const-evaluating + checking `uperms::{{constant}}#1`
#3 [typeck_tables_of] processing `uperms`
#4 [typeck_item_bodies] type-checking all item bodies
#5 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error


note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.39.0-nightly (4295eea90 2019-08-30) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

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

error: Could not compile `farklemaster`.

To learn more, run the command again with --verbose.

[goddessfreya]

Even smaller example:

#![feature(const_generics)]

fn uperms<const DICE_NUM: usize>() -> Vec<[u8; DICE_NUM]> {
    if DICE_NUM <= 1 {
        panic!();
    } else {
        let pperms = uperms::<{ DICE_NUM - 1 }>();
        panic!();
    }
}

fn main() {
    let perms1 = uperms::<{ 1 }>();
}

And the smallest of course:

#![feature(const_generics)]

fn uperms<const DICE_NUM: usize>() -> Vec<[u8; DICE_NUM]> {
    let pperms = uperms::<{ DICE_NUM - 1 }>();
}

fn main() {
    let perms1 = uperms::<{ 1 }>();
}

[hellow554]

This isn't bound to subtraction or number literals, but to every modification to the generic parameter, e.g.

#![feature(const_generics)]

pub fn uperms<const DICE_NUM: usize, const BAR: usize>() {
    uperms::<{ DICE_NUM + BAR }, { BAR }>();
}

fn main() {}

[goddessfreya]

Looks to be a dupe of #58987. Closing.