ICE `const variables should not be hashed: Var(_#0c)` inside async context on nightly

[matheus-consoli]

Code

I think the problem here is derived from rustc not knowing the array length. I'm using futures-concurrency crate to .await an array of streams, merge() returns a Merge<S, const N: usize> struct, but the N is not known at this point because we don't limit the array::from_fn.

use core::array;

use futures_concurrency::prelude::*;
use futures_lite::{future::block_on, stream, StreamExt};

fn main() {
    block_on(async {
        // works, the array length is known
        // let s = from_fn::<_, 5, _>(stream::once)
        let s = array::from_fn(stream::once)
            .merge()
            .fold(0, |acc, n| acc + n)
            .await;

        assert_eq!(10, s);
    });
}

this is much like

array::from_fn(identity).collect();

except that the latter doesn't panic, and gives a helpful error message.

Also, this seems to only occur within an async context.

Meta

rustc --version --verbose:

>> rustc --version --verbose
rustc 1.67.0-nightly (ff8c8dfbe 2022-11-22)
binary: rustc
commit-hash: ff8c8dfbe66701531e3e5e335c28c544d0fbc945
commit-date: 2022-11-22
host: x86_64-unknown-linux-gnu
release: 1.67.0-nightly
LLVM version: 15.0.4

Error output

>> RUST_BACKTRACE=1 cargo r
   Compiling f v0.1.0 (/home/consoli/projects/rust/contrib/f)
error[E0282]: type annotations needed
  --> src/main.rs:11:14
   |
11 |             .merge()
   |              ^^^^^
   |
help: try using a fully qualified path to specify the expected types
   |
10 |         let s = <[futures_lite::stream::Once<usize>; _] as futures_concurrency::stream::Merge>::merge(from_fn(stream::once))
   |                 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++                     ~

thread '<unnamed>' panicked at 'const variables should not be hashed: Var(_#0c)', /rustc/ff8c8dfbe66701531e3e5e335c28c544d0fbc945/compiler/rustc_middle/src/ty/consts/kind.rs:122:35

Backtrace

thread '<unnamed>' panicked at 'const variables should not be hashed: Var(_#0c)', /rustc/ff8c8dfbe66701531e3e5e335c28c544d0fbc945/compiler/rustc_middle/src/ty/consts/kind.rs:122:35
stack backtrace:
   0: rust_begin_unwind
             at /rustc/ff8c8dfbe66701531e3e5e335c28c544d0fbc945/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/ff8c8dfbe66701531e3e5e335c28c544d0fbc945/library/core/src/panicking.rs:65:14
   2: <rustc_middle::ty::consts::kind::InferConst as rustc_data_structures::stable_hasher::HashStable<rustc_query_system::ich::hcx::StableHashingContext>>::hash_stable
   3: <rustc_data_structures::intern::Interned<rustc_data_structures::intern::WithStableHash<rustc_middle::ty::TyS>> as rustc_data_structures::stable_hasher::HashStable<rustc_query_system::ich::hcx::StableHashingContext>>::hash_stable
   4: <[rustc_middle::ty::context::GeneratorInteriorTypeCause] as rustc_data_structures::stable_hasher::HashStable<rustc_query_system::ich::hcx::StableHashingContext>>::hash_stable
   5: rustc_query_system::dep_graph::graph::hash_result::<&rustc_middle::ty::context::TypeckResults>
   6: <rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::with_task::<rustc_middle::ty::context::TyCtxt, rustc_span::def_id::LocalDefId, &rustc_middle::ty::context::TypeckResults>
   7: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::LocalDefId, &rustc_middle::ty::context::TypeckResults>>
   8: rustc_data_structures::sync::par_for_each_in::<&[rustc_span::def_id::LocalDefId], <rustc_middle::hir::map::Map>::par_body_owners<rustc_hir_typeck::typeck_item_bodies::{closure#0}>::{closure#0}>
   9: rustc_hir_typeck::typeck_item_bodies
  10: <rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::with_task::<rustc_middle::ty::context::TyCtxt, (), ()>
  11: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<(), ()>>
  12: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::typeck_item_bodies, rustc_query_impl::plumbing::QueryCtxt>
  13: <rustc_session::session::Session>::time::<(), rustc_hir_analysis::check_crate::{closure#7}>
  14: rustc_hir_analysis::check_crate
  15: rustc_interface::passes::analysis
  16: <rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::with_task::<rustc_middle::ty::context::TyCtxt, (), core::result::Result<(), rustc_errors::ErrorGuaranteed>>
  17: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<(), core::result::Result<(), rustc_errors::ErrorGuaranteed>>>
  18: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::analysis, rustc_query_impl::plumbing::QueryCtxt>
  19: <rustc_interface::passes::QueryContext>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}::{closure#3}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>
  20: <rustc_interface::interface::Compiler>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_errors::ErrorGuaranteed>>
  21: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>::{closure#0}::{closure#1}>
  22: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Sorry if this isn't a true ICE, the compiler panics, but the usual message "this is an ICE" is not shown