Const generics: internal compiler error: collection encountered polymorphic constant

[is8ac]

Source:

#![feature(const_generics)]

pub trait BitLen: Sized {
    const BIT_LEN: usize;
}

impl<const L: usize> BitLen for [u8; L] {
    const BIT_LEN: usize = 8 * L;
}

fn main() {
    let foo = <[u8; 2]>::BIT_LEN;
}

Panic:

error: internal compiler error: src/librustc_mir/monomorphize/collector.rs:1289: collection encountered polymorphic constant
 --> src/bin/bitlen_demo.rs:4:5
  |
4 |     const BIT_LEN: usize;
  |     ^^^^^^^^^^^^^^^^^^^^^

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:585:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.29/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.29/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:212
   6: rustc::util::common::panic_hook
   7: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:479
   8: std::panicking::begin_panic
   9: rustc_errors::Handler::span_bug
  10: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  11: rustc::ty::context::tls::with_opt::{{closure}}
  12: rustc::ty::context::tls::with_context_opt
  13: rustc::ty::context::tls::with_opt
  14: rustc::util::bug::opt_span_bug_fmt
  15: rustc::util::bug::span_bug_fmt
  16: rustc_mir::monomorphize::collector::collect_const
  17: <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc::mir::visit::Visitor>::visit_rvalue
  18: rustc_mir::monomorphize::collector::collect_items_rec
  19: rustc_mir::monomorphize::collector::collect_crate_mono_items::{{closure}}
  20: rustc::util::common::time
  21: rustc_mir::monomorphize::collector::collect_crate_mono_items
  22: rustc::util::common::time
  23: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  24: rustc::ty::query::__query_compute::collect_and_partition_mono_items
  25: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_and_partition_mono_items>::compute
  26: rustc::dep_graph::graph::DepGraph::with_task_impl
  27: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  28: rustc_codegen_ssa::base::codegen_crate
  29: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  30: rustc::util::common::time
  31: rustc_interface::passes::start_codegen
  32: rustc::ty::context::tls::enter_global
  33: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  34: rustc_interface::passes::create_global_ctxt::{{closure}}
  35: rustc_interface::passes::BoxedGlobalCtxt::enter
  36: rustc_interface::queries::Query<T>::compute
  37: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::ongoing_codegen
  38: rustc_interface::interface::run_compiler_in_existing_thread_pool
  39: std::thread::local::LocalKey<T>::with
  40: scoped_tls::ScopedKey<T>::set
  41: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
#0 [collect_and_partition_mono_items] collect_and_partition_mono_items
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 1.37.0-nightly (d3e2cec29 2019-06-26) 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 `const_generics_demo`.

Caused by:
  process didn't exit successfully: `rustc --edition=2018 --crate-name bitlen_demo src/bin/bitlen_demo.rs --color always --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=aa6088263213b6e1 -C extra-filename=-aa6088263213b6e1 --out-dir /home/isaac/go/src/github.com/is8ac/const_generics_demo/target/debug/deps -C incremental=/home/isaac/go/src/github.com/is8ac/const_generics_demo/target/debug/incremental -L dependency=/home/isaac/go/src/github.com/is8ac/const_generics_demo/target/debug/deps` (exit code: 101)

Meta

[isaac@d6-arch const_generics_demo]$ rustc --version --verbose
rustc 1.37.0-nightly (d3e2cec29 2019-06-26)
binary: rustc
commit-hash: d3e2cec29225a46298ec4ebf082f34ebd7cfeecf
commit-date: 2019-06-26
host: x86_64-unknown-linux-gnu
release: 1.37.0-nightly
LLVM version: 8.0

[shamatar]

Same is valid for traits (abstract incomplete example)

pub trait FullMultiplication {
    type Multiplicand;
    type MulResult;
}

impl<const N: usize> FullMultiplication for BigintRepresentation<{N}> 
{
    type Multiplicand = BigintRepresentation<{N}>;
    type MulResult = BigintRepresentation<{N*2}>;
}

[vadixidav]

Also encountered this issue here.

[alex]

Does anyone have a sense of what would be involved in fixing this?

[JohnTitor]

Cannot see the ICE with the above example on latest nightly (playground)

[tesuji]

Should we add a regression test for this (and #62536)?

[Mark-Simulacrum]

I think we'll worry more about tests as we head towards more effective const generics; right now I think it's not really in a state where tests would be super helpful. cc @varkor though

[varkor]

Although we know a lot isn't working with const generics, I'd really like to expand the test suite, so it's more comprehensive as we move towards stabilisation. Let's try to add regression tests where possible. I'll add one for this issue and #62536.