ICE when checking size_of of closure in const generics

[iliakonnov]

In some cases compiler panicks when, I believe, computing const generic argument containing std::mem::size_of::<T>(), where T is some closure type. Interestingly, it works well when I put closure into associated type, so expression looks like size_of::<T::Assoc>(). Changing closure type to something more simple makes everything fine.

Code

This gives panic: (playground)

#![feature(const_generics)]
#![feature(const_evaluatable_checked)]

pub trait IsTrue<const T: bool> {}
impl IsTrue<true> for () {}

pub trait IsZST {}

impl<T> IsZST for T
where
    (): IsTrue<{ std::mem::size_of::<T>() == 0 }>
{}

fn func() -> impl IsZST {
    // Changing closure to unit type makes code to compile
    || {}
}

Interestingly, following code also requires closure type to be ZST, but compiles well: (playground)

#![feature(const_generics)]
#![feature(const_evaluatable_checked)]

pub trait IsTrue<const T: bool> {}
impl IsTrue<true> for () {}

pub trait HaveAssociatedType {
    type Assoc;
}

pub struct SomeStruct<T>(T);

impl<T> HaveAssociatedType for SomeStruct<T> {
    type Assoc = T;
}

pub trait AssocIsZST {}

impl<T> AssocIsZST for T
where
    T: HaveAssociatedType,
    (): IsTrue<{ std::mem::size_of::<T::Assoc>() == 0 }>
{}

fn func() -> impl AssocIsZST {
    SomeStruct(|| {})
}

Meta

rustc --version --verbose:

rustc 1.54.0-nightly (c79419af0 2021-06-04)
binary: rustc
commit-hash: c79419af0721c614d050f09b95f076da09d37b0d
commit-date: 2021-06-04
host: x86_64-unknown-linux-gnu
release: 1.54.0-nightly
LLVM version: 12.0.1

Error output

error: internal compiler error: compiler/rustc_middle/src/ty/sty.rs:396:19: Unexpected representation of upvar types tuple Bound(DebruijnIndex(0), BoundTy { var: 1, kind: Anon })

Backtrace

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:1007:9
stack backtrace:
   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::bug
   3: rustc_errors::Handler::bug
   4: rustc_middle::ty::context::tls::with_opt
   5: rustc_middle::util::bug::opt_span_bug_fmt
   6: rustc_middle::util::bug::bug_fmt
   7: rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt>::layout_raw_uncached
   8: rustc_middle::ty::layout::layout_raw
   9: rustc_query_impl::<impl rustc_query_system::query::config::QueryAccessors<rustc_query_impl::plumbing::QueryCtxt> for rustc_query_impl::queries::layout_raw>::compute
  10: rustc_query_system::query::plumbing::get_query_impl
  11: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::layout_raw
  12: <rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::query::TyCtxtAt> as rustc_target::abi::LayoutOf>::layout_of
  13: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_rvalue_into_place
  14: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::run
  15: rustc_mir::const_eval::eval_queries::eval_to_allocation_raw_provider
  16: rustc_query_impl::<impl rustc_query_system::query::config::QueryAccessors<rustc_query_impl::plumbing::QueryCtxt> for rustc_query_impl::queries::eval_to_allocation_raw>::compute
  17: rustc_query_system::query::plumbing::get_query_impl
  18: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::eval_to_allocation_raw
  19: rustc_mir::const_eval::eval_queries::eval_to_const_value_raw_provider
  20: rustc_query_impl::<impl rustc_query_system::query::config::QueryAccessors<rustc_query_impl::plumbing::QueryCtxt> for rustc_query_impl::queries::eval_to_const_value_raw>::compute
  21: rustc_query_system::query::plumbing::get_query_impl
  22: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::eval_to_const_value_raw
  23: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_global_id
  24: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_resolve
  25: rustc_infer::infer::InferCtxt::const_eval_resolve
  26: rustc_trait_selection::traits::fulfill::FulfillProcessor::progress_changed_obligations::{{closure}}
  27: rustc_trait_selection::traits::fulfill::FulfillProcessor::progress_changed_obligations
  28: rustc_data_structures::obligation_forest::ObligationForest<O>::process_obligations
  29: <rustc_trait_selection::traits::fulfill::FulfillmentContext as rustc_infer::traits::engine::TraitEngine>::select_where_possible
  30: rustc_typeck::check::fn_ctxt::_impl::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::resolve_vars_with_obligations
  31: rustc_typeck::check::coercion::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::try_coerce
  32: rustc_typeck::check::coercion::CoerceMany<E>::coerce_inner
  33: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_return_expr
  34: rustc_typeck::check::check::check_fn
  35: rustc_typeck::check::inherited::InheritedBuilder::enter
  36: rustc_typeck::check::typeck
  37: rustc_query_system::query::plumbing::get_query_impl
  38: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::typeck
  39: rustc_middle::ty::context::TyCtxt::typeck_opt_const_arg
  40: rustc_mir_build::build::mir_built
  41: rustc_query_system::query::plumbing::get_query_impl
  42: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_built
  43: rustc_mir::transform::check_unsafety::unsafety_check_result
  44: core::ops::function::FnOnce::call_once
  45: rustc_query_system::query::plumbing::get_query_impl
  46: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::unsafety_check_result
  47: rustc_mir::transform::mir_const
  48: rustc_query_system::query::plumbing::get_query_impl
  49: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_const
  50: rustc_mir::transform::mir_promoted
  51: rustc_query_system::query::plumbing::get_query_impl
  52: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_promoted
  53: rustc_mir::borrow_check::mir_borrowck
  54: core::ops::function::FnOnce::call_once
  55: rustc_query_system::query::plumbing::get_query_impl
  56: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::mir_borrowck
  57: rustc_typeck::collect::type_of::type_of
  58: rustc_query_system::query::plumbing::get_query_impl
  59: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::type_of
  60: rustc_typeck::check::check::check_item_type
  61: rustc_middle::hir::map::Map::visit_item_likes_in_module
  62: rustc_typeck::check::check::check_mod_item_types
  63: rustc_query_system::query::plumbing::get_query_impl
  64: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::check_mod_item_types
  65: rustc_session::utils::<impl rustc_session::session::Session>::time
  66: rustc_typeck::check_crate
  67: rustc_interface::passes::analysis
  68: rustc_query_system::query::plumbing::get_query_impl
  69: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
  70: rustc_interface::passes::QueryContext::enter
  71: rustc_span::with_session_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.54.0-nightly (c79419af0 2021-06-04) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type lib

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

query stack during panic:
#0 [layout_raw] computing layout of `[closure@src/lib.rs:16:5: 16:10]`
#1 [eval_to_allocation_raw] const-evaluating + checking `<impl at src/lib.rs:9:1: 12:3>::{constant#0}`
#2 [eval_to_const_value_raw] simplifying constant for the type system `<impl at src/lib.rs:9:1: 12:3>::{constant#0}`
#3 [typeck] type-checking `func`
#4 [mir_built] building MIR for `func`
#5 [unsafety_check_result] unsafety-checking `func`
#6 [mir_const] processing MIR for `func`
#7 [mir_promoted] processing `func`
#8 [mir_borrowck] borrow-checking `func`
#9 [type_of] computing type of `func::{opaque#0}`
#10 [check_mod_item_types] checking item types in top-level module
#11 [analysis] running analysis passes on this crate
end of query stack

[inquisitivecrystal]

@rustbot label +A-closures +A-zst

[iliakonnov]

@bookofportals, sorry for bothering you, but probably ZST is not a real cause of a problem. I'm very far from compiler internals, but it looks like issue is coming from using mem::size_of::<{closure}> in const generics using incomplete feature const_evaluatable_checked. Sorry if title was inaccurate. Unfortunately I do not know what tags may be more suitable here and even can't edit them.

Upd: Making closure non-ZST still crashes the compiler: playground

[iliakonnov]

After googling a bit I finally learned how to become a better contributor :)

@rustbot label +F-const_evaluatable_checked +A-const-eval +F-const_generics +A-const-generics -A-zst

[inquisitivecrystal]

That's quite alright! You don't need to worry about bothering me. I appreciate any advice on how to make my categorizations more helpful. That said, I might suggest editing the title and opening post to make the probable source of the problem a bit more clear. :)

@rustbot label +requires-nightly