Normalization error with `ToOwned`

[Jerald]

I've recently been messing around with Cow and discovered the very weird behaviour associated with it, rooted in ToOwned. In an attempt to make an enum that can hold a Cow of itself, I ran into an internal compiler error! Very fun.

I've been developing on nightly, but I tested it on stable and the error exists there as well. I included both versions I tested with below.

The code below seems to be the minimal version that will even compile, let alone produce the error. There is likely more distant forms of complex reduction that could be done to get to a more core version of the error, but that's outside the scope of what I found.

I found a few other issues related to similar weirdness with ToOwned and/or normalization issues that seem to be similar (but not directly):

• Recursive type with Cow, 'static and slice triggers "overflow evaluating the requirement" #47032
• Using Cow with the Vec/slice ToOwned impl for creating a recursive type fails to typecheck due to overflow #38962
• RFC 1214 problems with bounds on associated type of type parameter #30311

Code

use std::borrow::{Borrow, Cow};
use std::ops::{Deref, DerefMut};

enum Recursive<'a>
where
    Self: ToOwned<Owned=Box<Self>>
{
    Variant(MyCow<'a, Recursive<'a>>),
}

pub struct Wrapper<T>(T);

pub struct MyCow<'a, T: ToOwned<Owned=Box<T>> + 'a>(Wrapper<Cow<'a, T>>);

Meta

rustc --version --verbose:

• Stable:

rustc 1.42.0 (b8cedc004 2020-03-09)
binary: rustc
commit-hash: b8cedc00407a4c56a3bda1ed605c6fc166655447
commit-date: 2020-03-09
host: x86_64-unknown-linux-gnu
release: 1.42.0
LLVM version: 9.0

• Nightly:

rustc 1.44.0-nightly (94d346360 2020-04-09)
binary: rustc
commit-hash: 94d346360da50f159e0dc777dc9bc3c5b6b51a00
commit-date: 2020-04-09
host: x86_64-unknown-linux-gnu
release: 1.44.0-nightly
LLVM version: 9.0

Error output

error: internal compiler error: src/librustc_traits/normalize_erasing_regions.rs:37: could not fully normalize `<Recursive as std::borrow::ToOwned>::Owned`

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:880:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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 1.44.0-nightly (94d346360 2020-04-09) running on x86_64-unknown-linux-gnu

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

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

Backtrace

error: internal compiler error: src/librustc_traits/normalize_erasing_regions.rs:37: could not fully normalize `<Recursive as std::borrow::ToOwned>::Owned`

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:880:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1069
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1504
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:218
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:515
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc_middle::ty::context::tls::with_opt::{{closure}}
  17: rustc_middle::ty::context::tls::with_opt
  18: rustc_middle::util::bug::opt_span_bug_fmt
  19: rustc_middle::util::bug::bug_fmt
  20: rustc_middle::ty::context::GlobalCtxt::enter_local
  21: rustc_traits::normalize_erasing_regions::normalize_generic_arg_after_erasing_regions
  22: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::normalize_generic_arg_after_erasing_regions>::compute
  23: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  24: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  25: rustc_query_system::query::plumbing::get_query
  26: rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::normalize_erasing_regions
  27: <rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt> as rustc_target::abi::LayoutOf>::layout_of
  28: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  29: <core::iter::adapters::ResultShunt<I,E> as core::iter::traits::iterator::Iterator>::next
  30: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  31: rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt>::layout_raw_uncached
  32: rustc_middle::ty::layout::layout_raw
  33: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::layout_raw>::compute
  34: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  35: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  36: rustc_query_system::query::plumbing::get_query
  37: <rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt> as rustc_target::abi::LayoutOf>::layout_of
  38: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  39: <core::iter::adapters::ResultShunt<I,E> as core::iter::traits::iterator::Iterator>::next
  40: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  41: rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt>::layout_raw_uncached
  42: rustc_middle::ty::layout::layout_raw
  43: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::layout_raw>::compute
  44: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  45: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  46: rustc_query_system::query::plumbing::get_query
  47: <rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt> as rustc_target::abi::LayoutOf>::layout_of
  48: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  49: <core::iter::adapters::ResultShunt<I,E> as core::iter::traits::iterator::Iterator>::next
  50: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  51: rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt>::layout_raw_uncached
  52: rustc_middle::ty::layout::layout_raw
  53: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::layout_raw>::compute
  54: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  55: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  56: rustc_query_system::query::plumbing::get_query
  57: <rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt> as rustc_target::abi::LayoutOf>::layout_of
  58: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  59: <core::iter::adapters::ResultShunt<I,E> as core::iter::traits::iterator::Iterator>::next
  60: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  61: rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt>::layout_raw_uncached
  62: rustc_middle::ty::layout::layout_raw
  63: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::layout_raw>::compute
  64: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  65: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  66: rustc_query_system::query::plumbing::get_query
  67: <rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt> as rustc_target::abi::LayoutOf>::layout_of
  68: <rustc_lint::types::VariantSizeDifferences as rustc_lint::passes::LateLintPass>::check_item
  69: <rustc_lint::BuiltinCombinedModuleLateLintPass as rustc_lint::passes::LateLintPass>::check_item
  70: rustc_hir::intravisit::Visitor::visit_nested_item
  71: rustc_hir::intravisit::walk_mod
  72: rustc_lint::late::late_lint_mod
  73: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::lint_mod>::compute
  74: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  75: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  76: rustc_query_system::query::plumbing::get_query
  77: rustc_query_system::query::plumbing::ensure_query
  78: rustc_session::utils::<impl rustc_session::session::Session>::time
  79: rustc_data_structures::sync::join
  80: rustc_interface::passes::analysis::{{closure}}::{{closure}}
  81: rustc_session::utils::<impl rustc_session::session::Session>::time
  82: rustc_interface::passes::analysis
  83: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  84: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  85: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  86: rustc_query_system::query::plumbing::get_query
  87: rustc_middle::ty::context::tls::enter_global
  88: rustc_interface::interface::run_compiler_in_existing_thread_pool
  89: scoped_tls::ScopedKey<T>::set
  90: rustc_ast::attr::with_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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.44.0-nightly (94d346360 2020-04-09) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `<Recursive as std::borrow::ToOwned>::Owned`
#1 [layout_raw] computing layout of `std::borrow::Cow<Recursive>`
#2 [layout_raw] computing layout of `DummyHolder<std::borrow::Cow<Recursive>>`
#3 [layout_raw] computing layout of `Cow<Recursive>`
#4 [layout_raw] computing layout of `Recursive`
#5 [lint_mod] linting top-level module
#6 [analysis] running analysis passes on this crate
end of query stack

[spastorino]

The ICE starts to happen in 1.32 but I'm getting the following error in 1.31 ...

error[E0411]: cannot find type `Self` in this scope
 --> <source>:6:5
  |
6 |     Self: ToOwned<Owned=Box<Self>>
  |     ^^^^ `Self` is only available in traits and impls

error[E0411]: cannot find type `Self` in this scope
 --> <source>:6:29
  |
6 |     Self: ToOwned<Owned=Box<Self>>
  |                             ^^^^ `Self` is only available in traits and impls

warning: unused import: `Borrow`
 --> <source>:1:19
  |
1 | use std::borrow::{Borrow, Cow};
  |                   ^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

warning: unused imports: `DerefMut`, `Deref`
 --> <source>:2:16
  |
2 | use std::ops::{Deref, DerefMut};
  |                ^^^^^  ^^^^^^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0411`.

[spastorino]

Ok, I was guessing that this may have been because the compiler stops before exercising the code that produces the ICE and I've just confirmed that.

use std::borrow::{Borrow, Cow};
use std::ops::{Deref, DerefMut};

enum Recursive<'a>
where
    Recursive<'a>: ToOwned<Owned=Box<Recursive<'a>>>
{
    Variant(MyCow<'a, Recursive<'a>>),
}

pub struct Wrapper<T>(T);

pub struct MyCow<'a, T: ToOwned<Owned=Box<T>> + 'a>(Wrapper<Cow<'a, T>>);

Also ICEs in 1.31

[spastorino]

Assigning P-medium as discussed as part of the Prioritization Working Group process and removing I-prioritize.