Trait alias: Add trait + dyn causes compiler panic

[jonaspleyer]

Code

This problem resulted from the aim to store a Trait Object with generics in another object. When using the dyn keyword to store the trait object, the compiler panics, but only when having used a trait alias in defining the trait. If the trait is implemented in a standard (stable complying) manner, compilation succedes.

Here is a MWE of a compiler-panic version and a compiling version. This suggests that the used trait alias might be responsible. It seems that the Rust compiler does something different than just syntactic replacements when compiling a trait alias.
https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=05f0dd96c14a4f5ff5a301b21b006be4

I am still a beginner when it comes to Rust so any feedback is highly appreciated!

#![feature(trait_alias)]
use core::ops::Add;

pub trait DoSome<T> {}

// Option 1 - Compiler panic
pub trait Cell<T: Add<T, Output=T>> = DoSome<T>;

// Option 2 - Compiles
// pub trait Cell<T: Add<T, Output=T>>: DoSome<T> {}
// impl<T,A> Cell<T> for A
// where
//     T: Add<T,Output=T>,
//     A: DoSome<T>,
// {}

struct _Container<T> {
    pub cells: dyn Cell<T>,
}


fn main() {}

Meta

Since the trait alias feature is nightly only, this compiler was only tested.

rustc --version --verbose:

rustc 1.68.0-nightly (92c1937a9 2022-12-27)
binary: rustc
commit-hash: 92c1937a90e5b6f20fa6e87016d6869da363972e
commit-date: 2022-12-27
host: x86_64-unknown-linux-gnu
release: 1.68.0-nightly
LLVM version: 15.0.6

Error output

   Compiling playground v0.0.1 (/playground)
thread 'rustc' panicked at 'assertion failed: `(left == right)`
  left: `T`,
 right: `FreshTy(0)`', compiler/rustc_hir_analysis/src/astconv/mod.rs:1510:17
stack backtrace:
   0:     0x7fd5f64ee3da - std::backtrace_rs::backtrace::libunwind::trace::hd9920c45cab02727
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7fd5f64ee3da - std::backtrace_rs::backtrace::trace_unsynchronized::h0e8ba232432f278b
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7fd5f64ee3da - std::sys_common::backtrace::_print_fmt::h70b1bf152d50d826
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/sys_common/backtrace.rs:65:5
   3:     0x7fd5f64ee3da - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h0f8a00c5ac0ea98c
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7fd5f655162e - core::fmt::write::h75194d37b0340e77
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/core/src/fmt/mod.rs:1208:17
   5:     0x7fd5f64de885 - std::io::Write::write_fmt::h019baaeffa3c233d
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/io/mod.rs:1682:15
   6:     0x7fd5f64ee1a5 - std::sys_common::backtrace::_print::hd2cb40128730a76d
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/sys_common/backtrace.rs:47:5
   7:     0x7fd5f64ee1a5 - std::sys_common::backtrace::print::h30c90b179f5588ff
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/sys_common/backtrace.rs:34:9
   8:     0x7fd5f64f0f6f - std::panicking::default_hook::{{closure}}::hb4605418c1ac9dde
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/panicking.rs:267:22
   9:     0x7fd5f64f0cab - std::panicking::default_hook::h911627d2c5874d29
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/panicking.rs:286:9
  10:     0x7fd5f97f5e4f - rustc_driver[be31376d096510e8]::DEFAULT_HOOK::{closure#0}::{closure#0}
  11:     0x7fd5f64f17ad - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h30886b880f16ecd7
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/alloc/src/boxed.rs:2032:9
  12:     0x7fd5f64f17ad - std::panicking::rust_panic_with_hook::h7c2b44024a401155
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/panicking.rs:692:13
  13:     0x7fd5f64f1529 - std::panicking::begin_panic_handler::{{closure}}::h8e64ae5b7bafe3c0
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/panicking.rs:579:13
  14:     0x7fd5f64ee88c - std::sys_common::backtrace::__rust_end_short_backtrace::hd43ee2e6b4a0d8d6
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/sys_common/backtrace.rs:137:18
  15:     0x7fd5f64f1232 - rust_begin_unwind
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/panicking.rs:575:5
  16:     0x7fd5f654e013 - core::panicking::panic_fmt::hfd723e1f10b23e9d
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/core/src/panicking.rs:64:14
  17:     0x7fd5f654e40f - core::panicking::assert_failed_inner::h43b7b0de29dad9f3
  18:     0x7fd5f99261ec - core[468623a6da7d9ec9]::panicking::assert_failed::<rustc_middle[f940356e05fab334]::ty::Ty, rustc_middle[f940356e05fab334]::ty::Ty>
  19:     0x7fd5f86840a0 - <rustc_middle[f940356e05fab334]::ty::sty::Binder<rustc_middle[f940356e05fab334]::ty::ProjectionPredicate>>::map_bound::<<dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::conv_object_ty_poly_trait_ref::{closure#13}::{closure#0}, rustc_middle[f940356e05fab334]::ty::sty::ExistentialProjection>
  20:     0x7fd5f8683e24 - <core[468623a6da7d9ec9]::iter::adapters::chain::Chain<core[468623a6da7d9ec9]::iter::adapters::chain::Chain<core[468623a6da7d9ec9]::iter::adapters::map::Map<core[468623a6da7d9ec9]::iter::adapters::map::Map<core[468623a6da7d9ec9]::slice::iter::Iter<rustc_trait_selection[2eda08b4c5fdc05b]::traits::util::TraitAliasExpansionInfo>, <dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::conv_object_ty_poly_trait_ref::{closure#12}>, <dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::conv_object_ty_poly_trait_ref::{closure#14}>, core[468623a6da7d9ec9]::iter::adapters::map::Map<core[468623a6da7d9ec9]::iter::adapters::map::Map<core[468623a6da7d9ec9]::slice::iter::Iter<(rustc_middle[f940356e05fab334]::ty::sty::Binder<rustc_middle[f940356e05fab334]::ty::ProjectionPredicate>, rustc_span[414a16377e6cb62b]::span_encoding::Span)>, <dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::conv_object_ty_poly_trait_ref::{closure#13}>, <dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::conv_object_ty_poly_trait_ref::{closure#16}>>, core[468623a6da7d9ec9]::iter::adapters::map::Map<alloc[c40375f0f7ff5c91]::vec::into_iter::IntoIter<rustc_trait_selection[2eda08b4c5fdc05b]::traits::util::TraitAliasExpansionInfo>, <dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::conv_object_ty_poly_trait_ref::{closure#15}>> as core[468623a6da7d9ec9]::iter::traits::iterator::Iterator>::next
  21:     0x7fd5f86837a8 - <smallvec[8f35afdb4c070a9c]::SmallVec<[rustc_middle[f940356e05fab334]::ty::sty::Binder<rustc_middle[f940356e05fab334]::ty::sty::ExistentialPredicate>; 8usize]> as core[468623a6da7d9ec9]::iter::traits::collect::Extend<rustc_middle[f940356e05fab334]::ty::sty::Binder<rustc_middle[f940356e05fab334]::ty::sty::ExistentialPredicate>>>::extend::<core[468623a6da7d9ec9]::iter::adapters::chain::Chain<core[468623a6da7d9ec9]::iter::adapters::chain::Chain<core[468623a6da7d9ec9]::iter::adapters::map::Map<core[468623a6da7d9ec9]::iter::adapters::map::Map<core[468623a6da7d9ec9]::slice::iter::Iter<rustc_trait_selection[2eda08b4c5fdc05b]::traits::util::TraitAliasExpansionInfo>, <dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::conv_object_ty_poly_trait_ref::{closure#12}>, <dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::conv_object_ty_poly_trait_ref::{closure#14}>, core[468623a6da7d9ec9]::iter::adapters::map::Map<core[468623a6da7d9ec9]::iter::adapters::map::Map<core[468623a6da7d9ec9]::slice::iter::Iter<(rustc_middle[f940356e05fab334]::ty::sty::Binder<rustc_middle[f940356e05fab334]::ty::ProjectionPredicate>, rustc_span[414a16377e6cb62b]::span_encoding::Span)>, <dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::conv_object_ty_poly_trait_ref::{closure#13}>, <dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::conv_object_ty_poly_trait_ref::{closure#16}>>, core[468623a6da7d9ec9]::iter::adapters::map::Map<alloc[c40375f0f7ff5c91]::vec::into_iter::IntoIter<rustc_trait_selection[2eda08b4c5fdc05b]::traits::util::TraitAliasExpansionInfo>, <dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::conv_object_ty_poly_trait_ref::{closure#15}>>>
  22:     0x7fd5f867f202 - <dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::conv_object_ty_poly_trait_ref
  23:     0x7fd5f8655895 - <dyn rustc_hir_analysis[9ee823d6a4dae130]::astconv::AstConv>::ast_ty_to_ty_inner::{closure#0}
  24:     0x7fd5f8fe8915 - rustc_hir_analysis[9ee823d6a4dae130]::collect::type_of::type_of
  25:     0x7fd5f81970c6 - rustc_query_system[5425389c485e711a]::query::plumbing::get_query::<rustc_query_impl[c6ef26a2c565c2a5]::queries::type_of, rustc_query_impl[c6ef26a2c565c2a5]::plumbing::QueryCtxt, rustc_middle[f940356e05fab334]::dep_graph::dep_node::DepKind>
  26:     0x7fd5f8d37911 - rustc_hir_analysis[9ee823d6a4dae130]::outlives::implicit_infer::infer_predicates
  27:     0x7fd5f8d36bc6 - rustc_hir_analysis[9ee823d6a4dae130]::outlives::inferred_outlives_crate
  28:     0x7fd5f8fded2b - rustc_query_system[5425389c485e711a]::query::plumbing::try_execute_query::<rustc_query_impl[c6ef26a2c565c2a5]::plumbing::QueryCtxt, rustc_query_system[5425389c485e711a]::query::caches::ArenaCache<(), rustc_middle[f940356e05fab334]::ty::CratePredicatesMap>>
  29:     0x7fd5f8fdea10 - rustc_query_system[5425389c485e711a]::query::plumbing::get_query::<rustc_query_impl[c6ef26a2c565c2a5]::queries::inferred_outlives_crate, rustc_query_impl[c6ef26a2c565c2a5]::plumbing::QueryCtxt, rustc_middle[f940356e05fab334]::dep_graph::dep_node::DepKind>
  30:     0x7fd5f80adad9 - rustc_query_system[5425389c485e711a]::query::plumbing::get_query::<rustc_query_impl[c6ef26a2c565c2a5]::queries::inferred_outlives_of, rustc_query_impl[c6ef26a2c565c2a5]::plumbing::QueryCtxt, rustc_middle[f940356e05fab334]::dep_graph::dep_node::DepKind>
  31:     0x7fd5f81cc64b - rustc_hir_analysis[9ee823d6a4dae130]::collect::predicates_defined_on
  32:     0x7fd5f7e315d7 - rustc_query_system[5425389c485e711a]::query::plumbing::get_query::<rustc_query_impl[c6ef26a2c565c2a5]::queries::predicates_defined_on, rustc_query_impl[c6ef26a2c565c2a5]::plumbing::QueryCtxt, rustc_middle[f940356e05fab334]::dep_graph::dep_node::DepKind>
  33:     0x7fd5f8142232 - rustc_query_system[5425389c485e711a]::query::plumbing::get_query::<rustc_query_impl[c6ef26a2c565c2a5]::queries::predicates_of, rustc_query_impl[c6ef26a2c565c2a5]::plumbing::QueryCtxt, rustc_middle[f940356e05fab334]::dep_graph::dep_node::DepKind>
  34:     0x7fd5f8141a53 - <rustc_query_impl[c6ef26a2c565c2a5]::Queries as rustc_middle[f940356e05fab334]::ty::query::QueryEngine>::predicates_of
  35:     0x7fd5f8135347 - rustc_hir_analysis[9ee823d6a4dae130]::collect::convert_item
  36:     0x7fd5f812f766 - <rustc_middle[f940356e05fab334]::hir::map::Map>::visit_item_likes_in_module::<rustc_hir_analysis[9ee823d6a4dae130]::collect::CollectItemTypesVisitor>
  37:     0x7fd5f812f55c - rustc_hir_analysis[9ee823d6a4dae130]::collect::collect_mod_item_types
  38:     0x7fd5f8395700 - rustc_query_system[5425389c485e711a]::query::plumbing::try_execute_query::<rustc_query_impl[c6ef26a2c565c2a5]::plumbing::QueryCtxt, rustc_query_system[5425389c485e711a]::query::caches::VecCache<rustc_span[414a16377e6cb62b]::def_id::LocalDefId, ()>>
  39:     0x7fd5f8c04b43 - rustc_query_system[5425389c485e711a]::query::plumbing::get_query::<rustc_query_impl[c6ef26a2c565c2a5]::queries::collect_mod_item_types, rustc_query_impl[c6ef26a2c565c2a5]::plumbing::QueryCtxt, rustc_middle[f940356e05fab334]::dep_graph::dep_node::DepKind>
  40:     0x7fd5f7c93ea5 - <rustc_middle[f940356e05fab334]::hir::map::Map>::for_each_module::<rustc_hir_analysis[9ee823d6a4dae130]::check_crate::{closure#0}::{closure#0}::{closure#0}>
  41:     0x7fd5f7c93678 - <rustc_session[484a5cd1f43b0798]::session::Session>::track_errors::<rustc_hir_analysis[9ee823d6a4dae130]::check_crate::{closure#0}, ()>
  42:     0x7fd5f7c9336f - rustc_hir_analysis[9ee823d6a4dae130]::check_crate
  43:     0x7fd5f7c9308b - rustc_interface[e3f740b58bbe8d54]::passes::analysis
  44:     0x7fd5f8fb01c7 - rustc_query_system[5425389c485e711a]::query::plumbing::try_execute_query::<rustc_query_impl[c6ef26a2c565c2a5]::plumbing::QueryCtxt, rustc_query_system[5425389c485e711a]::query::caches::DefaultCache<(), core[468623a6da7d9ec9]::result::Result<(), rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>>>
  45:     0x7fd5f8fafec0 - rustc_query_system[5425389c485e711a]::query::plumbing::get_query::<rustc_query_impl[c6ef26a2c565c2a5]::queries::analysis, rustc_query_impl[c6ef26a2c565c2a5]::plumbing::QueryCtxt, rustc_middle[f940356e05fab334]::dep_graph::dep_node::DepKind>
  46:     0x7fd5f8a93953 - <rustc_interface[e3f740b58bbe8d54]::passes::QueryContext>::enter::<rustc_driver[be31376d096510e8]::run_compiler::{closure#1}::{closure#2}::{closure#2}, core[468623a6da7d9ec9]::result::Result<(), rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>>
  47:     0x7fd5f8a8fd03 - <rustc_interface[e3f740b58bbe8d54]::interface::Compiler>::enter::<rustc_driver[be31376d096510e8]::run_compiler::{closure#1}::{closure#2}, core[468623a6da7d9ec9]::result::Result<core[468623a6da7d9ec9]::option::Option<rustc_interface[e3f740b58bbe8d54]::queries::Linker>, rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>>
  48:     0x7fd5f8a8ad18 - rustc_span[414a16377e6cb62b]::with_source_map::<core[468623a6da7d9ec9]::result::Result<(), rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>, rustc_interface[e3f740b58bbe8d54]::interface::run_compiler<core[468623a6da7d9ec9]::result::Result<(), rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>, rustc_driver[be31376d096510e8]::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
  49:     0x7fd5f8a8a805 - <scoped_tls[b199c6e77ce176e9]::ScopedKey<rustc_span[414a16377e6cb62b]::SessionGlobals>>::set::<rustc_interface[e3f740b58bbe8d54]::interface::run_compiler<core[468623a6da7d9ec9]::result::Result<(), rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>, rustc_driver[be31376d096510e8]::run_compiler::{closure#1}>::{closure#0}, core[468623a6da7d9ec9]::result::Result<(), rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>>
  50:     0x7fd5f8a89df2 - std[f1a6b9449c89a13c]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[e3f740b58bbe8d54]::util::run_in_thread_pool_with_globals<rustc_interface[e3f740b58bbe8d54]::interface::run_compiler<core[468623a6da7d9ec9]::result::Result<(), rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>, rustc_driver[be31376d096510e8]::run_compiler::{closure#1}>::{closure#0}, core[468623a6da7d9ec9]::result::Result<(), rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[468623a6da7d9ec9]::result::Result<(), rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>>
  51:     0x7fd5f90b759a - <<std[f1a6b9449c89a13c]::thread::Builder>::spawn_unchecked_<rustc_interface[e3f740b58bbe8d54]::util::run_in_thread_pool_with_globals<rustc_interface[e3f740b58bbe8d54]::interface::run_compiler<core[468623a6da7d9ec9]::result::Result<(), rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>, rustc_driver[be31376d096510e8]::run_compiler::{closure#1}>::{closure#0}, core[468623a6da7d9ec9]::result::Result<(), rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[468623a6da7d9ec9]::result::Result<(), rustc_errors[78faa046e0c9ca7a]::ErrorGuaranteed>>::{closure#1} as core[468623a6da7d9ec9]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  52:     0x7fd5f64fbc53 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h633e4f7ed41b131b
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/alloc/src/boxed.rs:2000:9
  53:     0x7fd5f64fbc53 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h46d52a363ffe776a
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/alloc/src/boxed.rs:2000:9
  54:     0x7fd5f64fbc53 - std::sys::unix::thread::Thread::new::thread_start::h6d9ed7daf86174ef
                               at /rustc/92c1937a90e5b6f20fa6e87016d6869da363972e/library/std/src/sys/unix/thread.rs:108:17
  55:     0x7fd5f63bf609 - start_thread
  56:     0x7fd5f62e2133 - clone
  57:                0x0 - <unknown>

error: internal compiler error: unexpected panic

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 1.68.0-nightly (92c1937a9 2022-12-27) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -C opt-level=3 -C embed-bitcode=no -C codegen-units=1

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

query stack during panic:
#0 [type_of] computing type of `_Container::cells`
#1 [inferred_outlives_crate] computing the inferred outlives predicates for items in this crate
#2 [inferred_outlives_of] computing inferred outlives predicates of `_Container`
#3 [predicates_defined_on] computing predicates of `_Container`
#4 [predicates_of] computing predicates of `_Container`
#5 [collect_mod_item_types] collecting item types in top-level module
#6 [analysis] running analysis passes on this crate
end of query stack
error: could not compile `playground`

[fmease]

Potential duplicate of #104719 which would be fixed by #104745.