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ICE - Mixing Async & GAT traits (invalid code)

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A-GATs C-bug I-ICE S-bug-has-test S-has-mcve T-compiler T-types
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Description

Code
pub trait AsyncLend {
    type From<'a>
    where
        Self: 'a;
    type Into<'a>
    where
        Self: 'a;

    async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
}

pub struct AsyncLend2<T, F, I>(T, core::marker::PhantomData<fn() -> (F, I)>);

impl<T, F, I> AsyncLend for AsyncLend2<T, F, I>
where
    T: AsyncFnMut(&mut F) -> I,
{
    type From<'a> = F;
    type Into<'a> = I;

    async fn lend(self, from: Self::From<'_>) -> Self::Into<'_> {
        (self.0)(from).await
    }
}
Meta

Exists on stable/beta/nightly

rustc --version --verbose:

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-unknown-linux-gnu
release: 1.86.0
LLVM version: 19.1.7
Error output
   Compiling ice2 v0.1.0 (/var/home/jeron/Code/test/ice2)
error[E0309]: the parameter type `T` may not live long enough
  --> src/lib.rs:18:21
   |
18 |     type From<'a> = F;
   |               --    ^ ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds
   |               |
   |               the parameter type `T` must be valid for the lifetime `'a` as defined here...
   |
help: consider adding an explicit lifetime bound
   |
18 |     type From<'a> = F where T: 'a;
   |                       +++++++++++

error[E0309]: the parameter type `F` may not live long enough
  --> src/lib.rs:18:21
   |
18 |     type From<'a> = F;
   |               --    ^ ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds
   |               |
   |               the parameter type `F` must be valid for the lifetime `'a` as defined here...
   |
help: consider adding an explicit lifetime bound
   |
18 |     type From<'a> = F where F: 'a;
   |                       +++++++++++

error[E0309]: the parameter type `I` may not live long enough
  --> src/lib.rs:18:21
   |
18 |     type From<'a> = F;
   |               --    ^ ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds
   |               |
   |               the parameter type `I` must be valid for the lifetime `'a` as defined here...
   |
help: consider adding an explicit lifetime bound
   |
18 |     type From<'a> = F where I: 'a;
   |                       +++++++++++

error[E0309]: the parameter type `T` may not live long enough
  --> src/lib.rs:19:21
   |
19 |     type Into<'a> = I;
   |               --    ^ ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds
   |               |
   |               the parameter type `T` must be valid for the lifetime `'a` as defined here...
   |
help: consider adding an explicit lifetime bound
   |
19 |     type Into<'a> = I where T: 'a;
   |                       +++++++++++

error[E0309]: the parameter type `F` may not live long enough
  --> src/lib.rs:19:21
   |
19 |     type Into<'a> = I;
   |               --    ^ ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds
   |               |
   |               the parameter type `F` must be valid for the lifetime `'a` as defined here...
   |
help: consider adding an explicit lifetime bound
   |
19 |     type Into<'a> = I where F: 'a;
   |                       +++++++++++

error[E0309]: the parameter type `I` may not live long enough
  --> src/lib.rs:19:21
   |
19 |     type Into<'a> = I;
   |               --    ^ ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds
   |               |
   |               the parameter type `I` must be valid for the lifetime `'a` as defined here...
   |
help: consider adding an explicit lifetime bound
   |
19 |     type Into<'a> = I where I: 'a;
   |                       +++++++++++

error[E0308]: mismatched types
  --> src/lib.rs:22:18
   |
14 | impl<T, F, I> AsyncLend for AsyncLend2<T, F, I>
   |         - found this type parameter
...
22 |         (self.0)(from).await
   |         -------- ^^^^ expected `&mut F`, found type parameter `F`
   |         |
   |         arguments to this function are incorrect
   |
   = note: expected mutable reference `&mut _`
                 found type parameter `_`
note: method defined here
  --> /rustc/05f9846f893b09a1be1fc8560e33fc3c815cfecb/library/core/src/ops/async_function.rs:34:27
help: consider mutably borrowing here
   |
22 |         (self.0)(&mut from).await
   |                  ++++

error[E0311]: the parameter type `T` may not live long enough
 --> src/lib.rs:9:31
  |
9 |     async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
  |                               ^^^^^^^^^^^--^
  |                               |          |
  |                               |          the parameter type `T` must be valid for the anonymous lifetime as defined here...
  |                               ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds...
  |
note: ...that is required by this bound
 --> src/lib.rs:4:15
  |
4 |         Self: 'a;
  |               ^^
help: consider adding an explicit lifetime bound
  |
9 -     async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
9 +     async fn lend<'a>(self, from: Self::From<'a>) -> Self::Into<'a> where T: 'a;
  |

error[E0311]: the parameter type `F` may not live long enough
 --> src/lib.rs:9:31
  |
9 |     async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
  |                               ^^^^^^^^^^^--^
  |                               |          |
  |                               |          the parameter type `F` must be valid for the anonymous lifetime as defined here...
  |                               ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds...
  |
note: ...that is required by this bound
 --> src/lib.rs:4:15
  |
4 |         Self: 'a;
  |               ^^
help: consider adding an explicit lifetime bound
  |
9 -     async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
9 +     async fn lend<'a>(self, from: Self::From<'a>) -> Self::Into<'a> where F: 'a;
  |

error[E0311]: the parameter type `I` may not live long enough
 --> src/lib.rs:9:31
  |
9 |     async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
  |                               ^^^^^^^^^^^--^
  |                               |          |
  |                               |          the parameter type `I` must be valid for the anonymous lifetime as defined here...
  |                               ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds...
  |
note: ...that is required by this bound
 --> src/lib.rs:4:15
  |
4 |         Self: 'a;
  |               ^^
help: consider adding an explicit lifetime bound
  |
9 -     async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
9 +     async fn lend<'a>(self, from: Self::From<'a>) -> Self::Into<'a> where I: 'a;
  |

error: internal compiler error: compiler/rustc_middle/src/ty/generics.rs:249:17: expected lifetime parameter, but found another generic parameter: GenericParamDef {
                                    name: "F",
                                    def_id: DefId(0:19 ~ ice2[b533]::{impl#0}::F),
                                    index: 1,
                                    pure_wrt_drop: false,
                                    kind: Type {
                                        has_default: false,
                                        synthetic: false,
                                    },
                                }


thread 'rustc' panicked at compiler/rustc_middle/src/ty/generics.rs:249:17:
Box<dyn Any>
stack backtrace:
   0:     0x7f4f75dd6e40 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h6d42cc84fc840290
   1:     0x7f4f7662867c - core::fmt::write::h5af61a909e3ec64d
   2:     0x7f4f7792ca51 - std::io::Write::write_fmt::h5a7b54aa6e4a315d
   3:     0x7f4f75dd6ca2 - std::sys::backtrace::BacktraceLock::print::h555579e7396c26ac
   4:     0x7f4f75dd9122 - std::panicking::default_hook::{{closure}}::h9128866118196224
   5:     0x7f4f75dd8faa - std::panicking::default_hook::h52e9e7314e0255f6
   6:     0x7f4f74f33449 - std[d9e466a2d75004a2]::panicking::update_hook::<alloc[54bfe2542ace865d]::boxed::Box<rustc_driver_impl[680c351c5444d7cd]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7f4f75dd9ca3 - std::panicking::rust_panic_with_hook::h541791bcc774ef34
   8:     0x7f4f74f6e3b1 - std[d9e466a2d75004a2]::panicking::begin_panic::<rustc_errors[90b6e0e1196d63f0]::ExplicitBug>::{closure#0}
   9:     0x7f4f74f63326 - std[d9e466a2d75004a2]::sys::backtrace::__rust_end_short_backtrace::<std[d9e466a2d75004a2]::panicking::begin_panic<rustc_errors[90b6e0e1196d63f0]::ExplicitBug>::{closure#0}, !>
  10:     0x7f4f74f6330d - std[d9e466a2d75004a2]::panicking::begin_panic::<rustc_errors[90b6e0e1196d63f0]::ExplicitBug>
  11:     0x7f4f74f78381 - <rustc_errors[90b6e0e1196d63f0]::diagnostic::BugAbort as rustc_errors[90b6e0e1196d63f0]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x7f4f7554ff23 - rustc_middle[a78c8f6c75429410]::util::bug::opt_span_bug_fmt::<rustc_span[15ded62729b25549]::span_encoding::Span>::{closure#0}
  13:     0x7f4f75538d6a - rustc_middle[a78c8f6c75429410]::ty::context::tls::with_opt::<rustc_middle[a78c8f6c75429410]::util::bug::opt_span_bug_fmt<rustc_span[15ded62729b25549]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7f4f75538bfb - rustc_middle[a78c8f6c75429410]::ty::context::tls::with_context_opt::<rustc_middle[a78c8f6c75429410]::ty::context::tls::with_opt<rustc_middle[a78c8f6c75429410]::util::bug::opt_span_bug_fmt<rustc_span[15ded62729b25549]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7f4f73ebcf90 - rustc_middle[a78c8f6c75429410]::util::bug::bug_fmt
  16:     0x7f4f77cea415 - <rustc_middle[a78c8f6c75429410]::ty::generics::Generics>::region_param.cold
  17:     0x7f4f72c0e7d7 - <rustc_middle[a78c8f6c75429410]::ty::region::Region>::opt_param_def_id
  18:     0x7f4f75539b18 - <rustc_middle[a78c8f6c75429410]::ty::context::TyCtxt>::is_suitable_region
  19:     0x7f4f75bf0a66 - <rustc_trait_selection[2bf3b2046bc969f2]::error_reporting::TypeErrCtxt>::construct_generic_bound_failure
  20:     0x7f4f75bec78e - <rustc_trait_selection[2bf3b2046bc969f2]::error_reporting::TypeErrCtxt>::report_region_errors
  21:     0x7f4f76ed0436 - <rustc_trait_selection[2bf3b2046bc969f2]::traits::engine::ObligationCtxt<rustc_trait_selection[2bf3b2046bc969f2]::traits::FulfillmentError>>::resolve_regions_and_report_errors::<indexmap[86021827789b9189]::set::IndexSet<rustc_middle[a78c8f6c75429410]::ty::Ty, core[46aa9df3d3dcdeb1]::hash::BuildHasherDefault<rustc_hash[33e007ae09964f4c]::FxHasher>>>
  22:     0x7f4f76c5c968 - rustc_hir_analysis[f9dd0ea7be29e3be]::check::compare_impl_item::check_type_bounds
  23:     0x7f4f76c52f3f - rustc_hir_analysis[f9dd0ea7be29e3be]::check::compare_impl_item::compare_impl_item
  24:     0x7f4f76c51515 - rustc_query_impl[abff21d8349146d]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[abff21d8349146d]::query_impl::compare_impl_item::dynamic_query::{closure#2}::{closure#0}, rustc_middle[a78c8f6c75429410]::query::erase::Erased<[u8; 1usize]>>
  25:     0x7f4f76a9442b - rustc_query_system[9464b8c501ef87f3]::query::plumbing::try_execute_query::<rustc_query_impl[abff21d8349146d]::DynamicConfig<rustc_data_structures[6e992f72641bf6c6]::vec_cache::VecCache<rustc_span[15ded62729b25549]::def_id::LocalDefId, rustc_middle[a78c8f6c75429410]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[9464b8c501ef87f3]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[abff21d8349146d]::plumbing::QueryCtxt, true>
  26:     0x7f4f76a9664b - rustc_query_impl[abff21d8349146d]::query_impl::compare_impl_item::get_query_incr::__rust_end_short_backtrace
  27:     0x7f4f7762fc2d - rustc_hir_analysis[f9dd0ea7be29e3be]::check::check::check_item_type
  28:     0x7f4f76c70ccd - rustc_hir_analysis[f9dd0ea7be29e3be]::check::wfcheck::check_well_formed
  29:     0x7f4f76c6f8d1 - rustc_query_impl[abff21d8349146d]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[abff21d8349146d]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[a78c8f6c75429410]::query::erase::Erased<[u8; 1usize]>>
  30:     0x7f4f76a94827 - rustc_query_system[9464b8c501ef87f3]::query::plumbing::try_execute_query::<rustc_query_impl[abff21d8349146d]::DynamicConfig<rustc_data_structures[6e992f72641bf6c6]::vec_cache::VecCache<rustc_span[15ded62729b25549]::def_id::LocalDefId, rustc_middle[a78c8f6c75429410]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[9464b8c501ef87f3]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[abff21d8349146d]::plumbing::QueryCtxt, true>
  31:     0x7f4f76a93cae - rustc_query_impl[abff21d8349146d]::query_impl::check_well_formed::get_query_incr::__rust_end_short_backtrace
  32:     0x7f4f76c6c96c - rustc_hir_analysis[f9dd0ea7be29e3be]::check::wfcheck::check_mod_type_wf
  33:     0x7f4f76c6c78b - rustc_query_impl[abff21d8349146d]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[abff21d8349146d]::query_impl::check_mod_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[a78c8f6c75429410]::query::erase::Erased<[u8; 1usize]>>
  34:     0x7f4f7756855a - rustc_query_system[9464b8c501ef87f3]::query::plumbing::try_execute_query::<rustc_query_impl[abff21d8349146d]::DynamicConfig<rustc_query_system[9464b8c501ef87f3]::query::caches::DefaultCache<rustc_span[15ded62729b25549]::def_id::LocalModDefId, rustc_middle[a78c8f6c75429410]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[abff21d8349146d]::plumbing::QueryCtxt, true>
  35:     0x7f4f77568f9a - rustc_query_impl[abff21d8349146d]::query_impl::check_mod_type_wf::get_query_incr::__rust_end_short_backtrace
  36:     0x7f4f76a079aa - rustc_hir_analysis[f9dd0ea7be29e3be]::check_crate
  37:     0x7f4f76a028ab - rustc_interface[84ab11ffe8dacd23]::passes::run_required_analyses
  38:     0x7f4f77118f1e - rustc_interface[84ab11ffe8dacd23]::passes::analysis
  39:     0x7f4f77118eef - rustc_query_impl[abff21d8349146d]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[abff21d8349146d]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[a78c8f6c75429410]::query::erase::Erased<[u8; 0usize]>>
  40:     0x7f4f7764ef93 - rustc_query_system[9464b8c501ef87f3]::query::plumbing::try_execute_query::<rustc_query_impl[abff21d8349146d]::DynamicConfig<rustc_query_system[9464b8c501ef87f3]::query::caches::SingleCache<rustc_middle[a78c8f6c75429410]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[abff21d8349146d]::plumbing::QueryCtxt, true>
  41:     0x7f4f7764e915 - rustc_query_impl[abff21d8349146d]::query_impl::analysis::get_query_incr::__rust_end_short_backtrace
  42:     0x7f4f7759aea9 - rustc_interface[84ab11ffe8dacd23]::passes::create_and_enter_global_ctxt::<core[46aa9df3d3dcdeb1]::option::Option<rustc_interface[84ab11ffe8dacd23]::queries::Linker>, rustc_driver_impl[680c351c5444d7cd]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  43:     0x7f4f775ab726 - rustc_interface[84ab11ffe8dacd23]::interface::run_compiler::<(), rustc_driver_impl[680c351c5444d7cd]::run_compiler::{closure#0}>::{closure#1}
  44:     0x7f4f774e0744 - std[d9e466a2d75004a2]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[84ab11ffe8dacd23]::util::run_in_thread_with_globals<rustc_interface[84ab11ffe8dacd23]::util::run_in_thread_pool_with_globals<rustc_interface[84ab11ffe8dacd23]::interface::run_compiler<(), rustc_driver_impl[680c351c5444d7cd]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  45:     0x7f4f774e0419 - <<std[d9e466a2d75004a2]::thread::Builder>::spawn_unchecked_<rustc_interface[84ab11ffe8dacd23]::util::run_in_thread_with_globals<rustc_interface[84ab11ffe8dacd23]::util::run_in_thread_pool_with_globals<rustc_interface[84ab11ffe8dacd23]::interface::run_compiler<(), rustc_driver_impl[680c351c5444d7cd]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[46aa9df3d3dcdeb1]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  46:     0x7f4f774dfbaf - std::sys::pal::unix::thread::Thread::new::thread_start::hcc5ed016d554f327
  47:     0x7f4f7167f1d4 - start_thread
  48:     0x7f4f71701cec - __clone3
  49:                0x0 - <unknown>

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 1.86.0 (05f9846f8 2025-03-31) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED]

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

query stack during panic:
#0 [compare_impl_item] checking assoc item `<impl at src/lib.rs:14:1: 16:32>::{synthetic#0}` is compatible with trait definition
#1 [check_well_formed] checking that `<impl at src/lib.rs:14:1: 16:32>` is well-formed
... and 2 other queries... use `env RUST_BACKTRACE=1` to see the full query stack
Some errors have detailed explanations: E0308, E0309, E0311.
For more information about an error, try `rustc --explain E0308`.
error: could not compile `ice2` (lib) due to 10 previous errors
Backtrace 

   Compiling ice2 v0.1.0 (/var/home/jeron/Code/test/ice2)
error[E0309]: the parameter type `T` may not live long enough
  --> src/lib.rs:18:21
   |
18 |     type From<'a> = F;
   |               --    ^ ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds
   |               |
   |               the parameter type `T` must be valid for the lifetime `'a` as defined here...
   |
help: consider adding an explicit lifetime bound
   |
18 |     type From<'a> = F where T: 'a;
   |                       +++++++++++

error[E0309]: the parameter type `F` may not live long enough
  --> src/lib.rs:18:21
   |
18 |     type From<'a> = F;
   |               --    ^ ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds
   |               |
   |               the parameter type `F` must be valid for the lifetime `'a` as defined here...
   |
help: consider adding an explicit lifetime bound
   |
18 |     type From<'a> = F where F: 'a;
   |                       +++++++++++

error[E0309]: the parameter type `I` may not live long enough
  --> src/lib.rs:18:21
   |
18 |     type From<'a> = F;
   |               --    ^ ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds
   |               |
   |               the parameter type `I` must be valid for the lifetime `'a` as defined here...
   |
help: consider adding an explicit lifetime bound
   |
18 |     type From<'a> = F where I: 'a;
   |                       +++++++++++

error[E0309]: the parameter type `T` may not live long enough
  --> src/lib.rs:19:21
   |
19 |     type Into<'a> = I;
   |               --    ^ ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds
   |               |
   |               the parameter type `T` must be valid for the lifetime `'a` as defined here...
   |
help: consider adding an explicit lifetime bound
   |
19 |     type Into<'a> = I where T: 'a;
   |                       +++++++++++

error[E0309]: the parameter type `F` may not live long enough
  --> src/lib.rs:19:21
   |
19 |     type Into<'a> = I;
   |               --    ^ ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds
   |               |
   |               the parameter type `F` must be valid for the lifetime `'a` as defined here...
   |
help: consider adding an explicit lifetime bound
   |
19 |     type Into<'a> = I where F: 'a;
   |                       +++++++++++

error[E0309]: the parameter type `I` may not live long enough
  --> src/lib.rs:19:21
   |
19 |     type Into<'a> = I;
   |               --    ^ ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds
   |               |
   |               the parameter type `I` must be valid for the lifetime `'a` as defined here...
   |
help: consider adding an explicit lifetime bound
   |
19 |     type Into<'a> = I where I: 'a;
   |                       +++++++++++

error[E0308]: mismatched types
  --> src/lib.rs:22:18
   |
14 | impl<T, F, I> AsyncLend for AsyncLend2<T, F, I>
   |         - found this type parameter
...
22 |         (self.0)(from).await
   |         -------- ^^^^ expected `&mut F`, found type parameter `F`
   |         |
   |         arguments to this function are incorrect
   |
   = note: expected mutable reference `&mut _`
                 found type parameter `_`
note: method defined here
  --> /rustc/05f9846f893b09a1be1fc8560e33fc3c815cfecb/library/core/src/ops/async_function.rs:34:27
help: consider mutably borrowing here
   |
22 |         (self.0)(&mut from).await
   |                  ++++

error[E0311]: the parameter type `T` may not live long enough
 --> src/lib.rs:9:31
  |
9 |     async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
  |                               ^^^^^^^^^^^--^
  |                               |          |
  |                               |          the parameter type `T` must be valid for the anonymous lifetime as defined here...
  |                               ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds...
  |
note: ...that is required by this bound
 --> src/lib.rs:4:15
  |
4 |         Self: 'a;
  |               ^^
help: consider adding an explicit lifetime bound
  |
9 -     async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
9 +     async fn lend<'a>(self, from: Self::From<'a>) -> Self::Into<'a> where T: 'a;
  |

error[E0311]: the parameter type `F` may not live long enough
 --> src/lib.rs:9:31
  |
9 |     async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
  |                               ^^^^^^^^^^^--^
  |                               |          |
  |                               |          the parameter type `F` must be valid for the anonymous lifetime as defined here...
  |                               ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds...
  |
note: ...that is required by this bound
 --> src/lib.rs:4:15
  |
4 |         Self: 'a;
  |               ^^
help: consider adding an explicit lifetime bound
  |
9 -     async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
9 +     async fn lend<'a>(self, from: Self::From<'a>) -> Self::Into<'a> where F: 'a;
  |

error[E0311]: the parameter type `I` may not live long enough
 --> src/lib.rs:9:31
  |
9 |     async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
  |                               ^^^^^^^^^^^--^
  |                               |          |
  |                               |          the parameter type `I` must be valid for the anonymous lifetime as defined here...
  |                               ...so that the type `AsyncLend2<T, F, I>` will meet its required lifetime bounds...
  |
note: ...that is required by this bound
 --> src/lib.rs:4:15
  |
4 |         Self: 'a;
  |               ^^
help: consider adding an explicit lifetime bound
  |
9 -     async fn lend(self, from: Self::From<'_>) -> Self::Into<'_>;
9 +     async fn lend<'a>(self, from: Self::From<'a>) -> Self::Into<'a> where I: 'a;
  |

error: internal compiler error: compiler/rustc_middle/src/ty/generics.rs:249:17: expected lifetime parameter, but found another generic parameter: GenericParamDef {
                                    name: "F",
                                    def_id: DefId(0:19 ~ ice2[b533]::{impl#0}::F),
                                    index: 1,
                                    pure_wrt_drop: false,
                                    kind: Type {
                                        has_default: false,
                                        synthetic: false,
                                    },
                                }


thread 'rustc' panicked at compiler/rustc_middle/src/ty/generics.rs:249:17:
Box<dyn Any>
stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: <rustc_errors::diagnostic::BugAbort as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
   2: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
   3: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
   4: rustc_middle::ty::context::tls::with_context_opt::<rustc_middle::ty::context::tls::with_opt<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
   5: rustc_middle::util::bug::bug_fmt
   6: <rustc_middle::ty::generics::Generics>::region_param.cold
   7: <rustc_middle::ty::region::Region>::opt_param_def_id
   8: <rustc_middle::ty::context::TyCtxt>::is_suitable_region
   9: <rustc_trait_selection::error_reporting::TypeErrCtxt>::construct_generic_bound_failure
  10: <rustc_trait_selection::error_reporting::TypeErrCtxt>::report_region_errors
  11: <rustc_trait_selection::traits::engine::ObligationCtxt<rustc_trait_selection::traits::FulfillmentError>>::resolve_regions_and_report_errors::<indexmap::set::IndexSet<rustc_middle::ty::Ty, core::hash::BuildHasherDefault<rustc_hash::FxHasher>>>
  12: rustc_hir_analysis::check::compare_impl_item::check_type_bounds
  13: rustc_hir_analysis::check::compare_impl_item::compare_impl_item
      [... omitted 1 frame ...]
  14: rustc_hir_analysis::check::check::check_item_type
  15: rustc_hir_analysis::check::wfcheck::check_well_formed
      [... omitted 1 frame ...]
  16: rustc_hir_analysis::check::wfcheck::check_mod_type_wf
      [... omitted 1 frame ...]
  17: rustc_hir_analysis::check_crate
  18: rustc_interface::passes::run_required_analyses
  19: rustc_interface::passes::analysis
      [... omitted 1 frame ...]
  20: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  21: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 1.86.0 (05f9846f8 2025-03-31) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED]

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

query stack during panic:
#0 [compare_impl_item] checking assoc item `<impl at src/lib.rs:14:1: 16:32>::{synthetic#0}` is compatible with trait definition
#1 [check_well_formed] checking that `<impl at src/lib.rs:14:1: 16:32>` is well-formed
#2 [check_mod_type_wf] checking that types are well-formed in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack
Some errors have detailed explanations: E0308, E0309, E0311.
For more information about an error, try `rustc --explain E0308`.
error: could not compile `ice2` (lib) due to 10 previous errors

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the minimized example on the stable, beta, and nightly toolchains first. Read compiler/rustc_middle/src/ty/generics.rs around line 249 and the rustc_hir_analysis compare_impl_item path shown in the backtrace; the work is complete when this invalid code produces diagnostics without an internal compiler error.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
38/100

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