[ICE]: lifetime parameter for associated type in Trait impl for an Enum

[SleighPenguin22]

I was busy writing an impl for some trait that has an associated type.
I had one impl for this trait where the associated type had a lifetime parameter, e.g. type AssocType = FooStruct<'a>
By experimenting i found the following things:

• the panic occurs when the implementor is an Enum (a Struct implementor compiles just fine)
• the panic does not occur when the associated type has the 'static lifetime
• the panic occurs when the impl block has at least one generic lifetime, but also occurs when more lifetime and/or type parameters are added

The root issues seems the generic lifetime in the impl Trait block. When using some type with a lifetime param, i.e.

struct FooStruct<'a> {
    some_ref: &'a str
}

and using that as the associated type for the Enum like this:

impl<'a> Foo for FooEnum {
    type Bar = FooStruct<'a>;
}

also causes a panic.

In short, as long as a trait impl has a lifetime parameter, and that parameter is used for an associated type, I get a panic.
The panic does not occur in Beta or Nightly, only in Stable. I do not see it listed in the patch notes on releases.rs.

I might try to find the exact commit that caused it, it would be my first time trying something like that, would that be doable? I would love to contribute

Code

use std::marker::PhantomData;

pub trait Foo {
    type Bar;
}

pub enum FooEnum {}

impl<'a> Foo for FooEnum {
    type Bar = PhantomData<&'a ()>;
}

Meta

rustc --version --verbose:

rustc 1.97.1 (8bab26f4f 2026-07-14)
binary: rustc
commit-hash: 8bab26f4f68e0e26f0bb7960be334d5b520ea452
commit-date: 2026-07-14
host: x86_64-unknown-linux-gnu
release: 1.97.1
LLVM version: 22.1.6

Error output

   Compiling fuckaround v0.1.0 (/home/wannie/Desktop/fuckaround)

thread 'rustc' (133938) panicked at /rustc-dev/8bab26f4f68e0e26f0bb7960be334d5b520ea452/compiler/rustc_hir_analysis/src/errors/remove_or_use_generic.rs:154:85:
expect_struct: found Item { owner_id: DefId(0:6 ~ fuckaround[d31c]::FooEnum), kind: Enum(FooEnum#0, Generics { params: [], predicates: [], has_where_clause_predicates: false, where_clause_span: src/lib.rs:7:17: 7:17 (#0), span: src/lib.rs:7:17: 7:17 (#0) }, EnumDef { variants: [] }), span: src/lib.rs:7:1: 7:20 (#0), vis_span: src/lib.rs:7:1: 7:4 (#0), has_delayed_lints: false, eii: false }
stack backtrace:
   0:     0x7fbc5c4901f9 - <<std[1e3c4ec04c5261a9]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[37f591cfbe66b0b1]::fmt::Display>::fmt
   1:     0x7fbc5cc20cc8 - core[37f591cfbe66b0b1]::fmt::write
   2:     0x7fbc5e00d936 - <std[1e3c4ec04c5261a9]::sys::stdio::unix::Stderr as std[1e3c4ec04c5261a9]::io::Write>::write_fmt
   3:     0x7fbc5c4663fe - std[1e3c4ec04c5261a9]::panicking::default_hook::{closure#0}
   4:     0x7fbc5c483b33 - std[1e3c4ec04c5261a9]::panicking::default_hook
   5:     0x7fbc5b276c31 - std[1e3c4ec04c5261a9]::panicking::update_hook::<alloc[24b8200b0946867]::boxed::Box<rustc_driver_impl[4e5b0da0f4005eeb]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fbc5c483e12 - std[1e3c4ec04c5261a9]::panicking::panic_with_hook
   7:     0x7fbc5c4664c2 - std[1e3c4ec04c5261a9]::panicking::panic_handler::{closure#0}
   8:     0x7fbc5c45a9a9 - std[1e3c4ec04c5261a9]::sys::backtrace::__rust_end_short_backtrace::<std[1e3c4ec04c5261a9]::panicking::panic_handler::{closure#0}, !>
   9:     0x7fbc5c467f2d - __rustc[6ef78fba13ac776c]::rust_begin_unwind
  10:     0x7fbc58f192dc - core[37f591cfbe66b0b1]::panicking::panic_fmt
  11:     0x7fbc5b333a08 - rustc_hir[ffb6a75957ec3d1e]::hir::expect_failed::<&rustc_hir[ffb6a75957ec3d1e]::hir::Item>
  12:     0x7fbc5b335b8e - <rustc_hir[ffb6a75957ec3d1e]::hir::Item>::expect_struct
  13:     0x7fbc59b15145 - rustc_hir_analysis[1956eea595525479]::errors::remove_or_use_generic::suggest_to_remove_or_use_generic
  14:     0x7fbc5d3cc961 - rustc_hir_analysis[1956eea595525479]::check::check::check_item_type
  15:     0x7fbc5d3c1be6 - rustc_hir_analysis[1956eea595525479]::check::wfcheck::check_well_formed
  16:     0x7fbc5d3c1bc7 - rustc_query_impl[bdb42d22f02e134a]::query_impl::check_well_formed::invoke_provider_fn::__rust_begin_short_backtrace
  17:     0x7fbc5ce4d378 - rustc_query_impl[bdb42d22f02e134a]::execution::try_execute_query::<rustc_data_structures[6314a90323e5c320]::vec_cache::VecCache<rustc_span[d86c6dd34b0962f8]::def_id::LocalDefId, rustc_middle[8a940fbd8c5d50e6]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[8a940fbd8c5d50e6]::dep_graph::graph::DepNodeIndex>, true>
  18:     0x7fbc5ce4c9ba - rustc_query_impl[bdb42d22f02e134a]::query_impl::check_well_formed::execute_query_incr::__rust_end_short_backtrace
  19:     0x7fbc5d3bdc29 - rustc_hir_analysis[1956eea595525479]::check::wfcheck::check_type_wf
  20:     0x7fbc5d3bdb39 - rustc_query_impl[bdb42d22f02e134a]::query_impl::check_type_wf::invoke_provider_fn::__rust_begin_short_backtrace
  21:     0x7fbc5dfe80f1 - rustc_query_impl[bdb42d22f02e134a]::execution::try_execute_query::<rustc_middle[8a940fbd8c5d50e6]::query::caches::SingleCache<rustc_middle[8a940fbd8c5d50e6]::query::erase::ErasedData<[u8; 1usize]>>, true>
  22:     0x7fbc5dfe790b - rustc_query_impl[bdb42d22f02e134a]::query_impl::check_type_wf::execute_query_incr::__rust_end_short_backtrace
  23:     0x7fbc5cebb469 - rustc_hir_analysis[1956eea595525479]::check_crate
  24:     0x7fbc5ceb6e33 - rustc_interface[6818d3a8678689ee]::passes::analysis
  25:     0x7fbc5dd66c35 - rustc_query_impl[bdb42d22f02e134a]::execution::try_execute_query::<rustc_middle[8a940fbd8c5d50e6]::query::caches::SingleCache<rustc_middle[8a940fbd8c5d50e6]::query::erase::ErasedData<[u8; 0usize]>>, true>
  26:     0x7fbc5dd665ea - rustc_query_impl[bdb42d22f02e134a]::query_impl::analysis::execute_query_incr::__rust_end_short_backtrace
  27:     0x7fbc5dd26f1b - rustc_interface[6818d3a8678689ee]::interface::run_compiler::<(), rustc_driver_impl[4e5b0da0f4005eeb]::run_compiler::{closure#0}>::{closure#1}
  28:     0x7fbc5dd5407a - std[1e3c4ec04c5261a9]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[6818d3a8678689ee]::util::run_in_thread_with_globals<rustc_interface[6818d3a8678689ee]::util::run_in_thread_pool_with_globals<rustc_interface[6818d3a8678689ee]::interface::run_compiler<(), rustc_driver_impl[4e5b0da0f4005eeb]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  29:     0x7fbc5dd5476d - <std[1e3c4ec04c5261a9]::thread::lifecycle::spawn_unchecked<rustc_interface[6818d3a8678689ee]::util::run_in_thread_with_globals<rustc_interface[6818d3a8678689ee]::util::run_in_thread_pool_with_globals<rustc_interface[6818d3a8678689ee]::interface::run_compiler<(), rustc_driver_impl[4e5b0da0f4005eeb]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[37f591cfbe66b0b1]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  30:     0x7fbc5dd5556c - <std[1e3c4ec04c5261a9]::sys::thread::unix::Thread>::new::thread_start
  31:     0x7fbc5729af30 - start_thread
  32:     0x7fbc5731ec2c - __clone3
  33:                0x0 - <unknown>

error: 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 1.97.1 (8bab26f4f 2026-07-14) 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 [check_well_formed] checking that `<impl at src/lib.rs:9:1: 9:25>` is well-formed
#1 [check_type_wf] checking that types are well-formed
... and 1 other queries... use `env RUST_BACKTRACE=1` to see the full query stack
error: could not compile `fuckaround` (lib)

Caused by:
  process didn't exit successfully: `/home/wannie/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/rustc --crate-name fuckaround --edition=2024 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=270 --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=b197715107c73955 -C extra-filename=-01acd1037278aaa7 --out-dir /home/wannie/Desktop/fuckaround/target/debug/deps -C incremental=/home/wannie/Desktop/fuckaround/target/debug/incremental -L dependency=/home/wannie/Desktop/fuckaround/target/debug/deps --extern log=/home/wannie/Desktop/fuckaround/target/debug/deps/liblog-8994e55502ea410c.rmeta` (exit status: 101)

Backtrace

   Compiling fuckaround v0.1.0 (/home/wannie/Desktop/fuckaround)

thread 'rustc' (132770) panicked at /rustc-dev/8bab26f4f68e0e26f0bb7960be334d5b520ea452/compiler/rustc_hir_analysis/src/errors/remove_or_use_generic.rs:154:85:
expect_struct: found Item { owner_id: DefId(0:9 ~ fuckaround[d31c]::FooEnum), kind: Enum(FooEnum#0, Generics { params: [], predicates: [], has_where_clause_predicates: false, where_clause_span: src/lib.rs:11:17: 11:17 (#0), span: src/lib.rs:11:17: 11:17 (#0) }, EnumDef { variants: [Variant { ident: Var1#0, hir_id: HirId(DefId(0:9 ~ fuckaround[d31c]::FooEnum).1), def_id: DefId(0:10 ~ fuckaround[d31c]::FooEnum::Var1), data: Unit(HirId(DefId(0:9 ~ fuckaround[d31c]::FooEnum).2), DefId(0:11 ~ fuckaround[d31c]::FooEnum::Var1::{constructor#0})), disr_expr: None, span: src/lib.rs:12:5: 12:9 (#0) }] }), span: src/lib.rs:11:1: 13:2 (#0), vis_span: src/lib.rs:11:1: 11:4 (#0), has_delayed_lints: false, eii: false }
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: rustc_hir::hir::expect_failed::<&rustc_hir::hir::Item>
   3: <rustc_hir::hir::Item>::expect_struct
   4: rustc_hir_analysis::errors::remove_or_use_generic::suggest_to_remove_or_use_generic
   5: rustc_hir_analysis::check::check::check_item_type
   6: rustc_hir_analysis::check::wfcheck::check_well_formed
      [... omitted 1 frame ...]
   7: rustc_hir_analysis::check::wfcheck::check_type_wf
      [... omitted 1 frame ...]
   8: rustc_hir_analysis::check_crate
   9: rustc_interface::passes::analysis
  10: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 0]>>, true>
  11: 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.

error: 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 1.97.1 (8bab26f4f 2026-07-14) 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 [check_well_formed] checking that `<impl at src/lib.rs:15:1: 15:25>` is well-formed
#1 [check_type_wf] checking that types are well-formed
#2 [analysis] running analysis passes on crate `fuckaround`
end of query stack
error: could not compile `fuckaround` (lib)

Caused by:
  process didn't exit successfully: `/home/wannie/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/rustc --crate-name fuckaround --edition=2024 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=270 --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=b197715107c73955 -C extra-filename=-01acd1037278aaa7 --out-dir /home/wannie/Desktop/fuckaround/target/debug/deps -C incremental=/home/wannie/Desktop/fuckaround/target/debug/incremental -L dependency=/home/wannie/Desktop/fuckaround/target/debug/deps --extern log=/home/wannie/Desktop/fuckaround/target/debug/deps/liblog-8994e55502ea410c.rmeta` (exit status: 101)

[theemathas]

I might try to find the exact commit that caused it

Use https://rust-lang.github.io/cargo-bisect-rustc/. I'd recommend just trying to find the commit that fixed it though, which might indicate whether the problem is truly fixed or if it just papered over the problem. Fixes like this usually do not show up in the release notes.

[SleighPenguin22]

Thanks!

[bjorn3]

Was this fixed by #157846?

[SleighPenguin22]

Yes it was! Thanks