[ICE]: "DefId::expect_local: … isn't local" in phantom-lifetime heavy traits

[chrysn]

Code

A lot, and possibly depending on out-of-crate traits. Right now it's in https://github.com/chrysn-pull-requests/ariel-os/blob/client-lifetime-ice/src/lib/coapcore/src/client_standalone.rs; to reproduce, check it out, go to src/lib/coapcore and run cargo check there.

Note that the 'ws lifetimes in there don't necessarily make sense – I'm fighting the b^W^W^W doing pair programming with the compiler, and think I'm falling in the "just one more lifetime will fix everything" trap (it usually doesn't fix anything). But AIU, ICE is a bug no matter how stupid my code is.

It's 01:20 in the morning and I can't minify this today; I'll try to get it small tom... today.

I have a small bit of hope that just from the error output – not expecting anyone would look at unminified code – someone manages to already find that this is a duplicate of something else, so I don't have to minify, but unless that happens, minification it is. I've looked at open issues and found #143498, but that appears to be about unstable features, whereas my crash is reproducible on beta (couldn't try stable yet b/c the workspace has a few quirks).

Meta

rustc --version --verbose:

rustc 1.96.0-nightly (ec818fda3 2026-03-02)
binary: rustc
commit-hash: ec818fda361ca216eb186f5cf45131bd9c776bb4
commit-date: 2026-03-02
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.0

but also on beta (but all other outputs are from nightly):

rustc 1.94.0-beta.6 (ec6e98837 2026-02-26)
binary: rustc
commit-hash: ec6e9883776ff64b7c97a33931f85cc02c31d05f
commit-date: 2026-02-26
host: x86_64-unknown-linux-gnu
release: 1.94.0-beta.6
LLVM version: 21.1.8

Error output

thread 'rustc' (4168347) panicked at /rustc-dev/ec818fda361ca216eb186f5cf45131bd9c776bb4/compiler/rustc_trait_selection/src/errors/note_and_explain.rs:26:22:
DefId::expect_local: `DefId(44:11 ~ coap_request[448f]::Request::build_request)` isn't local

Backtrace

(I didn't need to turn up backtraces; seems ICEs give full backtraces by default now).

    Checking coapcore v0.1.1 (/home/chrysn/git/crates/ariel-os-worktrees/client-lifetime-ice/src/lib/coapcore)

thread 'rustc' (4167884) panicked at /rustc-dev/ec818fda361ca216eb186f5cf45131bd9c776bb4/compiler/rustc_trait_selection/src/errors/note_and_explain.rs:26:22:
DefId::expect_local: `DefId(44:11 ~ coap_request[448f]::Request::build_request)` isn't local
stack backtrace:
   0:     0x7fd2b09532bb - <<std[eb110dcb1455ebbb]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[89784ade3600f19b]::fmt::Display>::fmt
   1:     0x7fd2b10202c8 - core[89784ade3600f19b]::fmt::write
   2:     0x7fd2b096a2c6 - <std[eb110dcb1455ebbb]::sys::stdio::unix::Stderr as std[eb110dcb1455ebbb]::io::Write>::write_fmt
   3:     0x7fd2b0929398 - std[eb110dcb1455ebbb]::panicking::default_hook::{closure#0}
   4:     0x7fd2b0946723 - std[eb110dcb1455ebbb]::panicking::default_hook
   5:     0x7fd2af93125c - std[eb110dcb1455ebbb]::panicking::update_hook::<alloc[29fb76bce34b892e]::boxed::Box<rustc_driver_impl[fb3bb71bcb51a2dc]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fd2b0946a02 - std[eb110dcb1455ebbb]::panicking::panic_with_hook
   7:     0x7fd2b0929458 - std[eb110dcb1455ebbb]::panicking::panic_handler::{closure#0}
   8:     0x7fd2b0920189 - std[eb110dcb1455ebbb]::sys::backtrace::__rust_end_short_backtrace::<std[eb110dcb1455ebbb]::panicking::panic_handler::{closure#0}, !>
   9:     0x7fd2b092aebd - __rustc[b5c62ea7d35cbfc4]::rust_begin_unwind
  10:     0x7fd2ad4ce2fc - core[89784ade3600f19b]::panicking::panic_fmt
  11:     0x7fd2b074fa1a - <rustc_trait_selection[bf9027be1a6a20ec]::errors::note_and_explain::RegionExplanation>::new
  12:     0x7fd2b071eb95 - <rustc_trait_selection[bf9027be1a6a20ec]::error_reporting::TypeErrCtxt>::report_concrete_failure
  13:     0x7fd2b071d068 - <rustc_trait_selection[bf9027be1a6a20ec]::error_reporting::TypeErrCtxt>::report_region_errors
  14:     0x7fd2b193e00c - <rustc_trait_selection[bf9027be1a6a20ec]::traits::engine::ObligationCtxt<rustc_trait_selection[bf9027be1a6a20ec]::traits::FulfillmentError>>::resolve_regions_and_report_errors::<indexmap[777de5bf500a086]::set::IndexSet<rustc_middle[35e622c0033dd948]::ty::Ty, rustc_hash[5d99512a11e6cdb]::FxBuildHasher>>
  15:     0x7fd2b195e0b8 - rustc_hir_analysis[eb8396faa04d2a8f]::check::compare_impl_item::check_type_bounds
  16:     0x7fd2b1951efa - rustc_hir_analysis[eb8396faa04d2a8f]::check::compare_impl_item::compare_impl_item
  17:     0x7fd2b194f3e9 - rustc_query_impl[52bc1cae8cb1f0f9]::query_impl::compare_impl_item::invoke_provider_fn::__rust_begin_short_backtrace
  18:     0x7fd2b1906119 - rustc_query_impl[52bc1cae8cb1f0f9]::execution::try_execute_query::<rustc_data_structures[d0096a8ffe6a0514]::vec_cache::VecCache<rustc_span[566afffa9b4ced27]::def_id::LocalDefId, rustc_middle[35e622c0033dd948]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[35e622c0033dd948]::dep_graph::graph::DepNodeIndex>, false>
  19:     0x7fd2b1905c0e - rustc_query_impl[52bc1cae8cb1f0f9]::query_impl::compare_impl_item::execute_query_non_incr::__rust_end_short_backtrace
  20:     0x7fd2b19090b2 - rustc_hir_analysis[eb8396faa04d2a8f]::check::check::check_item_type
  21:     0x7fd2b1906ba6 - rustc_hir_analysis[eb8396faa04d2a8f]::check::wfcheck::check_well_formed
  22:     0x7fd2b1906b87 - rustc_query_impl[52bc1cae8cb1f0f9]::query_impl::check_well_formed::invoke_provider_fn::__rust_begin_short_backtrace
  23:     0x7fd2b1906119 - rustc_query_impl[52bc1cae8cb1f0f9]::execution::try_execute_query::<rustc_data_structures[d0096a8ffe6a0514]::vec_cache::VecCache<rustc_span[566afffa9b4ced27]::def_id::LocalDefId, rustc_middle[35e622c0033dd948]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[35e622c0033dd948]::dep_graph::graph::DepNodeIndex>, false>
  24:     0x7fd2b1905e7b - rustc_query_impl[52bc1cae8cb1f0f9]::query_impl::check_well_formed::execute_query_non_incr::__rust_end_short_backtrace
  25:     0x7fd2b1902d7d - rustc_hir_analysis[eb8396faa04d2a8f]::check::wfcheck::check_type_wf
  26:     0x7fd2b1902c87 - rustc_query_impl[52bc1cae8cb1f0f9]::query_impl::check_type_wf::invoke_provider_fn::__rust_begin_short_backtrace
  27:     0x7fd2b221d097 - rustc_query_impl[52bc1cae8cb1f0f9]::execution::try_execute_query::<rustc_middle[35e622c0033dd948]::query::caches::SingleCache<rustc_middle[35e622c0033dd948]::query::erase::ErasedData<[u8; 1usize]>>, false>
  28:     0x7fd2b221cd69 - rustc_query_impl[52bc1cae8cb1f0f9]::query_impl::check_type_wf::execute_query_non_incr::__rust_end_short_backtrace
  29:     0x7fd2b1a08455 - rustc_hir_analysis[eb8396faa04d2a8f]::check_crate
  30:     0x7fd2b1174f93 - rustc_interface[e910a3c7e0b41bdb]::passes::analysis
  31:     0x7fd2b221ecda - rustc_query_impl[52bc1cae8cb1f0f9]::execution::try_execute_query::<rustc_middle[35e622c0033dd948]::query::caches::SingleCache<rustc_middle[35e622c0033dd948]::query::erase::ErasedData<[u8; 0usize]>>, false>
  32:     0x7fd2b221eaab - rustc_query_impl[52bc1cae8cb1f0f9]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  33:     0x7fd2b21de26d - rustc_interface[e910a3c7e0b41bdb]::interface::run_compiler::<(), rustc_driver_impl[fb3bb71bcb51a2dc]::run_compiler::{closure#0}>::{closure#1}
  34:     0x7fd2b220d4be - std[eb110dcb1455ebbb]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[e910a3c7e0b41bdb]::util::run_in_thread_with_globals<rustc_interface[e910a3c7e0b41bdb]::util::run_in_thread_pool_with_globals<rustc_interface[e910a3c7e0b41bdb]::interface::run_compiler<(), rustc_driver_impl[fb3bb71bcb51a2dc]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  35:     0x7fd2b220db20 - <std[eb110dcb1455ebbb]::thread::lifecycle::spawn_unchecked<rustc_interface[e910a3c7e0b41bdb]::util::run_in_thread_with_globals<rustc_interface[e910a3c7e0b41bdb]::util::run_in_thread_pool_with_globals<rustc_interface[e910a3c7e0b41bdb]::interface::run_compiler<(), rustc_driver_impl[fb3bb71bcb51a2dc]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[89784ade3600f19b]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  36:     0x7fd2b220e9ac - <std[eb110dcb1455ebbb]::sys::thread::unix::Thread>::new::thread_start
  37:     0x7fd2abc9cb7b - start_thread
                               at ./nptl/pthread_create.c:448:8
  38:     0x7fd2abd1a7b8 - __GI___clone3
                               at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:78:0
  39:                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: please make sure that you have updated to the latest nightly

note: please attach the file at `/home/chrysn/git/crates/ariel-os-worktrees/client-lifetime-ice/rustc-ice-2026-03-04T00_22_23-4167882.txt` to your bug report

note: rustc 1.96.0-nightly (ec818fda3 2026-03-02) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C opt-level=s -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 -C debug-assertions=on

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

query stack during panic:
#0 [compare_impl_item] checking assoc item `client_standalone::<impl at src/lib/coapcore/src/client_standalone.rs:104:1: 107:15>::build_request::{anon_assoc#0}` is compatible with trait definition
#1 [check_well_formed] checking that `client_standalone::<impl at src/lib/coapcore/src/client_standalone.rs:104:1: 107:15>` is well-formed
... and 2 other queries... use `env RUST_BACKTRACE=1` to see the full query stack
error: could not compile `coapcore` (lib)

Caused by:
  process didn't exit successfully: `/home/chrysn/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --crate-name coapcore --edition=2024 src/lib/coapcore/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=419 --crate-type lib --emit=dep-info,metadata -C opt-level=s -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --warn=rust_2018_idioms '--warn=clippy::pedantic' '--warn=clippy::unused_trait_names' '--deny=clippy::unused_result_ok' --deny=unused_must_use --deny=unsafe_op_in_unsafe_fn --deny=unsafe_code --allow=unexpected_cfgs '--warn=clippy::undocumented_unsafe_blocks' '--allow=clippy::too_many_lines' '--warn=clippy::too_long_first_doc_paragraph' --deny=private_interfaces --deny=private_bounds '--warn=clippy::missing_safety_doc' '--deny=clippy::indexing_slicing' '--warn=clippy::doc_paragraphs_missing_punctuation' '--warn=rustdoc::broken_intra_doc_links' -C debug-assertions=on --cfg 'feature="default"' --cfg 'feature="liboscore-defaults"' --cfg 'feature="liboscore-provide-abort"' --cfg 'feature="liboscore-provide-assert"' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("_nightly_docs", "_test", "default", "defmt", "liboscore-defaults", "liboscore-provide-abort", "liboscore-provide-assert", "log"))' -C metadata=8add47b4ef55fe8e -C extra-filename=-99c774bea8fe35b1 --out-dir /home/chrysn-cache/dot-cache/cargo-target/debug/deps -L dependency=/home/chrysn-cache/dot-cache/cargo-target/debug/deps --extern aes=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libaes-5bb9b23fbec4baa9.rmeta --extern arrayvec=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libarrayvec-a96e5f78150f7a0f.rmeta --extern ccm=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libccm-b10ac976b898d0b3.rmeta --extern coap_handler=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libcoap_handler-bc57d5491cddc9bc.rmeta --extern coap_message=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libcoap_message-e63988729416cfbb.rmeta --extern coap_message_implementations=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libcoap_message_implementations-396d4207cca59a52.rmeta --extern coap_message_utils=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libcoap_message_utils-5682998dc694705b.rmeta --extern coap_numbers=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libcoap_numbers-319a138ce99b34df.rmeta --extern coap_request=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libcoap_request-1e433c13c4af4691.rmeta --extern defmt_or_log=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libdefmt_or_log-ee06f1955b7e8f87.rmeta --extern document_features=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libdocument_features-52bac9d899dfb4ab.so --extern heapless=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libheapless-9fd7ae1cf814d250.rmeta --extern lakers=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/liblakers-6e8c9cf37ba6cec6.rmeta --extern lakers_crypto_rustcrypto=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/liblakers_crypto_rustcrypto-878c3a188a5f6f2b.rmeta --extern liboscore=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libliboscore-8f6a448b5896f77f.rmeta --extern minicbor=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libminicbor-bacc655ff0892c6b.rmeta --extern minicbor_adapters=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libminicbor_adapters-b0f99c0a8cb0afb9.rmeta --extern p256=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/libp256-81e9ddfc504871f3.rmeta --extern rand_core=/home/chrysn-cache/dot-cache/cargo-target/debug/deps/librand_core-93f2d37683537f78.rmeta -L native=/home/chrysn-cache/dot-cache/cargo-target/debug/build/liboscore-57651a036eee9ac4/out` (exit status: 101)

rustc-ice-2026-03-04T00_22_23-4167882.txt

[chrysn]

I've minified it as far as I could without creating forks of upstream crates -- and there are still dependencies in: If I move the lib.rs content of coap-request in to a module, all of a sudden the ICE is gone.