ICE: autodiff: `segments.len() == 1`

[matthiaskrgr]

auto-reduced (treereduce-rust):

#[core::autodiff::autodiff_forward(::autodiff)]
fn main() {}

original:

#![feature(autodiff)]
#[core::autodiff::autodiff_forward(::autodiff)]
fn main() {}

Version information

rustc 1.94.0-nightly (2ca7bcd03 2025-12-23)
binary: rustc
commit-hash: 2ca7bcd03b87b52f7055a59b817443b0ac4a530d
commit-date: 2025-12-23
host: x86_64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.8

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output

error[E0658]: use of unstable library feature `autodiff`
 --> /tmp/icemaker_global_tempdir.4K5QMhZtihrW/rustc_testrunner_tmpdir_reporting.eRqldW7npjLk/mvce.rs:1:3
  |
1 | #[core::autodiff::autodiff_forward(::autodiff)]
  |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #124509 <https://github.com/rust-lang/rust/issues/124509> for more information
  = help: add `#![feature(autodiff)]` to the crate attributes to enable
  = note: this compiler was built on 2025-12-23; consider upgrading it if it is out of date


thread 'rustc' (3569183) panicked at /rustc-dev/2ca7bcd03b87b52f7055a59b817443b0ac4a530d/compiler/rustc_builtin_macros/src/autodiff.rs:59:9:
assertion failed: segments.len() == 1
stack backtrace:
   0:     0x7f7310c2c3a3 - <<std[243ef3771c46e43a]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[c088ff38143eb53e]::fmt::Display>::fmt
   1:     0x7f7311210648 - core[c088ff38143eb53e]::fmt::write
   2:     0x7f7310c42af6 - <std[243ef3771c46e43a]::sys::stdio::unix::Stderr as std[243ef3771c46e43a]::io::Write>::write_fmt
   3:     0x7f7310c02b48 - std[243ef3771c46e43a]::panicking::default_hook::{closure#0}
   4:     0x7f7310c20213 - std[243ef3771c46e43a]::panicking::default_hook
   5:     0x7f730fc10d77 - std[243ef3771c46e43a]::panicking::update_hook::<alloc[baa86124b2c3e6f2]::boxed::Box<rustc_driver_impl[e986b619aabff65e]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f7310c204f2 - std[243ef3771c46e43a]::panicking::panic_with_hook
   7:     0x7f7310c02c3a - std[243ef3771c46e43a]::panicking::panic_handler::{closure#0}
   8:     0x7f7310bf9c19 - std[243ef3771c46e43a]::sys::backtrace::__rust_end_short_backtrace::<std[243ef3771c46e43a]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f7310c046fd - __rustc[6d0b6af41dd8415b]::rust_begin_unwind
  10:     0x7f730dde611c - core[c088ff38143eb53e]::panicking::panic_fmt
  11:     0x7f730d3bd872 - core[c088ff38143eb53e]::panicking::panic
  12:     0x7f730f9a21ca - rustc_builtin_macros[c43dc9e402b49fb2]::autodiff::llvm_enzyme::first_ident
  13:     0x7f730f9a5a9c - rustc_builtin_macros[c43dc9e402b49fb2]::autodiff::llvm_enzyme::expand_with_mode
  14:     0x7f730f9b8198 - <rustc_builtin_macros[c43dc9e402b49fb2]::autodiff::llvm_enzyme::expand_forward as rustc_expand[7ba574a324a285b6]::base::MultiItemModifier>::expand
  15:     0x7f731281a288 - <rustc_expand[7ba574a324a285b6]::expand::MacroExpander>::fully_expand_fragment
  16:     0x7f73127fec12 - <rustc_expand[7ba574a324a285b6]::expand::MacroExpander>::expand_crate
  17:     0x7f73119715f3 - rustc_interface[577997324878ea43]::passes::configure_and_expand
  18:     0x7f731241bad8 - rustc_interface[577997324878ea43]::passes::resolver_for_lowering_raw
  19:     0x7f731241b84d - rustc_query_impl[3af702bc24a35b84]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[3af702bc24a35b84]::query_impl::resolver_for_lowering_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle[395a2484b1db1a27]::query::erase::Erased<[u8; 16usize]>>
  20:     0x7f731241b827 - <rustc_query_impl[3af702bc24a35b84]::query_impl::resolver_for_lowering_raw::dynamic_query::{closure#2} as core[c088ff38143eb53e]::ops::function::FnOnce<(rustc_middle[395a2484b1db1a27]::ty::context::TyCtxt, ())>>::call_once
  21:     0x7f73123cb35f - rustc_query_system[fae14337e0c02060]::query::plumbing::try_execute_query::<rustc_query_impl[3af702bc24a35b84]::DynamicConfig<rustc_query_system[fae14337e0c02060]::query::caches::SingleCache<rustc_middle[395a2484b1db1a27]::query::erase::Erased<[u8; 16usize]>>, false, false, false>, rustc_query_impl[3af702bc24a35b84]::plumbing::QueryCtxt, false>
  22:     0x7f73123caec4 - rustc_query_impl[3af702bc24a35b84]::query_impl::resolver_for_lowering_raw::get_query_non_incr::__rust_end_short_backtrace
  23:     0x7f73125a90f8 - <rustc_interface[577997324878ea43]::passes::create_and_enter_global_ctxt<core[c088ff38143eb53e]::option::Option<rustc_interface[577997324878ea43]::queries::Linker>, rustc_driver_impl[e986b619aabff65e]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[c088ff38143eb53e]::ops::function::FnOnce<(&rustc_session[9908f9ef7e13fd8d]::session::Session, rustc_middle[395a2484b1db1a27]::ty::context::CurrentGcx, alloc[baa86124b2c3e6f2]::sync::Arc<rustc_data_structures[75601f20e3d68749]::jobserver::Proxy>, &std[243ef3771c46e43a]::sync::once_lock::OnceLock<rustc_middle[395a2484b1db1a27]::ty::context::GlobalCtxt>, &rustc_data_structures[75601f20e3d68749]::sync::worker_local::WorkerLocal<rustc_middle[395a2484b1db1a27]::arena::Arena>, &rustc_data_structures[75601f20e3d68749]::sync::worker_local::WorkerLocal<rustc_hir[38bedddefeb6b9a6]::Arena>, rustc_driver_impl[e986b619aabff65e]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  24:     0x7f731243cc8a - rustc_interface[577997324878ea43]::interface::run_compiler::<(), rustc_driver_impl[e986b619aabff65e]::run_compiler::{closure#0}>::{closure#1}
  25:     0x7f731238ffce - std[243ef3771c46e43a]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[577997324878ea43]::util::run_in_thread_with_globals<rustc_interface[577997324878ea43]::util::run_in_thread_pool_with_globals<rustc_interface[577997324878ea43]::interface::run_compiler<(), rustc_driver_impl[e986b619aabff65e]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  26:     0x7f731238fda0 - <std[243ef3771c46e43a]::thread::lifecycle::spawn_unchecked<rustc_interface[577997324878ea43]::util::run_in_thread_with_globals<rustc_interface[577997324878ea43]::util::run_in_thread_pool_with_globals<rustc_interface[577997324878ea43]::interface::run_compiler<(), rustc_driver_impl[e986b619aabff65e]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[c088ff38143eb53e]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  27:     0x7f7312395b38 - <std[243ef3771c46e43a]::sys::thread::unix::Thread>::new::thread_start
  28:     0x7f730bc9698b - <unknown>
  29:     0x7f730bd1a9cc - <unknown>
  30:                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: rustc 1.94.0-nightly (2ca7bcd03 2025-12-23) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [resolver_for_lowering_raw] getting the resolver for lowering
end of query stack
error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0658`.

@rustbot label +F-autodiff