[ICE]: `entered unreachable code: This method is not called in impls, as patterns are always allowed there`

auto-reduced (treereduce-rust):

struct S;

impl S {
    fn f(... a: u8) -> u8 {}
}

original:

struct S;

impl S {
    fn f(... a: u8) -> u8 {}
}

fn main() {}

Version information

rustc 1.99.0-nightly (c9ff49689 2026-08-03)
binary: rustc
commit-hash: c9ff496891c278ad660bc0ab85c1f0b72059464a
commit-date: 2026-08-03
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 22.1.8

Possibly related line of code:
https://github.com/rust-lang/rust/blob/c9ff496891c278ad660bc0ab85c1f0b72059464a/compiler/rustc_parse/src/parser/diagnostics.rs#L2395-L2407

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

error: range-to patterns with `...` are not allowed
 --> /tmp/icemaker_global_tempdir.xoIgQidNADMx/rustc_testrunner_tmpdir_reporting.SYwayaaFypGP/mvce.rs:4:10
  |
4 |     fn f(... a: u8) -> u8 {}
  |          ^^^
  |
help: use `..=` instead
  |
4 -     fn f(... a: u8) -> u8 {}
4 +     fn f(..= a: u8) -> u8 {}
  |


thread 'rustc' (3424648) panicked at /rustc-dev/c9ff496891c278ad660bc0ab85c1f0b72059464a/compiler/rustc_parse/src/parser/diagnostics.rs:2401:36:
internal error: entered unreachable code: This method is not called in impls, as patterns are always allowed there
stack backtrace:
   0:     0x7f0120ccdb86 - <<std[6680dfaa64cc66c5]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[f17540a71317f7a8]::fmt::Display>::fmt
   1:     0x7f012140a948 - core[f17540a71317f7a8]::fmt::write
   2:     0x7f0120ce2cac - <std[6680dfaa64cc66c5]::sys::stdio::unix::Stderr as core[f17540a71317f7a8]::io::write::Write>::write_fmt
   3:     0x7f0120ca109a - std[6680dfaa64cc66c5]::panicking::default_hook::{closure#0}
   4:     0x7f0120cc1b33 - std[6680dfaa64cc66c5]::panicking::default_hook
   5:     0x7f011fc2529b - std[6680dfaa64cc66c5]::panicking::update_hook::<alloc[d5bd4e042c235362]::boxed::Box<rustc_driver_impl[27b6e2229d67d5da]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f0120cc1fd2 - std[6680dfaa64cc66c5]::panicking::panic_with_hook
   7:     0x7f0120ca1184 - std[6680dfaa64cc66c5]::panicking::panic_handler::{closure#0}
   8:     0x7f0120c98d69 - std[6680dfaa64cc66c5]::sys::backtrace::__rust_end_short_backtrace::<std[6680dfaa64cc66c5]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f0120ca2b4d - __rustc[4d6acb9b5637ad95]::rust_begin_unwind
  10:     0x7f011da34fac - core[f17540a71317f7a8]::panicking::panic_fmt
  11:     0x7f01204d9a51 - <rustc_parse[5338ae46bc8a9eee]::parser::Parser>::recover_arg_parse
  12:     0x7f01214d00c6 - <rustc_parse[5338ae46bc8a9eee]::parser::Parser>::parse_param_general::{closure#0}
  13:     0x7f01214cc31a - <rustc_parse[5338ae46bc8a9eee]::parser::Parser>::parse_param_general
  14:     0x7f01214c7016 - <rustc_parse[5338ae46bc8a9eee]::parser::Parser>::parse_fn_params
  15:     0x7f012150c751 - <rustc_parse[5338ae46bc8a9eee]::parser::Parser>::parse_fn
  16:     0x7f0121591ced - <rustc_parse[5338ae46bc8a9eee]::parser::Parser>::parse_item_kind
  17:     0x7f012158d363 - <rustc_parse[5338ae46bc8a9eee]::parser::Parser>::parse_item_common
  18:     0x7f01214c057c - <rustc_parse[5338ae46bc8a9eee]::parser::Parser>::parse_item_impl
  19:     0x7f0121591efd - <rustc_parse[5338ae46bc8a9eee]::parser::Parser>::parse_item_kind
  20:     0x7f012158d363 - <rustc_parse[5338ae46bc8a9eee]::parser::Parser>::parse_item_common
  21:     0x7f0121540bd4 - <rustc_parse[5338ae46bc8a9eee]::parser::Parser>::parse_mod
  22:     0x7f01223c7f3d - rustc_interface[f3e11f48256f7348]::passes::parse
  23:     0x7f01225d0e0e - rustc_interface[f3e11f48256f7348]::interface::run_compiler::<(), rustc_driver_impl[27b6e2229d67d5da]::run_compiler::{closure#0}>::{closure#2}
  24:     0x7f012261a924 - std[6680dfaa64cc66c5]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[f3e11f48256f7348]::util::run_in_thread_with_globals<rustc_interface[f3e11f48256f7348]::util::run_in_thread_pool_with_globals<rustc_interface[f3e11f48256f7348]::interface::run_compiler<(), rustc_driver_impl[27b6e2229d67d5da]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  25:     0x7f012261a6ed - <std[6680dfaa64cc66c5]::thread::lifecycle::spawn_unchecked<rustc_interface[f3e11f48256f7348]::util::run_in_thread_with_globals<rustc_interface[f3e11f48256f7348]::util::run_in_thread_pool_with_globals<rustc_interface[f3e11f48256f7348]::interface::run_compiler<(), rustc_driver_impl[27b6e2229d67d5da]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[f17540a71317f7a8]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  26:     0x7f0122626986 - <std[6680dfaa64cc66c5]::sys::thread::unix::Thread>::new::thread_start
  27:     0x7f011ba97739 - <unknown>
  28:     0x7f011bb1bedc - <unknown>
  29:                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 1.99.0-nightly (c9ff49689 2026-08-03) running on x86_64-unknown-linux-gnu

query stack during panic:
end of query stack
error: aborting due to 1 previous error