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

[matthiaskrgr]

snippet:

fn main(... : ...)

Version information

rustc 1.99.0-nightly (c47bb2f39 2026-08-01)
binary: rustc
commit-hash: c47bb2f39658c00b109693df9611d7238bedf6fa
commit-date: 2026-08-01
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 22.1.8

Possibly related line of code:

rust/compiler/rustc_parse/src/parser/diagnostics.rs

Lines 2394 to 2406 in c47bb2f

	let ty = self.parse_ty()?;
	self.dcx().emit_err(PatternMethodParamWithoutBody {
	span: pat.span,
	target: match context {
	FnContext::Trait => "methods without bodies",
	FnContext::FunctionPtrType => "function pointer types",
	FnContext::Free => unreachable!("This method is not called in free functions, as patterns are always allowed there"),
	FnContext::Impl => unreachable!("This method is not called in impls, as patterns are always allowed there"),
	},
	});
	// Pretend the pattern is `_`, to avoid duplicate errors from AST validation.
	let pat = Box::new(Pat { kind: PatKind::Wild, span: pat.span, id: ast::DUMMY_NODE_ID });

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

Program output

error: unexpected `...`
 --> /tmp/icemaker_global_tempdir.CelWPqCMIeKv/rustc_testrunner_tmpdir_reporting.UYziVYwkqkUH/mvce.rs:1:9
  |
1 | fn main(... : ...)
  |         ^^^ not a valid pattern
  |
help: for a rest pattern, use `..` instead of `...`
  |
1 - fn main(... : ...)
1 + fn main(.. : ...)
  |

error: unexpected `...`
 --> /tmp/icemaker_global_tempdir.CelWPqCMIeKv/rustc_testrunner_tmpdir_reporting.UYziVYwkqkUH/mvce.rs:1:15
  |
1 | fn main(... : ...)
  |               ^^^
  |
  = note: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list


thread 'rustc' (3744479) panicked at /rustc-dev/c47bb2f39658c00b109693df9611d7238bedf6fa/compiler/rustc_parse/src/parser/diagnostics.rs:2400:36:
internal error: entered unreachable code: This method is not called in free functions, as patterns are always allowed there
stack backtrace:
   0:     0x7fbae0ce9da6 - <<std[5df1c8dca6d21b98]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[ce0eec471ef52918]::fmt::Display>::fmt
   1:     0x7fbae140a9d8 - core[ce0eec471ef52918]::fmt::write
   2:     0x7fbae0cfeeec - <std[5df1c8dca6d21b98]::sys::stdio::unix::Stderr as core[ce0eec471ef52918]::io::write::Write>::write_fmt
   3:     0x7fbae0cbd24a - std[5df1c8dca6d21b98]::panicking::default_hook::{closure#0}
   4:     0x7fbae0cddd53 - std[5df1c8dca6d21b98]::panicking::default_hook
   5:     0x7fbadfc4d13b - std[5df1c8dca6d21b98]::panicking::update_hook::<alloc[e7e89e2ae0ae1dde]::boxed::Box<rustc_driver_impl[253a96ca5a13625c]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fbae0cde1f2 - std[5df1c8dca6d21b98]::panicking::panic_with_hook
   7:     0x7fbae0cbd334 - std[5df1c8dca6d21b98]::panicking::panic_handler::{closure#0}
   8:     0x7fbae0cb4f19 - std[5df1c8dca6d21b98]::sys::backtrace::__rust_end_short_backtrace::<std[5df1c8dca6d21b98]::panicking::panic_handler::{closure#0}, !>
   9:     0x7fbae0cbecfd - __rustc[17c44f708f0ce7f4]::rust_begin_unwind
  10:     0x7fbadd9f86ec - core[ce0eec471ef52918]::panicking::panic_fmt
  11:     0x7fbae05006d1 - <rustc_parse[b37aa5865a8ae0b4]::parser::Parser>::recover_arg_parse
  12:     0x7fbae14f6ad9 - <rustc_parse[b37aa5865a8ae0b4]::parser::Parser>::parse_param_general::{closure#0}
  13:     0x7fbae14ec931 - <rustc_parse[b37aa5865a8ae0b4]::parser::Parser>::parse_fn_params
  14:     0x7fbae152ba51 - <rustc_parse[b37aa5865a8ae0b4]::parser::Parser>::parse_fn
  15:     0x7fbae15b122c - <rustc_parse[b37aa5865a8ae0b4]::parser::Parser>::parse_item_kind
  16:     0x7fbae15ac85a - <rustc_parse[b37aa5865a8ae0b4]::parser::Parser>::parse_item_common
  17:     0x7fbae155ffd4 - <rustc_parse[b37aa5865a8ae0b4]::parser::Parser>::parse_mod
  18:     0x7fbae243357d - rustc_interface[5ce7df55cbbf2612]::passes::parse
  19:     0x7fbae25e014e - rustc_interface[5ce7df55cbbf2612]::interface::run_compiler::<(), rustc_driver_impl[253a96ca5a13625c]::run_compiler::{closure#0}>::{closure#2}
  20:     0x7fbae261e764 - std[5df1c8dca6d21b98]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[5ce7df55cbbf2612]::util::run_in_thread_with_globals<rustc_interface[5ce7df55cbbf2612]::util::run_in_thread_pool_with_globals<rustc_interface[5ce7df55cbbf2612]::interface::run_compiler<(), rustc_driver_impl[253a96ca5a13625c]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  21:     0x7fbae261e52d - <std[5df1c8dca6d21b98]::thread::lifecycle::spawn_unchecked<rustc_interface[5ce7df55cbbf2612]::util::run_in_thread_with_globals<rustc_interface[5ce7df55cbbf2612]::util::run_in_thread_pool_with_globals<rustc_interface[5ce7df55cbbf2612]::interface::run_compiler<(), rustc_driver_impl[253a96ca5a13625c]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[ce0eec471ef52918]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  22:     0x7fbae2621cc6 - <std[5df1c8dca6d21b98]::sys::thread::unix::Thread>::new::thread_start
  23:     0x7fbadba97739 - <unknown>
  24:     0x7fbadbb1bedc - <unknown>
  25:                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 (c47bb2f39 2026-08-01) running on x86_64-unknown-linux-gnu

query stack during panic:
end of query stack
error: aborting due to 2 previous errors

[matthiaskrgr]

cc #160282

[matthiaskrgr]

also:

struct Baz where U : fn(() : bool)