ICE unpretty-printing `c_variadic`

[matthiaskrgr]

auto-reduced (treereduce-rust):

//@compile-flags: -Zunpretty=thir-tree --crate-type=lib
#![feature(c_variadic)]
unsafe extern "C" fn foo(_: (), ...) {}

original:

#![feature(c_variadic)]
unsafe extern "C" fn foo(_: (), ...) {}

Version information

rustc 1.92.0-nightly (8111a2d6d 2025-10-06)
binary: rustc
commit-hash: 8111a2d6da405e9684a8a83c2c9d69036bf23f12
commit-date: 2025-10-06
host: x86_64-unknown-linux-gnu
release: 1.92.0-nightly
LLVM version: 21.1.2

Possibly related line of code:

rust/compiler/rustc_mir_build/src/thir/print.rs

Lines 686 to 698 in 8111a2d

	}
	fn print_pat_kind(&mut self, pat_kind: &PatKind<'tcx>, depth_lvl: usize) {
	print_indented!(self, "kind: PatKind {", depth_lvl);
	match pat_kind {
	PatKind::Missing => unreachable!(),
	PatKind::Wild => {
	print_indented!(self, "Wild", depth_lvl + 1);
	}
	PatKind::Never => {
	print_indented!(self, "Never", depth_lvl + 1);
	}

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zunpretty=thir-tree --crate-type=lib

Program output

warning: function `foo` is never used
 --> /tmp/icemaker_global_tempdir.8hd3231DSQ7F/rustc_testrunner_tmpdir_reporting.T8tRFQYfH1Lj/mvce.rs:2:22
  |
2 | unsafe extern "C" fn foo(_: (), ...) {}
  |                      ^^^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: `extern` fn uses type `()`, which is not FFI-safe
 --> /tmp/icemaker_global_tempdir.8hd3231DSQ7F/rustc_testrunner_tmpdir_reporting.T8tRFQYfH1Lj/mvce.rs:2:29
  |
2 | unsafe extern "C" fn foo(_: (), ...) {}
  |                             ^^ not FFI-safe
  |
  = help: consider using a struct instead
  = note: tuples have unspecified layout
  = note: `#[warn(improper_ctypes_definitions)]` on by default


thread 'rustc' (1183049) panicked at compiler/rustc_mir_build/src/thir/print.rs:692:33:
internal error: entered unreachable code
stack backtrace:
   0:     0x7f6e923c5933 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h9d493bf7fa709445
   1:     0x7f6e92a01fbc - core::fmt::write::hc6c08c30b818d85a
   2:     0x7f6e9237a333 - std::io::Write::write_fmt::h1a8070f00bb47620
   3:     0x7f6e9238b8f2 - std::sys::backtrace::BacktraceLock::print::he5d95542d2b86573
   4:     0x7f6e92391969 - std::panicking::default_hook::{{closure}}::hf509239352864b0b
   5:     0x7f6e92391493 - std::panicking::default_hook::h41a205ba0e6bad2c
   6:     0x7f6e913a6af7 - std[5bd9112d56c9e542]::panicking::update_hook::<alloc[8dd70e2de2dd10a]::boxed::Box<rustc_driver_impl[799d235ef8173497]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7f6e92391d8f - std::panicking::panic_with_hook::h423ed9a60c1bb0bf
   8:     0x7f6e92391b16 - std::panicking::panic_handler::{{closure}}::h380789c533e4be75
   9:     0x7f6e9238ba39 - std::sys::backtrace::__rust_end_short_backtrace::h6043c6eeb7d07ee0
  10:     0x7f6e9236c56d - __rustc[79156e1635b10208]::rust_begin_unwind
  11:     0x7f6e8f3b7230 - core::panicking::panic_fmt::h566dcbe782a35b3a
  12:     0x7f6e8ed30afc - core::panicking::panic::h89c36c3165d18322
  13:     0x7f6e91a6c086 - <rustc_mir_build[d887d91fdf65e771]::thir::print::ThirPrinter>::print_pat
  14:     0x7f6e91a71c5f - rustc_mir_build[d887d91fdf65e771]::thir::print::thir_tree
  15:     0x7f6e913b1133 - rustc_driver_impl[799d235ef8173497]::pretty::print
  16:     0x7f6e913a93cd - rustc_interface[8b029a4f81d57bf6]::passes::create_and_enter_global_ctxt::<(), rustc_driver_impl[799d235ef8173497]::run_compiler::{closure#0}::{closure#1}>::{closure#2}::{closure#0}
  17:     0x7f6e913ad3cd - <rustc_interface[8b029a4f81d57bf6]::passes::create_and_enter_global_ctxt<(), rustc_driver_impl[799d235ef8173497]::run_compiler::{closure#0}::{closure#1}>::{closure#2} as core[16b3fef4fa94656]::ops::function::FnOnce<(&rustc_session[62811e3f1498e52e]::session::Session, rustc_middle[244f91c2aa28f1b4]::ty::context::CurrentGcx, alloc[8dd70e2de2dd10a]::sync::Arc<rustc_data_structures[70eb418aee26b76d]::jobserver::Proxy>, &std[5bd9112d56c9e542]::sync::once_lock::OnceLock<rustc_middle[244f91c2aa28f1b4]::ty::context::GlobalCtxt>, &rustc_data_structures[70eb418aee26b76d]::sync::worker_local::WorkerLocal<rustc_middle[244f91c2aa28f1b4]::arena::Arena>, &rustc_data_structures[70eb418aee26b76d]::sync::worker_local::WorkerLocal<rustc_hir[b3b90dd82afea593]::Arena>, rustc_driver_impl[799d235ef8173497]::run_compiler::{closure#0}::{closure#1})>>::call_once::{shim:vtable#0}
  18:     0x7f6e93bf8b20 - rustc_interface[8b029a4f81d57bf6]::interface::run_compiler::<(), rustc_driver_impl[799d235ef8173497]::run_compiler::{closure#0}>::{closure#1}
  19:     0x7f6e93b096c7 - std[5bd9112d56c9e542]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[8b029a4f81d57bf6]::util::run_in_thread_with_globals<rustc_interface[8b029a4f81d57bf6]::util::run_in_thread_pool_with_globals<rustc_interface[8b029a4f81d57bf6]::interface::run_compiler<(), rustc_driver_impl[799d235ef8173497]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  20:     0x7f6e93b093a8 - <<std[5bd9112d56c9e542]::thread::Builder>::spawn_unchecked_<rustc_interface[8b029a4f81d57bf6]::util::run_in_thread_with_globals<rustc_interface[8b029a4f81d57bf6]::util::run_in_thread_pool_with_globals<rustc_interface[8b029a4f81d57bf6]::interface::run_compiler<(), rustc_driver_impl[799d235ef8173497]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[16b3fef4fa94656]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  21:     0x7f6e93b0facd - std::sys::thread::unix::Thread::new::thread_start::h41545b42ecfc43e3
  22:     0x7f6e8d4969cb - <unknown>
  23:     0x7f6e8d51aa0c - <unknown>
  24:                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 1.92.0-nightly (8111a2d6d 2025-10-06) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z unpretty=thir-tree --crate-type lib -Z dump-mir-dir=dir

query stack during panic:
end of query stack
warning: 2 warnings emitted

@rustbot label +F-c_variadic

[folkertdev]

#143619 will introduce a warning for the bare ... in function definitions (suggesting _ : ... instead), so really this case should not come up organically.

We can fix this issue by turning Missing into Wild when the pattern type is a VaList. Alternatively bare ... could be its own PatKind. But I'm not sure that is worth it really.

cc @beetrees @workingjubilee if you have thoughts?

[fmease]

Just replace it with

{ print_indented!(self, "Missing", depth_lvl + 1); }

In PR #139035, nnethercote just added an unreachable!() because nothing in the test suite failed when doing so. For context, that's a common thing he does during his refactorings and it's very much intended that it may lead to theoretical breakages; once any breakages are identified (often via fuzzing as is the case here), someone can come in and fix them plus add a regression test to increase the test coverage (which was obviously lacking before).