[ICE]: expected branch, got Leaf

[matthiaskrgr]

auto-reduced (treereduce-rust):

#![feature(macroless_generic_const_args)]
#![feature(generic_const_args)]
const TUPLE: (&'static str, &'static str) = ("a", true);
fn main() {
    TUPLE;
}

original:

#![feature(macroless_generic_const_args)]
#![feature(generic_const_args, min_generic_const_args)]
const TUPLE: (&'static str, &'static str) = ("a", true);
fn main() {
    TUPLE;
}

Version information

rustc 1.100.0-nightly (da47efd27 2026-09-06)
binary: rustc
commit-hash: da47efd27287edb02eed0b5a178a165c8f258e37
commit-date: 2026-09-06
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.1

Possibly related line of code:

rust/compiler/rustc_type_ir/src/const_kind.rs

Lines 273 to 285 in da47efd

	/// Converts to a `ValTreeKind::Branch` value, `panic`'ing
	/// if this valtree is some other kind.
	#[inline]
	pub fn to_branch(&self) -> I::Consts {
	match self {
	ValTreeKind::Branch(branch) => *branch,
	ValTreeKind::Leaf(..) => panic!("expected branch, got {:?}", self),
	}
	}
	/// Attempts to convert to a `ValTreeKind::Leaf` value.
	pub fn try_to_leaf(&self) -> Option<I::ScalarInt> {
	match self {

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

Program output

error: `generic_const_args` requires `min_generic_const_args` to be enabled
 --> /tmp/icemaker_global_tempdir.BxFyzlIakPYN/rustc_testrunner_tmpdir_reporting.ZoQ9TkLdcYq6/mvce.rs:2:12
  |
2 | #![feature(generic_const_args)]
  |            ^^^^^^^^^^^^^^^^^^
  |
  = help: enable all of these features

error: `macroless_generic_const_args` requires `min_generic_const_args` to be enabled
 --> /tmp/icemaker_global_tempdir.BxFyzlIakPYN/rustc_testrunner_tmpdir_reporting.ZoQ9TkLdcYq6/mvce.rs:1:12
  |
1 | #![feature(macroless_generic_const_args)]
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: enable all of these features

warning: the feature `macroless_generic_const_args` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.BxFyzlIakPYN/rustc_testrunner_tmpdir_reporting.ZoQ9TkLdcYq6/mvce.rs:1:12
  |
1 | #![feature(macroless_generic_const_args)]
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #159006 <https://github.com/rust-lang/rust/issues/159006> for more information
  = note: `#[warn(incomplete_features)]` on by default

warning: the feature `generic_const_args` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.BxFyzlIakPYN/rustc_testrunner_tmpdir_reporting.ZoQ9TkLdcYq6/mvce.rs:2:12
  |
2 | #![feature(generic_const_args)]
  |            ^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #151972 <https://github.com/rust-lang/rust/issues/151972> for more information

error[E0658]: use of unstable library feature `unsized_const_params`
 --> /tmp/icemaker_global_tempdir.BxFyzlIakPYN/rustc_testrunner_tmpdir_reporting.ZoQ9TkLdcYq6/mvce.rs:3:1
  |
3 | const TUPLE: (&'static str, &'static str) = ("a", true);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add `#![feature(unsized_const_params)]` to the crate attributes to enable
  = note: this compiler was built on 2026-09-06; consider upgrading it if it is out of date
  = note: required for `str` to implement `ConstParamTy_`
  = note: 2 redundant requirements hidden
  = note: required for `(&'static str, &'static str)` to implement `ConstParamTy_`


thread 'rustc' (2696937) panicked at /rustc-dev/da47efd27287edb02eed0b5a178a165c8f258e37/compiler/rustc_type_ir/src/const_kind.rs:279:38:
expected branch, got Leaf(0x01)
stack backtrace:
   0:     0x7fc310cbde86 - <<std[7b8d936ff0591b4d]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[1338a38dbbf8640b]::fmt::Display>::fmt
   1:     0x7fc31140829d - core[1338a38dbbf8640b]::fmt::write
   2:     0x7fc310cd2f5c - <std[7b8d936ff0591b4d]::sys::stdio::unix::Stderr as core[1338a38dbbf8640b]::io::write::Write>::write_fmt
   3:     0x7fc310c9072a - std[7b8d936ff0591b4d]::panicking::default_hook::{closure#0}
   4:     0x7fc310cb2183 - std[7b8d936ff0591b4d]::panicking::default_hook
   5:     0x7fc30fbfc55f - std[7b8d936ff0591b4d]::panicking::update_hook::<alloc[c8046bf86c4c1cc9]::boxed::Box<rustc_driver_impl[542a485098bab399]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fc310cb2622 - std[7b8d936ff0591b4d]::panicking::panic_with_hook
   7:     0x7fc310c907e2 - std[7b8d936ff0591b4d]::panicking::panic_handler::{closure#0}
   8:     0x7fc310c87659 - std[7b8d936ff0591b4d]::sys::backtrace::__rust_end_short_backtrace::<std[7b8d936ff0591b4d]::panicking::panic_handler::{closure#0}, !>
   9:     0x7fc310c923bd - __rustc[e24c0e0bed34fbce]::rust_begin_unwind
  10:     0x7fc30d9a3e8c - core[1338a38dbbf8640b]::panicking::panic_fmt
  11:     0x7fc311adfb1c - rustc_const_eval[74340a00061fdc10]::const_eval::valtrees::create_valtree_place
  12:     0x7fc311ad4ecd - rustc_const_eval[74340a00061fdc10]::const_eval::valtrees::valtree_to_ref
  13:     0x7fc311ad55af - rustc_const_eval[74340a00061fdc10]::const_eval::valtrees::valtree_into_mplace
  14:     0x7fc311ad534a - rustc_const_eval[74340a00061fdc10]::const_eval::valtrees::valtree_into_mplace
  15:     0x7fc30e49276b - rustc_const_eval[74340a00061fdc10]::const_eval::valtrees::valtree_to_const_value
  16:     0x7fc312757aeb - <rustc_const_eval[74340a00061fdc10]::provide::{closure#1} as core[1338a38dbbf8640b]::ops::function::FnOnce<(rustc_middle[3307db12b2d0bcfe]::ty::context::TyCtxt, rustc_middle[3307db12b2d0bcfe]::ty::consts::valtree::Value)>>::call_once
  17:     0x7fc312757a9c - rustc_query_impl[a795fc8e0dec0451]::query_vtables::valtree_to_const_val::invoke_provider_fn::__rust_begin_short_backtrace
  18:     0x7fc312757314 - rustc_query_impl[a795fc8e0dec0451]::execution::try_execute_query::<rustc_middle[3307db12b2d0bcfe]::query::caches::DefaultCache<rustc_middle[3307db12b2d0bcfe]::ty::consts::valtree::Value, rustc_middle[3307db12b2d0bcfe]::query::erase::ErasedData<[u8; 24usize]>>, false>
  19:     0x7fc31275703e - rustc_query_impl[a795fc8e0dec0451]::query_vtables::valtree_to_const_val::execute_query_non_incr::__rust_end_short_backtrace
  20:     0x7fc31202132c - <rustc_mir_transform[5ffca57c3928c519]::known_panics_lint::ConstPropagator as rustc_middle[3307db12b2d0bcfe]::mir::visit::Visitor>::visit_assign
  21:     0x7fc312019a8e - <rustc_mir_transform[5ffca57c3928c519]::known_panics_lint::ConstPropagator as rustc_middle[3307db12b2d0bcfe]::mir::visit::Visitor>::visit_body
  22:     0x7fc312014a6a - <rustc_mir_transform[5ffca57c3928c519]::known_panics_lint::KnownPanicsLint as rustc_mir_transform[5ffca57c3928c519]::pass_manager::MirLint>::run_lint
  23:     0x7fc3114185ba - rustc_mir_transform[5ffca57c3928c519]::run_analysis_to_runtime_passes
  24:     0x7fc311db4089 - rustc_mir_transform[5ffca57c3928c519]::mir_drops_elaborated_and_const_checked
  25:     0x7fc311bc7b35 - rustc_query_impl[a795fc8e0dec0451]::execution::try_execute_query::<rustc_data_structures[a20bf6f691e5a6d4]::vec_cache::VecCache<rustc_span[be290dbee5d4923]::def_id::LocalDefId, rustc_middle[3307db12b2d0bcfe]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[3307db12b2d0bcfe]::dep_graph::graph::DepNodeIndex>, false>
  26:     0x7fc311bc7667 - rustc_query_impl[a795fc8e0dec0451]::query_vtables::mir_drops_elaborated_and_const_checked::execute_query_non_incr::__rust_end_short_backtrace
  27:     0x7fc311bca516 - <rustc_middle[3307db12b2d0bcfe]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[55d539fc721ed361]::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  28:     0x7fc311bc964e - rustc_interface[55d539fc721ed361]::passes::analysis
  29:     0x7fc31263084a - rustc_query_impl[a795fc8e0dec0451]::execution::try_execute_query::<rustc_middle[3307db12b2d0bcfe]::query::caches::SingleCache<rustc_middle[3307db12b2d0bcfe]::query::erase::ErasedData<[u8; 0usize]>>, false>
  30:     0x7fc31263062b - rustc_query_impl[a795fc8e0dec0451]::query_vtables::analysis::execute_query_non_incr::__rust_end_short_backtrace
  31:     0x7fc31271f78c - rustc_interface[55d539fc721ed361]::interface::run_compiler::<(), rustc_driver_impl[542a485098bab399]::run_compiler::{closure#0}>::{closure#2}
  32:     0x7fc31282f564 - std[7b8d936ff0591b4d]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[55d539fc721ed361]::util::run_in_thread_with_globals<rustc_interface[55d539fc721ed361]::util::run_in_thread_pool_with_globals<rustc_interface[55d539fc721ed361]::interface::run_compiler<(), rustc_driver_impl[542a485098bab399]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  33:     0x7fc31282f30b - <std[7b8d936ff0591b4d]::thread::lifecycle::spawn_unchecked<rustc_interface[55d539fc721ed361]::util::run_in_thread_with_globals<rustc_interface[55d539fc721ed361]::util::run_in_thread_pool_with_globals<rustc_interface[55d539fc721ed361]::interface::run_compiler<(), rustc_driver_impl[542a485098bab399]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[1338a38dbbf8640b]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  34:     0x7fc31282cd47 - <std[7b8d936ff0591b4d]::sys::thread::unix::Thread>::new::thread_start
  35:     0x7fc30b8980a2 - <unknown>
  36:     0x7fc30b92080c - <unknown>
  37:                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 1.100.0-nightly (da47efd27 2026-09-06) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [valtree_to_const_val] converting type-level constant value to MIR constant value
#1 [mir_drops_elaborated_and_const_checked] elaborating drops for `main`
#2 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 3 previous errors; 2 warnings emitted

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

@rustbot label +F-generic_const_args +F-min_generic_const_args

[malezjaa]

searched nightlies: from nightly-2026-09-04 to nightly-2026-09-08
regressed nightly: nightly-2026-09-06
searched commit range: 0ed41eb...f248f40
regressed commit: 0f819a1

bisected with cargo-bisect-rustc v0.6.11

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --end=2026-09-08 --regress=ice --without-cargo

[malezjaa]

I think its #162179 exactly