lint len_zero panicked: index out of bounds: the len is 0 but the index is 0

[eareimu]

Summary

Code: genmeta/gm-quic@7a6ecb5

Run: cargo clippy

Version

rustc 1.91.0-nightly (565a9ca63 2025-09-10)
binary: rustc
commit-hash: 565a9ca63e9df4b223fed0da01f15e578acfb538
commit-date: 2025-09-10
host: aarch64-apple-darwin
release: 1.91.0-nightly
LLVM version: 21.1.0

also tested: 

rustc 1.91.0-nightly (565a9ca63 2025-09-10)
binary: rustc
commit-hash: 565a9ca63e9df4b223fed0da01f15e578acfb538
commit-date: 2025-09-10
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0

Error output

Backtrace

yiyue@eam ~/c/gm-quic (feat/client_auth) [101]> RUST_BACKTRACE=1 cargo clippy
  Checking qbase v0.3.0 (/Users/yiyue/code/gm-quic/qbase)

thread 'rustc' (3498856) panicked at src/tools/clippy/clippy_lints/src/len_zero.rs:374:36:
index out of bounds: the len is 0 but the index is 0
stack backtrace:
 0: __rustc::rust_begin_unwind
 1: core::panicking::panic_fmt
 2: core::panicking::panic_bounds_check
 3: clippy_lints::len_zero::parse_len_output
 4: <clippy_lints::len_zero::LenZero as rustc_lint::passes::LateLintPass>::check_impl_item
 5: <rustc_lint::late::LateContextAndPass<rustc_lint::late::RuntimeCombinedLateLintPass>>::with_param_env::<<rustc_lint::late::LateContextAndPass<rustc_lint::late::RuntimeCombinedLateLintPass> as rustc_hir::intravisit::Visitor>::visit_item::{closure#0}::{closure#0}>
 6: <rustc_lint::late::LateContextAndPass<rustc_lint::late::RuntimeCombinedLateLintPass> as rustc_hir::intravisit::Visitor>::visit_item
 7: <rustc_lint::late::LateContextAndPass<rustc_lint::late::RuntimeCombinedLateLintPass>>::with_param_env::<<rustc_lint::late::LateContextAndPass<rustc_lint::late::RuntimeCombinedLateLintPass> as rustc_hir::intravisit::Visitor>::visit_item::{closure#0}::{closure#0}>
 8: <rustc_lint::late::LateContextAndPass<rustc_lint::late::RuntimeCombinedLateLintPass> as rustc_hir::intravisit::Visitor>::visit_item
 9: <rustc_lint::late::LateContextAndPass<rustc_lint::late::RuntimeCombinedLateLintPass>>::with_param_env::<<rustc_lint::late::LateContextAndPass<rustc_lint::late::RuntimeCombinedLateLintPass> as rustc_hir::intravisit::Visitor>::visit_item::{closure#0}::{closure#0}>
10: <rustc_lint::late::LateContextAndPass<rustc_lint::late::RuntimeCombinedLateLintPass> as rustc_hir::intravisit::Visitor>::visit_item
11: rustc_lint::late::check_crate::{closure#0}
12: rustc_lint::late::check_crate
13: rustc_interface::passes::analysis::{closure#0}
14: rustc_interface::passes::analysis
    [... omitted 1 frame ...]
15: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>
16: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust-clippy/issues/new?template=ice.yml

note: please make sure that you have updated to the latest nightly

note: please attach the file at `/Users/yiyue/code/gm-quic/rustc-ice-2025-09-11T03_03_51-26132.txt` to your bug report

note: compiler flags: --crate-type lib -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked -C incremental=[REDACTED]

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [analysis] running analysis passes on this crate
end of query stack
note: Clippy version: clippy 0.1.91 (565a9ca63e 2025-09-10)

error: could not compile `qbase` (lib)

Caused by:
process didn't exit successfully: `/Users/yiyue/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/clippy-driver /Users/yiyue/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/rustc --crate-name qbase --edition=2021 qbase/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=119 --crate-type lib --emit=dep-info,metadata -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=ba03792c2091dc91 -C extra-filename=-e8bd6c931d5ca3f8 --out-dir /Users/yiyue/code/gm-quic/target/debug/deps -C incremental=/Users/yiyue/code/gm-quic/target/debug/incremental -L dependency=/Users/yiyue/code/gm-quic/target/debug/deps --extern bitflags=/Users/yiyue/code/gm-quic/target/debug/deps/libbitflags-fa1dd98d52e2f7fd.rmeta --extern bytes=/Users/yiyue/code/gm-quic/target/debug/deps/libbytes-c2ab4c1d764ea135.rmeta --extern derive_more=/Users/yiyue/code/gm-quic/target/debug/deps/libderive_more-3c7b206559d21828.rmeta --extern enum_dispatch=/Users/yiyue/code/gm-quic/target/debug/deps/libenum_dispatch-5715449272b2b5e5.dylib --extern futures=/Users/yiyue/code/gm-quic/target/debug/deps/libfutures-24ad6447a527dcd1.rmeta --extern getset=/Users/yiyue/code/gm-quic/target/debug/deps/libgetset-6d7ac4a7ad849b26.dylib --extern http=/Users/yiyue/code/gm-quic/target/debug/deps/libhttp-cd184cc70f6abb3d.rmeta --extern netdev=/Users/yiyue/code/gm-quic/target/debug/deps/libnetdev-b5cdcd98d5985501.rmeta --extern nom=/Users/yiyue/code/gm-quic/target/debug/deps/libnom-8752c7748262418d.rmeta --extern qmacro=/Users/yiyue/code/gm-quic/target/debug/deps/libqmacro-5fc38a5f30026bf8.dylib --extern rand=/Users/yiyue/code/gm-quic/target/debug/deps/librand-ead936e766e5b8ce.rmeta --extern rustls=/Users/yiyue/code/gm-quic/target/debug/deps/librustls-9d18a5aff5b1761c.rmeta --extern serde=/Users/yiyue/code/gm-quic/target/debug/deps/libserde-bd5579fc1cf9ca3b.rmeta --extern thiserror=/Users/yiyue/code/gm-quic/target/debug/deps/libthiserror-bddbdb9f87ec71c9.rmeta --extern tokio=/Users/yiyue/code/gm-quic/target/debug/deps/libtokio-62c14cf1c6618ade.rmeta --extern tracing=/Users/yiyue/code/gm-quic/target/debug/deps/libtracing-2cf5e9fce403c75d.rmeta --cfg tokio_unstable` (exit status: 101)
yiyue@eam ~/c/gm-quic (feat/client_auth) [101]>

[y21]

Minimized: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=5c485c81021c5530a0214b1c62b3f8f4 (Tools -> Clippy shows the ICE)

Regressed in 37b6237, cc @ada4a

Before the refactor, type_at(0) was only evaluated after checking that the type is Option/Result which always has a generic parameter, whereas now it's always evaluated and ICEs if the type does not have any generic parameters.

[ada4a]

Oh no, sorry! Thanks for the reproducer @y21, the PR is up