[ICE]: diagnostics: index out of bounds `let target_param = &struct_generics.params[idx]`

[matthiaskrgr]

auto-reduced (treereduce-rust):

struct A;
impl A<B> {}

original:

struct A;
impl A<B> {}
fn main() {}

Version information

rustc 1.96.0-nightly (44e662074 2026-03-21)
binary: rustc
commit-hash: 44e662074f187af2723a26182f06ffff17d833e6
commit-date: 2026-03-21
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.0

Possibly related line of code:

rust/compiler/rustc_resolve/src/late/diagnostics.rs

Lines 3378 to 3390 in 44e6620

	source,
	) && let Some(resolve) = resolve.full_res()
	&& let Res::Def(_, def_id) = resolve
	&& def_id.is_local()
	&& let Some(local_def_id) = def_id.as_local()
	&& let Some(struct_generics) = self.r.struct_generics.get(&local_def_id)
	&& let target_param = &struct_generics.params[idx]
	&& let GenericParamKind::Const { ty, .. } = &target_param.kind
	&& let TyKind::Path(_, path) = &ty.kind
	{
	let full_type = path
	.segments
	.iter()

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

Program output

thread 'rustc' (1695678) panicked at /rustc-dev/44e662074f187af2723a26182f06ffff17d833e6/compiler/rustc_resolve/src/late/diagnostics.rs:3384:36:
index out of bounds: the len is 0 but the index is 0
stack backtrace:
   0:     0x7f735a7a4efb - <<std[67bae6351216b547]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[b30d1e4e5a844562]::fmt::Display>::fmt
   1:     0x7f735ae2e148 - core[b30d1e4e5a844562]::fmt::write
   2:     0x7f735a7bbf06 - <std[67bae6351216b547]::sys::stdio::unix::Stderr as std[67bae6351216b547]::io::Write>::write_fmt
   3:     0x7f735a77af08 - std[67bae6351216b547]::panicking::default_hook::{closure#0}
   4:     0x7f735a798303 - std[67bae6351216b547]::panicking::default_hook
   5:     0x7f735979117c - std[67bae6351216b547]::panicking::update_hook::<alloc[6c4275e16e664f15]::boxed::Box<rustc_driver_impl[f15c86420af2366]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f735a7985e2 - std[67bae6351216b547]::panicking::panic_with_hook
   7:     0x7f735a77afc8 - std[67bae6351216b547]::panicking::panic_handler::{closure#0}
   8:     0x7f735a771cf9 - std[67bae6351216b547]::sys::backtrace::__rust_end_short_backtrace::<std[67bae6351216b547]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f735a77ca2d - __rustc[cf5e8c4616c45f27]::rust_begin_unwind
  10:     0x7f73578076ac - core[b30d1e4e5a844562]::panicking::panic_fmt
  11:     0x7f7357780e72 - core[b30d1e4e5a844562]::panicking::panic_bounds_check
  12:     0x7f735a31994f - <rustc_resolve[71896364d49c6596]::late::LateResolutionVisitor>::detect_and_suggest_const_parameter_error
  13:     0x7f73571fbc97 - <rustc_resolve[71896364d49c6596]::late::LateResolutionVisitor>::smart_resolve_path_fragment::{closure#0}
  14:     0x7f735bb541fb - <rustc_resolve[71896364d49c6596]::late::LateResolutionVisitor>::smart_resolve_path_fragment
  15:     0x7f735bb521a9 - <rustc_resolve[71896364d49c6596]::late::LateResolutionVisitor as rustc_ast[f3e7ecc0246e0ee7]::visit::Visitor>::visit_ty
  16:     0x7f735bb524f2 - <rustc_resolve[71896364d49c6596]::late::LateResolutionVisitor as rustc_ast[f3e7ecc0246e0ee7]::visit::Visitor>::visit_ty
  17:     0x7f735bb6a3b4 - <rustc_resolve[71896364d49c6596]::late::LateResolutionVisitor>::resolve_item
  18:     0x7f735bb690c3 - rustc_ast[f3e7ecc0246e0ee7]::visit::visit_items::<rustc_resolve[71896364d49c6596]::late::LateResolutionVisitor>
  19:     0x7f735b3c0be4 - <rustc_resolve[71896364d49c6596]::Resolver>::resolve_crate::{closure#0}
  20:     0x7f735b3be6c7 - <rustc_resolve[71896364d49c6596]::Resolver>::resolve_crate
  21:     0x7f735b3a59c9 - rustc_interface[adb13c4b1c58cd5]::passes::configure_and_expand
  22:     0x7f735c048678 - rustc_interface[adb13c4b1c58cd5]::passes::resolver_for_lowering_raw
  23:     0x7f735c0483f3 - rustc_query_impl[971313cfa79974c6]::query_impl::resolver_for_lowering_raw::invoke_provider_fn::__rust_begin_short_backtrace
  24:     0x7f735c026229 - rustc_query_impl[971313cfa79974c6]::execution::try_execute_query::<rustc_middle[69765d1c068851e5]::query::caches::SingleCache<rustc_middle[69765d1c068851e5]::query::erase::ErasedData<[u8; 16usize]>>, false>
  25:     0x7f735c025d8e - rustc_query_impl[971313cfa79974c6]::query_impl::resolver_for_lowering_raw::execute_query_non_incr::__rust_end_short_backtrace
  26:     0x7f735bffc0be - rustc_interface[adb13c4b1c58cd5]::interface::run_compiler::<(), rustc_driver_impl[f15c86420af2366]::run_compiler::{closure#0}>::{closure#1}
  27:     0x7f735bfe55fe - std[67bae6351216b547]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[adb13c4b1c58cd5]::util::run_in_thread_with_globals<rustc_interface[adb13c4b1c58cd5]::util::run_in_thread_pool_with_globals<rustc_interface[adb13c4b1c58cd5]::interface::run_compiler<(), rustc_driver_impl[f15c86420af2366]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  28:     0x7f735bfe5ea0 - <std[67bae6351216b547]::thread::lifecycle::spawn_unchecked<rustc_interface[adb13c4b1c58cd5]::util::run_in_thread_with_globals<rustc_interface[adb13c4b1c58cd5]::util::run_in_thread_pool_with_globals<rustc_interface[adb13c4b1c58cd5]::interface::run_compiler<(), rustc_driver_impl[f15c86420af2366]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[b30d1e4e5a844562]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  29:     0x7f735bfe6d2c - <std[67bae6351216b547]::sys::thread::unix::Thread>::new::thread_start
  30:     0x7f7355aa597a - <unknown>
  31:     0x7f7355b292bc - <unknown>
  32:                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 1.96.0-nightly (44e662074 2026-03-21) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [resolver_for_lowering_raw] getting the resolver for lowering
end of query stack

[matthiaskrgr]

since #152913 cc @Unique-Usman

[Unique-Usman]

@matthiaskrgr this pr will fix this -> #154182

Thanks for catching this.