ICE when coercing a too large array to a trait object

[theemathas]

Code

#![feature(try_as_dyn)]

trait Trait {}
impl Trait for [u8; 1<<63] {}

pub fn foo(x: &[u8; 1<<63]) {
    let _ = std::any::try_as_dyn::<[u8; 1<<63], dyn Trait>(x);
}

Tracking issue for try_as_dyn: #144361

Meta

Reproducible on the playground with version 1.95.0-nightly (2026-02-02 f60a0f1bcc5a2a6dd8eb)

Error output

thread 'rustc' (61) panicked at /rustc-dev/f60a0f1bcc5a2a6dd8eb2d2021da79b40f5fbae9/compiler/rustc_middle/src/ty/vtable.rs:104:10:
failed to build vtable representation: SizeOverflow([u8; 9223372036854775808_usize])

Backtrace

stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::result::unwrap_failed
   3: rustc_middle::ty::vtable::vtable_allocation_provider
      [... omitted 2 frames ...]
   4: rustc_monomorphize::collector::collect_alloc
   5: rustc_monomorphize::collector::items_of_instance
      [... omitted 1 frame ...]
   6: rustc_monomorphize::collector::collect_items_rec
   7: rustc_monomorphize::collector::collect_items_rec
   8: rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure#0}
   9: rustc_monomorphize::partitioning::collect_and_partition_mono_items
      [... omitted 2 frames ...]
  10: rustc_codegen_ssa::back::symbol_export::exported_generic_symbols_provider_local
      [... omitted 2 frames ...]
  11: <rustc_metadata::rmeta::encoder::EncodeContext>::encode_crate_root
  12: <rustc_metadata::rmeta::encoder::encode_metadata::{closure#3} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, &std::path::Path)>>::call_once
  13: rustc_metadata::rmeta::encoder::encode_metadata
  14: rustc_metadata::fs::encode_and_write_metadata
  15: <rustc_interface::queries::Linker>::codegen_and_build_linker
  16: <rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core::ops::function::FnOnce<(&rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  17: 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/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: please attach the file at `/playground/rustc-ice-2026-02-03T08_23_40-59.txt` to your bug report

note: rustc 1.95.0-nightly (f60a0f1bc 2026-02-02) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

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

query stack during panic:
#0 [vtable_allocation] vtable const allocation for <[u8; ValTree(Leaf(0x8000000000000000): usize)] as Trait>
#1 [items_of_instance] collecting items used by `core::any::try_as_dyn::<[u8; ValTree(Leaf(0x8000000000000000): usize)], dyn Trait>`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
#3 [exported_generic_symbols] collecting exported generic symbols for crate `0`
end of query stack

[theemathas]

Note that try_as_dyn already has a reasonable-ish error when faced with a struct that contains itself.

Code and error

#![feature(try_as_dyn)]

trait Apply {
    type Output<T>;
}
struct Identity;
impl Apply for Identity {
    type Output<T> = T;
}

struct Thing<A: Apply>(<A as Apply>::Output<Self>);

/*
Effectively:
struct Thing_Identity(Thing_Identity);
*/

trait Trait {}
impl Trait for Thing<Identity> {}

pub fn foo(x: &Thing<Identity>) {
    let _ = std::any::try_as_dyn::<Thing<Identity>, dyn Trait>(x);
}

error: reached the recursion limit finding the struct tail for `<Identity as Apply>::Output<Thing<Identity>>`
  |
  = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]`

[bushrat011899]

Replacing the call to try_as_dyn with Some(x as &dyn Trait) yields the expected error message:

   Compiling playground v0.0.1 (/playground)
error: values of the type `[u8; 9223372036854775808]` are too big for the target architecture

error: could not compile `playground` (lib) due to 1 previous error

I assume the fix here for try_as_dyn to throw the same (or similar) compilation error instead?

[meithecatte]

@rustbot claim

[meithecatte]

Looks like try_as_dyn is not the actual culprit, and you can already reproduce it on stable as long as you have debuginfo disabled:

//@ compile-flags: --crate-type lib --emit=mir
pub trait Trait {}
impl<T> Trait for T {}

//~? ERROR: values of the type `[u8; usize::MAX]` are too big for the target architecture
pub fn foo(x: &[u8; usize::MAX]) -> &dyn Trait {
    x as &dyn Trait
}

---

Regression in nightly-2023-08-02

---

get_commits_between returning commits, len: 10
commit[0] 2023-07-31: Auto merge of #114307 - matthiaskrgr:rollup-8k5rq16, r=matthiaskrgr
commit[1] 2023-07-31: Auto merge of #114308 - matthiaskrgr:rollup-m64bkm7, r=matthiaskrgr
commit[2] 2023-08-01: Auto merge of #114310 - flip1995:clippyup, r=Manishearth
commit[3] 2023-08-01: Auto merge of #111753 - cjgillot:simp-place-conflict, r=compiler-errors
commit[4] 2023-08-01: Auto merge of #114318 - matthiaskrgr:rollup-c7gcw18, r=matthiaskrgr
commit[5] 2023-08-01: Auto merge of #112849 - m-ou-se:panic-message-format, r=thomcc
commit[6] 2023-08-01: Auto merge of #114331 - matthiaskrgr:rollup-rnrmwcx, r=matthiaskrgr
commit[7] 2023-08-01: Auto merge of #113854 - klensy:aarch64-msvc-remove-hack, r=Mark-Simulacrum
commit[8] 2023-08-01: Auto merge of #105545 - erikdesjardins:ptrclean, r=bjorn3
commit[9] 2023-08-01: Auto merge of #114273 - nnethercote:move-doc-comment-desugaring, r=petrochenkov
ERROR: no CI builds available between db7ff98 and d12c6e9 within last 167 days

---

@rustbot label +regression-from-stable-to-stable -T-libs-api