[ICE]: `BoundsCheckFailed` in `valtree_to_const_value` on array const arg

A version of #155168 where the expected len of the array is an assoc const whose self ty is generic. #158587 fixes the case where the type can be normalized, but if T is a generic param, then <T as Trait>::LEN can't be normalized, causing us to ICE if the provided array is too long.

See https://github.com/rust-lang/rust/pull/158587#discussion_r3671288068.

Code

//@ compile-flags: -Copt-level=0
#![allow(incomplete_features)]
#![feature(adt_const_params, min_generic_const_args, macroless_generic_const_args)]
#![feature(generic_const_parameter_types)]

trait Trait {
    type const LEN: usize;
}

struct S;
impl Trait for S {
    type const LEN: usize = 2;
}

fn foo<T: Trait, const A: [u8; <T as Trait>::LEN]>() -> [u8; <T as Trait>::LEN] {
    A
}

fn bar<T: Trait>() -> [u8; <T as Trait>::LEN] {
    foo::<T, { [1, 2, 3] }>()
}

fn main() {
    bar::<S>();
}

Or if the array is too short, causes UB:

//@ compile-flags: -Copt-level=0
#![allow(incomplete_features)]
#![feature(adt_const_params, min_generic_const_args, macroless_generic_const_args)]
#![feature(generic_const_parameter_types)]

trait Trait {
    type const LEN: usize;
}

struct S;
impl Trait for S {
    type const LEN: usize = 2;
}

fn foo<T: Trait, const A: [u8; <T as Trait>::LEN]>() -> [u8; <T as Trait>::LEN] {
    A
}

fn bar<T: Trait>() -> [u8; <T as Trait>::LEN] {
    foo::<T, { [] }>()
}

fn main() {
    println!("{:?}", bar::<S>());
}

Meta

rustc --version --verbose:

rustc 1.99.0-nightly (26ae60a9e 2026-07-28)
binary: rustc
commit-hash: 26ae60a9eeb20b4935be49d7a931a650fa1d2923
commit-date: 2026-07-28
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 22.1.8

also ICEs/has UB on #158587's stage1.

Error output

thread 'rustc' panicked at compiler/rustc_const_eval/src/const_eval/valtrees.rs:459:60:
called `Result::unwrap()` on an `Err` value: InterpErrorInfo(InterpErrorInfoInner { kind: UndefinedBehavior(BoundsCheckFailed { len: 2, index: 2 }), backtrace: InterpErrorBacktrace { backtrace: None } })

query stack during panic:
#0 [valtree_to_const_val] converting type-level constant value to MIR constant value
#1 [items_of_instance] collecting items used by `foo::<S, ValTree(Branch([ValTree(Leaf(0x01): u8), ValTree(Leaf(0x02): u8), ValTree(Leaf(0x03): u8)]): [u8; ValTree(Leaf(0x0000000000000002): usize)])>`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

Backtrace

thread 'rustc' (1013477) panicked at /rustc-dev/26ae60a9eeb20b4935be49d7a931a650fa1d2923/compiler/rustc_const_eval/src/const_eval/valtrees.rs:459:60:
called `Result::unwrap()` on an `Err` value: InterpErrorInfo(InterpErrorInfoInner { kind: UndefinedBehavior(BoundsCheckFailed { len: 2, index: 2 }), backtrace: InterpErrorBacktrace { backtrace: None } })
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::result::unwrap_failed
   3: rustc_const_eval::const_eval::valtrees::valtree_into_mplace
   4: rustc_const_eval::const_eval::valtrees::valtree_to_const_value
   5: <rustc_const_eval::provide::{closure#1} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, rustc_middle::ty::consts::valtree::Value)>>::call_once
      [... omitted 1 frame ...]
   6: rustc_monomorphize::collector::items_of_instance
      [... omitted 1 frame ...]
   7: rustc_monomorphize::collector::collect_items_rec
   8: rustc_monomorphize::collector::collect_items_rec
   9: rustc_monomorphize::collector::collect_items_rec
  10: rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure#0}
  11: rustc_monomorphize::partitioning::collect_and_partition_mono_items
      [... omitted 1 frame ...]
  12: rustc_codegen_ssa::base::codegen_crate::<rustc_codegen_llvm::LlvmCodegenBackend, rustc_codegen_llvm::ModuleLlvm>
  13: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  14: <rustc_interface::queries::Linker>::codegen_and_build_linker
  15: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.