`<usize as SliceIndex<[T]>>::get_unchecked` reports "`assume` called with `false`" in const-eval instead of the out-of-bounds precondition message

[bardiharborow]

Code

#![feature(const_index, const_trait_impl)]

const ARRAY: [u32; 5] = [1, 2, 3, 4, 5];
const OUT_OF_BOUNDS: &u32 = unsafe { ARRAY.get_unchecked(9) };

Current output

error[E0080]: `assume` called with `false`
 --> src/lib.rs:4:38
  |
4 | const OUT_OF_BOUNDS: &u32 = unsafe { ARRAY.get_unchecked(9) };
  |                                      ^^^^^^^^^^^^^^^^^^^^^^ evaluation of `OUT_OF_BOUNDS` failed here

For more information about this error, try `rustc --explain E0080`.
error: could not compile `playground` (lib) due to 1 previous error

Desired output

error[E0080]: evaluation panicked: unsafe precondition(s) violated: slice::get_unchecked requires that the index is within the slice
              
              This indicates a bug in the program. This Undefined Behavior check is optional, and cannot be relied on for safety.
 --> src/lib.rs:4:38
  |
4 | const OUT_OF_BOUNDS: &u32 = unsafe { ARRAY.get_unchecked(9) };
  |                                      ^^^^^^^^^^^^^^^^^^^^^^ evaluation of `OUT_OF_BOUNDS` failed here

For more information about this error, try `rustc --explain E0080`.
error: could not compile `playground` (lib) due to 1 previous error

Rationale and extra context

No response

Other cases

#![feature(const_index, const_trait_impl)]

const MUT: () = unsafe {
    let mut array: [u32; 5] = [1, 2, 3, 4, 5];
    let _ = array.get_unchecked_mut(9);
};

correctly errors with:

error[E0080]: evaluation panicked: unsafe precondition(s) violated: slice::get_unchecked_mut requires that the index is within the slice
              
              This indicates a bug in the program. This Undefined Behavior check is optional, and cannot be relied on for safety.
 --> src/lib.rs:5:13
  |
5 |     let _ = array.get_unchecked_mut(9);
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `MUT` failed here

For more information about this error, try `rustc --explain E0080`.
error: could not compile `playground` (lib) due to 1 previous error

Rust Version

Nightly version: 1.100.0-nightly (2026-08-22 c54751567b19c4ceb08b)

Anything else?

Ref #143775

[bardiharborow]

@rustbot label +A-const-eval +A-slice +T-libs -T-compiler +requires-nightly

[bardiharborow]

@rustbot label +D-confusing

[Aditya-PS-05]

@rustbot claim

[bjorn3]

a PR was already opened by someone else

@rustbot release