Autoharness ICE: SIMD codegen unwraps vector length for a generic helper instantiated with a scalar #4919

Summary

kani autoharness crashes on a generic function whose body calls SIMD intrinsics but whose T has no SIMD bound. Autoharness picks T = i32, and the SIMD comparison codegen then unwraps the vector length of a scalar:

thread 'rustc' panicked at kani-compiler/src/codegen_cprover_gotoc/codegen/intrinsic.rs:1548:29:
called `Option::unwrap()` on a `None` value
[Kani] current codegen item: codegen_function: lib::imin::<i32>

Reproducer

#![feature(core_intrinsics)]
#![allow(internal_features)]

pub unsafe fn imin<T: Copy>(a: T, b: T) -> T {
    let mask: T = core::intrinsics::simd::simd_lt(a, b);
    core::intrinsics::simd::simd_select(mask, a, b)
}

$ kani autoharness -Z autoharness lib.rs --only-codegen
thread 'rustc' panicked at kani-compiler/src/codegen_cprover_gotoc/codegen/intrinsic.rs:1548:29:
called `Option::unwrap()` on a `None` value

Line 1548 is unchanged on main.

Cause

if arg1.typ().len().unwrap() != ret_typ.len().unwrap() {

len() is None for a non-vector type. rustc itself rejects simd_lt::<i32> at monomorphization, so this instantiation can never be real code — Kani should report it, not crash.

Suggested fix

Either:

• Have autoharness skip candidates that instantiate a SIMD intrinsic with a non-SIMD type argument (same idea as the const-block skip in Autoharness: skip const-generic fns with const-block preconditions #4820), or
• Turn the unwrap()s in the SIMD codegen paths into a graceful unsupported-construct / codegen error.

The first avoids the dead instantiation; the second stops any other unbounded generic SIMD helper from aborting a whole run.

Why this matters for the standard library

core::core_arch::simd::simd_imin<T: Copy> (stdarch, nightly-2026-08-21) has exactly this shape. With #4819, #4848, #4869 and #4801 applied, autoharness over the whole standard library gets far enough to reach it, and core then fails codegen. This is what still blocks the unfiltered autoharness path — the Kani Metrics, Kani List and autoharness-analyzer jobs in model-checking/verify-rust-std#687, which pass no --include-pattern. I searched for existing issues and PRs on this and found none.