bug: Compiler ICE in `cairo-lang-sierra-generator-2.18.0/src/db.rs:370:9` on `.span()` of any fixed-size array of zero-sized type

[anon-researchers-123]

Bug Report

Cairo version:

scarb 2.18.0 (e6144df0f 2026-04-21)
cairo: 2.18.0 (https://crates.io/crates/cairo-lang-compiler/2.18.0)
sierra: 1.8.0
arch: aarch64-apple-darwin

Current behavior:

Calling .span() on a fixed-size array [U; N] (N ≥ 1) where U is a zero-sized type causes a Rust-side panic! at sierra-generator/src/db.rs:370 instead of a Cairo diagnostic.

Expected behavior:

A 6-line program that constructs a fixed-size array of an empty struct and calls .span() on it should compile cleanly. Either the program runs (returning the array length as felt252), or the compiler produces a localized Cairo diagnostic explaining that Array<T> does not support zero-sized T. However, the compiler panics with a Rust backtrace.

Steps to reproduce:

scarb new repro --no-vcs --test-runner none
cd repro
(then, Replace src/lib.cairo with the snippet under "Related code" below)
scarb build

   Compiling scratch v0.1.0 (Scarb.toml)

thread 'scarb compile scratch-j7r3ccbvc5mek' (130744) panicked at /Users/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cairo-lang-sierra-generator-2.18.0/src/db.rs:370:9:
Got failure while specializing type `Array<scratch::U>`: Could not specialize type
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

thread 'main' (130733) panicked at scarb/src/ops/compile.rs:238:14:
Compiler thread has panicked.: Any { .. }

Related code:

#[derive(Drop, Copy)] struct U {}
#[executable]
fn main() -> felt252 {
    let arr: [U; 2] = [U {}, U {}];
    let s = arr.span();
    s.len().into()
}

Other information:

These following variants also trigger this ICE bug:

• let arr: [U; 2] = [U {}, U {}]; arr.span().len()
• let arr: [U; 1] = [U {}]; arr.span().len()
• let arr: [U; 4] = [...]; arr.span().len()
• let s: Span<U> = arr.span();
• arr.span().pop_front()