AILang/crates/ail/tests/show_no_instance_e2e.rs

//! Pins the Show-aware `no-instance` diagnostic shipped in milestone 24.3.
//!
//! Property protected: calling `print` on a function type fires the
//! `no-instance` diagnostic with a Show-aware message that
//! cross-references DESIGN.md §"Prelude (built-in) classes", so the
//! LLM author immediately learns which types ship with built-in Show
//! and how to declare their own instance for a user type.

use ailang_check::check_workspace;
use ailang_surface::load_workspace;
use std::path::PathBuf;

fn fixture(name: &str) -> PathBuf {
    PathBuf::from(env!("CARGO_MANIFEST_DIR"))
        .join("../../examples")
        .join(name)
}

#[test]
fn print_on_fn_type_fires_show_aware_no_instance() {
    let ws = load_workspace(&fixture("show_no_instance.ail")).expect("load");
    let diags = check_workspace(&ws);

    assert!(
        !diags.is_empty(),
        "expected NoInstance Show diagnostic, got no diagnostics"
    );

    let no_inst = diags
        .iter()
        .find(|d| d.code == "no-instance")
        .unwrap_or_else(|| {
            panic!("expected diagnostic with code 'no-instance', got: {diags:#?}")
        });

    // Must mention Show.
    assert!(
        no_inst.message.contains("Show"),
        "expected Show-aware NoInstance diagnostic, got message: {:?}",
        no_inst.message
    );

    // Must cross-reference DESIGN.md §"Prelude (built-in) classes"
    // verbatim — that's the canonical anchor naming which types
    // ship with Show.
    assert!(
        no_inst.message.contains("Prelude (built-in) classes"),
        "expected DESIGN.md §Prelude (built-in) classes cross-reference, got message: {:?}",
        no_inst.message
    );
}