AILang/crates/ailang-core/tests/design_schema_drift.rs

//! Drift detection between ast.rs and `design/contracts/0002-data-model.md`.
//!
//! `design/contracts/0002-data-model.md` is the canonical schema
//! source-of-truth. Every AST enum (`Term`, `Pattern`, `Type`, `Def`,
//! `Literal`, `ParamMode`) must have a JSON-schema anchor (e.g.
//! `"t": "lit"`, `"k": "fn"`) present in that document. These tests
//! enforce the property.
//!
//! The exhaustive `match` per enum is the load-bearing mechanism: adding a
//! new variant without a matching arm fails compilation before the test runs.
//! Once the variant is matched, the test asserts the anchor is present in
//! `design/contracts/0002-data-model.md`. The whole file is the data-model
//! contract, so the file boundary now bounds the section — the former
//! `## Data model` … `## Pipeline` slicer is no longer needed (the split
//! gave the section its own file).

use std::collections::BTreeMap;

use ailang_core::ast::{
    ClassDef, ClassMethod, Constraint, ConstDef, Ctor, Def, FnDef, InstanceDef,
    InstanceMethod, Literal, Module, NewArg, Pattern, ParamMode, Suppress, Term, Type, TypeDef,
};

const DATA_MODEL: &str = include_str!("../../../design/contracts/0002-data-model.md");

/// Scoped substring match: returns `true` iff `anchor` appears inside a
/// fenced code block (``` or ~~~) of `md`. Info-string `jsonc` / `json` /
/// unspecified all count — fence-toggling is by fence-marker line alone,
/// mirroring the inverse `strip_fences` pattern in
/// `crates/ailang-core/tests/design_index_pin.rs`. The whole-file
/// `.contains()` was fidelity-widened: anchors mentioned in surrounding
/// prose (footnotes, inline references, historical notes) counted as
/// present, so a future edit could delete the canonical schema entry for
/// a variant from inside a fenced block and the drift test would still
/// pass. This helper closes that surface.
fn anchor_in_jsonc_block(md: &str, anchor: &str) -> bool {
    let mut in_fence = false;
    for line in md.lines() {
        let t = line.trim_start();
        if t.starts_with("```") || t.starts_with("~~~") {
            in_fence = !in_fence;
            continue;
        }
        if in_fence && line.contains(anchor) {
            return true;
        }
    }
    false
}

/// Every `Term` variant must have its JSON-schema anchor present in
/// design/contracts/0002-data-model.md. An LLM author cannot produce a term variant
/// whose `"t"` tag is absent from the canonical schema document.
#[test]
fn design_md_anchors_every_term_variant() {
    let exemplars: Vec<(&str, Term)> = vec![
        (
            r#""t": "lit""#,
            Term::Lit { lit: Literal::Unit },
        ),
        (
            r#""t": "var""#,
            Term::Var { name: "x".into() },
        ),
        (
            r#""t": "app""#,
            Term::App {
                callee: Box::new(Term::Var { name: "f".into() }),
                args: vec![],
                tail: false,
            },
        ),
        (
            r#""t": "let""#,
            Term::Let {
                name: "x".into(),
                value: Box::new(Term::Lit { lit: Literal::Int { value: 0 } }),
                body: Box::new(Term::Var { name: "x".into() }),
            },
        ),
        (
            r#""t": "letrec""#,
            Term::LetRec {
                name: "f".into(),
                ty: Type::fn_implicit(vec![], Type::int(), vec![]),
                params: vec![],
                body: Box::new(Term::Lit { lit: Literal::Int { value: 0 } }),
                in_term: Box::new(Term::Var { name: "f".into() }),
            },
        ),
        (
            r#""t": "if""#,
            Term::If {
                cond: Box::new(Term::Lit { lit: Literal::Bool { value: true } }),
                then: Box::new(Term::Lit { lit: Literal::Int { value: 1 } }),
                else_: Box::new(Term::Lit { lit: Literal::Int { value: 0 } }),
            },
        ),
        (
            r#""t": "do""#,
            Term::Do {
                op: "io/print_str".into(),
                args: vec![],
                tail: false,
            },
        ),
        (
            r#""t": "ctor""#,
            Term::Ctor {
                type_name: "List".into(),
                ctor: "Nil".into(),
                args: vec![],
            },
        ),
        (
            r#""t": "match""#,
            Term::Match {
                scrutinee: Box::new(Term::Var { name: "x".into() }),
                arms: vec![],
            },
        ),
        (
            r#""t": "lam""#,
            Term::Lam {
                params: vec![],
                param_tys: vec![],
                ret_ty: Box::new(Type::int()),
                effects: vec![],
                body: Box::new(Term::Lit { lit: Literal::Int { value: 0 } }),
            },
        ),
        (
            r#""t": "seq""#,
            Term::Seq {
                lhs: Box::new(Term::Var { name: "a".into() }),
                rhs: Box::new(Term::Var { name: "b".into() }),
            },
        ),
        (
            r#""t": "clone""#,
            Term::Clone {
                value: Box::new(Term::Var { name: "x".into() }),
            },
        ),
        (
            r#""t": "reuse-as""#,
            Term::ReuseAs {
                source: Box::new(Term::Var { name: "x".into() }),
                body: Box::new(Term::Var { name: "y".into() }),
            },
        ),
        (
            r#""t": "loop""#,
            Term::Loop {
                binders: Vec::new(),
                body: Box::new(Term::Lit { lit: Literal::Unit }),
            },
        ),
        (
            r#""t": "recur""#,
            Term::Recur { args: vec![] },
        ),
        (
            r#""t": "new""#,
            Term::New {
                type_name: "T".into(),
                args: vec![],
            },
        ),
        (r#""t": "intrinsic""#, Term::Intrinsic),
    ];

    for (anchor, term) in exemplars {
        // Exhaustive match: compiler rejects this file if a new Term
        // variant lacks an arm, catching drift at compile time.
        let _: &'static str = match term {
            Term::Lit { .. } => "lit",
            Term::Var { .. } => "var",
            Term::App { .. } => "app",
            Term::Let { .. } => "let",
            Term::LetRec { .. } => "letrec",
            Term::If { .. } => "if",
            Term::Do { .. } => "do",
            Term::Ctor { .. } => "ctor",
            Term::Match { .. } => "match",
            Term::Lam { .. } => "lam",
            Term::Seq { .. } => "seq",
            Term::Clone { .. } => "clone",
            Term::ReuseAs { .. } => "reuse-as",
            Term::Loop { .. } => "loop",
            Term::Recur { .. } => "recur",
            Term::New { .. } => "new",
            Term::Intrinsic => "intrinsic",
        };
        assert!(
            anchor_in_jsonc_block(DATA_MODEL, anchor),
            "design/contracts/0002-data-model.md is missing anchor `{anchor}` for a Term variant — \
             add it to design/contracts/0002-data-model.md"
        );
    }

    // The two Term::Lam compound-key tags are kebab-case (closes #30).
    // Pin the spellings inside data-model.md's fenced jsonc blocks so a
    // future edit cannot silently revert the contract document to
    // camelCase while ast.rs ships kebab.
    for anchor in [r#""param-types""#, r#""ret-type""#] {
        assert!(
            anchor_in_jsonc_block(DATA_MODEL, anchor),
            "design/contracts/0002-data-model.md is missing Term::Lam kebab-key anchor `{anchor}` — \
             check the `lam` fenced JSON block"
        );
    }
}

/// Every `Pattern` variant must have its JSON-schema anchor present in
/// design/contracts/0002-data-model.md. Missing anchors mean an LLM cannot produce
/// the corresponding pattern form.
#[test]
fn design_md_anchors_every_pattern_variant() {
    let exemplars: Vec<(&str, Pattern)> = vec![
        (r#""p": "wild""#, Pattern::Wild),
        (r#""p": "var""#, Pattern::Var { name: "x".into() }),
        (r#""p": "lit""#, Pattern::Lit { lit: Literal::Int { value: 0 } }),
        (
            r#""p": "ctor""#,
            Pattern::Ctor { ctor: "Nil".into(), fields: vec![] },
        ),
    ];

    for (anchor, pat) in exemplars {
        let _: &'static str = match pat {
            Pattern::Wild => "wild",
            Pattern::Var { .. } => "var",
            Pattern::Lit { .. } => "lit",
            Pattern::Ctor { .. } => "ctor",
        };
        assert!(
            anchor_in_jsonc_block(DATA_MODEL, anchor),
            "design/contracts/0002-data-model.md is missing anchor `{anchor}` for a Pattern variant"
        );
    }
}

/// Every `Type` variant must have its JSON-schema anchor present in
/// design/contracts/0002-data-model.md. The `"k"` discriminator is load-bearing for
/// the codegen and typechecker; an LLM must know all four forms.
#[test]
fn design_md_anchors_every_type_variant() {
    let exemplars: Vec<(&str, Type)> = vec![
        (r#""k": "con""#, Type::int()),
        (r#""k": "fn""#, Type::fn_implicit(vec![], Type::unit(), vec![])),
        (r#""k": "var""#, Type::Var { name: "a".into() }),
        (
            r#""k": "forall""#,
            Type::Forall {
                vars: vec!["a".into()],
                constraints: vec![],
                body: Box::new(Type::Var { name: "a".into() }),
            },
        ),
    ];

    for (anchor, ty) in exemplars {
        let _: &'static str = match ty {
            Type::Con { .. } => "con",
            Type::Fn { .. } => "fn",
            Type::Var { .. } => "var",
            Type::Forall { .. } => "forall",
        };
        assert!(
            anchor_in_jsonc_block(DATA_MODEL, anchor),
            "design/contracts/0002-data-model.md is missing anchor `{anchor}` for a Type variant"
        );
    }
}

/// Every `Literal` variant must have its JSON-schema anchor present in
/// design/contracts/0002-data-model.md. The `"kind"` discriminator identifies the
/// literal type; an LLM cannot produce a literal it hasn't seen in the
/// schema.
#[test]
fn design_md_anchors_every_literal_variant() {
    let exemplars: Vec<(&str, Literal)> = vec![
        (r#""kind": "int""#, Literal::Int { value: 0 }),
        (r#""kind": "bool""#, Literal::Bool { value: true }),
        (r#""kind": "str""#, Literal::Str { value: "x".into() }),
        (r#""kind": "unit""#, Literal::Unit),
        (r#""kind": "float""#, Literal::Float { bits: 0 }),
    ];

    for (anchor, lit) in exemplars {
        let _: &'static str = match lit {
            Literal::Int { .. } => "int",
            Literal::Bool { .. } => "bool",
            Literal::Str { .. } => "str",
            Literal::Unit => "unit",
            Literal::Float { .. } => "float",
        };
        assert!(
            anchor_in_jsonc_block(DATA_MODEL, anchor),
            "design/contracts/0002-data-model.md is missing anchor `{anchor}` for a Literal variant"
        );
    }
}

/// Every `Def` kind must have its JSON-schema anchor present in
/// design/contracts/0002-data-model.md. All five kinds (`fn`, `const`, `type`,
/// `class`, `instance`) must be documented so an LLM can write
/// any kind of top-level definition.
#[test]
fn design_md_anchors_every_def_kind() {
    let fn_def = FnDef {
        name: "f".into(),
        doc: None,
        suppress: vec![],
        ty: Type::fn_implicit(vec![], Type::int(), vec![]),
        params: vec![],
        body: Term::Lit { lit: Literal::Int { value: 0 } },
    export: None,
    };
    let const_def = ConstDef {
        name: "k".into(),
        doc: None,
        ty: Type::int(),
        value: Term::Lit { lit: Literal::Int { value: 0 } },
    };
    let type_def = TypeDef {
        name: "T".into(),
        doc: None,
        vars: vec![],
        ctors: vec![Ctor { name: "C".into(), fields: vec![] }],
        drop_iterative: false,
        param_in: BTreeMap::new(),
    };
    let class_def = ClassDef {
        name: "Show".into(),
        param: "a".into(),
        superclass: None,
        methods: vec![ClassMethod {
            name: "show".into(),
            ty: Type::fn_implicit(vec![Type::Var { name: "a".into() }], Type::str_(), vec![]),
            default: None,
        }],
        doc: None,
    };
    let instance_def = InstanceDef {
        class: "Show".into(),
        type_: Type::int(),
        methods: vec![InstanceMethod {
            name: "show".into(),
            body: Term::Lit { lit: Literal::Str { value: "0".into() } },
        }],
        doc: None,
    };

    let exemplars: Vec<(&str, Def)> = vec![
        (r#""kind": "fn""#, Def::Fn(fn_def)),
        (r#""kind": "const""#, Def::Const(const_def)),
        (r#""kind": "type""#, Def::Type(type_def)),
        (r#""kind": "class""#, Def::Class(class_def)),
        (r#""kind": "instance""#, Def::Instance(instance_def)),
    ];

    for (anchor, def) in exemplars {
        let _: &'static str = match def {
            Def::Fn(_) => "fn",
            Def::Const(_) => "const",
            Def::Type(_) => "type",
            Def::Class(_) => "class",
            Def::Instance(_) => "instance",
        };
        assert!(
            anchor_in_jsonc_block(DATA_MODEL, anchor),
            "design/contracts/0002-data-model.md is missing anchor `{anchor}` for a Def kind"
        );
    }
}

/// Every `ParamMode` variant must have its serialized string form present
/// in design/contracts/0002-data-model.md. The mode annotations are load-bearing for
/// ownership checking; an LLM author must know all three forms.
#[test]
fn design_md_anchors_every_parammode_variant() {
    let exemplars: Vec<(&str, ParamMode)> = vec![
        (r#""implicit""#, ParamMode::Implicit),
        (r#""own""#, ParamMode::Own),
        (r#""borrow""#, ParamMode::Borrow),
    ];

    for (anchor, mode) in exemplars {
        let _: &'static str = match mode {
            ParamMode::Implicit => "implicit",
            ParamMode::Own => "own",
            ParamMode::Borrow => "borrow",
        };
        assert!(
            anchor_in_jsonc_block(DATA_MODEL, anchor),
            "design/contracts/0002-data-model.md is missing anchor `{anchor}` for a ParamMode variant"
        );
    }
}

/// The anchor-presence check must scope its substring match to ```jsonc```
/// (and ``` / ```json) fenced code blocks. The whole-file `.contains()` is
/// fidelity-widened: a future edit can delete the canonical schema entry for
/// a variant from inside a fenced block while leaving the anchor string
/// mentioned in surrounding prose (e.g. a "design rationale" footnote, an
/// inline reference like `the "t": "lit" form`, or a historical-note
/// section). The drift test would silently pass, and downstream LLM authors
/// would lose the canonical schema for a variant the AST still produces.
///
/// This test pins the property in two directions:
///   1. A markdown string with the anchor present ONLY in prose (no fenced
///      block) must report ABSENT under the scoped helper. The unscoped
///      `.contains()` reports PRESENT for the same input — that mismatch is
///      the bug surface.
///   2. A markdown string with the anchor present ONLY inside a ```jsonc```
///      block must report PRESENT under the scoped helper.
///
/// Gitea issue #10. The helper `anchor_in_jsonc_block` does not yet exist —
/// this test is RED until the `implement` skill (mini-mode dispatch) adds it
/// and re-routes the six existing call sites onto it.
#[test]
fn anchor_presence_check_is_scoped_to_jsonc_blocks() {
    // Anchor mentioned only in prose — the false-pass surface. The whole
    // sentence after the heading is plain markdown body text; no fenced
    // block exists in this fixture at all.
    let prose_only = r#"# Data model

The `"t": "lit"` form used to be the canonical literal anchor. See
historical note below.

## Historical note

Older drafts pinned the literal schema as `"t": "lit"`; the current
schema uses a different shape (see commit log).
"#;

    // Anchor present only inside a fenced jsonc block — the true-positive
    // surface. Prose outside the block does not mention the anchor.
    let jsonc_only = r#"# Data model

The literal form is canonical.

```jsonc
{ "t": "lit", "lit": Literal }
```

End.
"#;

    // Unscoped substring match — what the file does today. Both inputs
    // report PRESENT, and that is the bug: prose-only must not count.
    assert!(prose_only.contains(r#""t": "lit""#));
    assert!(jsonc_only.contains(r#""t": "lit""#));

    // Scoped helper — what the file must do. Prose-only is ABSENT,
    // jsonc-only is PRESENT. This call is the RED: the helper does not
    // yet exist, so this test fails to compile until `implement` mini-mode
    // factors it out and re-routes the six call sites onto it.
    assert!(
        !anchor_in_jsonc_block(prose_only, r#""t": "lit""#),
        "scoped helper must NOT count an anchor that lives only in prose"
    );
    assert!(
        anchor_in_jsonc_block(jsonc_only, r#""t": "lit""#),
        "scoped helper MUST count an anchor that lives inside a ```jsonc``` block"
    );

    // Live document: the helper must also report PRESENT for the live
    // data-model.md (proves the helper does not over-narrow and break
    // the existing six checks).
    assert!(
        anchor_in_jsonc_block(DATA_MODEL, r#""t": "lit""#),
        "scoped helper must find live anchors inside data-model.md ```jsonc``` blocks"
    );
}

/// Schema-shape pin: the JSON serialisation of `Term::Lam` must
/// emit kebab-case tags `"param-types"` and `"ret-type"`, and
/// must NOT emit the old camelCase `"paramTypes"` / `"retType"`.
/// Pinned because the closes-#30 milestone made the rename a
/// schema-stability invariant: a future regression that flips
/// the `#[serde(rename)]` string back to camelCase must fire RED
/// here, not slip through.
#[test]
fn lam_serialises_with_kebab_keys() {
    let lam = Term::Lam {
        params: vec!["x".into()],
        param_tys: vec![Type::int()],
        ret_ty: Box::new(Type::int()),
        effects: vec![],
        body: Box::new(Term::Var { name: "x".into() }),
    };
    let v = serde_json::to_value(&lam).expect("Term::Lam serialises");
    let obj = v.as_object().expect("Term::Lam serialises as object");

    assert!(
        obj.contains_key("param-types"),
        "Term::Lam must emit `param-types` key; got keys {:?}",
        obj.keys().collect::<Vec<_>>(),
    );
    assert!(
        obj.contains_key("ret-type"),
        "Term::Lam must emit `ret-type` key; got keys {:?}",
        obj.keys().collect::<Vec<_>>(),
    );
    assert!(
        !obj.contains_key("paramTypes"),
        "Term::Lam must NOT emit old camelCase `paramTypes` key",
    );
    assert!(
        !obj.contains_key("retType"),
        "Term::Lam must NOT emit old camelCase `retType` key",
    );
}

/// Nested struct key anchors must be present in design/contracts/0002-data-model.md.
/// These keys appear inside `Suppress`, `ClassMethod`, `InstanceMethod`,
/// and `Type::Forall` — they are not discriminators but they ARE part
/// of the canonical JSON schema and must remain documented.
#[test]
fn design_md_anchors_nested_struct_keys() {
    // Constructors exercised here ensure ast.rs field names are correct.
    let _ = Suppress { code: "x".into(), because: "y".into() };
    let _ = ClassMethod {
        name: "m".into(),
        ty: Type::fn_implicit(vec![], Type::int(), vec![]),
        default: None,
    };
    let _ = InstanceMethod {
        name: "m".into(),
        body: Term::Lit { lit: Literal::Unit },
    };
    let _ = Constraint {
        class: "Show".into(),
        type_: Type::int(),
    };

    let anchors = [
        r#""code""#,
        r#""because""#,
        r#""methods""#,
        r#""constraints""#,
    ];

    for anchor in anchors {
        assert!(
            anchor_in_jsonc_block(DATA_MODEL, anchor),
            "design/contracts/0002-data-model.md is missing nested-struct-key anchor `{anchor}`"
        );
    }
}

/// prep.2 (kernel-extension-mechanics): pin the JSON byte-shape of
/// `Term::New` with a single `NewArg::Value`. Property protected:
/// the tag is `t: "new"`, the type-name field is serialised as
/// `"type"` (not `"type_name"` or any camelCase variant), the args
/// list serialises each `NewArg::Value` as `{kind: "value", value:
/// <Term JSON>}`, and the whole shape round-trips through
/// `serde_json` without information loss. A future edit that
/// re-renames any of these keys breaks the pin and the data-model
/// document must be updated in lockstep.
#[test]
fn term_new_round_trips() {
    let t = Term::New {
        type_name: "Counter".into(),
        args: vec![NewArg::Value(Term::Lit {
            lit: Literal::Int { value: 42 },
        })],
    };
    let json = serde_json::to_value(&t).expect("serialise Term::New");
    assert_eq!(json["t"], "new", "Term discriminator must be `t: \"new\"`");
    assert_eq!(json["type"], "Counter", "type_name must serialise as `\"type\"`");
    let args = json["args"].as_array().expect("args must be an array");
    assert_eq!(args.len(), 1);
    assert_eq!(
        args[0]["kind"], "value",
        "NewArg::Value discriminator must be `kind: \"value\"`"
    );
    assert!(
        args[0]["value"].is_object(),
        "value-arg's payload is the inlined Term JSON object"
    );
    let recovered: Term =
        serde_json::from_value(json).expect("Term::New round-trips through serde");
    match recovered {
        Term::New { type_name, args } => {
            assert_eq!(type_name, "Counter");
            assert_eq!(args.len(), 1);
            assert!(matches!(args[0], NewArg::Value(_)));
        }
        other => panic!("expected Term::New after round-trip, got {other:?}"),
    }
}

/// prep.2 (kernel-extension-mechanics): pin the JSON byte-shape of
/// `NewArg::Type` (a Type-positional arg). Property protected: the
/// inner `value` carries a full `Type` JSON object (here a
/// `(k = "con", name = "Float")`), and the round-trip preserves both
/// the discriminator and the embedded Type's own discriminator. A
/// future edit that changes `NewArg`'s serde tag/content shape (or
/// renames `kind`/`value`) breaks the pin in lockstep with the
/// data-model document.
#[test]
fn term_new_type_arg_round_trips() {
    let t = Term::New {
        type_name: "Series".into(),
        args: vec![
            NewArg::Type(Type::Con {
                name: "Float".into(),
                args: vec![],
            }),
            NewArg::Value(Term::Lit {
                lit: Literal::Int { value: 3 },
            }),
        ],
    };
    let json = serde_json::to_value(&t).expect("serialise Term::New with mixed args");
    let args = json["args"].as_array().expect("args must be an array");
    assert_eq!(args.len(), 2);
    assert_eq!(args[0]["kind"], "type", "first arg is a Type-positional");
    assert_eq!(args[0]["value"]["k"], "con", "Type-arg's value carries the Type tag");
    assert_eq!(args[0]["value"]["name"], "Float");
    assert_eq!(args[1]["kind"], "value", "second arg is a Value-positional");
    let recovered: Term =
        serde_json::from_value(json).expect("mixed Term::New round-trips through serde");
    if let Term::New { args, .. } = recovered {
        assert!(matches!(args[0], NewArg::Type(Type::Con { ref name, .. }) if name == "Float"));
        assert!(matches!(args[1], NewArg::Value(_)));
    } else {
        panic!("expected Term::New after round-trip");
    }
}



/// prep.3 (kernel-extension-mechanics): pin the JSON byte-shape of
/// `Module.kernel`. Property protected: when `true`, the key
/// serialises as the bare `"kernel": true` flag adjacent to
/// `"name"`; when `false`, it is omitted entirely from canonical
/// JSON (preserving the bit-identical hash of every pre-existing
/// fixture). A future edit that renames the field, drops the
/// `skip_serializing_if`, or changes its boolean shape breaks the
/// pin in lockstep with the data-model document.
#[test]
fn module_kernel_flag_round_trips() {
    let m_on = Module {
        schema: ailang_core::SCHEMA.to_string(),
        name: "k".into(),
        kernel: true,
        imports: vec![],
        defs: vec![],
    };
    let json_on = serde_json::to_value(&m_on).expect("serialise kernel: true");
    assert_eq!(
        json_on["kernel"], true,
        "kernel: true must appear as a bare boolean key"
    );

    let m_off = Module {
        schema: ailang_core::SCHEMA.to_string(),
        name: "k".into(),
        kernel: false,
        imports: vec![],
        defs: vec![],
    };
    let json_off = serde_json::to_value(&m_off).expect("serialise kernel: false");
    assert!(
        json_off.get("kernel").is_none(),
        "kernel: false must be omitted from canonical JSON (hash-stability)"
    );

    let recovered: Module =
        serde_json::from_value(json_on).expect("Module with kernel: true round-trips");
    assert!(recovered.kernel);
}

/// prep.3 (kernel-extension-mechanics): pin the JSON byte-shape of
/// `TypeDef.param_in`. Property protected: when non-empty, the map
/// serialises under the kebab-case key `"param-in"`; when empty,
/// it is omitted entirely (preserving the bit-identical hash of
/// every fixture that does not restrict). The BTreeMap/BTreeSet
/// choice gives deterministic iteration order (alphabetical) so
/// the canonical-JSON bytes are reproducible. A future edit that
/// renames the field, drops the `skip_serializing_if`, or changes
/// the inner collection type breaks the pin in lockstep with the
/// data-model document.
#[test]
fn typedef_param_in_round_trips() {
    use std::collections::BTreeSet;

    let mut restrict: BTreeMap<String, BTreeSet<String>> = BTreeMap::new();
    restrict.insert(
        "a".into(),
        ["Int".to_string(), "Float".to_string()].into_iter().collect(),
    );

    let td_on = TypeDef {
        name: "T".into(),
        vars: vec!["a".into()],
        ctors: vec![],
        doc: None,
        drop_iterative: false,
        param_in: restrict.clone(),
    };
    let json_on = serde_json::to_value(&td_on).expect("serialise param_in non-empty");
    let pi = json_on.get("param-in").expect(
        "non-empty param_in serialises under kebab key `param-in`",
    );
    let allowed = pi["a"].as_array().expect("inner value is a Type-name array");
    assert_eq!(allowed.len(), 2);
    // BTreeSet iteration order is alphabetical:
    assert_eq!(allowed[0], "Float");
    assert_eq!(allowed[1], "Int");

    let td_off = TypeDef {
        name: "T".into(),
        vars: vec![],
        ctors: vec![],
        doc: None,
        drop_iterative: false,
        param_in: BTreeMap::new(),
    };
    let json_off = serde_json::to_value(&td_off).expect("serialise param_in empty");
    assert!(
        json_off.get("param-in").is_none(),
        "empty param_in must be omitted from canonical JSON (hash-stability)"
    );

    let recovered: TypeDef =
        serde_json::from_value(json_on).expect("TypeDef with param_in round-trips");
    assert_eq!(recovered.param_in, restrict);
}

/// prep.3 (kernel-extension-mechanics): pin the JSON byte-shape of
/// the kernel-stub module — exercises all three new schema items
/// in a single fixture (`Module.kernel = true`, one TypeDef with
/// `param-in`, one ctor). A future edit that breaks any of the new
/// additive schema fields will trip this pin in lockstep with the
/// per-field round-trip tests (`module_kernel_flag_round_trips`,
/// `typedef_param_in_round_trips`).
#[test]
fn kernel_stub_module_round_trips() {
    let m = ailang_surface::parse_kernel_stub();
    assert!(m.kernel, "stub module is kernel-tier");
    assert_eq!(m.name, "kernel_stub");

    let json = serde_json::to_value(&m).expect("serialise stub module");
    assert_eq!(json["kernel"], true);
    assert_eq!(json["name"], "kernel_stub");

    let recovered: Module =
        serde_json::from_value(json).expect("stub module round-trips through serde");
    assert!(recovered.kernel);
    assert_eq!(recovered.name, "kernel_stub");

    // Locate the StubT TypeDef and confirm param_in is intact.
    let td = recovered.defs.iter().find_map(|d| match d {
        Def::Type(t) if t.name == "StubT" => Some(t),
        _ => None,
    }).expect("StubT TypeDef present in stub module");
    let allowed = td.param_in.get("a").expect("StubT.a restricted");
    assert!(allowed.contains("Int"));
    assert!(allowed.contains("Float"));
}