bug: Rust verification - generated output fails `cargo check`

[apoorv7g]

Generated Rust from Concerto CTO models is written by RustVisitor (lib/codegen/fromcto/rust/rustvisitor.js). Snapshot/unit tests do not compile the output; test/verification/rust.compile.test.js can run cargo check, but hr_base / hr_integration are skipped because they fail.

Related: bug: GraphQL verification bugs (generated output fails graphql-js) #248

Unlike Proto3, Option<HashMap<...>> is valid Rust - so the Proto “optional map” bug does not apply. The HR fixtures fail for different reasons.

---

Bug 1: Custom scalars referenced in map aliases, but never emitted

Cause

hr_base.cto / hr.cto define scalars (SSN, Time, KinName, KinTelephone) and use them as map keys/values:

map EmployeeTShirtSizes {
    o SSN
    o TShirtSizeType
}

map EmployeeDirectory {
    o SSN
    o Employee
}

RustVisitor.visit() no-ops scalar declarations:

} else if (thing.isScalarDeclaration?.()) {
    return;
}

visitMapDeclaration still emits the scalar name as a Rust type:

visitMapDeclaration(mapDeclaration, parameters) {
    const mapKeyType = mapDeclaration.getKey().getType();
    const mapValueType = mapDeclaration.getValue().getType();

    const keyType = this.toRustType(mapKeyType);
    const valueType = this.toRustType(mapValueType);

    parameters.fileWriter.writeLine(
        0,
        `pub type ${mapDeclaration.getName()} = HashMap<${keyType}, ${valueType}>;`
    );

So the crate names types that were never defined.

Reproduce

From the repo root (after npm ci, with Rust toolchain installed). Written for PowerShell:

@'
const fs = require("fs");
const path = require("path");
const { execFileSync } = require("child_process");
const { dir } = require("tmp-promise");
const { ModelManager } = require("@accordproject/concerto-core");
const { FileWriter } = require("@accordproject/concerto-util");
const RustVisitor = require("./lib/codegen/fromcto/rust/rustvisitor.js");

process.env.ENABLE_MAP_TYPE = "true";
process.env.IMPORT_ALIASING = "true";

const MODEL_DIR = "./test/codegen/fromcto/data/model";

(async () => {
  const mm = new ModelManager();
  mm.addCTOModel(fs.readFileSync(path.join(MODEL_DIR, "hr_base.cto"), "utf8"), "hr_base.cto");
  mm.addCTOModel(fs.readFileSync(path.join(MODEL_DIR, "hr.cto"), "utf8"), "hr.cto");

  const { path: out, cleanup } = await dir({ unsafeCleanup: true });
  const src = path.join(out, "src");
  fs.mkdirSync(src, { recursive: true });
  mm.accept(new RustVisitor(), {
    fileWriter: new FileWriter(src),
    showCompositionRelationships: true,
  });
  fs.renameSync(path.join(src, "mod.rs"), path.join(src, "lib.rs"));
  fs.writeFileSync(path.join(out, "Cargo.toml"), [
    "[package]", 'name = "verify"', 'version = "0.1.0"', 'edition = "2021"',
    "", "[lib]", 'path = "src/lib.rs"', "",
    "[dependencies]",
    'serde = { version = "1", features = ["derive"] }',
    'chrono = { version = "0.4", features = ["serde"] }', "",
  ].join("\n"));

  try {
    execFileSync("cargo", ["check", "--quiet"], {
      cwd: out, encoding: "utf-8", stdio: ["ignore", "pipe", "pipe"],
    });
    console.log("OK");
  } catch (err) {
    console.error([err.stdout, err.stderr].filter(Boolean).join("\n").trim());
    process.exit(1);
  } finally {
    await cleanup();
  }
})();
'@ | node

Expected errors (among others)

error[E0425]: cannot find type `SSN` in this scope
error[E0425]: cannot find type `Time` in this scope
error[E0425]: cannot find type `KinName` in this scope
error[E0425]: cannot find type `KinTelephone` in this scope

Generated fragments

pub type EmployeeTShirtSizes = HashMap<SSN, TShirtSizeType>;
pub type EmployeeLoginTimes = HashMap<String, Time>;
pub type EmployeeSocialSecurityNumbers = HashMap<String, SSN>;
pub type NextOfKin = HashMap<KinName, KinTelephone>;
pub type EmployeeDirectory = HashMap<SSN, Employee>;

Minimal repro:

@'
use std::collections::HashMap;
pub type EmployeeDirectory = HashMap<SSN, Employee>;
'@ | Set-Content tmp.rs; rustc --crate-type lib tmp.rs

→ cannot find type 'SSN' in this scope

---

Bug 2: Inconsistent scalar handling (aliases vs field sites)

Cause

When a field is a map type, visitField unwraps scalars to the underlying primitive:

} else if (ModelUtil.isScalar(mapDeclaration.getKey())) {
    const scalarDeclaration = mapDeclaration
        .getModelFile()
        .getType(mapDeclaration.getKey().getType());
    const scalarType = scalarDeclaration.getType();
    rustKeyType = this.toRustType(scalarType);

So the same model produces:

Site	Generated type
Map alias	HashMap<SSN, Employee>
Field on Company	Option<HashMap<String, Employee>>

Field sites compile (after unwrapping). The type aliases do not. C# emits scalars (visitScalarDeclaration); Rust does not.

Generated contrast

// type alias - broken
pub type EmployeeDirectory = HashMap<SSN, Employee>;

// field - scalar unwrapped to String
pub employee_directory: Option<HashMap<String, Employee>>,

---

Bug 3: Recursive relationship types without indirection

Cause

Employee / Manager have relationships to Manager. visitRelationshipDeclaration emits the named type directly (Option<Manager>), not String/Box<Manager>.

Manager extends Employee therefore contains manager: Option<Manager> - infinite size in Rust.

Expected error

error[E0072]: recursive type `Manager` has infinite size
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle

Generated fragment

pub struct Manager {
    // ... inherited from Employee ...
    pub manager: Option<Manager>,  // needs Option<Box<Manager>>
}

---

Related code

File	Role
lib/codegen/fromcto/rust/rustvisitor.js	Emits Rust (visitMapDeclaration, scalar no-op, relationships)
test/codegen/fromcto/rust/rustvisitor.js	Unit tests with mocks, not full HR fixtures
test/verification/rust.compile.test.js	cargo check verification (skips hr_*)
test/verification/cases.js	Documents skip: scalar map keys/values not emitted