ci: Add Hardhat tooling integration test (#516)

## Description

Adds a CI workflow that builds a small Hardhat project with
`hardhat-polkadot`, exercising the standard JSON interface between
`resolc` and `hardhat-polkadot` end-to-end.

## Resolved Issues

Closes #514

.github/workflows/hardhat.yml

@@ -0,0 +1,47 @@
+# Tests compilation and E2E integration with hardhat-polkadot, but not runtime correctness.
+
+name: Hardhat Integration
+
+on:
+  workflow_call:
+
+env:
+  # Workflow-internal hardhat envs are prefixed with `CI_` to not conflict with the
+  # `HARDHAT_*` namespace, which hardhat itself will interpret for its configuration.
+  CI_HARDHAT_PROJECT_ROOT: tooling-projects/hardhat/erc20
+
+jobs:
+  use-hardhat:
+    runs-on: ubuntu-24.04
+    steps:
+      - name: Checkout revive
+        uses: actions/checkout@v4
+
+      - name: Download resolc Binary
+        uses: actions/download-artifact@v4
+        with:
+          name: resolc-x86_64-unknown-linux-musl
+          path: resolc-bin
+
+      - name: Export resolc Path
+        run: |
+          chmod +x resolc-bin/resolc-x86_64-unknown-linux-musl
+          # This env is used by `hardhat.config.js`.
+          echo "RESOLC_PATH=$(pwd)/resolc-bin/resolc-x86_64-unknown-linux-musl" >> $GITHUB_ENV
+
+      - name: Set Up Node.js
+        uses: actions/setup-node@v6
+        with:
+          node-version: "24"
+
+      - name: Install Project Dependencies
+        working-directory: ${{ env.CI_HARDHAT_PROJECT_ROOT }}
+        run: npm ci
+
+      - name: Compile Project
+        working-directory: ${{ env.CI_HARDHAT_PROJECT_ROOT }}
+        run: npx hardhat compile
+
+      - name: Verify Output
+        working-directory: ${{ env.CI_HARDHAT_PROJECT_ROOT }}
+        run: bash verify-compiler-output.sh

.github/workflows/test.yml

@@ -65,3 +65,7 @@ jobs:
     # Only run this if `test` successfully completes.
     needs: test
     uses: ./.github/workflows/differential-tests.yml
+
+  run-hardhat-tests:
+    needs: run-differential-tests
+    uses: ./.github/workflows/hardhat.yml

tooling-projects/hardhat/erc20/contracts/MyToken.sol

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: MIT
+// Compatible with OpenZeppelin Contracts ^5.0.0
+pragma solidity ^0.8.22;
+
+import { ERC20 }   from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
+import { Ownable } from "@openzeppelin/contracts/access/Ownable.sol";
+
+contract MyToken is ERC20, Ownable {
+    constructor(address initialOwner)
+        ERC20("MyToken", "MTK")
+        Ownable(initialOwner)
+    {}
+
+    function mint(address to, uint256 amount) public onlyOwner {
+        _mint(to, amount);
+    }
+}

tooling-projects/hardhat/erc20/hardhat.config.js

@@ -0,0 +1,33 @@
+require("@nomicfoundation/hardhat-toolbox");
+require("@parity/hardhat-polkadot");
+
+const RESOLC_PATH_ENV_NAME = "RESOLC_PATH";
+
+function getEnv(name) {
+    const value = process.env[name];
+    if (!value) {
+        throw new Error(`Required environment variable '${name}' is missing`);
+    }
+
+    return value;
+}
+
+/** @type {import('hardhat/config').HardhatUserConfig} */
+module.exports = {
+    solidity: "0.8.35",
+    resolc: {
+        compilerSource: "binary",
+        settings: {
+            resolcPath: getEnv(RESOLC_PATH_ENV_NAME),
+            optimizer: {
+                enabled: true,
+                parameters: "z"
+            },
+        },
+    },
+    networks: {
+        hardhat: {
+            polkadot: true
+        },
+    },
+};

tooling-projects/hardhat/erc20/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "erc20-hardhat",
+  "private": true,
+  "devDependencies": {
+    "@nomicfoundation/hardhat-toolbox": "^5.0.0",
+    "@parity/hardhat-polkadot": "^0.3.0",
+    "hardhat": "^2.26.0"
+  },
+  "dependencies": {
+    "@openzeppelin/contracts": "^5.3.0"
+  }
+}

tooling-projects/hardhat/erc20/verify-compiler-output.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+# Asserts that the compiled project contains the expected compiler output.
+# Run from the project's root.
+
+set -euxo pipefail
+
+shopt -s nullglob
+build_info_files=(artifacts/build-info/*.json)
+if [[ ${#build_info_files[@]} -ne 1 ]]; then
+  echo "expected exactly one build-info file under artifacts/build-info/, found ${#build_info_files[@]}" >&2
+  exit 1
+fi
+
+build_info=${build_info_files[0]}
+contract='.output.contracts["contracts/MyToken.sol"].MyToken'
+source='.output.sources["contracts/MyToken.sol"]'
+
+jq -er "$contract.evm.bytecode.object"         "$build_info" | grep -q '^50564d'
+jq -er "$contract.evm.deployedBytecode.object" "$build_info" | grep -q '^50564d'
+jq -e "$contract.evm.methodIdentifiers         | length > 0" "$build_info" > /dev/null
+jq -e "$contract.abi                           | length > 0" "$build_info" > /dev/null
+jq -e "$contract.storageLayout.storage         | length > 0" "$build_info" > /dev/null
+jq -e "$contract.metadata                      | length > 0" "$build_info" > /dev/null
+jq -e "$source.ast                             | length > 0" "$build_info" > /dev/null
+jq -e '.output.sources                         | length > 1' "$build_info" > /dev/null
+
+echo "all checks passed"