Show warning if HardhatConfig returns undefined/empty object

This is usually indicative of a mistake in HardhatConfig, hence we should warn users.

Relates to NomicFoundation#1490.

.changeset/healthy-cows-ring.md

@@ -0,0 +1,5 @@
+---
+"hardhat": patch
+---
+
+Show warning if user doesn't export or exports empty object from Hardhat config file (issue #1490)

packages/hardhat-core/src/internal/cli/cli.ts

@@ -126,10 +126,13 @@ async function main() {
 
     let taskName = parsedTaskName ?? TASK_HELP;
 
+    const showEmptyWarnings = true;
     const showSolidityConfigWarnings = taskName === TASK_COMPILE;
 
     const ctx = HardhatContext.createHardhatContext();
+
     const config = loadConfigAndTasks(hardhatArguments, {
+      showEmptyWarnings,
       showSolidityConfigWarnings,
     });
 

packages/hardhat-core/src/internal/core/config/config-loading.ts

@@ -38,7 +38,16 @@ export function resolveConfigPath(configPath: string | undefined) {
 
 export function loadConfigAndTasks(
   hardhatArguments?: Partial<HardhatArguments>,
-  { showSolidityConfigWarnings } = { showSolidityConfigWarnings: false }
+  {
+    showEmptyWarnings = false,
+    showSolidityConfigWarnings = false,
+  }: {
+    showEmptyWarnings?: boolean;
+    showSolidityConfigWarnings?: boolean;
+  } = {
+    showEmptyWarnings: false,
+    showSolidityConfigWarnings: false,
+  }
 ): HardhatConfig {
   let configPath =
     hardhatArguments !== undefined ? hardhatArguments.config : undefined;
@@ -73,6 +82,10 @@ export function loadConfigAndTasks(
     ctx.setConfigLoadingAsFinished();
   }
 
+  if (showEmptyWarnings) {
+    checkEmptyConfig(userConfig, { showSolidityConfigWarnings });
+  }
+
   validateConfig(userConfig);
 
   if (showSolidityConfigWarnings) {
@@ -220,6 +233,25 @@ function readPackageJson(packageName: string): PackageJson | undefined {
   }
 }
 
+function checkEmptyConfig(
+  userConfig: any,
+  {
+    showSolidityConfigWarnings = false,
+  }: { showSolidityConfigWarnings: boolean } = {
+    showSolidityConfigWarnings: false,
+  }
+) {
+  if (userConfig === undefined || Object.keys(userConfig).length === 0) {
+    let warning = `Hardhat config is returning an empty config object, check the export from the config file if this is unexpected.\n`;
+
+    if (!showSolidityConfigWarnings) {
+      warning += `Learn more about compiler configuration at https://hardhat.org/config"\n`;
+    }
+
+    console.warn(chalk.yellow(warning));
+  }
+}
+
 function checkMissingSolidityConfig(userConfig: any) {
   if (userConfig.solidity === undefined) {
     console.warn(

packages/hardhat-core/test/fixture-projects/solidity-config-warnings/config-without-solidity.js

@@ -1 +1 @@
-module.exports = {};
+module.exports = { solidity: undefined };

packages/hardhat-core/test/fixture-projects/solidity-config-warnings/empty-config.js

@@ -0,0 +1 @@
+module.exports = {};

packages/hardhat-core/test/internal/core/config/config-loading.ts

@@ -371,6 +371,21 @@ Hardhat plugin instead.`
       resetHardhatContext();
     });
 
+    it("should emit a warning if config is the empty object", function () {
+      loadConfigAndTasks(
+        {
+          config: "empty-config.js",
+        },
+        { showEmptyWarnings: true }
+      );
+
+      assert.equal(consoleWarnStub.callCount, 1);
+      assert.include(
+        consoleWarnStub.args[0][0],
+        "Hardhat config is returning an empty config object, check the export from the config file if this is unexpected."
+      );
+    });
+
     it("should emit a warning if there's no configured solidity", function () {
       const config = loadConfigAndTasks(
         {