Add helper plugin for 11ty/eleventy#3373

.eleventy.js

@@ -15,7 +15,8 @@ module.exports = function(eleventyConfig) {
   console.log(chalk.blue(`[${pkg.name}] ---`));
 
   // Full list of issues: https://github.com/11ty/eleventy/issues?q=milestone%3A%22Eleventy+3.0.0%22+is%3Aclosed+label%3Abreaking-change
-  eleventyConfig.addPlugin(require("./src/node-version"));
+  eleventyConfig.addPlugin(require("./src/node-version.js"));
+  eleventyConfig.addPlugin(require("./src/explicit-config-file.js"));
 
   eleventyConfig.on("eleventy.after", () => {
     console.log(chalk.blue(`[${pkg.name}] This plugin is intended for temporary use: once you’re satisfied please remove this plugin from your project!`));

package.json

@@ -31,6 +31,7 @@
   "homepage": "https://www.11ty.dev/",
   "dependencies": {
     "kleur": "^4.1.5",
+    "minimist": "^1.2.8",
     "semver": "^7.6.3"
   },
   "11ty": {

src/explicit-config-file.js

@@ -0,0 +1,20 @@
+const fs = require("node:fs");
+const chalk = require("kleur");
+const pkg = require("../package.json");
+
+const minimist = require("minimist");
+
+const argv = minimist(process.argv.slice(2), {
+	string: ["config"]
+});
+
+module.exports = function(eleventyConfig) {
+	let type = chalk.blue("NOTICE");
+
+	if(argv.config && !fs.existsSync(argv.config)) {
+		// We’ll never get here because the configuration file has to work to run the upgrade plugin.
+		type = chalk.red("ERROR");
+	}
+
+	console.log(chalk.blue(`[${pkg.name}]`), type, `Eleventy will fail with an error when you point \`--config\` to a configuration file that does not exist. Previous versions ran as-is (without application configuration). Read more: https://github.com/11ty/eleventy/issues/3373`);
+};