Fix: Corrected notice for invalid (:) plugin names

lib/cli-engine/config-array-factory.js

@@ -286,14 +286,15 @@ function loadESLintIgnoreFile(filePath) {
  * Creates an error to notify about a missing config to extend from.
  * @param {string} configName The name of the missing config.
  * @param {string} importerName The name of the config that imported the missing config
+ * @param {string} messageTemplate The text template to source error strings from.
  * @returns {Error} The error object to throw
  * @private
  */
-function configMissingError(configName, importerName) {
+function configInvalidError(configName, importerName, messageTemplate) {
     return Object.assign(
         new Error(`Failed to load config "${configName}" to extend from.`),
         {
-            messageTemplate: "extend-config-missing",
+            messageTemplate,
             messageData: { configName, importerName }
         }
     );
@@ -802,7 +803,7 @@ class ConfigArrayFactory {
             });
         }
 
-        throw configMissingError(extendName, ctx.name);
+        throw configInvalidError(extendName, ctx.name, "extend-config-missing");
     }
 
     /**
@@ -814,6 +815,11 @@ class ConfigArrayFactory {
      */
     _loadExtendedPluginConfig(extendName, ctx) {
         const slashIndex = extendName.lastIndexOf("/");
+
+        if (slashIndex === -1) {
+            throw configInvalidError(extendName, ctx.filePath, "plugin-invalid");
+        }
+
         const pluginName = extendName.slice("plugin:".length, slashIndex);
         const configName = extendName.slice(slashIndex + 1);
 
@@ -834,7 +840,7 @@ class ConfigArrayFactory {
             });
         }
 
-        throw plugin.error || configMissingError(extendName, ctx.filePath);
+        throw plugin.error || configInvalidError(extendName, ctx.filePath, "extend-config-missing");
     }
 
     /**
@@ -867,7 +873,7 @@ class ConfigArrayFactory {
         } catch (error) {
             /* istanbul ignore else */
             if (error && error.code === "MODULE_NOT_FOUND") {
-                throw configMissingError(extendName, ctx.filePath);
+                throw configInvalidError(extendName, ctx.filePath, "extend-config-missing");
             }
             throw error;
         }

messages/plugin-invalid.txt

@@ -0,0 +1,8 @@
+"<%= configName %>" is invalid syntax for a config specifier.
+
+* If your intention is to extend from a configuration exported from the plugin, add the configuration name after a slash: e.g. "<%= configName %>/myConfig".
+* If this is the name of a shareable config instead of a plugin, remove the "plugin:" prefix: i.e. "<%= configName.slice("plugin:".length) %>".
+
+'"<%- configName %>"' was referenced from the config file in "<%- importerName %>".
+
+If you still can't figure out the problem, please stop by https://eslint.org/chat/help to chat with the team.

tests/lib/cli-engine/config-array-factory.js

@@ -1515,6 +1515,23 @@ describe("ConfigArrayFactory", () => {
             }, /Failed to load config "plugin:invalid-config\/bar" to extend from./u);
         });
 
+        it("should throw an error with a message template when a plugin referenced for a plugin config is invalid", () => {
+            try {
+                applyExtends({
+                    extends: "plugin:nonexistent-plugin",
+                    rules: { eqeqeq: 2 }
+                });
+            } catch (err) {
+                assert.strictEqual(err.messageTemplate, "plugin-invalid");
+                assert.deepStrictEqual(err.messageData, {
+                    configName: "plugin:nonexistent-plugin",
+                    importerName: path.join(process.cwd(), "whatever")
+                });
+                return;
+            }
+            assert.fail("Expected to throw an error");
+        });
+
         it("should throw an error with a message template when a plugin referenced for a plugin config is not found", () => {
             try {
                 applyExtends({