Change warning to be specific to --experimental-loader

Author: GeoffreyBooth

lib/internal/modules/esm/loader.js

@@ -498,6 +498,7 @@ class CustomizedModuleLoader {
   }
 }
 
+let emittedLoaderFlagWarning = false;
 /**
  * A loader instance is used as the main entry point for loading ES modules. Currently, this is a singleton; there is
  * only one used for loading the main module and everything in its dependency graph, though separate instances of this
@@ -513,6 +514,13 @@ function createModuleLoader(useCustomLoadersIfPresent = true) {
       !require('internal/modules/esm/utils').isLoaderWorker()) {
     const userLoaderPaths = getOptionValue('--experimental-loader');
     if (userLoaderPaths.length > 0) {
+      if (!emittedLoaderFlagWarning) {
+        process.emitWarning(
+          '`--experimental-loader` may be removed in the future; instead use `--import` to reference a file that calls `register()`',
+          'ExperimentalWarning',
+        );
+        emittedLoaderFlagWarning = true;
+      }
       customizations = new CustomizedModuleLoader();
     }
   }

test/es-module/test-esm-experimental-warnings.mjs

@@ -25,6 +25,10 @@ describe('ESM: warn for obsolete hooks provided', { concurrency: true }, () => {
   describe('experimental warnings for enabled experimental feature', () => {
     for (
       const [experiment, arg] of [
+        [
+          /`--experimental-loader` may be removed in the future/,
+          `--experimental-loader=${fileURL('es-module-loaders', 'hooks-custom.mjs')}`
+        ],
         [/Network Imports/, '--experimental-network-imports'],
       ]
     ) {