Handle --enable-proposed-api without extension IDs as "enable all"

Fixes #48521

src/vs/platform/environment/node/argv.ts

@@ -160,7 +160,7 @@ const extensionsHelp: { [name: string]: string; } = {
 	'--show-versions': localize('showVersions', "Show versions of installed extensions, when using --list-extension."),
 	'--install-extension (<extension-id> | <extension-vsix-path>)': localize('installExtension', "Installs an extension."),
 	'--uninstall-extension (<extension-id> | <extension-vsix-path>)': localize('uninstallExtension', "Uninstalls an extension."),
-	'--enable-proposed-api <extension-id>': localize('experimentalApis', "Enables proposed API features for an extension.")
+	'--enable-proposed-api (<extension-id>)': localize('experimentalApis', "Enables proposed API features for extensions. Can receive one or more extension IDs to enable individually.")
 };
 
 const troubleshootingHelp: { [name: string]: string; } = {

src/vs/workbench/services/extensions/electron-browser/extensionService.ts

@@ -544,9 +544,12 @@ export class ExtensionService extends Disposable implements IExtensionService {
 				const extensionsToDisable: IExtensionIdentifier[] = [];
 				const userMigratedSystemExtensions: IExtensionIdentifier[] = [{ id: BetterMergeId }];
 
-				const enableProposedApiForAll = !this._environmentService.isBuilt || (!!this._environmentService.extensionDevelopmentPath && product.nameLong.indexOf('Insiders') >= 0);
 				const enableProposedApiFor: string | string[] = this._environmentService.args['enable-proposed-api'] || [];
 
+				const enableProposedApiForAll = !this._environmentService.isBuilt ||
+					(!!this._environmentService.extensionDevelopmentPath && product.nameLong.indexOf('Insiders') >= 0) ||
+					(enableProposedApiFor.length === 0 && 'enable-proposed-api' in this._environmentService.args);
+
 				for (const extension of allExtensions) {
 					const isExtensionUnderDevelopment = this._environmentService.isExtensionDevelopment && extension.extensionLocation.scheme === Schemas.file && extension.extensionLocation.fsPath.indexOf(this._environmentService.extensionDevelopmentPath) === 0;
 					// Do not disable extensions under development