Better support monorepos by allowing users to opt into autoconfiguring "root"

packages/babel-core/src/config/config-chain.js

@@ -49,7 +49,7 @@ export type PresetInstance = {
 export type ConfigContext = {
   filename: string | void,
   cwd: string,
-  root: string,
+  root: string | false,
   envName: string,
   caller: CallerMetadata | void,
 };
@@ -150,7 +150,7 @@ export function buildRootChain(
       context.envName,
       context.caller,
     );
-  } else if (opts.configFile !== false) {
+  } else if (opts.configFile !== false && context.root !== false) {
     configFile = findRootConfig(context.root, context.envName, context.caller);
   }
 
@@ -237,6 +237,8 @@ function babelrcLoadEnabled(
   // Fast path to avoid having to match patterns if the babelrc is just
   // loading in the standard root directory.
   if (babelrcRoots === undefined) {
+    if (absoluteRoot === false) return false;
+
     return pkgData.directories.indexOf(absoluteRoot) !== -1;
   }
 
@@ -248,7 +250,11 @@ function babelrcLoadEnabled(
 
   // Fast path to avoid having to match patterns if the babelrc is just
   // loading in the standard root directory.
-  if (babelrcPatterns.length === 1 && babelrcPatterns[0] === absoluteRoot) {
+  if (
+    absoluteRoot !== false &&
+    babelrcPatterns.length === 1 &&
+    babelrcPatterns[0] === absoluteRoot
+  ) {
     return pkgData.directories.indexOf(absoluteRoot) !== -1;
   }
 
@@ -661,9 +667,9 @@ function matchesPatterns(
 }
 
 function matchPattern(
-  pattern,
-  dirname,
-  pathToTest,
+  pattern: string | RegExp | Function,
+  dirname: string,
+  pathToTest: string | void,
   context: ConfigContext,
 ): boolean {
   if (typeof pattern === "function") {

packages/babel-core/src/config/files/configuration.js

@@ -24,6 +24,24 @@ const BABELRC_FILENAME = ".babelrc";
 const BABELRC_JS_FILENAME = ".babelrc.js";
 const BABELIGNORE_FILENAME = ".babelignore";
 
+export function findConfigRoot(cwd: string): string {
+  let dirname = cwd;
+  while (true) {
+    if (fs.existsSync(path.join(dirname, BABEL_CONFIG_JS_FILENAME))) {
+      return dirname;
+    }
+
+    const nextDir = path.dirname(dirname);
+    if (dirname === nextDir) break;
+    dirname = nextDir;
+  }
+
+  throw new Error(
+    `Babel was run with root:true but a root babel.config.js could not ` +
+      `be found when searching from "${dirname}"`,
+  );
+}
+
 export function findRelativeConfig(
   packageData: FilePackageData,
   envName: string,

packages/babel-core/src/config/files/index-browser.js

@@ -11,6 +11,14 @@ import type { CallerMetadata } from "../validation/options";
 
 export type { ConfigFile, IgnoreFile, RelativeConfig, FilePackageData };
 
+export function findConfigRoot(
+  cwd: string, // eslint-disable-line no-unused-vars
+): string {
+  throw new Error(
+    `Cannot search for filesystem config root when running in a browser`,
+  );
+}
+
 export function findPackageData(filepath: string): FilePackageData {
   return {
     filepath,

packages/babel-core/src/config/files/index.js

@@ -10,6 +10,7 @@ import typeof * as indexType from "./index";
 export { findPackageData } from "./package";
 
 export {
+  findConfigRoot,
   findRelativeConfig,
   findRootConfig,
   loadConfig,

packages/babel-core/src/config/partial.js

@@ -8,7 +8,7 @@ import { buildRootChain, type ConfigContext } from "./config-chain";
 import { getEnv } from "./helpers/environment";
 import { validate, type ValidatedOptions } from "./validation/options";
 
-import type { ConfigFile, IgnoreFile } from "./files";
+import { findConfigRoot, type ConfigFile, type IgnoreFile } from "./files";
 
 export default function loadPrivatePartialConfig(
   inputOpts: mixed,
@@ -30,7 +30,12 @@ export default function loadPrivatePartialConfig(
 
   const { envName = getEnv(), cwd = ".", root: rootDir = ".", caller } = args;
   const absoluteCwd = path.resolve(cwd);
-  const absoluteRootDir = path.resolve(absoluteCwd, rootDir);
+  const absoluteRootDir =
+    typeof rootDir === "boolean"
+      ? rootDir
+        ? findConfigRoot(absoluteCwd)
+        : false
+      : path.resolve(absoluteCwd, rootDir);
 
   const context: ConfigContext = {
     filename:

packages/babel-core/src/config/validation/option-assertions.js

@@ -15,6 +15,7 @@ import type {
   RootInputSourceMapOption,
   NestingPath,
   CallerMetadata,
+  ProjectRoot,
 } from "./options";
 
 export type ValidatorSet = {
@@ -77,6 +78,17 @@ export function assertSourceMaps(
   return value;
 }
 
+export function assertRoot(loc: OptionPath, value: mixed): ProjectRoot | void {
+  if (
+    value !== undefined &&
+    typeof value !== "boolean" &&
+    typeof value !== "string"
+  ) {
+    throw new Error(`${msg(loc)} must be a string, false, or undefined`);
+  }
+  return value;
+}
+
 export function assertCompact(
   loc: OptionPath,
   value: mixed,

packages/babel-core/src/config/validation/options.js

@@ -22,14 +22,15 @@ import {
   assertSourceMaps,
   assertCompact,
   assertSourceType,
+  assertRoot,
   type ValidatorSet,
   type Validator,
   type OptionPath,
 } from "./option-assertions";
 
 const ROOT_VALIDATORS: ValidatorSet = {
   cwd: (assertString: Validator<$PropertyType<ValidatedOptions, "cwd">>),
-  root: (assertString: Validator<$PropertyType<ValidatedOptions, "root">>),
+  root: (assertRoot: Validator<$PropertyType<ValidatedOptions, "root">>),
   configFile: (assertConfigFileSearch: Validator<
     $PropertyType<ValidatedOptions, "configFile">,
   >),
@@ -175,7 +176,7 @@ export type ValidatedOptions = {
   babelrc?: boolean,
   babelrcRoots?: BabelrcSearch,
   configFile?: ConfigFileSearch,
-  root?: string,
+  root?: ProjectRoot,
   code?: boolean,
   ast?: boolean,
   inputSourceMap?: RootInputSourceMapOption,
@@ -260,6 +261,7 @@ export type SourceMapsOption = boolean | "inline" | "both";
 export type SourceTypeOption = "module" | "script" | "unambiguous";
 export type CompactOption = boolean | "auto";
 export type RootInputSourceMapOption = {} | boolean;
+export type ProjectRoot = string | boolean;
 
 export type OptionsSource =
   | "arguments"