add showConfig support (#11588)

* draft: showConfig support

* feat: pass through showConfig command options

* update test file

* refactor: add createLogger to makeChainWalker

* serializing dynamic plugin instance

* fix flow errors

* chore: add tests on extended config

* fix: do not print empty presets

* add more test cases

* add windows testcases

* address review comments

* throw error when showConfigPath does not exist

* print reason when showConfig is targetting an ignored file

* remove showConfig: boolean

* refactor: simplify environment flag name

* rename test fixtures

* fix: throw when SHOW_CONFIG_FOR is not a regular file

* cleanup test fixtures

* add test on only

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

Co-authored-by: Brian Ng <bng412@gmail.com>

* address review comments

* update test fixtures

* feat: sort config items in ascending priority

Co-authored-by: Brian Ng <bng412@gmail.com>

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with ignored file/in-files/babel.config.json

@@ -0,0 +1,14 @@
+{
+  "sourceType": "script",
+  "overrides": [
+    {
+      "test": "src/index.js",
+      "sourceType": "module"
+    }
+  ],
+  "env": {
+    "test": {
+      "ignore": ["./src/index.js"]
+    }
+  }
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with ignored file/options.json

@@ -0,0 +1,7 @@
+{
+  "args": ["./src/index.js"],
+  "env": {
+    "BABEL_SHOW_CONFIG_FOR": "./src/index.js"
+  },
+  "os": ["darwin", "linux"]
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with ignored file/stdout.txt

@@ -0,0 +1 @@
+No config is applied to "<CWD>/src/index.js" because it matches one of `ignore: ["./src/index.js"]` from "<CWD>"

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with multiple files-windows/in-files/babel.config.json

@@ -0,0 +1,13 @@
+{
+  "sourceType": "script",
+  "overrides": [
+    {
+      "test": "src/foo.js",
+      "sourceType": "module"
+    },
+    {
+      "test": "src/bar.js",
+      "sourceType": "script"
+    }
+  ]
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with multiple files-windows/options.json

@@ -0,0 +1,7 @@
+{
+  "args": ["./src", "--out-file", "./test.js"],
+  "os": ["win32"],
+  "env": {
+    "BABEL_SHOW_CONFIG_FOR": "./src/foo.js"
+  }
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with multiple files-windows/out-files/test.js

@@ -0,0 +1 @@
+"use strict";

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with multiple files-windows/stdout.txt

@@ -0,0 +1,31 @@
+Babel configs on "<CWD>/src/foo.js" (ascending priority):
+config <CWD>/babel.config.json
+{
+  "sourceType": "script"
+}
+
+config <CWD>/babel.config.json .overrides[0]
+{
+  "test": "src/foo.js",
+  "sourceType": "module"
+}
+
+config <CWD>/.babelrc
+{}
+
+programmatic options from @babel/cli
+{
+  "sourceFileName": "src/foo.js",
+  "presets": [
+    "<ROOTDIR>//packages//babel-preset-react"
+  ],
+  "plugins": [
+    "<ROOTDIR>//packages//babel-plugin-transform-arrow-functions",
+    "<ROOTDIR>//packages//babel-plugin-transform-strict-mode",
+    "<ROOTDIR>//packages//babel-plugin-transform-modules-commonjs"
+  ],
+  "caller": {
+    "name": "@babel/cli"
+  },
+  "filename": "src//foo.js"
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with multiple files/in-files/babel.config.json

@@ -0,0 +1,13 @@
+{
+  "sourceType": "script",
+  "overrides": [
+    {
+      "test": "src/foo.js",
+      "sourceType": "module"
+    },
+    {
+      "test": "src/bar.js",
+      "sourceType": "script"
+    }
+  ]
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with multiple files/options.json

@@ -0,0 +1,7 @@
+{
+  "args": ["./src", "--out-file", "./test.js"],
+  "os": ["darwin", "linux"],
+  "env": {
+    "BABEL_SHOW_CONFIG_FOR": "./src/foo.js"
+  }
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with multiple files/out-files/test.js

@@ -0,0 +1 @@
+"use strict";

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with multiple files/stdout.txt

@@ -0,0 +1,31 @@
+Babel configs on "<CWD>/src/foo.js" (ascending priority):
+config <CWD>/babel.config.json
+{
+  "sourceType": "script"
+}
+
+config <CWD>/babel.config.json .overrides[0]
+{
+  "test": "src/foo.js",
+  "sourceType": "module"
+}
+
+config <CWD>/.babelrc
+{}
+
+programmatic options from @babel/cli
+{
+  "sourceFileName": "src/foo.js",
+  "presets": [
+    "<ROOTDIR>/packages/babel-preset-react"
+  ],
+  "plugins": [
+    "<ROOTDIR>/packages/babel-plugin-transform-arrow-functions",
+    "<ROOTDIR>/packages/babel-plugin-transform-strict-mode",
+    "<ROOTDIR>/packages/babel-plugin-transform-modules-commonjs"
+  ],
+  "caller": {
+    "name": "@babel/cli"
+  },
+  "filename": "src/foo.js"
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with only file/in-files/babel.config.json

@@ -0,0 +1,14 @@
+{
+  "sourceType": "script",
+  "overrides": [
+    {
+      "test": "src/index.js",
+      "sourceType": "module"
+    }
+  ],
+  "env": {
+    "test": {
+      "only": ["./src/unicorn.js"]
+    }
+  }
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with only file/options.json

@@ -0,0 +1,7 @@
+{
+  "args": ["./src/index.js"],
+  "env": {
+    "BABEL_SHOW_CONFIG_FOR": "./src/index.js"
+  },
+  "os": ["darwin", "linux"]
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR with only file/stdout.txt

@@ -0,0 +1 @@
+No config is applied to "<CWD>/src/index.js" because it fails to match one of `only: ["./src/unicorn.js"]` from "<CWD>"

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR-windows/in-files/babel.config.json

@@ -0,0 +1,13 @@
+{
+  "sourceType": "script",
+  "overrides": [
+    {
+      "test": "src/foo.js",
+      "sourceType": "module"
+    },
+    {
+      "test": "src/bar.js",
+      "sourceType": "script"
+    }
+  ]
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR-windows/options.json

@@ -0,0 +1,7 @@
+{
+  "args": ["./src", "-d", "lib"],
+  "env": {
+    "BABEL_SHOW_CONFIG_FOR": "./src/foo.js"
+  },
+  "os": ["win32"]
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR-windows/out-files/lib/bar.js

@@ -0,0 +1 @@
+"use strict";

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR-windows/stdout.txt

@@ -0,0 +1,32 @@
+Babel configs on "<CWD>/src/foo.js" (ascending priority):
+config <CWD>/babel.config.json
+{
+  "sourceType": "script"
+}
+
+config <CWD>/babel.config.json .overrides[0]
+{
+  "test": "src/foo.js",
+  "sourceType": "module"
+}
+
+config <CWD>/.babelrc
+{}
+
+programmatic options from @babel/cli
+{
+  "sourceFileName": "../src/foo.js",
+  "presets": [
+    "<ROOTDIR>//packages//babel-preset-react"
+  ],
+  "plugins": [
+    "<ROOTDIR>//packages//babel-plugin-transform-arrow-functions",
+    "<ROOTDIR>//packages//babel-plugin-transform-strict-mode",
+    "<ROOTDIR>//packages//babel-plugin-transform-modules-commonjs"
+  ],
+  "caller": {
+    "name": "@babel/cli"
+  },
+  "filename": "src//foo.js"
+}
+Successfully compiled 1 file with Babel (123ms).

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR/in-files/my-config.js

@@ -0,0 +1,40 @@
+module.exports = {
+  sourceType: "script",
+  plugins: [require("@foo/babel-plugin-1")],
+  extends: "./my-extended.js",
+  overrides: [
+    {
+      test: "src/index.js",
+      plugins: [["@foo/babel-plugin-2", { noDocumentAll: true }]],
+      env: {
+        test: {
+          plugins: [
+            "@foo/babel-plugin-1",
+            [
+              { name: "@foo/inline-babel-plugin-1", visitor: { Program() {} } },
+              { noDocumentAll: true },
+            ],
+          ],
+        },
+      },
+    },
+    {
+      exclude: "src/index.js",
+      plugins: ["@foo/babel-plugin-4"],
+    },
+  ],
+  env: {
+    test: {
+      plugins: [
+        [
+          "@foo/babel-plugin-3",
+          { noDocumentAll: true },
+          "@foo/babel-plugin-three",
+        ],
+      ],
+    },
+    development: {
+      plugins: ["@foo/babel-plugin-4"],
+    },
+  },
+};

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR/in-files/my-extended.js

@@ -0,0 +1,47 @@
+module.exports = {
+  sourceMaps: false,
+  presets: ["@foo/babel-preset-1"],
+  overrides: [
+    {
+      test: "src/index.js",
+      presets: [["@foo/babel-preset-2", { noDocumentAll: true }]],
+      env: {
+        test: {
+          presets: [
+            "@foo/babel-preset-1",
+            [
+              {
+                name: "@foo/inline-babel-preset-1",
+                plugins: [
+                  {
+                    name: "@foo/inline-babel-plugin-1",
+                    visitor: { Program() {} },
+                  },
+                ],
+              },
+              { noDocumentAll: true },
+            ],
+          ],
+        },
+      },
+    },
+    {
+      exclude: "src/index.js",
+      presets: ["@foo/babel-preset-4"],
+    },
+  ],
+  env: {
+    test: {
+      presets: [
+        [
+          "@foo/babel-preset-3",
+          { noDocumentAll: true },
+          "@foo/babel-preset-three",
+        ],
+      ],
+    },
+    development: {
+      presets: ["@foo/babel-preset-4"],
+    },
+  },
+};

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR/in-files/node_modules/@foo/babel-plugin-1/index.js

@@ -0,0 +1,6 @@
+module.exports = (api) => ({
+  name: "@foo/" + __dirname,
+  visitor: {
+    Program() {}
+  }
+})

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR/in-files/node_modules/@foo/babel-plugin-2/index.js

@@ -0,0 +1,6 @@
+module.exports = (api) => ({
+  name: "@foo/" + __dirname,
+  visitor: {
+    Program() {}
+  }
+})

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR/in-files/node_modules/@foo/babel-plugin-3/index.js

@@ -0,0 +1,6 @@
+module.exports = (api) => ({
+  name: "@foo/" + __dirname,
+  visitor: {
+    Program() {}
+  }
+})

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR/in-files/node_modules/@foo/babel-plugin-4/index.js

@@ -0,0 +1,6 @@
+module.exports = (api) => ({
+  name: "@foo/" + __dirname,
+  visitor: {
+    Program() {}
+  }
+})

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR/in-files/node_modules/@foo/babel-preset-1/index.js

@@ -0,0 +1,4 @@
+module.exports = (api) => ({
+  name: "@foo/" + __dirname,
+  plugins: ["@foo/" + __dirname.replace("preset", "plugin")]
+})

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR/in-files/node_modules/@foo/babel-preset-2/index.js

@@ -0,0 +1,4 @@
+module.exports = (api) => ({
+  name: "@foo/" + __dirname,
+  plugins: ["@foo/" + __dirname.replace("preset", "plugin")]
+})

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR/in-files/node_modules/@foo/babel-preset-3/index.js

@@ -0,0 +1,4 @@
+module.exports = (api) => ({
+  name: "@foo/" + __dirname,
+  plugins: ["@foo/" + __dirname.replace("preset", "plugin")]
+})

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR/in-files/node_modules/@foo/babel-preset-4/index.js

@@ -0,0 +1,4 @@
+module.exports = (api) => ({
+  name: "@foo/" + __dirname,
+  plugins: ["@foo/" + __dirname.replace("preset", "plugin")]
+})

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR/options.json

@@ -0,0 +1,8 @@
+{
+  "args": ["--config-file", "./my-config.js", "./src/index.js"],
+  "env": {
+    "BABEL_ENV": "test",
+    "BABEL_SHOW_CONFIG_FOR": "./src/index.js"
+  },
+  "os": ["darwin", "linux"]
+}

packages/babel-cli/test/fixtures/babel/env - SHOW_CONFIG_FOR/stdout.txt

@@ -0,0 +1,127 @@
+Babel configs on "<CWD>/src/index.js" (ascending priority):
+config <CWD>/my-extended.js
+{
+  "sourceMaps": false,
+  "presets": [
+    "@foo/babel-preset-1"
+  ]
+}
+
+config <CWD>/my-extended.js .env["test"]
+{
+  "presets": [
+    [
+      "@foo/babel-preset-3",
+      {
+        "noDocumentAll": true
+      },
+      "@foo/babel-preset-three"
+    ]
+  ]
+}
+
+config <CWD>/my-extended.js .overrides[0]
+{
+  "test": "src/index.js",
+  "presets": [
+    [
+      "@foo/babel-preset-2",
+      {
+        "noDocumentAll": true
+      }
+    ]
+  ]
+}
+
+config <CWD>/my-extended.js .overrides[0].env["test"]
+{
+  "presets": [
+    "@foo/babel-preset-1",
+    [
+      {
+        "name": "@foo/inline-babel-preset-1",
+        "plugins": [
+          {
+            "name": "@foo/inline-babel-plugin-1",
+            "visitor": {}
+          }
+        ]
+      },
+      {
+        "noDocumentAll": true
+      }
+    ]
+  ]
+}
+
+config <CWD>/my-config.js
+{
+  "sourceType": "script",
+  "plugins": [
+    "[Function: (api) => ({/n  name: /"@foo//" + __dirname,/n  visitor ... ]"
+  ],
+  "extends": "./my-extended.js"
+}
+
+config <CWD>/my-config.js .env["test"]
+{
+  "plugins": [
+    [
+      "@foo/babel-plugin-3",
+      {
+        "noDocumentAll": true
+      },
+      "@foo/babel-plugin-three"
+    ]
+  ]
+}
+
+config <CWD>/my-config.js .overrides[0]
+{
+  "test": "src/index.js",
+  "plugins": [
+    [
+      "@foo/babel-plugin-2",
+      {
+        "noDocumentAll": true
+      }
+    ]
+  ]
+}
+
+config <CWD>/my-config.js .overrides[0].env["test"]
+{
+  "plugins": [
+    "@foo/babel-plugin-1",
+    [
+      {
+        "name": "@foo/inline-babel-plugin-1",
+        "visitor": {}
+      },
+      {
+        "noDocumentAll": true
+      }
+    ]
+  ]
+}
+
+config <CWD>/.babelrc
+{}
+
+programmatic options from @babel/cli
+{
+  "sourceFileName": "./src/index.js",
+  "presets": [
+    "<ROOTDIR>/packages/babel-preset-react"
+  ],
+  "plugins": [
+    "<ROOTDIR>/packages/babel-plugin-transform-arrow-functions",
+    "<ROOTDIR>/packages/babel-plugin-transform-strict-mode",
+    "<ROOTDIR>/packages/babel-plugin-transform-modules-commonjs"
+  ],
+  "configFile": "./my-config.js",
+  "caller": {
+    "name": "@babel/cli"
+  },
+  "filename": "./src/index.js"
+}

packages/babel-cli/test/fixtures/babel/error - SHOW_CONFIG_FOR is a folder-windows/in-files/babel.config.json

@@ -0,0 +1,13 @@
+{
+  "sourceType": "script",
+  "overrides": [
+    {
+      "test": "src/foo.js",
+      "sourceType": "module"
+    },
+    {
+      "test": "src/bar.js",
+      "sourceType": "script"
+    }
+  ]
+}

packages/babel-cli/test/fixtures/babel/error - SHOW_CONFIG_FOR is a folder-windows/options.json

@@ -0,0 +1,8 @@
+{
+  "args": ["./src", "-d", "lib"],
+  "env": {
+    "BABEL_SHOW_CONFIG_FOR": "./src"
+  },
+  "os": ["win32"],
+  "stderrContains": true
+}

packages/babel-cli/test/fixtures/babel/error - SHOW_CONFIG_FOR is a folder-windows/stderr.txt

@@ -0,0 +1 @@
+Error: <CWD>\src: BABEL_SHOW_CONFIG_FOR must refer to a regular file, directories are not supported.

packages/babel-cli/test/fixtures/babel/error - SHOW_CONFIG_FOR is a folder/in-files/babel.config.json

@@ -0,0 +1,13 @@
+{
+  "sourceType": "script",
+  "overrides": [
+    {
+      "test": "src/foo.js",
+      "sourceType": "module"
+    },
+    {
+      "test": "src/bar.js",
+      "sourceType": "script"
+    }
+  ]
+}

packages/babel-cli/test/fixtures/babel/error - SHOW_CONFIG_FOR is a folder/options.json

@@ -0,0 +1,8 @@
+{
+  "args": ["./src", "-d", "lib"],
+  "env": {
+    "BABEL_SHOW_CONFIG_FOR": "./src"
+  },
+  "os": ["darwin", "linux"],
+  "stderrContains": true
+}

packages/babel-cli/test/fixtures/babel/error - SHOW_CONFIG_FOR is a folder/stderr.txt

@@ -0,0 +1 @@
+Error: <CWD>/src: BABEL_SHOW_CONFIG_FOR must refer to a regular file, directories are not supported.

packages/babel-cli/test/index.js

@@ -3,12 +3,14 @@ const helper = require("@babel/helper-fixtures");
 const rimraf = require("rimraf");
 const { sync: makeDirSync } = require("make-dir");
 const child = require("child_process");
+const escapeRegExp = require("lodash/escapeRegExp");
 const merge = require("lodash/merge");
 const path = require("path");
 const fs = require("fs");
 
 const fixtureLoc = path.join(__dirname, "fixtures");
 const tmpLoc = path.join(__dirname, "tmp");
+const rootDir = path.resolve(__dirname, "../../..");
 
 const fileFilter = function (x) {
   return x !== ".DS_Store";
@@ -19,12 +21,12 @@ const outputFileSync = function (filePath, data) {
   fs.writeFileSync(filePath, data);
 };
 
-const presetLocs = [path.join(__dirname, "../../babel-preset-react")];
+const presetLocs = [path.join(rootDir, "./packages/babel-preset-react")];
 
 const pluginLocs = [
-  path.join(__dirname, "/../../babel-plugin-transform-arrow-functions"),
-  path.join(__dirname, "/../../babel-plugin-transform-strict-mode"),
-  path.join(__dirname, "/../../babel-plugin-transform-modules-commonjs"),
+  path.join(rootDir, "./packages/babel-plugin-transform-arrow-functions"),
+  path.join(rootDir, "./packages/babel-plugin-transform-strict-mode"),
+  path.join(rootDir, "./packages/babel-plugin-transform-modules-commonjs"),
 ].join(",");
 
 const readDir = function (loc, filter) {
@@ -50,13 +52,21 @@ const saveInFiles = function (files) {
 };
 
 const normalizeOutput = function (str, cwd) {
-  let prev;
-  do {
-    prev = str;
-    str = str.replace(cwd, "<CWD>");
-  } while (str !== prev);
-
-  return str.replace(/\(\d+ms\)/g, "(123ms)");
+  let result = str
+    .replace(/\(\d+ms\)/g, "(123ms)")
+    .replace(new RegExp(escapeRegExp(cwd), "g"), "<CWD>")
+    // (non-win32) /foo/babel/packages -> <CWD>/packages
+    // (win32) C:\foo\babel\packages -> <CWD>\packages
+    .replace(new RegExp(escapeRegExp(rootDir), "g"), "<ROOTDIR>");
+  if (process.platform === "win32") {
+    result = result
+      // C:\\foo\\babel\\packages -> <CWD>\\packages (in js string literal)
+      .replace(
+        new RegExp(escapeRegExp(rootDir.replace(/\\/g, "\\\\")), "g"),
+        "<ROOTDIR>",
+      );
+  }
+  return result;
 };
 
 const assertTest = function (stdout, stderr, opts, cwd) {
@@ -132,8 +142,9 @@ const buildTest = function (binName, testName, opts) {
     }
 
     args = args.concat(opts.args);
+    const env = { ...process.env, ...opts.env };
 
-    const spawn = child.spawn(process.execPath, args);
+    const spawn = child.spawn(process.execPath, args, { env });
 
     let stderr = "";
     let stdout = "";

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

@@ -12,6 +12,7 @@ import {
   type CallerMetadata,
 } from "./validation/options";
 import pathPatternToRegex from "./pattern-to-regex";
+import { ConfigPrinter, ChainFormatter } from "./printer";
 
 const debug = buildDebug("babel:config:config-chain");
 
@@ -53,6 +54,7 @@ export type ConfigContext = {
   root: string,
   envName: string,
   caller: CallerMetadata | void,
+  showConfig: boolean,
 };
 
 /**
@@ -81,6 +83,7 @@ export const buildPresetChainWalker: (
   overrides: (preset, index) => loadPresetOverridesDescriptors(preset)(index),
   overridesEnv: (preset, index, envName) =>
     loadPresetOverridesEnvDescriptors(preset)(index)(envName),
+  createLogger: () => () => {}, // Currently we don't support logging how preset is expanded
 });
 const loadPresetDescriptors = makeWeakCacheSync((preset: PresetInstance) =>
   buildRootDescriptors(preset, preset.alias, createUncachedDescriptors),
@@ -134,14 +137,19 @@ export function* buildRootChain(
   opts: ValidatedOptions,
   context: ConfigContext,
 ): Handler<RootConfigChain | null> {
+  let configReport, babelRcReport;
+  const programmaticLogger = new ConfigPrinter();
   const programmaticChain = yield* loadProgrammaticChain(
     {
       options: opts,
       dirname: context.cwd,
     },
     context,
+    undefined,
+    programmaticLogger,
   );
   if (!programmaticChain) return null;
+  const programmaticReport = programmaticLogger.output();
 
   let configFile;
   if (typeof opts.configFile === "string") {
@@ -163,10 +171,17 @@ export function* buildRootChain(
   let babelrcRootsDirectory = context.cwd;
 
   const configFileChain = emptyChain();
+  const configFileLogger = new ConfigPrinter();
   if (configFile) {
     const validatedFile = validateConfigFile(configFile);
-    const result = yield* loadFileChain(validatedFile, context);
+    const result = yield* loadFileChain(
+      validatedFile,
+      context,
+      undefined,
+      configFileLogger,
+    );
     if (!result) return null;
+    configReport = configFileLogger.output();
 
     // Allow config files to toggle `.babelrc` resolution on and off and
     // specify where the roots are.
@@ -208,16 +223,32 @@ export function* buildRootChain(
     }
 
     if (babelrcFile) {
+      const validatedFile = validateBabelrcFile(babelrcFile);
+      const babelrcLogger = new ConfigPrinter();
       const result = yield* loadFileChain(
-        validateBabelrcFile(babelrcFile),
+        validatedFile,
         context,
+        undefined,
+        babelrcLogger,
       );
       if (!result) return null;
+      babelRcReport = babelrcLogger.output();
 
       mergeChain(fileChain, result);
     }
   }
 
+  if (context.showConfig) {
+    console.log(
+      // $FlowIgnore: context.showConfig implies context.filename is not null
+      `Babel configs on "${context.filename}" (ascending priority):\n` +
+        // print config by the order of ascending priority
+        [configReport, babelRcReport, programmaticReport]
+          .filter(x => !!x)
+          .join("\n\n"),
+    );
+    return null;
+  }
   // Insert file chain in front so programmatic options have priority
   // over configuration file chain items.
   const chain = mergeChain(
@@ -317,6 +348,8 @@ const loadProgrammaticChain = makeChainWalker({
       index,
       envName,
     ),
+  createLogger: (input, context, baseLogger) =>
+    buildProgrammaticLogger(input, context, baseLogger),
 });
 
 /**
@@ -328,6 +361,8 @@ const loadFileChain = makeChainWalker({
   overrides: (file, index) => loadFileOverridesDescriptors(file)(index),
   overridesEnv: (file, index, envName) =>
     loadFileOverridesEnvDescriptors(file)(index)(envName),
+  createLogger: (file, context, baseLogger) =>
+    buildFileLogger(file.filepath, context, baseLogger),
 });
 const loadFileDescriptors = makeWeakCacheSync((file: ValidatedFile) =>
   buildRootDescriptors(file, file.filepath, createUncachedDescriptors),
@@ -367,10 +402,32 @@ const loadFileOverridesEnvDescriptors = makeWeakCacheSync(
     ),
 );
 
+function buildFileLogger(
+  filepath: string,
+  context: ConfigContext,
+  baseLogger: ConfigPrinter | void,
+) {
+  if (!baseLogger) {
+    return () => {};
+  }
+  return baseLogger.configure(context.showConfig, ChainFormatter.Config, {
+    filepath,
+  });
+}
+
 function buildRootDescriptors({ dirname, options }, alias, descriptors) {
   return descriptors(dirname, options, alias);
 }
 
+function buildProgrammaticLogger(_, context, baseLogger: ConfigPrinter | void) {
+  if (!baseLogger) {
+    return () => {};
+  }
+  return baseLogger.configure(context.showConfig, ChainFormatter.Programmatic, {
+    callerName: context.caller?.name,
+  });
+}
+
 function buildEnvDescriptors(
   { dirname, options },
   alias,
@@ -418,41 +475,68 @@ function makeChainWalker<ArgT: { options: ValidatedOptions, dirname: string }>({
   env,
   overrides,
   overridesEnv,
+  createLogger,
 }: {|
   root: ArgT => OptionsAndDescriptors,
   env: (ArgT, string) => OptionsAndDescriptors | null,
   overrides: (ArgT, number) => OptionsAndDescriptors,
   overridesEnv: (ArgT, number, string) => OptionsAndDescriptors | null,
+  createLogger: (
+    ArgT,
+    ConfigContext,
+    ConfigPrinter | void,
+  ) => (OptionsAndDescriptors, ?number, ?string) => void,
 |}): (
   ArgT,
   ConfigContext,
-  Set<ConfigFile> | void,
+  files?: Set<ConfigFile> | void,
+  baseLogger: ConfigPrinter | void,
 ) => Handler<ConfigChain | null> {
-  return function* (input, context, files = new Set()) {
+  return function* (input, context, files = new Set(), baseLogger) {
     const { dirname } = input;
 
-    const flattenedConfigs = [];
+    const flattenedConfigs: Array<{|
+      config: OptionsAndDescriptors,
+      index: ?number,
+      envName: ?string,
+    |}> = [];
 
     const rootOpts = root(input);
     if (configIsApplicable(rootOpts, dirname, context)) {
-      flattenedConfigs.push(rootOpts);
+      flattenedConfigs.push({
+        config: rootOpts,
+        envName: undefined,
+        index: undefined,
+      });
 
       const envOpts = env(input, context.envName);
       if (envOpts && configIsApplicable(envOpts, dirname, context)) {
-        flattenedConfigs.push(envOpts);
+        flattenedConfigs.push({
+          config: envOpts,
+          envName: context.envName,
+          index: undefined,
+        });
       }
 
       (rootOpts.options.overrides || []).forEach((_, index) => {
         const overrideOps = overrides(input, index);
         if (configIsApplicable(overrideOps, dirname, context)) {
-          flattenedConfigs.push(overrideOps);
+          flattenedConfigs.push({
+            config: overrideOps,
+            index,
+            envName: undefined,
+          });
 
           const overrideEnvOpts = overridesEnv(input, index, context.envName);
           if (
             overrideEnvOpts &&
             configIsApplicable(overrideEnvOpts, dirname, context)
           ) {
-            flattenedConfigs.push(overrideEnvOpts);
+            flattenedConfigs.push({
+              config: overrideEnvOpts,
+              index,
+              envName: context.envName,
+            });
           }
         }
       });
@@ -462,7 +546,7 @@ function makeChainWalker<ArgT: { options: ValidatedOptions, dirname: string }>({
     // that we don't do extra work loading extended configs if a file is
     // ignored.
     if (
-      flattenedConfigs.some(({ options: { ignore, only } }) =>
+      flattenedConfigs.some(({ config: { options: { ignore, only } } }) =>
         shouldIgnore(context, ignore, only, dirname),
       )
     ) {
@@ -470,15 +554,24 @@ function makeChainWalker<ArgT: { options: ValidatedOptions, dirname: string }>({
     }
 
     const chain = emptyChain();
+    const logger = createLogger(input, context, baseLogger);
 
-    for (const op of flattenedConfigs) {
+    for (const { config, index, envName } of flattenedConfigs) {
       if (
-        !(yield* mergeExtendsChain(chain, op.options, dirname, context, files))
+        !(yield* mergeExtendsChain(
+          chain,
+          config.options,
+          dirname,
+          context,
+          files,
+          baseLogger,
+        ))
       ) {
         return null;
       }
 
-      mergeChainOpts(chain, op);
+      logger(config, index, envName);
+      mergeChainOpts(chain, config);
     }
     return chain;
   };
@@ -490,6 +583,7 @@ function* mergeExtendsChain(
   dirname: string,
   context: ConfigContext,
   files: Set<ConfigFile>,
+  baseLogger: ConfigPrinter | void,
 ): Handler<boolean> {
   if (opts.extends === undefined) return true;
 
@@ -513,6 +607,7 @@ function* mergeExtendsChain(
     validateExtendFile(file),
     context,
     files,
+    baseLogger,
   );
   files.delete(file);
 
@@ -650,22 +745,28 @@ function shouldIgnore(
   dirname: string,
 ): boolean {
   if (ignore && matchesPatterns(context, ignore, dirname)) {
-    debug(
-      "Ignored %o because it matched one of %O from %o",
-      context.filename,
+    const message = `No config is applied to "${
+      context.filename ?? "(unknown)"
+    }" because it matches one of \`ignore: ${JSON.stringify(
       ignore,
-      dirname,
-    );
+    )}\` from "${dirname}"`;
+    debug(message);
+    if (context.showConfig) {
+      console.log(message);
+    }
     return true;
   }
 
   if (only && !matchesPatterns(context, only, dirname)) {
-    debug(
-      "Ignored %o because it failed to match one of %O from %o",
-      context.filename,
+    const message = `No config is applied to "${
+      context.filename ?? "(unknown)"
+    }" because it fails to match one of \`only: ${JSON.stringify(
       only,
-      dirname,
-    );
+    )}\` from "${dirname}"`;
+    debug(message);
+    if (context.showConfig) {
+      console.log(message);
+    }
     return true;
   }
 

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

@@ -299,6 +299,23 @@ const readIgnoreConfig = makeStaticFileCache((filepath, content) => {
   };
 });
 
+export function* resolveShowConfigPath(
+  dirname: string,
+): Handler<string | null> {
+  const targetPath = process.env.BABEL_SHOW_CONFIG_FOR;
+  if (targetPath != null) {
+    const absolutePath = path.resolve(dirname, targetPath);
+    const stats = yield* fs.stat(absolutePath);
+    if (!stats.isFile()) {
+      throw new Error(
+        `${absolutePath}: BABEL_SHOW_CONFIG_FOR must refer to a regular file, directories are not supported.`,
+      );
+    }
+    return absolutePath;
+  }
+  return null;
+}
+
 function throwConfigError(): empty {
   throw new Error(`\
 Caching was left unconfigured. Babel's plugins, presets, and .babelrc.js files can be configured

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

@@ -58,6 +58,13 @@ export function* loadConfig(
   throw new Error(`Cannot load ${name} relative to ${dirname} in a browser`);
 }
 
+// eslint-disable-next-line require-yield
+export function* resolveShowConfigPath(
+  dirname: string, // eslint-disable-line no-unused-vars
+): Handler<string | null> {
+  return null;
+}
+
 export const ROOT_CONFIG_FILENAMES = [];
 
 // eslint-disable-next-line no-unused-vars

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

@@ -14,6 +14,7 @@ export {
   findRelativeConfig,
   findRootConfig,
   loadConfig,
+  resolveShowConfigPath,
   ROOT_CONFIG_FILENAMES,
 } from "./configuration";
 export type {

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

@@ -15,6 +15,7 @@ import {
 
 import {
   findConfigUpwards,
+  resolveShowConfigPath,
   ROOT_CONFIG_FILENAMES,
   type ConfigFile,
   type IgnoreFile,
@@ -89,15 +90,20 @@ export default function* loadPrivatePartialConfig(
     rootMode,
   );
 
+  const filename =
+    typeof args.filename === "string"
+      ? path.resolve(cwd, args.filename)
+      : undefined;
+
+  const showConfigPath = yield* resolveShowConfigPath(absoluteCwd);
+
   const context: ConfigContext = {
-    filename:
-      typeof args.filename === "string"
-        ? path.resolve(cwd, args.filename)
-        : undefined,
+    filename,
     cwd: absoluteCwd,
     root: absoluteRootDir,
     envName,
     caller,
+    showConfig: showConfigPath === filename,
   };
 
   const configChain = yield* buildRootChain(args, context);

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

@@ -0,0 +1,133 @@
+// @flow
+
+import type {
+  OptionsAndDescriptors,
+  UnloadedDescriptor,
+} from "./config-descriptors";
+
+// todo: Use flow enums when @babel/transform-flow-types supports it
+export const ChainFormatter = {
+  Programmatic: 0,
+  Config: 1,
+};
+
+type PrintableConfig = {
+  content: OptionsAndDescriptors,
+  type: $Values<typeof ChainFormatter>,
+  callerName: ?string,
+  filepath: ?string,
+  index: ?number,
+  envName: ?string,
+};
+
+const Formatter = {
+  title(
+    type: $Values<typeof ChainFormatter>,
+    callerName: ?string,
+    filepath: ?string,
+  ): string {
+    let title = "";
+    if (type === ChainFormatter.Programmatic) {
+      title = "programmatic options";
+      if (callerName) {
+        title += " from " + callerName;
+      }
+    } else {
+      // $FlowIgnore
+      title = "config " + filepath;
+    }
+    return title;
+  },
+  loc(index: ?number, envName: ?string): string {
+    let loc = "";
+    if (index != null) {
+      loc += `.overrides[${index}]`;
+    }
+    if (envName != null) {
+      loc += `.env["${envName}"]`;
+    }
+    return loc;
+  },
+
+  optionsAndDescriptors(opt: OptionsAndDescriptors) {
+    const content = { ...opt.options };
+    // overrides and env will be printed as separated config items
+    delete content.overrides;
+    delete content.env;
+    // resolve to descriptors
+    const pluginDescriptors = [...opt.plugins()];
+    if (pluginDescriptors.length) {
+      content.plugins = pluginDescriptors.map(d => descriptorToConfig(d));
+    }
+    const presetDescriptors = [...opt.presets()];
+    if (presetDescriptors.length) {
+      content.presets = [...presetDescriptors].map(d => descriptorToConfig(d));
+    }
+    return JSON.stringify(content, undefined, 2);
+  },
+};
+
+function descriptorToConfig(d: UnloadedDescriptor): string | {} | Array<mixed> {
+  let name = d.file?.request;
+  if (name == null) {
+    if (typeof d.value === "object") {
+      name = d.value;
+    } else if (typeof d.value === "function") {
+      // If the unloaded descriptor is a function, i.e. `plugins: [ require("my-plugin") ]`,
+      // we print the first 50 characters of the function source code and hopefully we can see
+      // `name: 'my-plugin'` in the source
+      name = `[Function: ${d.value.toString().substr(0, 50)} ... ]`;
+    }
+  }
+  if (name == null) {
+    name = "[Unknown]";
+  }
+  if (d.options === undefined) {
+    return name;
+  } else if (d.name == null) {
+    return [name, d.options];
+  } else {
+    return [name, d.options, d.name];
+  }
+}
+
+export class ConfigPrinter {
+  _stack: Array<PrintableConfig> = [];
+  configure(
+    enabled: boolean,
+    type: $Values<typeof ChainFormatter>,
+    { callerName, filepath }: { callerName?: string, filepath?: string },
+  ) {
+    if (!enabled) return () => {};
+    return (
+      content: OptionsAndDescriptors,
+      index: ?number,
+      envName: ?string,
+    ) => {
+      this._stack.push({
+        type,
+        callerName,
+        filepath,
+        content,
+        index,
+        envName,
+      });
+    };
+  }
+  static format(config: PrintableConfig): string {
+    let title = Formatter.title(
+      config.type,
+      config.callerName,
+      config.filepath,
+    );
+    const loc = Formatter.loc(config.index, config.envName);
+    if (loc) title += ` ${loc}`;
+    const content = Formatter.optionsAndDescriptors(config.content);
+    return `${title}\n${content}`;
+  }
+
+  output(): string {
+    if (this._stack.length === 0) return "";
+    return this._stack.map(s => ConfigPrinter.format(s)).join("\n\n");
+  }
+}

packages/babel-core/src/gensync-utils/fs.js

@@ -19,3 +19,8 @@ export const exists = gensync<[string], boolean>({
   },
   errback: (path, cb) => fs.access(path, undefined, err => cb(null, !err)),
 });
+
+export const stat = gensync<[string], *>({
+  sync: fs.statSync,
+  errback: fs.stat,
+});