Strengthen Flow typing around config objects

Summary:
Make `InputConfigT` read-only at top-level (note that this is already the case in TypeScript), and nested objects of `WatcherConfigT` read-only, consistent with `ResolverConfigT`, etc.

This removes a bunch of issues around passing `ConfigT` as an `InputConfigT` and makes several internal uses of `mergeConfig` and `runServer` type-safe.

Changelog: Internal

Reviewed By: huntie

Differential Revision: D62707211

fbshipit-source-id: 6dda78cf31fe504480ac6fff777bbb66b5f8bd05

Author: robhogan

packages/metro-config/src/configTypes.flow.js

@@ -195,37 +195,39 @@ type SymbolicatorConfigT = {
 
 type WatcherConfigT = {
   additionalExts: $ReadOnlyArray<string>,
-  healthCheck: {
+  healthCheck: $ReadOnly<{
     enabled: boolean,
     interval: number,
     timeout: number,
     filePrefix: string,
-  },
+  }>,
   unstable_workerThreads: boolean,
-  watchman: {
+  watchman: $ReadOnly<{
     deferStates: $ReadOnlyArray<string>,
-  },
+  }>,
 };
 
-export type InputConfigT = Partial<{
-  ...MetalConfigT,
-  ...$ReadOnly<{
-    cacheStores:
-      | $ReadOnlyArray<CacheStore<TransformResult<>>>
-      | (MetroCache => $ReadOnlyArray<CacheStore<TransformResult<>>>),
-    resolver: $ReadOnly<Partial<ResolverConfigT>>,
-    server: $ReadOnly<Partial<ServerConfigT>>,
-    serializer: $ReadOnly<Partial<SerializerConfigT>>,
-    symbolicator: $ReadOnly<Partial<SymbolicatorConfigT>>,
-    transformer: $ReadOnly<Partial<TransformerConfigT>>,
-    watcher: $ReadOnly<
-      Partial<{
-        ...WatcherConfigT,
-        healthCheck?: $ReadOnly<Partial<WatcherConfigT['healthCheck']>>,
-      }>,
-    >,
+export type InputConfigT = $ReadOnly<
+  Partial<{
+    ...MetalConfigT,
+    ...$ReadOnly<{
+      cacheStores:
+        | $ReadOnlyArray<CacheStore<TransformResult<>>>
+        | (MetroCache => $ReadOnlyArray<CacheStore<TransformResult<>>>),
+      resolver: $ReadOnly<Partial<ResolverConfigT>>,
+      server: $ReadOnly<Partial<ServerConfigT>>,
+      serializer: $ReadOnly<Partial<SerializerConfigT>>,
+      symbolicator: $ReadOnly<Partial<SymbolicatorConfigT>>,
+      transformer: $ReadOnly<Partial<TransformerConfigT>>,
+      watcher: $ReadOnly<
+        Partial<{
+          ...WatcherConfigT,
+          healthCheck?: $ReadOnly<Partial<WatcherConfigT['healthCheck']>>,
+        }>,
+      >,
+    }>,
   }>,
-}>;
+>;
 
 export type MetroConfig = InputConfigT;
 

packages/metro-config/src/loadConfig.js

@@ -196,22 +196,18 @@ async function loadMetroConfigFromDisk(
   const rootPath = dirname(filepath);
 
   const defaults = await getDefaultConfig(rootPath);
-  // $FlowFixMe[incompatible-variance]
-  // $FlowFixMe[incompatible-call]
+
   const defaultConfig: ConfigT = mergeConfig(defaults, defaultConfigOverrides);
 
   if (typeof configModule === 'function') {
     // Get a default configuration based on what we know, which we in turn can pass
     // to the function.
 
     const resultedConfig = await configModule(defaultConfig);
-    // $FlowFixMe[incompatible-call]
-    // $FlowFixMe[incompatible-variance]
+
     return mergeConfig(defaultConfig, resultedConfig);
   }
 
-  // $FlowFixMe[incompatible-variance]
-  // $FlowFixMe[incompatible-call]
   return mergeConfig(defaultConfig, configModule);
 }
 
@@ -220,17 +216,21 @@ function overrideConfigWithArguments(
   argv: YargArguments,
 ): ConfigT {
   // We override some config arguments here with the argv
-
-  const output: InputConfigT = {
+  const output: {
+    // Spread to remove invariance so that `output` is mutable.
+    ...Partial<ConfigT>,
+    resolver: {...Partial<ConfigT['resolver']>},
+    serializer: {...Partial<ConfigT['serializer']>},
+    server: {...Partial<ConfigT['server']>},
+    transformer: {...Partial<ConfigT['transformer']>},
+  } = {
     resolver: {},
     serializer: {},
     server: {},
     transformer: {},
   };
 
   if (argv.port != null) {
-    // $FlowFixMe[incompatible-use]
-    // $FlowFixMe[cannot-write]
     output.server.port = Number(argv.port);
   }
 
@@ -243,20 +243,14 @@ function overrideConfigWithArguments(
   }
 
   if (argv.assetExts != null) {
-    // $FlowFixMe[incompatible-use]
-    // $FlowFixMe[cannot-write]
     output.resolver.assetExts = argv.assetExts;
   }
 
   if (argv.sourceExts != null) {
-    // $FlowFixMe[incompatible-use]
-    // $FlowFixMe[cannot-write]
     output.resolver.sourceExts = argv.sourceExts;
   }
 
   if (argv.platforms != null) {
-    // $FlowFixMe[incompatible-use]
-    // $FlowFixMe[cannot-write]
     output.resolver.platforms = argv.platforms;
   }
 
@@ -265,8 +259,6 @@ function overrideConfigWithArguments(
   }
 
   if (argv.transformer != null) {
-    // $FlowFixMe[incompatible-use]
-    // $FlowFixMe[cannot-write]
     output.transformer.babelTransformerPath = argv.transformer;
   }
 
@@ -283,8 +275,6 @@ function overrideConfigWithArguments(
     // TODO: Ask if this is the way to go
   }
 
-  // $FlowFixMe[incompatible-variance]
-  // $FlowFixMe[incompatible-call]
   return mergeConfig(config, output);
 }
 
@@ -319,19 +309,11 @@ async function loadConfig(
   // Override the configuration with cli parameters
   const configWithArgs = overrideConfigWithArguments(configuration, argv);
 
-  const overriddenConfig: {[string]: mixed} = {};
-
-  overriddenConfig.watchFolders = [
-    configWithArgs.projectRoot,
-    ...configWithArgs.watchFolders,
-  ];
-
   // Set the watchfolders to include the projectRoot, as Metro assumes that is
   // the case
-  // $FlowFixMe[incompatible-variance]
-  // $FlowFixMe[incompatible-indexer]
-  // $FlowFixMe[incompatible-call]
-  return mergeConfig(configWithArgs, overriddenConfig);
+  return mergeConfig(configWithArgs, {
+    watchFolders: [configWithArgs.projectRoot, ...configWithArgs.watchFolders],
+  });
 }
 
 module.exports = {

packages/metro/src/index.flow.js

@@ -134,8 +134,6 @@ export type {MetroConfig};
 
 async function getConfig(config: InputConfigT): Promise<ConfigT> {
   const defaultConfig = await getDefaultConfig(config.projectRoot);
-  // $FlowFixMe[incompatible-variance]
-  // $FlowFixMe[incompatible-call]
   return mergeConfig(defaultConfig, config);
 }
 
@@ -191,8 +189,6 @@ const createConnectMiddleware = async function (
   config: ConfigT,
   options?: RunMetroOptions,
 ): Promise<MetroMiddleWare> {
-  // $FlowFixMe[incompatible-variance]
-  // $FlowFixMe[incompatible-call]
   const metroServer = await runMetro(config, options);
 
   let enhancedMiddleware: Middleware = metroServer.processRequest;
@@ -385,8 +381,6 @@ exports.runBuild = async (
   map: string,
   ...
 }> => {
-  // $FlowFixMe[incompatible-variance]
-  // $FlowFixMe[incompatible-call]
   const metroServer = await runMetro(config, {
     watch: false,
   });