per-locale bundle & inline locale (#16369)

* feat: split entry file by locale name

* chore: とりあえず transform hook で雑に分割

* chore: とりあえず transform 結果をいい感じに

* chore: concurrent buildで高速化

* chore: vite ではローケルのないものをビルドして後処理でどうにかするように

* chore: 後処理のためにi18n.jを単体になるように切り出す

* chore: use typescript

* chore: remove unref(i18n) in vite build process

* chore: inline variable

* fix: build error

* fix: i18n.ts.something.replaceAll() become error

* chore: ignore export specifier from error

* chore: support i18n.tsx as object

* chore: process literal for all files

* chore: split config and locale

* chore: inline locale name

* chore: remove updating locale in boot common

* chore: use top-level await to load locales

* chore: inline locale

* chore: remove loading locale from boot.js

* chore: remove loading locale from boot.js

* コメント追加

* fix test; fetchに失敗する

* import削除ログをdebugレベルに

* fix: watch pug

* chore: use hash for entry files

* chore: remove es-module-lexer from dependencies

* chore: move to frontend-builder

* chore: use inline locale in embed

* chore: refetch json on hot reload

* feat: store localization related to boot.js in backend in bootloaderLocales localstorage

* 応急処置を戻す

* fix spex

* fix `Using i18n identifier "e" directly. Skipping inlining.` warning

* refactor: use scriptsDir parameter

* chore: remove i18n from depmap

* chore: make build crash if errors

* error -> warn few conditions

* use inline object

* update localstorage keys

* remove accessing locale localstorage

* fix: failed to process i18n.tsx.aaa({x:i18n.bbb})

10 files changed, 942 insertions, 0 deletions

diff --git a/packages/frontend-builder/README.txt b/packages/frontend-builder/README.txt
new file mode 100644
index 0000000000..db878ffa83
--- /dev/null
+++ b/packages/frontend-builder/README.txt
@@ -0,0 +1,3 @@
+This package contains the common scripts that are used to build the frontend and frontend-embed packages.
+
+
diff --git a/packages/frontend-builder/eslint.config.js b/packages/frontend-builder/eslint.config.js
new file mode 100644
index 0000000000..5805c9924d
--- /dev/null
+++ b/packages/frontend-builder/eslint.config.js
@@ -0,0 +1,98 @@
+import globals from 'globals';
+import tsParser from '@typescript-eslint/parser';
+import pluginMisskey from '@misskey-dev/eslint-plugin';
+import sharedConfig from '../shared/eslint.config.js';
+
+// eslint-disable-next-line import/no-default-export
+export default [
+	...sharedConfig,
+	{
+		files: ['**/*.vue'],
+		...pluginMisskey.configs.typescript,
+	},
+	{
+		files: [
+			'@types/**/*.ts',
+			'js/**/*.ts',
+			'**/*.vue',
+		],
+		languageOptions: {
+			globals: {
+				...Object.fromEntries(Object.entries(globals.node).map(([key]) => [key, 'off'])),
+				...globals.browser,
+
+				// Node.js
+				module: false,
+				require: false,
+				__dirname: false,
+
+				// Misskey
+				_DEV_: false,
+				_LANGS_: false,
+				_VERSION_: false,
+				_ENV_: false,
+				_PERF_PREFIX_: false,
+			},
+			parserOptions: {
+				parser: tsParser,
+				project: ['./tsconfig.json'],
+				sourceType: 'module',
+				tsconfigRootDir: import.meta.dirname,
+			},
+		},
+		rules: {
+			'@typescript-eslint/no-unused-vars': 'off',
+			'@typescript-eslint/no-empty-interface': ['error', {
+				allowSingleExtends: true,
+			}],
+			'import/consistent-type-specifier-style': ['error', 'prefer-top-level'],
+			// window の禁止理由: グローバルスコープと衝突し、予期せぬ結果を招くため
+			// e の禁止理由: error や event など、複数のキーワードの頭文字であり分かりにくいため
+			'id-denylist': ['error', 'window', 'e'],
+			'no-shadow': ['warn'],
+			'vue/attributes-order': ['error', {
+				alphabetical: false,
+			}],
+			'vue/no-use-v-if-with-v-for': ['error', {
+				allowUsingIterationVar: false,
+			}],
+			'vue/no-ref-as-operand': 'error',
+			'vue/no-multi-spaces': ['error', {
+				ignoreProperties: false,
+			}],
+			'vue/no-v-html': 'warn',
+			'vue/order-in-components': 'error',
+			'vue/html-indent': ['warn', 'tab', {
+				attribute: 1,
+				baseIndent: 0,
+				closeBracket: 0,
+				alignAttributesVertically: true,
+				ignores: [],
+			}],
+			'vue/html-closing-bracket-spacing': ['warn', {
+				startTag: 'never',
+				endTag: 'never',
+				selfClosingTag: 'never',
+			}],
+			'vue/multi-word-component-names': 'warn',
+			'vue/require-v-for-key': 'warn',
+			'vue/no-unused-components': 'warn',
+			'vue/no-unused-vars': 'warn',
+			'vue/no-dupe-keys': 'warn',
+			'vue/valid-v-for': 'warn',
+			'vue/return-in-computed-property': 'warn',
+			'vue/no-setup-props-reactivity-loss': 'warn',
+			'vue/max-attributes-per-line': 'off',
+			'vue/html-self-closing': 'off',
+			'vue/singleline-html-element-content-newline': 'off',
+			'vue/v-on-event-hyphenation': ['error', 'never', {
+				autofix: true,
+			}],
+			'vue/attribute-hyphenation': ['error', 'never'],
+		},
+	},
+	{
+		ignores: [
+		],
+	},
+];
diff --git a/packages/frontend-builder/locale-inliner.ts b/packages/frontend-builder/locale-inliner.ts
new file mode 100644
index 0000000000..ce3f59a81c
--- /dev/null
+++ b/packages/frontend-builder/locale-inliner.ts
@@ -0,0 +1,145 @@
+import * as fs from 'fs/promises';
+import * as path from 'node:path';
+import { type Locale } from '../../locales/index.js';
+import type { Manifest as ViteManifest } from 'vite';
+import MagicString from 'magic-string';
+import { collectModifications } from './locale-inliner/collect-modifications.js';
+import { applyWithLocale } from './locale-inliner/apply-with-locale.js';
+import { blankLogger, type Logger } from './logger.js';
+
+export class LocaleInliner {
+	outputDir: string;
+	scriptsDir: string;
+	i18nFile: string;
+	i18nFileName: string;
+	logger: Logger;
+	chunks: ScriptChunk[];
+
+	static async create(options: {
+		outputDir: string,
+		scriptsDir: string,
+		i18nFile: string,
+		logger: Logger,
+	}): Promise<LocaleInliner> {
+		const manifest: ViteManifest = JSON.parse(await fs.readFile(`${options.outputDir}/manifest.json`, 'utf-8'));
+		return new LocaleInliner({ ...options, manifest });
+	}
+
+	constructor(options: {
+		outputDir: string,
+		scriptsDir: string,
+		i18nFile: string,
+		manifest: ViteManifest,
+		logger: Logger,
+	}) {
+		this.outputDir = options.outputDir;
+		this.scriptsDir = options.scriptsDir;
+		this.i18nFile = options.i18nFile;
+		this.i18nFileName = this.stripScriptDir(options.manifest[this.i18nFile].file);
+		this.logger = options.logger;
+		this.chunks = Object.values(options.manifest).filter(chunk => this.isScriptFile(chunk.file)).map(chunk => ({
+			fileName: this.stripScriptDir(chunk.file),
+			chunkName: chunk.name,
+		}));
+	}
+
+	async loadFiles() {
+		await Promise.all(this.chunks.map(async chunk => {
+			const filePath = path.join(this.outputDir, this.scriptsDir, chunk.fileName);
+			chunk.sourceCode = await fs.readFile(filePath, 'utf-8');
+		}));
+	}
+
+	collectsModifications() {
+		for (const chunk of this.chunks) {
+			if (!chunk.sourceCode) {
+				throw new Error(`Source code for ${chunk.fileName} is not loaded.`);
+			}
+			const fileLogger = this.logger.prefixed(`${chunk.fileName} (${chunk.chunkName}): `);
+			chunk.modifications = collectModifications(chunk.sourceCode, chunk.fileName, fileLogger, this);
+		}
+	}
+
+	async saveAllLocales(locales: Record<string, Locale>) {
+		const localeNames = Object.keys(locales);
+		for (const localeName of localeNames) {
+			await this.saveLocale(localeName, locales[localeName]);
+		}
+	}
+
+	async saveLocale(localeName: string, localeJson: Locale) {
+		// create directory
+		await fs.mkdir(path.join(this.outputDir, localeName), { recursive: true });
+		const localeLogger = localeName == 'ja-JP' ? this.logger : blankLogger; // we want to log for single locale only
+		for (const chunk of this.chunks) {
+			if (!chunk.sourceCode || !chunk.modifications) {
+				throw new Error(`Source code or modifications for ${chunk.fileName} is not available.`);
+			}
+			const fileLogger = localeLogger.prefixed(`${chunk.fileName} (${chunk.chunkName}): `);
+			const magicString = new MagicString(chunk.sourceCode);
+			applyWithLocale(magicString, chunk.modifications, localeName, localeJson, fileLogger);
+
+			await fs.writeFile(path.join(this.outputDir, localeName, chunk.fileName), magicString.toString());
+		}
+	}
+
+	isScriptFile(fileName: string) {
+		return fileName.startsWith(this.scriptsDir + '/') && fileName.endsWith('.js');
+	}
+
+	stripScriptDir(fileName: string) {
+		if (!fileName.startsWith(this.scriptsDir + '/')) {
+			throw new Error(`${fileName} does not start with ${this.scriptsDir}/`);
+		}
+		return fileName.slice(this.scriptsDir.length + 1);
+	}
+}
+
+interface ScriptChunk {
+	fileName: string;
+	chunkName?: string;
+	sourceCode?: string;
+	modifications?: TextModification[];
+}
+
+export type TextModification = {
+	type: 'delete';
+	begin: number;
+	end: number;
+	localizedOnly: boolean;
+} | {
+	// can be used later to insert '../scripts' for common files
+	type: 'insert';
+	begin: number;
+	text: string;
+	localizedOnly: boolean;
+} | {
+	type: 'replace';
+	begin: number;
+	end: number;
+	text: string;
+	localizedOnly: boolean;
+} | {
+	type: 'localized';
+	begin: number;
+	end: number;
+	localizationKey: string[];
+	localizedOnly: true;
+} | {
+	type: 'parameterized-function';
+	begin: number;
+	end: number;
+	localizationKey: string[];
+	localizedOnly: true;
+} | {
+	type: 'locale-name';
+	begin: number;
+	end: number;
+	literal: boolean;
+	localizedOnly: true;
+} | {
+	type: 'locale-json';
+	begin: number;
+	end: number;
+	localizedOnly: true;
+};
diff --git a/packages/frontend-builder/locale-inliner/apply-with-locale.ts b/packages/frontend-builder/locale-inliner/apply-with-locale.ts
new file mode 100644
index 0000000000..a79ebb4253
--- /dev/null
+++ b/packages/frontend-builder/locale-inliner/apply-with-locale.ts
@@ -0,0 +1,97 @@
+import MagicString from 'magic-string';
+import type { Locale } from '../../../locales/index.js';
+import { assertNever } from '../utils.js';
+import type { TextModification } from '../locale-inliner.js';
+import type { Logger } from '../logger.js';
+
+export function applyWithLocale(
+	sourceCode: MagicString,
+	modifications: TextModification[],
+	localeName: string,
+	localeJson: Locale,
+	fileLogger: Logger,
+) {
+	for (const modification of modifications) {
+		switch (modification.type) {
+			case "delete":
+				sourceCode.remove(modification.begin, modification.end);
+				break;
+			case "insert":
+				sourceCode.appendRight(modification.begin, modification.text);
+				break;
+			case "replace":
+				sourceCode.update(modification.begin, modification.end, modification.text);
+				break;
+			case "localized": {
+				const accessed = getPropertyByPath(localeJson, modification.localizationKey);
+				if (accessed == null) {
+					fileLogger.warn(`Cannot find localization key ${modification.localizationKey.join('.')}`);
+				}
+				sourceCode.update(modification.begin, modification.end, JSON.stringify(accessed));
+				break;
+			}
+			case "parameterized-function": {
+				const accessed = getPropertyByPath(localeJson, modification.localizationKey);
+				let replacement: string;
+				if (typeof accessed === 'string') {
+					replacement = formatFunction(accessed);
+				} else if (typeof accessed === 'object' && accessed !== null) {
+					replacement = `({${Object.entries(accessed).map(([key, value]) => `${JSON.stringify(key)}:${formatFunction(value)}`).join(',')}})`;
+				} else {
+					fileLogger.warn(`Cannot find localization key ${modification.localizationKey.join('.')}`);
+					replacement = '(() => "")'; // placeholder for missing locale
+				}
+				sourceCode.update(modification.begin, modification.end, replacement);
+				break;
+
+				function formatFunction(accessed: string): string {
+					const params = new Set<string>();
+					const components: string[] = [];
+					let lastIndex = 0;
+					for (const match of accessed.matchAll(/\{(.+?)}/g)) {
+						const [fullMatch, paramName] = match;
+						if (lastIndex < match.index) {
+							components.push(JSON.stringify(accessed.slice(lastIndex, match.index)));
+						}
+						params.add(paramName);
+						components.push(paramName);
+						lastIndex = match.index + fullMatch.length;
+					}
+					components.push(JSON.stringify(accessed.slice(lastIndex)));
+
+					// we replace with `(({name,count})=>(name+count+"some"))`
+					const paramList = Array.from(params).join(',');
+					let body = components.filter(x => x != '""').join('+');
+					if (body == '') body = '""'; // if the body is empty, we return empty string
+					return `(({${paramList}})=>(${body}))`;
+				}
+			}
+			case "locale-name": {
+				sourceCode.update(modification.begin, modification.end, modification.literal ? JSON.stringify(localeName) : localeName);
+				break;
+			}
+			case "locale-json": {
+				// locale-json is inlined to place where initialize module-level variable which is executed only once.
+				// In such case we can use JSON.parse to speed up the parsing script.
+				// https://v8.dev/blog/cost-of-javascript-2019#json
+				sourceCode.update(modification.begin, modification.end, `JSON.parse(${JSON.stringify(JSON.stringify(localeJson))})`);
+				break;
+			}
+			default: {
+				assertNever(modification);
+			}
+		}
+	}
+}
+
+function getPropertyByPath(localeJson: any, localizationKey: string[]): string | object | null {
+	if (localizationKey.length === 0) return localeJson;
+	let current: any = localeJson;
+	for (const key of localizationKey) {
+		if (typeof current !== 'object' || current === null || !(key in current)) {
+			return null; // Key not found
+		}
+		current = current[key];
+	}
+	return current ?? null;
+}
diff --git a/packages/frontend-builder/locale-inliner/collect-modifications.ts b/packages/frontend-builder/locale-inliner/collect-modifications.ts
new file mode 100644
index 0000000000..0cbf6a504e
--- /dev/null
+++ b/packages/frontend-builder/locale-inliner/collect-modifications.ts
@@ -0,0 +1,419 @@
+import type { AstNode, ProgramNode } from 'rollup';
+import { parseAst } from 'vite';
+import * as estreeWalker from 'estree-walker';
+import type * as estree from 'estree';
+import type { LocaleInliner, TextModification } from '../locale-inliner.js';
+import type { Logger } from '../logger.js'
+import { assertNever, assertType } from '../utils.js';
+
+// WalkerContext is not exported from estree-walker, so we define it here
+interface WalkerContext {
+	skip: () => void;
+}
+
+export function collectModifications(sourceCode: string, fileName: string, fileLogger: Logger, inliner: LocaleInliner): TextModification[] {
+	let programNode: ProgramNode;
+	try {
+		programNode = parseAst(sourceCode);
+	} catch (e) {
+		fileLogger.error(`Failed to parse source code: ${e}`);
+		return [];
+	}
+	if (programNode.sourceType !== 'module') {
+		fileLogger.error(`Source code is not a module.`);
+		return [];
+	}
+
+	const modifications: TextModification[] = [];
+
+	// first
+	// 1) replace all `scripts/` path literals with locale code
+	// 2) replace all `localStorage.getItem("lang")` with `localeName` variable
+	// 3) replace all `await window.fetch(`/assets/locales/${d}.${x}.json`).then(u=>u.json())` with `localeJson` variable
+	estreeWalker.walk(programNode, {
+		enter(this: WalkerContext, node: Node) {
+			assertType<AstNode>(node)
+
+			if (node.type === 'Literal' && typeof node.value === 'string' && node.raw) {
+				if (node.raw.substring(1).startsWith(inliner.scriptsDir)) {
+					// we find `scripts/\w+\.js` literal and replace 'scripts' part with locale code
+					fileLogger.debug(`${lineCol(sourceCode, node)}: found ${inliner.scriptsDir}/ path literal ${node.raw}`);
+					modifications.push({
+						type: 'locale-name',
+						begin: node.start + 1,
+						end: node.start + 1 + inliner.scriptsDir.length,
+						literal: false,
+						localizedOnly: true,
+					});
+				}
+				if (node.raw.substring(1, node.raw.length - 1) == `${inliner.scriptsDir}/${inliner.i18nFileName}`) {
+					// we find `scripts/i18n.ts` literal.
+					// This is tipically in depmap and replace with this file name to avoid unnecessary loading i18n script
+					fileLogger.debug(`${lineCol(sourceCode, node)}: found ${inliner.i18nFileName} path literal ${node.raw}`);
+					modifications.push({
+						type: 'replace',
+						begin: node.end - 1 - inliner.i18nFileName.length,
+						end: node.end - 1,
+						text: fileName,
+						localizedOnly: true,
+					});
+				}
+			}
+
+			if (isLocalStorageGetItemLang(node)) {
+				fileLogger.debug(`${lineCol(sourceCode, node)}: found localStorage.getItem("lang") call`);
+				modifications.push({
+					type: 'locale-name',
+					begin: node.start,
+					end: node.end,
+					literal: true,
+					localizedOnly: true,
+				});
+			}
+
+			if (isAwaitFetchLocaleThenJson(node)) {
+				// await window.fetch(`/assets/locales/${d}.${x}.json`).then(u=>u.json(), () => null)
+				fileLogger.debug(`${lineCol(sourceCode, node)}: found await window.fetch(\`/assets/locales/\${d}.\${x}.json\`).then(u=>u.json()) call`);
+				modifications.push({
+					type: 'locale-json',
+					begin: node.start,
+					end: node.end,
+					localizedOnly: true,
+				});
+			}
+		}
+	})
+
+	const importSpecifierResult = findImportSpecifier(programNode, inliner.i18nFileName, 'i18n');
+
+	switch (importSpecifierResult.type) {
+		case 'no-import':
+			fileLogger.debug(`No import of i18n found, skipping inlining.`);
+			return modifications;
+		case 'no-specifiers':
+			fileLogger.debug(`Importing i18n without specifiers, removing the import.`);
+			modifications.push({
+				type: 'delete',
+				begin: importSpecifierResult.importNode.start,
+				end: importSpecifierResult.importNode.end,
+				localizedOnly: false,
+			});
+			return modifications;
+		case 'unexpected-specifiers':
+			fileLogger.info(`Importing ${inliner.i18nFileName} found but with unexpected specifiers. Skipping inlining.`);
+			return modifications;
+		case 'specifier':
+			fileLogger.debug(`Found import i18n as ${importSpecifierResult.localI18nIdentifier}`);
+			break;
+	}
+
+	const i18nImport = importSpecifierResult.importNode;
+	const localI18nIdentifier = importSpecifierResult.localI18nIdentifier;
+
+	// Check if the identifier is already declared in the file.
+	// If it is, we may overwrite it and cause issues so we skip inlining
+	let isSupported = true;
+	estreeWalker.walk(programNode, {
+		enter(node) {
+			if (node.type == 'VariableDeclaration') {
+				assertType<estree.VariableDeclaration>(node);
+				for (let id of node.declarations.flatMap(x => declsOfPattern(x.id))) {
+					if (id == localI18nIdentifier) {
+						isSupported = false;
+					}
+				}
+			}
+		}
+	})
+
+	if (!isSupported) {
+		fileLogger.error(`Duplicated identifier "${localI18nIdentifier}" in variable declaration. Skipping inlining.`);
+		return modifications;
+	}
+
+	fileLogger.debug(`imports i18n as ${localI18nIdentifier}`);
+
+	// In case of substitution failure, we will preserve the import statement
+	// otherwise we will remove it.
+	let preserveI18nImport = false;
+
+	const toSkip = new Set();
+	toSkip.add(i18nImport);
+	estreeWalker.walk(programNode, {
+		enter(this: WalkerContext, node, parent, property) {
+			assertType<AstNode>(node)
+			assertType<AstNode>(parent)
+			if (toSkip.has(node)) {
+				// This is the import specifier, skip processing it
+				this.skip();
+				return;
+			}
+
+			// We don't care original name part of the import declaration
+			if (node.type == 'ImportDeclaration') this.skip();
+
+			if (node.type === 'Identifier') {
+				assertType<estree.Identifier>(node)
+				assertType<estree.Property | estree.MemberExpression | estree.ExportSpecifier>(parent)
+				if (parent.type === 'Property' && !parent.computed && property == 'key') return; // we don't care 'id' part of { id: expr }
+				if (parent.type === 'MemberExpression' && !parent.computed && property == 'property') return; // we don't care 'id' part of { id: expr }
+				if (parent.type === 'ExportSpecifier' && property == 'exported') return; // we don't care 'id' part of { id: expr }
+				if (node.name == localI18nIdentifier) {
+					fileLogger.error(`${lineCol(sourceCode, node)}: Using i18n identifier "${localI18nIdentifier}" directly. Skipping inlining.`);
+					preserveI18nImport = true;
+				}
+			} else if (node.type === 'MemberExpression') {
+				assertType<estree.MemberExpression>(node);
+				const i18nPath = parseI18nPropertyAccess(node);
+				if (i18nPath != null && i18nPath.length >= 2 && i18nPath[0] == 'ts') {
+					if (parent.type === 'CallExpression' && property == 'callee') return; // we don't want to process `i18n.ts.property.stringBuiltinMethod()`
+					if (i18nPath.at(-1)?.startsWith('_')) fileLogger.debug(`found i18n grouped property access ${i18nPath.join('.')}`);
+					else fileLogger.debug(`${lineCol(sourceCode, node)}: found i18n property access ${i18nPath.join('.')}`);
+					// it's i18n.ts.propertyAccess
+					// i18n.ts.* will always be resolved to string or object containing strings
+					modifications.push({
+						type: 'localized',
+						begin: node.start,
+						end: node.end,
+						localizationKey: i18nPath.slice(1), // remove 'ts' prefix
+						localizedOnly: true,
+					});
+					this.skip();
+				} else if (i18nPath != null && i18nPath.length >= 2 && i18nPath[0] == 'tsx') {
+					// it's parameterized locale substitution (`i18n.tsx.property(parameters)`)
+					// we expect the parameter to be an object literal
+					fileLogger.debug(`${lineCol(sourceCode, node)}: found i18n function access (object) ${i18nPath.join('.')}`);
+					modifications.push({
+						type: 'parameterized-function',
+						begin: node.start,
+						end: node.end,
+						localizationKey: i18nPath.slice(1), // remove 'tsx' prefix
+						localizedOnly: true,
+					});
+					this.skip();
+				}
+			} else if (node.type === 'ArrowFunctionExpression') {
+				assertType<estree.ArrowFunctionExpression>(node);
+				// If there is 'i18n' in the parameters, we care interior of the function
+				if (node.params.flatMap(param => declsOfPattern(param)).includes(localI18nIdentifier)) this.skip();
+			}
+		}
+	})
+
+	if (!preserveI18nImport) {
+		fileLogger.debug(`removing i18n import statement`);
+		modifications.push({
+			type: 'delete',
+			begin: i18nImport.start,
+			end: i18nImport.end,
+			localizedOnly: true,
+		});
+	}
+
+	function parseI18nPropertyAccess(node: estree.Expression | estree.Super): string[] | null {
+		if (node.type === 'Identifier' && node.name == localI18nIdentifier) return []; // i18n itself
+		if (node.type !== 'MemberExpression') return null;
+		// super.*
+		if (node.object.type === 'Super') return null;
+
+		// i18n?.property is not supported
+		if (node.optional) return null;
+
+
+		let id: string | null = null;
+		if (node.computed) {
+			if (node.property.type === 'Literal' && typeof node.property.value === 'string') {
+				id = node.property.value;
+			}
+		} else {
+			if (node.property.type === 'Identifier') {
+				id = node.property.name;
+			}
+		}
+		// non-constant property access
+		if (id == null) return null;
+
+		const parentAccess = parseI18nPropertyAccess(node.object);
+		if (parentAccess == null) return null;
+		return [...parentAccess, id];
+	}
+
+	return modifications;
+}
+
+function declsOfPattern(pattern: estree.Pattern | null): string[] {
+	if (pattern == null) return [];
+	switch (pattern?.type) {
+		case "Identifier":
+			return [pattern.name];
+		case "ObjectPattern":
+			return pattern.properties.flatMap(prop => {
+				switch (prop.type) {
+					case 'Property':
+						return declsOfPattern(prop.value);
+					case 'RestElement':
+						return declsOfPattern(prop.argument);
+					default:
+						assertNever(prop)
+				}
+			});
+		case "ArrayPattern":
+			return pattern.elements.flatMap(p => declsOfPattern(p));
+		case "RestElement":
+			return declsOfPattern(pattern.argument);
+		case "AssignmentPattern":
+			return declsOfPattern(pattern.left);
+		case "MemberExpression":
+			// assignment pattern so no new variable is declared
+			return [];
+		default:
+			assertNever(pattern);
+	}
+}
+
+function lineCol(sourceCode: string, node: estree.Node): string {
+	assertType<AstNode>(node);
+	const leading = sourceCode.slice(0, node.start);
+	const lines = leading.split('\n');
+	const line = lines.length;
+	const col = lines[lines.length - 1].length + 1; // +1 for 1-based index
+	return `(${line}:${col})`;
+}
+
+//region checker functions
+
+type Node =
+	| estree.AssignmentProperty
+	| estree.CatchClause
+	| estree.Class
+	| estree.ClassBody
+	| estree.Expression
+	| estree.Function
+	| estree.Identifier
+	| estree.Literal
+	| estree.MethodDefinition
+	| estree.ModuleDeclaration
+	| estree.ModuleSpecifier
+	| estree.Pattern
+	| estree.PrivateIdentifier
+	| estree.Program
+	| estree.Property
+	| estree.PropertyDefinition
+	| estree.SpreadElement
+	| estree.Statement
+	| estree.Super
+	| estree.SwitchCase
+	| estree.TemplateElement
+	| estree.VariableDeclarator
+	;
+
+// localStorage.getItem("lang")
+function isLocalStorageGetItemLang(getItemCall: Node): boolean {
+	if (getItemCall.type !== 'CallExpression') return false;
+	if (getItemCall.arguments.length !== 1) return false;
+
+	const langLiteral = getItemCall.arguments[0];
+	if (!isStringLiteral(langLiteral, 'lang')) return false;
+
+	const getItemFunction = getItemCall.callee;
+	if (!isMemberExpression(getItemFunction, 'getItem')) return false;
+
+	const localStorageObject = getItemFunction.object;
+	if (!isIdentifier(localStorageObject, 'localStorage')) return false;
+
+	return true;
+}
+
+// await window.fetch(`/assets/locales/${d}.${x}.json`).then(u => u.json(), ....)
+function isAwaitFetchLocaleThenJson(awaitNode: Node): boolean {
+	if (awaitNode.type !== 'AwaitExpression') return false;
+
+	const thenCall = awaitNode.argument;
+	if (thenCall.type !== 'CallExpression') return false;
+	if (thenCall.arguments.length < 1) return false;
+
+	const arrowFunction = thenCall.arguments[0];
+	if (arrowFunction.type !== 'ArrowFunctionExpression') return false;
+	if (arrowFunction.params.length !== 1) return false;
+
+	const arrowBodyCall = arrowFunction.body;
+	if (arrowBodyCall.type !== 'CallExpression') return false;
+
+	const jsonFunction = arrowBodyCall.callee;
+	if (!isMemberExpression(jsonFunction, 'json')) return false;
+
+	const thenFunction = thenCall.callee;
+	if (!isMemberExpression(thenFunction, 'then')) return false;
+
+	const fetchCall = thenFunction.object;
+	if (fetchCall.type !== 'CallExpression') return false;
+	if (fetchCall.arguments.length !== 1) return false;
+
+	// `/assets/locales/${d}.${x}.json`
+	const assetLocaleTemplate = fetchCall.arguments[0];
+	if (assetLocaleTemplate.type !== 'TemplateLiteral') return false;
+	if (assetLocaleTemplate.quasis.length !== 3) return false;
+	if (assetLocaleTemplate.expressions.length !== 2) return false;
+	if (assetLocaleTemplate.quasis[0].value.cooked !== '/assets/locales/') return false;
+	if (assetLocaleTemplate.quasis[1].value.cooked !== '.') return false;
+	if (assetLocaleTemplate.quasis[2].value.cooked !== '.json') return false;
+
+	const fetchFunction = fetchCall.callee;
+	if (!isMemberExpression(fetchFunction, 'fetch')) return false;
+	const windowObject = fetchFunction.object;
+	if (!isIdentifier(windowObject, 'window')) return false;
+
+	return true;
+}
+
+type SpecifierResult =
+	| { type: 'no-import' }
+	| { type: 'no-specifiers', importNode: estree.ImportDeclaration & AstNode }
+	| { type: 'unexpected-specifiers', importNode: estree.ImportDeclaration & AstNode }
+	| { type: 'specifier', localI18nIdentifier: string, importNode: estree.ImportDeclaration & AstNode }
+	;
+
+function findImportSpecifier(programNode: ProgramNode, i18nFileName: string, i18nSymbol: string): SpecifierResult {
+	const imports = programNode.body.filter(x => x.type === 'ImportDeclaration');
+	const importNode = imports.find(x => x.source.value === `./${i18nFileName}`) as estree.ImportDeclaration;
+	if (!importNode) return { type: 'no-import' };
+	assertType<AstNode>(importNode);
+
+	if (importNode.specifiers.length == 0) {
+		return { type: 'no-specifiers', importNode };
+	}
+
+	if (importNode.specifiers.length != 1) {
+		return { type: 'unexpected-specifiers', importNode };
+	}
+	const i18nImportSpecifier = importNode.specifiers[0];
+	if (i18nImportSpecifier.type !== 'ImportSpecifier') {
+		return { type: 'unexpected-specifiers', importNode };
+	}
+
+	if (i18nImportSpecifier.imported.type !== 'Identifier') {
+		return { type: 'unexpected-specifiers', importNode };
+	}
+
+	const importingIdentifier = i18nImportSpecifier.imported.name;
+	if (importingIdentifier !== i18nSymbol) {
+		return { type: 'unexpected-specifiers', importNode };
+	}
+	const localI18nIdentifier = i18nImportSpecifier.local.name;
+	return { type: 'specifier', localI18nIdentifier, importNode };
+}
+
+// checker helpers
+function isMemberExpression(node: Node, property: string): node is estree.MemberExpression {
+	return node.type === 'MemberExpression' && !node.computed && node.property.type === 'Identifier' && node.property.name === property;
+}
+
+function isStringLiteral(node: Node, value: string): node is estree.Literal {
+	return node.type === 'Literal' && typeof node.value === 'string' && node.value === value;
+}
+
+function isIdentifier(node: Node, name: string): node is estree.Identifier {
+	return node.type === 'Identifier' && node.name === name;
+}
+
+//endregion
diff --git a/packages/frontend-builder/logger.ts b/packages/frontend-builder/logger.ts
new file mode 100644
index 0000000000..a3f66730e2
--- /dev/null
+++ b/packages/frontend-builder/logger.ts
@@ -0,0 +1,66 @@
+const debug = false;
+
+export interface Logger {
+	debug(message: string): void;
+
+	warn(message: string): void;
+
+	error(message: string): void;
+
+	info(message: string): void;
+
+	prefixed(newPrefix: string): Logger;
+}
+
+interface RootLogger extends Logger {
+	warningCount: number;
+	errorCount: number;
+}
+
+export function createLogger(): RootLogger {
+	return loggerFactory('', {
+		warningCount: 0,
+		errorCount: 0,
+	});
+}
+
+type LogContext = {
+	warningCount: number;
+	errorCount: number;
+}
+
+function loggerFactory(prefix: string, context: LogContext): RootLogger {
+	return {
+		debug: (message: string) => {
+			if (debug) console.log(`[DBG] ${prefix}${message}`);
+		},
+		warn: (message: string) => {
+			context.warningCount++;
+			console.log(`${debug ? '[WRN]' : 'w:'} ${prefix}${message}`);
+		},
+		error: (message: string) => {
+			context.errorCount++;
+			console.error(`${debug ? '[ERR]' : 'e:'} ${prefix}${message}`);
+		},
+		info: (message: string) => {
+			console.error(`${debug ? '[INF]' : 'i:'} ${prefix}${message}`);
+		},
+		prefixed: (newPrefix: string) => {
+			return loggerFactory(`${prefix}${newPrefix}`, context);
+		},
+		get warningCount() {
+			return context.warningCount;
+		},
+		get errorCount() {
+			return context.errorCount;
+		},
+	};
+}
+
+export const blankLogger: Logger = {
+	debug: () => void 0,
+	warn: () => void 0,
+	error: () => void 0,
+	info: () => void 0,
+	prefixed: () => blankLogger,
+}
diff --git a/packages/frontend-builder/package.json b/packages/frontend-builder/package.json
new file mode 100644
index 0000000000..5fdd25b32d
--- /dev/null
+++ b/packages/frontend-builder/package.json
@@ -0,0 +1,25 @@
+{
+	"name": "frontend-builder",
+	"type": "module",
+	"scripts": {
+		"eslint": "eslint './**/*.{js,jsx,ts,tsx}'",
+		"typecheck": "tsc --noEmit",
+		"lint": "pnpm typecheck && pnpm eslint"
+	},
+	"exports": {
+		"./*": "./js/*"
+	},
+	"devDependencies": {
+		"@types/estree": "1.0.8",
+		"@types/node": "22.17.0",
+		"@typescript-eslint/eslint-plugin": "8.38.0",
+		"@typescript-eslint/parser": "8.38.0",
+		"rollup": "4.46.2",
+		"typescript": "5.9.2"
+	},
+	"dependencies": {
+		"estree-walker": "3.0.3",
+		"magic-string": "0.30.17",
+		"vite": "7.0.6"
+	}
+}
diff --git a/packages/frontend-builder/rollup-plugin-remove-unref-i18n.ts b/packages/frontend-builder/rollup-plugin-remove-unref-i18n.ts
new file mode 100644
index 0000000000..9010e6910c
--- /dev/null
+++ b/packages/frontend-builder/rollup-plugin-remove-unref-i18n.ts
@@ -0,0 +1,53 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import * as estreeWalker from 'estree-walker';
+import type { Plugin } from 'vite';
+import type { CallExpression, Expression, Program, } from 'estree';
+import MagicString from 'magic-string';
+import type { AstNode } from 'rollup';
+import { assertType } from './utils.js';
+
+// This plugin transforms `unref(i18n)` to `i18n` in the code, which is useful for removing unnecessary unref calls
+// and helps locale inliner runs after vite build to inline the locale data into the final build.
+//
+// locale inliner cannot know minifiedSymbol(i18n) is 'unref(i18n)' or 'otherFunctionsWithEffect(i18n)' so
+// it is necessary to remove unref calls before minification.
+export default function pluginRemoveUnrefI18n(
+	{
+		i18nSymbolName = 'i18n',
+	}: {
+		i18nSymbolName?: string
+	} = {}): Plugin {
+	return {
+		name: 'UnwindCssModuleClassName',
+		renderChunk(code) {
+			if (!code.includes('unref(i18n)')) return null;
+			const ast = this.parse(code) as Program;
+			const magicString = new MagicString(code);
+			estreeWalker.walk(ast, {
+				enter(node) {
+					if (node.type === 'CallExpression' && node.callee.type === 'Identifier' && node.callee.name === 'unref'
+						&& node.arguments.length === 1) {
+						// calls to unref with single argument
+						const arg = node.arguments[0];
+						if (arg.type === 'Identifier' && arg.name === i18nSymbolName) {
+							// this is unref(i18n) so replace it with i18n
+							// to replace, remove the 'unref(' and the trailing ')'
+							assertType<CallExpression & AstNode>(node);
+							assertType<Expression & AstNode>(arg);
+							magicString.remove(node.start, arg.start);
+							magicString.remove(arg.end, node.end);
+						}
+					}
+				}
+			});
+			return {
+				code: magicString.toString(),
+				map: magicString.generateMap({ hires: true }),
+			}
+		},
+	};
+}
diff --git a/packages/frontend-builder/tsconfig.json b/packages/frontend-builder/tsconfig.json
new file mode 100644
index 0000000000..9250b2f3da
--- /dev/null
+++ b/packages/frontend-builder/tsconfig.json
@@ -0,0 +1,29 @@
+{
+	"$schema": "https://json.schemastore.org/tsconfig",
+	"compilerOptions": {
+		"target": "ES2022",
+		"module": "nodenext",
+		"moduleResolution": "nodenext",
+		"declaration": true,
+		"declarationMap": true,
+		"sourceMap": false,
+		"noEmit": true,
+		"removeComments": true,
+		"resolveJsonModule": true,
+		"strict": true,
+		"strictFunctionTypes": true,
+		"strictNullChecks": true,
+		"experimentalDecorators": true,
+		"noImplicitReturns": true,
+		"esModuleInterop": true,
+		"verbatimModuleSyntax": true,
+		"baseUrl": ".",
+		"typeRoots": [
+			"./@types",
+			"./node_modules/@types"
+		],
+		"lib": [
+			"esnext"
+		]
+	}
+}
diff --git a/packages/frontend-builder/utils.ts b/packages/frontend-builder/utils.ts
new file mode 100644
index 0000000000..1ce9f7fc76
--- /dev/null
+++ b/packages/frontend-builder/utils.ts
@@ -0,0 +1,7 @@
+
+export function assertNever(x: never): never {
+	throw new Error(`Unexpected type: ${(x as any)?.type ?? x}`);
+}
+
+export function assertType<T>(node: unknown): asserts node is T {
+}