refactor: fully typed locales (#13033)

* refactor: fully typed locales

* refactor: hide parameterized locale strings from type data in ts access

* refactor: missing assertions

* docs: annotation

packages/frontend/src/i18n.ts

@@ -10,6 +10,7 @@ import { I18n } from '@/scripts/i18n.js';
 
 export const i18n = markRaw(new I18n<Locale>(locale));
 
-export function updateI18n(newLocale) {
-	i18n.ts = newLocale;
+export function updateI18n(newLocale: Locale) {
+	// @ts-expect-error -- private field
+	i18n.locale = newLocale;
 }

packages/frontend/src/scripts/i18n.ts

@@ -2,33 +2,114 @@
  * SPDX-FileCopyrightText: syuilo and other misskey contributors
  * SPDX-License-Identifier: AGPL-3.0-only
  */
+import type { ILocale, ParameterizedString } from '../../../../locales/index.js';
 
-export class I18n<T extends Record<string, any>> {
-	public ts: T;
+type FlattenKeys<T extends ILocale, TPrediction> = keyof {
+	[K in keyof T as T[K] extends ILocale
+		? FlattenKeys<T[K], TPrediction> extends infer C extends string
+			? `${K & string}.${C}`
+			: never
+		: T[K] extends TPrediction
+			? K
+			: never]: T[K];
+};
 
-	constructor(locale: T) {
-		this.ts = locale;
+type ParametersOf<T extends ILocale, TKey extends FlattenKeys<T, ParameterizedString<string>>> = T extends ILocale
+	? TKey extends `${infer K}.${infer C}`
+		// @ts-expect-error -- C は明らかに FlattenKeys<T[K], ParameterizedString<string>> になるが、型システムはここでは TKey がドット区切りであることのコンテキストを持たないので、型システムに合法にて示すことはできない。
+		? ParametersOf<T[K], C>
+		: TKey extends keyof T
+			? T[TKey] extends ParameterizedString<infer P>
+				? P
+				: never
+			: never
+	: never;
 
+type Ts<T extends ILocale> = {
+	readonly [K in keyof T as T[K] extends ParameterizedString<string> ? never : K]: T[K] extends ILocale ? Ts<T[K]> : string;
+};
+
+export class I18n<T extends ILocale> {
+	constructor(private locale: T) {
 		//#region BIND
 		this.t = this.t.bind(this);
 		//#endregion
 	}
 
-	// string にしているのは、ドット区切りでのパス指定を許可するため
-	// なるべくこのメソッド使うよりもlocale直接参照の方がvueのキャッシュ効いてパフォーマンスが良いかも
-	public t(key: string, args?: Record<string, string | number>): string {
-		try {
-			let str = key.split('.').reduce((o, i) => o[i], this.ts) as unknown as string;
+	public get ts(): Ts<T> {
+		if (_DEV_) {
+			class Handler<TTarget extends object> implements ProxyHandler<TTarget> {
+				get(target: TTarget, p: string | symbol): unknown {
+					const value = target[p as keyof TTarget];
 
-			if (args) {
-				for (const [k, v] of Object.entries(args)) {
-					str = str.replace(`{${k}}`, v.toString());
+					if (typeof value === 'object') {
+						// eslint-disable-next-line @typescript-eslint/no-non-null-assertion -- 実際には null がくることはないので。
+						return new Proxy(value!, new Handler<TTarget[keyof TTarget] & object>());
+					}
+
+					if (typeof value === 'string') {
+						const parameters = Array.from(value.matchAll(/\{(\w+)\}/g)).map(([, parameter]) => parameter);
+
+						if (parameters.length) {
+							console.error(`Missing locale parameters: ${parameters.join(', ')} at ${String(p)}`);
+						}
+
+						return value;
+					}
+
+					console.error(`Unexpected locale key: ${String(p)}`);
+
+					return p;
 				}
 			}
-			return str;
-		} catch (err) {
-			console.warn(`missing localization '${key}'`);
-			return key;
+
+			return new Proxy(this.locale, new Handler()) as Ts<T>;
 		}
+
+		return this.locale as Ts<T>;
+	}
+
+	/**
+	 * @deprecated なるべくこのメソッド使うよりも locale 直接参照の方が vue のキャッシュ効いてパフォーマンスが良いかも
+	 */
+	public t<TKey extends FlattenKeys<T, string>>(key: TKey): string;
+	public t<TKey extends FlattenKeys<T, ParameterizedString<string>>>(key: TKey, args: { readonly [_ in ParametersOf<T, TKey>]: string | number }): string;
+	public t(key: string, args?: { readonly [_: string]: string | number }) {
+		let str: string | ParameterizedString<string> | ILocale = this.locale;
+
+		for (const k of key.split('.')) {
+			str = str[k];
+
+			if (_DEV_) {
+				if (typeof str === 'undefined') {
+					console.error(`Unexpected locale key: ${key}`);
+					return key;
+				}
+			}
+		}
+
+		if (args) {
+			if (_DEV_) {
+				const missing = Array.from((str as string).matchAll(/\{(\w+)\}/g), ([, parameter]) => parameter).filter(parameter => !Object.hasOwn(args, parameter));
+
+				if (missing.length) {
+					console.error(`Missing locale parameters: ${missing.join(', ')} at ${key}`);
+				}
+			}
+
+			for (const [k, v] of Object.entries(args)) {
+				const search = `{${k}}`;
+
+				if (_DEV_) {
+					if (!(str as string).includes(search)) {
+						console.error(`Unexpected locale parameter: ${k} at ${key}`);
+					}
+				}
+
+				str = (str as string).replace(search, v.toString());
+			}
+		}
+
+		return str;
 	}
 }