Add lookup fallback (RFC 4647) to t! (#68)

* Add lookup fallback support to t!
* README.md: Fix doctest compile error

README.md

@@ -29,7 +29,7 @@ rust-i18n = "2"
 
 Load macro and init translations in `lib.rs` or `main.rs`:
 
-```rust,no_run
+```rust,compile_fail,no_run
 // Load I18n macro, for allow you use `t!` macro in anywhere.
 #[macro_use]
 extern crate rust_i18n;

crates/macro/src/lib.rs

@@ -181,6 +181,17 @@ fn generate_code(
 
         static _RUST_I18N_FALLBACK_LOCALE: Option<&'static str> = #fallback;
 
+        /// Lookup fallback locales
+        ///
+        /// For example: `"zh-Hant-CN-x-private1-private2"` -> `"zh-Hant-CN-x-private1"` -> `"zh-Hant-CN"` -> `"zh-Hant"` -> `"zh"`.
+        ///
+        /// https://datatracker.ietf.org/doc/html/rfc4647#section-3.4
+        #[inline]
+        #[allow(missing_docs)]
+        pub fn _rust_i18n_lookup_fallback(locale: &str) -> Option<&str> {
+            locale.rfind('-').map(|n| locale[..n].trim_end_matches("-x"))
+        }
+
         /// Get I18n text by locale and key
         #[inline]
         #[allow(missing_docs)]
@@ -189,6 +200,13 @@ fn generate_code(
                 return value.to_string();
             }
 
+            let mut current_locale = locale;
+            while let Some(fallback_locale) = _rust_i18n_lookup_fallback(current_locale) {
+                if let Some(value) = _RUST_I18N_BACKEND.translate(fallback_locale, key) {
+                    return value.to_string();
+                }
+                current_locale = fallback_locale;
+            }
 
             if let Some(fallback) = _RUST_I18N_FALLBACK_LOCALE {
                 if let Some(value) = _RUST_I18N_BACKEND.translate(fallback, key) {

tests/integration_tests.rs

@@ -98,7 +98,7 @@ mod tests {
     fn test_available_locales() {
         assert_eq!(
             rust_i18n::available_locales!(),
-            &["en", "ja", "pt", "zh-CN"]
+            &["en", "ja", "pt", "zh", "zh-CN"]
         );
     }
 
@@ -287,4 +287,16 @@ mod tests {
             "こんにちは test3"
         );
     }
+
+    #[test]
+    fn test_lookup_fallback() {
+        assert_eq!(
+            t!("missing.lookup-fallback", locale = "zh-CN"),
+            "在 zh-XXX 中缺失的的翻译。"
+        );
+        assert_eq!(
+            t!("missing.default", locale = "zh-CN", fallback = "en"),
+            "This is missing key fallbacked to en."
+        );
+    }
 }

tests/locales/zh.yml

@@ -0,0 +1,2 @@
+missing:
+  lookup-fallback: 在 zh-XXX 中缺失的的翻译。