Add a comment about using Locale.Components.icuIdentifier as the cache key

Author: itingliu

Sources/FoundationEssentials/Locale/Locale+Components.swift

@@ -87,6 +87,7 @@ extension Locale {
         }
 
         // Returns an ICU-style identifier like "de_DE@calendar=gregorian"
+        // Must include every component stored by a `Locale.Components`, and be kept in sync with `init(identifier:)`.
         package var icuIdentifier: String {
 
             var keywords = [(ICULegacyKey, String)]()

Sources/FoundationEssentials/Locale/Locale_Cache.swift

@@ -45,7 +45,7 @@ struct LocaleCache : Sendable, ~Copyable {
         }
 
         private var cachedFixedLocales: [String : any _LocaleProtocol] = [:]
-        private var cachedFixedComponentsLocales: [String : any _LocaleProtocol] = [:]
+        private var cachedFixedComponentsLocales: [String /*ICU identifier*/: any _LocaleProtocol] = [:]
 
 #if FOUNDATION_FRAMEWORK
         private var cachedFixedIdentifierToNSLocales: [String : _NSSwiftLocale] = [:]
@@ -99,13 +99,9 @@ struct LocaleCache : Sendable, ~Copyable {
 
 #endif // FOUNDATION_FRAMEWORK
 
-        func fixedComponents(_ comps: String) -> (any _LocaleProtocol)? {
-            cachedFixedComponentsLocales[comps]
-        }
-
         mutating func fixedComponentsWithCache(_ comps: Locale.Components) -> any _LocaleProtocol {
             let identifier = comps.icuIdentifier
-            if let l = fixedComponents(identifier) {
+            if let l = cachedFixedComponentsLocales[identifier] {
                 return l
             } else {
                 let new = _localeICUClass().init(components: comps)