Bug 1736875: Add FormatBuffer::toAsciiString(). r=allstarschh

This avoids an unnecessary call to `JS::FindSmallestEncoding()` in `NewStringCopyUTF8N()`
when the string is guaranteed to be ASCII-only.

Differential Revision: https://phabricator.services.mozilla.com/D129053

js/src/builtin/intl/FormatBuffer.h

@@ -9,6 +9,7 @@
 
 #include "mozilla/Assertions.h"
 #include "mozilla/Span.h"
+#include "mozilla/TextUtils.h"
 
 #include <stddef.h>
 #include <stdint.h>
@@ -97,6 +98,17 @@ class FormatBuffer {
     }
   }
 
+  /**
+   * Copies the buffer's data to a JSString. The buffer must contain only
+   * ASCII characters.
+   */
+  JSLinearString* toAsciiString(JSContext* cx) const {
+    static_assert(std::is_same_v<CharT, char>);
+
+    MOZ_ASSERT(mozilla::IsAscii(buffer_));
+    return NewStringCopyN<CanGC>(cx, buffer_.begin(), buffer_.length());
+  }
+
   /**
    * Extract this buffer's content as a null-terminated string.
    */

js/src/builtin/intl/IntlObject.cpp

@@ -377,7 +377,7 @@ bool js::intl_supportedLocaleOrFallback(JSContext* cx, unsigned argc,
       return false;
     }
 
-    candidate = buffer.toString(cx);
+    candidate = buffer.toAsciiString(cx);
     if (!candidate) {
       return false;
     }

js/src/builtin/intl/Locale.cpp

@@ -113,7 +113,7 @@ static LocaleObject* CreateLocaleObject(JSContext* cx, HandleObject prototype,
     return nullptr;
   }
 
-  RootedString tagStr(cx, buffer.toString(cx));
+  RootedString tagStr(cx, buffer.toAsciiString(cx));
   if (!tagStr) {
     return nullptr;
   }
@@ -1367,7 +1367,7 @@ bool js::intl_ValidateAndCanonicalizeLanguageTag(JSContext* cx, unsigned argc,
     return false;
   }
 
-  JSString* resultStr = buffer.toString(cx);
+  JSString* resultStr = buffer.toAsciiString(cx);
   if (!resultStr) {
     return false;
   }
@@ -1424,7 +1424,7 @@ bool js::intl_TryValidateAndCanonicalizeLanguageTag(JSContext* cx,
     return false;
   }
 
-  JSString* resultStr = buffer.toString(cx);
+  JSString* resultStr = buffer.toAsciiString(cx);
   if (!resultStr) {
     return false;
   }