-[^non_ascii_idents]: Non-ASCII characters in identifiers are currently feature
- gated. This is expected to improve soon.
+- The first character is in one of the following ranges `U+0041` to `U+005A`
+("A" to "Z"), `U+0061` to `U+007A` ("a" to "z"), or `U+005F` ("\_").
+- The remaining characters are in the range `U+0030` to `U+0039` ("0" to "9"),
+or any of the prior valid initial characters.