Non-ASCII chars in identifiers aren't supported yet

Author: brauliobz

src/identifiers.md

@@ -2,10 +2,34 @@
 
 > **<sup>Lexer:<sup>**  
 > IDENTIFIER :  
-> &nbsp;&nbsp; &nbsp;&nbsp; XID_start XID_continue<sup>\*</sup>  
-> &nbsp;&nbsp; | `_` XID_continue<sup>+</sup>  
+> &nbsp;&nbsp; &nbsp;&nbsp; [`a`-`z` `A`-`Z`]&nbsp;[`a`-`z` `A`-`Z` `0`-`9` `_`]<sup>\*</sup>  
+> &nbsp;&nbsp; | `_` [`a`-`z` `A`-`Z` `0`-`9` `_`]<sup>+</sup>  
 
-An identifier is any nonempty Unicode[^non_ascii_idents] string of the following form:
+An identifier is any nonempty ASCII[^non_ascii_idents] string of the following form:
+
+Either
+
+   * The first character is a letter
+   * The remaining characters are alphanumeric or `_`
+
+Or
+
+   * The first character is `_`
+   * The identifier is more than one character, `_` alone is not an identifier
+   * The remaining characters are alphanumeric or `_`
+
+[keywords]: keywords.html
+
+[^non_ascii_idents] Non-ASCII characters in identifiers are currently feature-gated. See [issue #28979](https://github.com/rust-lang/rust/issues/28979).
+
+<!-- When non-ascii identifiers are actually supported:
+
+ > **<sup>Lexer:<sup>**  
+ > IDENTIFIER :  
+ > &nbsp;&nbsp; &nbsp;&nbsp; XID_start XID_continue<sup>\*</sup>  
+ > &nbsp;&nbsp; | `_` XID_continue<sup>+</sup>  
+
+An identifier is any nonempty Unicode string of the following form:
 
 Either
 
@@ -23,11 +47,8 @@ that does _not_ occur in the set of [keywords].
 > **Note**: [`XID_start`] and [`XID_continue`] as character properties cover the
 > character ranges used to form the more familiar C and Java language-family
 > identifiers.
-
-
    
 [`XID_start`]:  http://unicode.org/cldr/utility/list-unicodeset.jsp?a=%5B%3AXID_Start%3A%5D&abb=on&g=&i=
 [`XID_continue`]: http://unicode.org/cldr/utility/list-unicodeset.jsp?a=%5B%3AXID_Continue%3A%5D&abb=on&g=&i=
-[keywords]: keywords.html
-[^non_ascii_idents]: Non-ASCII characters in identifiers are currently feature
-  gated. This is expected to improve soon.
+
+-->