Add test to check unicode identifier version

compiler/rustc_lexer/src/lib.rs

@@ -33,6 +33,7 @@ pub mod unescape;
 mod tests;
 
 use unicode_properties::UnicodeEmoji;
+pub use unicode_xid::UNICODE_VERSION;
 
 use self::LiteralKind::*;
 use self::TokenKind::*;

tests/ui-fulldeps/lexer/unicode-version.rs

@@ -0,0 +1,18 @@
+// This test is used to validate which version of Unicode is used for parsing
+// identifiers. If the Unicode version changes, it should also be updated in
+// the reference at
+// https://github.com/rust-lang/reference/blob/HEAD/src/identifiers.md.
+
+//@ run-pass
+//@ check-run-results
+//@ ignore-cross-compile
+//@ reference: ident.unicode
+
+#![feature(rustc_private)]
+
+extern crate rustc_driver;
+extern crate rustc_lexer;
+
+fn main() {
+    println!("Unicode version is: {:?}", rustc_lexer::UNICODE_VERSION);
+}

tests/ui-fulldeps/lexer/unicode-version.run.stdout

@@ -0,0 +1 @@
+Unicode version is: (16, 0, 0)

triagebot.toml

@@ -991,6 +991,10 @@ cc = ["@Zalathar"]
 [mentions."src/tools/opt-dist"]
 cc = ["@kobzol"]
 
+[mentions."tests/ui-fulldeps/lexer/unicode-version.run.stdout"]
+message = "Unicode version has changed."
+cc = ["@ehuss"]
+
 [assign]
 warn_non_default_branch = true
 contributing_url = "https://rustc-dev-guide.rust-lang.org/getting-started.html"