Update identifier Unicode character validation to match Python spec (#7209)

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_python_stdlib/src/identifiers.rs

@@ -1,3 +1,5 @@
+use unic_ucd_ident::{is_xid_continue, is_xid_start};
+
 use crate::keyword::is_keyword;
 
 /// Returns `true` if a string is a valid Python identifier (e.g., variable
@@ -5,12 +7,12 @@ use crate::keyword::is_keyword;
 pub fn is_identifier(name: &str) -> bool {
     // Is the first character a letter or underscore?
     let mut chars = name.chars();
-    if !chars.next().is_some_and(|c| c.is_alphabetic() || c == '_') {
+    if !chars.next().is_some_and(is_identifier_start) {
         return false;
     }
 
     // Are the rest of the characters letters, digits, or underscores?
-    if !chars.all(|c| c.is_alphanumeric() || c == '_') {
+    if !chars.all(is_identifier_continuation) {
         return false;
     }
 
@@ -22,6 +24,21 @@ pub fn is_identifier(name: &str) -> bool {
     true
 }
 
+// Checks if the character c is a valid starting character as described
+// in https://docs.python.org/3/reference/lexical_analysis.html#identifiers
+fn is_identifier_start(c: char) -> bool {
+    matches!(c, 'a'..='z' | 'A'..='Z' | '_') || is_xid_start(c)
+}
+
+// Checks if the character c is a valid continuation character as described
+// in https://docs.python.org/3/reference/lexical_analysis.html#identifiers
+fn is_identifier_continuation(c: char) -> bool {
+    match c {
+        'a'..='z' | 'A'..='Z' | '_' | '0'..='9' => true,
+        c => is_xid_continue(c),
+    }
+}
+
 /// Returns `true` if a string is a private identifier, such that, when the
 /// identifier is defined in a class definition, it will be mangled prior to
 /// code generation.
@@ -76,7 +93,25 @@ pub fn is_migration_name(name: &str) -> bool {
 
 #[cfg(test)]
 mod tests {
-    use crate::identifiers::{is_migration_name, is_module_name};
+    use crate::identifiers::{is_identifier, is_migration_name, is_module_name};
+
+    #[test]
+    fn valid_identifiers() {
+        assert!(is_identifier("_abc"));
+        assert!(is_identifier("abc"));
+        assert!(is_identifier("_"));
+        assert!(is_identifier("a_b_c"));
+        assert!(is_identifier("abc123"));
+        assert!(is_identifier("abc_123"));
+        assert!(is_identifier("漢字"));
+        assert!(is_identifier("ひらがな"));
+        assert!(is_identifier("العربية"));
+        assert!(is_identifier("кириллица"));
+        assert!(is_identifier("πr"));
+        assert!(!is_identifier(""));
+        assert!(!is_identifier("percentile_co³t"));
+        assert!(!is_identifier("HelloWorld❤️"));
+    }
 
     #[test]
     fn module_name() {