Allow underscores at the start of a variable

Author: Josh Dutton

crates/rune/src/ast/ident.rs

@@ -6,6 +6,7 @@ fn ast_parse() {
 
     rt::<ast::Ident>("foo");
     rt::<ast::Ident>("a42");
+    rt::<ast::Ident>("_ignored");
 }
 
 /// An identifier, like `foo` or `Hello`.

crates/rune/src/parse/lexer.rs

@@ -143,10 +143,17 @@ impl<'a> Lexer<'a> {
             self.iter.next();
         }
 
-        let (ident, span) = self.iter.source_from(start);
-        let kind = ast::Kind::from_keyword(ident)
-            .unwrap_or(ast::Kind::Ident(ast::LitSource::Text(self.source_id)));
-        Ok(Some(ast::Token { kind, span }))
+        match self.iter.source_from(start) {
+            ("_", span) => Ok(Some(ast::Token {
+                span,
+                kind: ast::Kind::Underscore,
+            })),
+            (ident, span) => {
+                let kind = ast::Kind::from_keyword(ident)
+                    .unwrap_or(ast::Kind::Ident(ast::LitSource::Text(self.source_id)));
+                Ok(Some(ast::Token { kind, span }))
+            }
+        }
     }
 
     /// Consume a number literal.
@@ -781,7 +788,6 @@ impl<'a> Lexer<'a> {
                     }
                     '[' => ast::Kind::Open(ast::Delimiter::Bracket),
                     ']' => ast::Kind::Close(ast::Delimiter::Bracket),
-                    '_' => ast::Kind::Underscore,
                     ',' => ast::Kind::Comma,
                     ':' => ast::Kind::Colon,
                     '#' => ast::Kind::Pound,
@@ -803,7 +809,7 @@ impl<'a> Lexer<'a> {
                     '@' => ast::Kind::At,
                     '$' => ast::Kind::Dollar,
                     '~' => ast::Kind::Tilde,
-                    'a'..='z' | 'A'..='Z' => {
+                    '_' | 'a'..='z' | 'A'..='Z' => {
                         return self.next_ident(start);
                     }
                     '0'..='9' => {

crates/rune/src/tests/vm_assign_exprs.rs

@@ -9,6 +9,24 @@ fn test_basic_assign() {
     assert_eq!(out, 42);
 }
 
+#[test]
+fn test_assign_underscore() {
+    let out: i64 = rune! {
+        pub fn main() { let _a = 0; _a = 42; _a }
+    };
+
+    assert_eq!(out, 42);
+}
+
+#[test]
+fn test_assign_underscores() {
+    let out: i64 = rune! {
+        pub fn main() { let ___ = 0; ___ = 42; ___ }
+    };
+
+    assert_eq!(out, 42);
+}
+
 #[test]
 fn test_assign_anon_object() {
     let out: i64 = rune! {