Support else-if expressions (but not statements yet)

src/parse.rs

@@ -524,9 +524,17 @@ impl<'a> IfExpr<'a> {
     pub fn build(lexer: &mut Lexer<'a>) -> Self {
         let test_expr = Box::new(Self::eat_test_expr(lexer));
         let then_body = BranchBody::build(lexer);
+
         lexer.eat_expect(Token::Else);
-        lexer.eat_expect(Token::BraceOpen);
-        let else_body = BranchBody::build(lexer);
+        let else_body = match lexer.next().unwrap() {
+            Token::BraceOpen => BranchBody::build(lexer),
+            Token::If => BranchBody {
+                statements: Vec::new(),
+                tail_expr: Some(Box::new(Expr::If(IfExpr::build(lexer)))),
+            },
+            _ => lexer.panic_message("Expected if or opening brace"),
+        };
+
         Self {
             test_expr,
             then_body,
@@ -674,6 +682,11 @@ mod tests {
             parse_expr("if true { 1 } else { 0 };");
         }
 
+        #[test]
+        fn else_if_expr() {
+            parse_expr("if a { 1 } else if b { 2 } else { c };");
+        }
+
         #[test]
         fn func_call() {
             parse_expr("func_call(1 + 2, 3, 4 * 5);");