expectWhileExpr

fn expectWhileExpr(p: *Parse) !Node.Index {
    const while_token = p.assertToken(.keyword_while);
    _ = try p.expectToken(.l_paren);
    const condition = try p.expectExpr();
    _ = try p.expectToken(.r_paren);
    _ = try p.parsePtrPayload();
    const cont_expr = try p.parseWhileContinueExpr();

    const then_expr = try p.expectExpr();
    _ = p.eatToken(.keyword_else) orelse {
        if (cont_expr == null) {
            return try p.addNode(.{
                .tag = .while_simple,
                .main_token = while_token,
                .data = .{ .node_and_node = .{
                    condition,
                    then_expr,
                } },
            });
        } else {
            return try p.addNode(.{
                .tag = .while_cont,
                .main_token = while_token,
                .data = .{ .node_and_extra = .{
                    condition,
                    try p.addExtra(Node.WhileCont{
                        .cont_expr = cont_expr.?,
                        .then_expr = then_expr,
                    }),
                } },
            });
        }
    };
    _ = try p.parsePayload();
    const else_expr = try p.expectExpr();
    return try p.addNode(.{
        .tag = .@"while",
        .main_token = while_token,
        .data = .{ .node_and_extra = .{
            condition,
            try p.addExtra(Node.While{
                .cont_expr = .fromOptional(cont_expr),
                .then_expr = then_expr,
                .else_expr = else_expr,
            }),
        } },
    });
}