parseForStatement

fn parseForStatement(p: *Parse) !?Node.Index {
    const for_token = p.eatToken(.keyword_for) orelse return null;

    const scratch_top = p.scratch.items.len;
    defer p.scratch.shrinkRetainingCapacity(scratch_top);
    const inputs = try p.forPrefix();

    var else_required = false;
    var seen_semicolon = false;
    const then_expr = blk: {
        const block_expr = try p.parseBlockExpr();
        if (block_expr) |block| break :blk block;
        const assign_expr = try p.parseAssignExpr() orelse {
            return p.fail(.expected_block_or_assignment);
        };
        if (p.eatToken(.semicolon)) |_| {
            seen_semicolon = true;
            break :blk assign_expr;
        }
        else_required = true;
        break :blk assign_expr;
    };
    var has_else = false;
    if (!seen_semicolon and p.eatToken(.keyword_else) != null) {
        try p.scratch.append(p.gpa, then_expr);
        const else_stmt = try p.expectStatement(false);
        try p.scratch.append(p.gpa, else_stmt);
        has_else = true;
    } else if (inputs == 1) {
        if (else_required) try p.warn(.expected_semi_or_else);
        return try p.addNode(.{
            .tag = .for_simple,
            .main_token = for_token,
            .data = .{ .node_and_node = .{
                p.scratch.items[scratch_top],
                then_expr,
            } },
        });
    } else {
        if (else_required) try p.warn(.expected_semi_or_else);
        try p.scratch.append(p.gpa, then_expr);
    }
    return try p.addNode(.{
        .tag = .@"for",
        .main_token = for_token,
        .data = .{ .@"for" = .{
            (try p.listToSpan(p.scratch.items[scratch_top..])).start,
            .{ .inputs = @intCast(inputs), .has_else = has_else },
        } },
    });
}