forPrefix

fn forPrefix(p: *Parse) Error!usize {
    const start = p.scratch.items.len;
    _ = try p.expectToken(.l_paren);

    while (true) {
        var input = try p.expectExpr();
        if (p.eatToken(.ellipsis2)) |ellipsis| {
            input = try p.addNode(.{
                .tag = .for_range,
                .main_token = ellipsis,
                .data = .{ .node_and_opt_node = .{
                    input,
                    .fromOptional(try p.parseExpr()),
                } },
            });
        }

        try p.scratch.append(p.gpa, input);
        switch (p.tokenTag(p.tok_i)) {
            .comma => p.tok_i += 1,
            .r_paren => {
                p.tok_i += 1;
                break;
            },
            .colon, .r_brace, .r_bracket => return p.failExpected(.r_paren),
            // Likely just a missing comma; give error but continue parsing.
            else => try p.warn(.expected_comma_after_for_operand),
        }
        if (p.eatToken(.r_paren)) |_| break;
    }
    const inputs = p.scratch.items.len - start;

    _ = p.eatToken(.pipe) orelse {
        try p.warn(.expected_loop_payload);
        return inputs;
    };

    while (true) {
        _ = p.eatToken(.asterisk);
        _ = try p.expectToken(.identifier);
        switch (p.tokenTag(p.tok_i)) {
            .comma => p.tok_i += 1,
            .pipe => {
                p.tok_i += 1;
                break;
            },
            // Likely just a missing comma; give error but continue parsing.
            else => try p.warn(.expected_comma_after_capture),
        }
        if (p.eatToken(.pipe)) |_| break;
    }

    return inputs;
}