当最终输出大小未知时，可以使用 std.Io.Writer.Allocating。它拥有一个可增长的缓冲区， 通过 written() 获取完整字节，并使用 deinit() 释放分配的内存。

//! Grow an output buffer through std.Io.Writer.Allocating.

const std = @import("std");

pub fn main(init: std.process.Init) !void {
    var output: std.Io.Writer.Allocating = .init(init.gpa);
    defer output.deinit();

    try output.writer.print("The output grows as needed: {d}\n", .{2026});
    try output.writer.writeAll("The completed bytes are available after writing.\n");

    std.debug.print("{s}", .{output.written()});
}