你不应该复用 strings.Builder

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// A Builder is used to efficiently build a string using [Builder.Write] methods.
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// It minimizes memory copying. The zero value is ready to use.
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// Do not copy a non-zero Builder.
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type Builder struct {
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  addr *Builder // of receiver, to detect copies by value
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  // External users should never get direct access to this buffer, since
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  // the slice at some point will be converted to a string using unsafe, also
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  // data between len(buf) and cap(buf) might be uninitialized.
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  buf []byte
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}
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// String returns the accumulated string.
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func (b *Builder) String() string [](){
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  return unsafe.String(unsafe.SliceData(b.buf), len(b.buf))
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}
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// Reset resets the [Builder] to be empty.
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func (b *Builder) Reset() {
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  b.addr = nil
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  b.buf = nil
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}
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// copyCheck implements a dynamic check to prevent modification after
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// copying a non-zero Builder, which would be unsafe (see #25907, #47276).
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//
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// We cannot add a noCopy field to Builder, to cause vet's copylocks
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// check to report copying, because copylocks cannot reliably
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// discriminate the zero and nonzero cases.
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func (b *Builder) copyCheck() {
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  if b.addr == nil {
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    // This hack works around a failing of Go's escape analysis
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    // that was causing b to escape and be heap allocated.
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    // See issue 23382.
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    // TODO: once issue 7921 is fixed, this should be reverted to
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    // just "b.addr = b".
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    b.addr = (*Builder)(abi.NoEscape(unsafe.Pointer(b)))
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  } else if b.addr != b {
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    panic("strings: illegal use of non-zero Builder copied by value")
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  }
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}

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func (b *Builder) String() string [](){
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  return unsafe.String(unsafe.SliceData(b.buf), len(b.buf))
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}

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package bench_test
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import (
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  "strings"
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  "sync"
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  "testing"
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)
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// 全局的 Pool，避免每次 benchmark 都重新创建
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var builderPool = sync.Pool{
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  New: func() any {
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    return new(strings.Builder)
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  },
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}
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// 为了公平，我们让两个 benchmark 都做同样多的写入工作
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const writeSize = 64 // 每次写 64 字节
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// 每次新建 Builder
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func BenchmarkWithoutReset(b *testing.B) {
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  b.ReportAllocs()
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  for b.Loop() {
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    var buf strings.Builder
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    for range 100 {
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      buf.WriteString("0123456789ABCDEF")
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      buf.WriteString("0123456789ABCDEF")
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      buf.WriteString("0123456789ABCDEF")
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      buf.WriteString("0123456789ABCDEF")
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    }
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    _ = buf.String()
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  }
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}
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// 复用 Builder 并 Reset
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func BenchmarkWithReset(b *testing.B) {
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  b.ReportAllocs()
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  for b.Loop() {
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    buf := builderPool.Get().(*strings.Builder)
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    buf.Reset() // 清零
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    for range 100 {
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      buf.WriteString("0123456789ABCDEF")
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      buf.WriteString("0123456789ABCDEF")
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      buf.WriteString("0123456789ABCDEF")
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      buf.WriteString("0123456789ABCDEF")
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    }
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    _ = buf.String()
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    builderPool.Put(buf) // 归还
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  }
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}
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// 测试非常小分配的情况下不调用 Reset 的情况
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func BenchmarkWithoutResetTiny(b *testing.B) {
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  const tiny = "x" // 1 字节
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  b.ReportAllocs()
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  for b.Loop() {
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    var buf strings.Builder
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    for range 100 {
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      buf.WriteString(tiny)
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    }
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    _ = buf.String()
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  }
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}
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// 测试非常小分配的情况下调用 Reset 的情况
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func BenchmarkWithResetTiny(b *testing.B) {
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  const tiny = "x"
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  b.ReportAllocs()
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  for b.Loop() {
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    buf := builderPool.Get().(*strings.Builder)
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    buf.Reset()
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    for range 100 {
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      buf.WriteString(tiny)
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    }
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    _ = buf.String()
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    builderPool.Put(buf)
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  }
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}
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// 测试非常小分配的情况下不调用 Reset 的情况（并行）
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func BenchmarkWithoutResetParallel(b *testing.B) {
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  const tiny = "x"
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  b.ReportAllocs()
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  b.RunParallel(func(pb *testing.PB) {
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    for pb.Next() {
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      var buf strings.Builder
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      for range 100 {
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        buf.WriteString(tiny)
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      }
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      _ = buf.String()
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    }
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  })
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}
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// 测试非常小分配的情况下调用 Reset 的情况（并行）
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func BenchmarkWithResetParallel(b *testing.B) {
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  const tiny = "x"
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  b.ReportAllocs()
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  b.RunParallel(func(pb *testing.PB) {
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    for pb.Next() {
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      buf := builderPool.Get().(*strings.Builder)
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      buf.Reset()
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      for range 100 {
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        buf.WriteString(tiny)
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      }
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      _ = buf.String()
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      builderPool.Put(buf)
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    }
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  })
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}