a.go

package main

import (
	"bytes"
	"fmt"
)

func main() {
	functionWithBuffer()
}

func functionWithBuffer() {
	var buf bytes.Buffer
	for i := 0; i < 100; i++ {
		buf.WriteByte(byte(i))
	}
	fmt.Println(buf.Len())
}

b.go

package main

import (
	"bytes"
	"fmt"
)

func main() {
	functionWithBuffer()
}

func functionWithBuffer() {
	buf := new(bytes.Buffer)
	for i := 0; i < 100; i++ {
		buf.WriteByte(byte(i))
	}
	fmt.Println(buf.Len())
}

output.txt

jackson@mba ~/src/scratch/goescape $ go build -gcflags '-m' a.go
# command-line-arguments
./a.go:17:21: inlining call to bytes.(*Buffer).Len
./a.go:15:6: buf escapes to heap
./a.go:13:6: moved to heap: buf
./a.go:17:21: buf.Len() escapes to heap
./a.go:17:21: functionWithBuffer buf does not escape
./a.go:17:13: functionWithBuffer ... argument does not escape

jackson@mba ~/src/scratch/goescape $ go build -gcflags '-m' b.go
# command-line-arguments
./b.go:17:21: inlining call to bytes.(*Buffer).Len
./b.go:13:12: new(bytes.Buffer) escapes to heap
./b.go:17:21: buf.Len() escapes to heap
./b.go:17:13: functionWithBuffer ... argument does not escape