% cat /tmp/x.go
package p

import (
	"fmt"
	"strings"
)

func main() {
	var b strings.Builder
	fmt.Fprintf(b, "hello")
}
% go tool compile -G=0 /tmp/x.go
/tmp/x.go:10:13: cannot use b (type strings.Builder) as type io.Writer in argument to fmt.Fprintf:
	strings.Builder does not implement io.Writer (Write method has pointer receiver)
% go tool compile /tmp/x.go
/tmp/x.go:10:14: cannot use b (variable of type strings.Builder) as type io.Writer in argument to fmt.Fprintf:
	strings.Builder does not implement io.Writer (Write method has pointer receiver) at $GOROOT/src/strings/builder.go:88:19
%