在Mutex的文档中，它说它实现了Send和Sync--这是有意义的，因为互斥体被设计为从多个线程访问，这些线程正在锁定、使用它保护的资源，然后解锁。
然而，在下面的代码中，我得到了一个编译器错误，据我所知，它抱怨互斥体没有实现Send/Sync：

error[E0599]: the method `try_init` exists for struct `SubscriberBuilder<DefaultFields, Format, tracing::level_filters::LevelFilter, std::sync::Mutex<MultiWriter>>`, but its trait bounds were not satisfied
   --> src/main.rs:131:10
    |
131 |           .try_init().expect("setting default subscriber failed");
    |            ^^^^^^^^ method cannot be called on `SubscriberBuilder<DefaultFields, Format, tracing::level_filters::LevelFilter, std::sync::Mutex<MultiWriter>>` due to unsatisfied trait bounds
    |
   ::: /Users/sean/.cargo/registry/src/github.com-1ecc6299db9ec823/tracing-subscriber-0.3.16/src/fmt/fmt_layer.rs:62:1
    |
62  | / pub struct Layer<
63  | |     S,
64  | |     N = format::DefaultFields,
65  | |     E = format::Format<format::Full>,
66  | |     W = fn() -> io::Stdout,
67  | | > {
    | | -
    | | |
    | |_doesn't satisfy `_: std::marker::Send`
    |   doesn't satisfy `_: std::marker::Sync`
    |
    = note: the following trait bounds were not satisfied:
            `tracing_subscriber::fmt::Layer<Registry, DefaultFields, Format, std::sync::Mutex<MultiWriter>>: std::marker::Send`
            `tracing_subscriber::fmt::Layer<Registry, DefaultFields, Format, std::sync::Mutex<MultiWriter>>: std::marker::Sync`

如果我从下面的代码中删除.with_writer(mw)行，错误就会消失。很明显，问题与作者有关，但我不确定如何正确地做到这一点。
代码的目标是将tracing框架中的日志写入stderr和dotenvy中指定的文件（如果指定了文件名，则为可选）。
注意：我使用的是最新的稳定版Rust和下面使用的每个机箱的发布版本，并在MacOS上用std，libc，alloc等（完整的Rust，而不是嵌入式）编译，但代码预计将在“多平台x86（_64）桌面”环境（Windows/MacOS/桌面Linux）下工作。

use std::fs::File;
use std::io::Write;
use std::sync::Mutex;

use dotenvy::var;
se std::sync::Arc;
use tracing::Level;

struct MultiWriter {
    writers: Vec<Arc<dyn Write>>,
}

impl Write for MultiWriter {
    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
        for writer in self.writers.iter_mut() {
            writer.write(buf)?;
        }
        Ok(buf.len())
    }

    fn flush(&mut self) -> std::io::Result<()> {
        for writer in self.writers.iter_mut() {
            writer.flush()?;
        }
        Ok(())
    }
}

#[tokio::main]
async fn main() -> anyhow::Result<()> {
    let mut writers: Vec<Arc<dyn Write>> = vec![(Arc::new(std::io::stderr()))];
    if let Some(log_file) = var("log_file").ok() {
        writers.push(Arc::new(File::create(log_file).unwrap()));
    }
    let mw = Mutex::new(MultiWriter { writers });

    let tsb = tracing_subscriber::FmtSubscriber::builder()
        .with_env_filter(tracing_subscriber::EnvFilter::from_default_env()).with_ansi(false)
        .with_writer(mw);

    if let Ok(log_level) = var("log_level") {
        match log_level.to_uppercase().as_str() {
            "TRACE" => tsb.with_max_level(Level::TRACE),
            "DEBUG" => tsb.with_max_level(Level::DEBUG),
            "INFO" => tsb.with_max_level(Level::INFO),
            "WARN" => tsb.with_max_level(Level::WARN),
            "ERROR" => tsb.with_max_level(Level::ERROR),
            _ => tsb.with_max_level(Level::INFO)
        }
        .try_init().expect("setting default subscriber failed");
    }   
}