如何在tracing中条件使用non_blocking writer替代stdout并解决类型错误？

我尝试用以下Rust代码初始化tracing subscriber，需求是仅在特定场景下使用non_blocking writer，其余场景用stdout，但出现了类型不匹配的编译错误，求解决方法：

use tracing_appender::non_blocking;
use tracing_log::LogTracer;
use tracing_subscriber::{fmt::Layer, prelude::*, EnvFilter, Registry};

pub fn init(tracing: &Tracing) -> non_blocking::WorkerGuard {
    LogTracer::init().expect("init LogTracer");

    let (non_blocking, non_blocking_guard) = non_blocking(std::io::stdout());

    let base_layer;

    if tracing.blocking {
        base_layer = Layer::default().with_writer(std::io::stdout);
    } else {
        base_layer = Layer::default().with_writer(non_blocking);
    }

    let mut layers = Vec::new();

    layers.push(if tracing.pretty {
        base_layer.pretty().with_file(false).boxed()
    } else {
        base_layer.json().boxed()
    });

    let registry = Registry::default()
        .with(EnvFilter::from(&tracing.level))
        .with(layers);

    tracing::subscriber::set_global_default(registry).unwrap();

    non_blocking_guard
}

编译错误信息：

error[E0308]: mismatched types
    |
18  |         base_layer = Layer::default().with_writer(non_blocking);
    |                                       ----------- ^^^^^^^^^^^^ expected fn item, found struct `tracing_appender::non_blocking::NonBlocking`
    |                                       |
    |                                       arguments to this function are incorrect
    |
    = note: expected fn item `fn() -> std::io::Stdout {std::io::stdout}`
                found struct `tracing_appender::non_blocking::NonBlocking`
note: associated function defined here
    |
177 |     pub fn with_writer<W2>(self, make_writer: W2) -> Layer<S, N, E, W2>
    |            ^^^^^^^^^^^

问题根源 #

两个分支中base_layer的类型不一致：

• 当tracing.blocking为true时，with_writer接收的是std::io::stdout（返回Stdout的函数），此时Layer的writer类型为fn() -> Stdout
• 当tracing.blocking为false时，with_writer接收的是NonBlocking结构体，此时Layer的writer类型为NonBlocking

Rust要求同一变量的类型必须统一，因此触发类型不匹配错误。

解决方案 #

利用tracing_subscriber::fmt::MakeWriter trait，将两种writer统一为Box<dyn MakeWriter> trait对象，让两个分支的Layer类型保持一致。

修正后的代码：

use tracing_appender::non_blocking;
use tracing_log::LogTracer;
use tracing_subscriber::{fmt::{Layer, MakeWriter}, prelude::*, EnvFilter, Registry};

pub fn init(tracing: &Tracing) -> non_blocking::WorkerGuard {
    LogTracer::init().expect("init LogTracer");

    let (non_blocking, non_blocking_guard) = non_blocking(std::io::stdout());

    // 统一writer类型为Box<dyn MakeWriter>
    let writer: Box<dyn MakeWriter> = if tracing.blocking {
        Box::new(std::io::stdout)
    } else {
        Box::new(non_blocking)
    };

    let base_layer = Layer::default().with_writer(writer);

    let mut layers = Vec::new();

    layers.push(if tracing.pretty {
        base_layer.pretty().with_file(false).boxed()
    } else {
        base_layer.json().boxed()
    });

    let registry = Registry::default()
        .with(EnvFilter::from(&tracing.level))
        .with(layers);

    tracing::subscriber::set_global_default(registry).unwrap();

    non_blocking_guard
}

说明 #

• MakeWriter是tracing_subscriber提供的标准trait，所有可作为日志输出目标的类型都实现了该trait，包括std::io::stdout函数和NonBlocking结构体
• 使用Box<dyn MakeWriter>将不同类型的writer封装为 trait 对象，确保Layer的writer类型统一，消除类型不匹配问题
• 即使在blocking模式下，我们依然创建了non_blocking writer，但此时不会被使用；WorkerGuard仍需返回，因为它负责管理后台线程的生命周期

内容的提问来源于stack exchange，提问作者Fred Hors