//! The [`Layer`] trait, a composable abstraction for building [`Subscriber`]s. //! //! The [`Subscriber`] trait in `tracing-core` represents the _complete_ set of //! functionality required to consume `tracing` instrumentation. This means that //! a single `Subscriber` instance is a self-contained implementation of a //! complete strategy for collecting traces; but it _also_ means that the //! `Subscriber` trait cannot easily be composed with other `Subscriber`s. //! //! In particular, [`Subscriber`]s are responsible for generating [span IDs] and //! assigning them to spans. Since these IDs must uniquely identify a span //! within the context of the current trace, this means that there may only be //! a single `Subscriber` for a given thread at any point in time — //! otherwise, there would be no authoritative source of span IDs. //! //! On the other hand, the majority of the [`Subscriber`] trait's functionality //! is composable: any number of subscribers may _observe_ events, span entry //! and exit, and so on, provided that there is a single authoritative source of //! span IDs. The [`Layer`] trait represents this composable subset of the //! [`Subscriber`] behavior; it can _observe_ events and spans, but does not //! assign IDs. //! //! # Composing Layers //! //! Since a [`Layer`] does not implement a complete strategy for collecting //! traces, it must be composed with a `Subscriber` in order to be used. The //! [`Layer`] trait is generic over a type parameter (called `S` in the trait //! definition), representing the types of `Subscriber` they can be composed //! with. Thus, a [`Layer`] may be implemented that will only compose with a //! particular `Subscriber` implementation, or additional trait bounds may be //! added to constrain what types implementing `Subscriber` a `Layer` can wrap. //! //! `Layer`s may be added to a `Subscriber` by using the [`SubscriberExt::with`] //! method, which is provided by `tracing-subscriber`'s [prelude]. This method //! returns a [`Layered`] struct that implements `Subscriber` by composing the //! `Layer` with the `Subscriber`. //! //! For example: //! ```rust //! use tracing_subscriber::Layer; //! use tracing_subscriber::prelude::*; //! use tracing::Subscriber; //! //! pub struct MyLayer { //! // ... //! } //! //! impl<S: Subscriber> Layer<S> for MyLayer { //! // ... //! } //! //! pub struct MySubscriber { //! // ... //! } //! //! # use tracing_core::{span::{Id, Attributes, Record}, Metadata, Event}; //! impl Subscriber for MySubscriber { //! // ... //! # fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(1) } //! # fn record(&self, _: &Id, _: &Record) {} //! # fn event(&self, _: &Event) {} //! # fn record_follows_from(&self, _: &Id, _: &Id) {} //! # fn enabled(&self, _: &Metadata) -> bool { false } //! # fn enter(&self, _: &Id) {} //! # fn exit(&self, _: &Id) {} //! } //! # impl MyLayer { //! # fn new() -> Self { Self {} } //! # } //! # impl MySubscriber { //! # fn new() -> Self { Self { }} //! # } //! //! let subscriber = MySubscriber::new() //! .with(MyLayer::new()); //! //! tracing::subscriber::set_global_default(subscriber); //! ``` //! //! Multiple `Layer`s may be composed in the same manner: //! ```rust //! # use tracing_subscriber::{Layer, layer::SubscriberExt}; //! # use tracing::Subscriber; //! pub struct MyOtherLayer { //! // ... //! } //! //! impl<S: Subscriber> Layer<S> for MyOtherLayer { //! // ... //! } //! //! pub struct MyThirdLayer { //! // ... //! } //! //! impl<S: Subscriber> Layer<S> for MyThirdLayer { //! // ... //! } //! # pub struct MyLayer {} //! # impl<S: Subscriber> Layer<S> for MyLayer {} //! # pub struct MySubscriber { } //! # use tracing_core::{span::{Id, Attributes, Record}, Metadata, Event}; //! # impl Subscriber for MySubscriber { //! # fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(1) } //! # fn record(&self, _: &Id, _: &Record) {} //! # fn event(&self, _: &Event) {} //! # fn record_follows_from(&self, _: &Id, _: &Id) {} //! # fn enabled(&self, _: &Metadata) -> bool { false } //! # fn enter(&self, _: &Id) {} //! # fn exit(&self, _: &Id) {} //! } //! # impl MyLayer { //! # fn new() -> Self { Self {} } //! # } //! # impl MyOtherLayer { //! # fn new() -> Self { Self {} } //! # } //! # impl MyThirdLayer { //! # fn new() -> Self { Self {} } //! # } //! # impl MySubscriber { //! # fn new() -> Self { Self { }} //! # } //! //! let subscriber = MySubscriber::new() //! .with(MyLayer::new()) //! .with(MyOtherLayer::new()) //! .with(MyThirdLayer::new()); //! //! tracing::subscriber::set_global_default(subscriber); //! ``` //! //! The [`Layer::with_subscriber`] constructs the [`Layered`] type from a //! [`Layer`] and [`Subscriber`], and is called by [`SubscriberExt::with`]. In //! general, it is more idiomatic to use [`SubscriberExt::with`], and treat //! [`Layer::with_subscriber`] as an implementation detail, as `with_subscriber` //! calls must be nested, leading to less clear code for the reader. //! //! ## Runtime Configuration With `Layer`s //! //! In some cases, a particular [`Layer`] may be enabled or disabled based on //! runtime configuration. This can introduce challenges, because the type of a //! layered [`Subscriber`] depends on which layers are added to it: if an `if` //! or `match` expression adds some [`Layer`] implementation in one branch, //! and other layers in another, the [`Subscriber`] values returned by those //! branches will have different types. For example, the following _will not_ //! work: //! //! ```compile_fail //! # fn docs() -> Result<(), Box<dyn std::error::Error + 'static>> { //! # struct Config { //! # is_prod: bool, //! # path: &'static str, //! # } //! # let cfg = Config { is_prod: false, path: "debug.log" }; //! use std::fs::File; //! use tracing_subscriber::{Registry, prelude::*}; //! //! let stdout_log = tracing_subscriber::fmt::layer().pretty(); //! let subscriber = Registry::default().with(stdout_log); //! //! // The compile error will occur here because the if and else //! // branches have different (and therefore incompatible) types. //! let subscriber = if cfg.is_prod { //! let file = File::create(cfg.path)?; //! let layer = tracing_subscriber::fmt::layer() //! .json() //! .with_writer(Arc::new(file)); //! layer.with(subscriber) //! } else { //! layer //! }; //! //! tracing::subscriber::set_global_default(subscriber) //! .expect("Unable to set global subscriber"); //! # Ok(()) } //! ``` //! //! However, a [`Layer`] wrapped in an [`Option`] [also implements the `Layer` //! trait][option-impl]. This allows individual layers to be enabled or disabled at //! runtime while always producing a [`Subscriber`] of the same type. For //! example: //! //! ``` //! # fn docs() -> Result<(), Box<dyn std::error::Error + 'static>> { //! # struct Config { //! # is_prod: bool, //! # path: &'static str, //! # } //! # let cfg = Config { is_prod: false, path: "debug.log" }; //! use std::fs::File; //! use tracing_subscriber::{Registry, prelude::*}; //! //! let stdout_log = tracing_subscriber::fmt::layer().pretty(); //! let subscriber = Registry::default().with(stdout_log); //! //! // if `cfg.is_prod` is true, also log JSON-formatted logs to a file. //! let json_log = if cfg.is_prod { //! let file = File::create(cfg.path)?; //! let json_log = tracing_subscriber::fmt::layer() //! .json() //! .with_writer(file); //! Some(json_log) //! } else { //! None //! }; //! //! // If `cfg.is_prod` is false, then `json` will be `None`, and this layer //! // will do nothing. However, the subscriber will still have the same type //! // regardless of whether the `Option`'s value is `None` or `Some`. //! let subscriber = subscriber.with(json_log); //! //! tracing::subscriber::set_global_default(subscriber) //! .expect("Unable to set global subscriber"); //! # Ok(()) } //! ``` //! //! If a [`Layer`] may be one of several different types, note that [`Box<dyn //! Layer<S> + Send + Sync>` implements `Layer`][box-impl]. //! This may be used to erase the type of a [`Layer`]. //! //! For example, a function that configures a [`Layer`] to log to one of //! several outputs might return a `Box<dyn Layer<S> + Send + Sync + 'static>`: //! ``` //! use tracing_subscriber::{ //! Layer, //! registry::LookupSpan, //! prelude::*, //! }; //! use std::{path::PathBuf, fs::File, io}; //! //! /// Configures whether logs are emitted to a file, to stdout, or to stderr. //! pub enum LogConfig { //! File(PathBuf), //! Stdout, //! Stderr, //! } //! //! impl LogConfig { //! pub fn layer<S>(self) -> Box<dyn Layer<S> + Send + Sync + 'static> //! where //! S: tracing_core::Subscriber, //! for<'a> S: LookupSpan<'a>, //! { //! // Shared configuration regardless of where logs are output to. //! let fmt = tracing_subscriber::fmt::layer() //! .with_target(true) //! .with_thread_names(true); //! //! // Configure the writer based on the desired log target: //! match self { //! LogConfig::File(path) => { //! let file = File::create(path).expect("failed to create log file"); //! Box::new(fmt.with_writer(file)) //! }, //! LogConfig::Stdout => Box::new(fmt.with_writer(io::stdout)), //! LogConfig::Stderr => Box::new(fmt.with_writer(io::stderr)), //! } //! } //! } //! //! let config = LogConfig::Stdout; //! tracing_subscriber::registry() //! .with(config.layer()) //! .init(); //! ``` //! //! The [`Layer::boxed`] method is provided to make boxing a `Layer` //! more convenient, but [`Box::new`] may be used as well. //! //! When the number of `Layer`s varies at runtime, note that a //! [`Vec<L> where L: Layer` also implements `Layer`][vec-impl]. This //! can be used to add a variable number of `Layer`s to a `Subscriber`: //! //! ``` //! use tracing_subscriber::{Layer, prelude::*}; //! struct MyLayer { //! // ... //! } //! # impl MyLayer { fn new() -> Self { Self {} }} //! //! impl<S: tracing_core::Subscriber> Layer<S> for MyLayer { //! // ... //! } //! //! /// Returns how many layers we need //! fn how_many_layers() -> usize { //! // ... //! # 3 //! } //! //! // Create a variable-length `Vec` of layers //! let mut layers = Vec::new(); //! for _ in 0..how_many_layers() { //! layers.push(MyLayer::new()); //! } //! //! tracing_subscriber::registry() //! .with(layers) //! .init(); //! ``` //! //! If a variable number of `Layer` is needed and those `Layer`s have //! different types, a `Vec` of [boxed `Layer` trait objects][box-impl] may //! be used. For example: //! //! ``` //! use tracing_subscriber::{filter::LevelFilter, Layer, prelude::*}; //! use std::fs::File; //! # fn main() -> Result<(), Box<dyn std::error::Error>> { //! struct Config { //! enable_log_file: bool, //! enable_stdout: bool, //! enable_stderr: bool, //! // ... //! } //! # impl Config { //! # fn from_config_file()-> Result<Self, Box<dyn std::error::Error>> { //! # // don't enable the log file so that the example doesn't actually create it //! # Ok(Self { enable_log_file: false, enable_stdout: true, enable_stderr: true }) //! # } //! # } //! //! let cfg = Config::from_config_file()?; //! //! // Based on our dynamically loaded config file, create any number of layers: //! let mut layers = Vec::new(); //! //! if cfg.enable_log_file { //! let file = File::create("myapp.log")?; //! let layer = tracing_subscriber::fmt::layer() //! .with_thread_names(true) //! .with_target(true) //! .json() //! .with_writer(file) //! // Box the layer as a type-erased trait object, so that it can //! // be pushed to the `Vec`. //! .boxed(); //! layers.push(layer); //! } //! //! if cfg.enable_stdout { //! let layer = tracing_subscriber::fmt::layer() //! .pretty() //! .with_filter(LevelFilter::INFO) //! // Box the layer as a type-erased trait object, so that it can //! // be pushed to the `Vec`. //! .boxed(); //! layers.push(layer); //! } //! //! if cfg.enable_stdout { //! let layer = tracing_subscriber::fmt::layer() //! .with_target(false) //! .with_filter(LevelFilter::WARN) //! // Box the layer as a type-erased trait object, so that it can //! // be pushed to the `Vec`. //! .boxed(); //! layers.push(layer); //! } //! //! tracing_subscriber::registry() //! .with(layers) //! .init(); //!# Ok(()) } //! ``` //! //! Finally, if the number of layers _changes_ at runtime, a `Vec` of //! subscribers can be used alongside the [`reload`](crate::reload) module to //! add or remove subscribers dynamically at runtime. //! //! [option-impl]: Layer#impl-Layer<S>-for-Option<L> //! [box-impl]: Layer#impl-Layer%3CS%3E-for-Box%3Cdyn%20Layer%3CS%3E%20+%20Send%20+%20Sync%3E //! [vec-impl]: Layer#impl-Layer<S>-for-Vec<L> //! [prelude]: crate::prelude //! //! # Recording Traces //! //! The [`Layer`] trait defines a set of methods for consuming notifications from //! tracing instrumentation, which are generally equivalent to the similarly //! named methods on [`Subscriber`]. Unlike [`Subscriber`], the methods on //! `Layer` are additionally passed a [`Context`] type, which exposes additional //! information provided by the wrapped subscriber (such as [the current span]) //! to the layer. //! //! # Filtering with `Layer`s //! //! As well as strategies for handling trace events, the `Layer` trait may also //! be used to represent composable _filters_. This allows the determination of //! what spans and events should be recorded to be decoupled from _how_ they are //! recorded: a filtering layer can be applied to other layers or //! subscribers. `Layer`s can be used to implement _global filtering_, where a //! `Layer` provides a filtering strategy for the entire subscriber. //! Additionally, individual recording `Layer`s or sets of `Layer`s may be //! combined with _per-layer filters_ that control what spans and events are //! recorded by those layers. //! //! ## Global Filtering //! //! A `Layer` that implements a filtering strategy should override the //! [`register_callsite`] and/or [`enabled`] methods. It may also choose to implement //! methods such as [`on_enter`], if it wishes to filter trace events based on //! the current span context. //! //! Note that the [`Layer::register_callsite`] and [`Layer::enabled`] methods //! determine whether a span or event is enabled *globally*. Thus, they should //! **not** be used to indicate whether an individual layer wishes to record a //! particular span or event. Instead, if a layer is only interested in a subset //! of trace data, but does *not* wish to disable other spans and events for the //! rest of the layer stack should ignore those spans and events in its //! notification methods. //! //! The filtering methods on a stack of `Layer`s are evaluated in a top-down //! order, starting with the outermost `Layer` and ending with the wrapped //! [`Subscriber`]. If any layer returns `false` from its [`enabled`] method, or //! [`Interest::never()`] from its [`register_callsite`] method, filter //! evaluation will short-circuit and the span or event will be disabled. //! //! ### Enabling Interest //! //! Whenever an tracing event (or span) is emitted, it goes through a number of //! steps to determine how and how much it should be processed. The earlier an //! event is disabled, the less work has to be done to process the event, so //! `Layer`s that implement filtering should attempt to disable unwanted //! events as early as possible. In order, each event checks: //! //! - [`register_callsite`], once per callsite (roughly: once per time that //! `event!` or `span!` is written in the source code; this is cached at the //! callsite). See [`Subscriber::register_callsite`] and //! [`tracing_core::callsite`] for a summary of how this behaves. //! - [`enabled`], once per emitted event (roughly: once per time that `event!` //! or `span!` is *executed*), and only if `register_callsite` registers an //! [`Interest::sometimes`]. This is the main customization point to globally //! filter events based on their [`Metadata`]. If an event can be disabled //! based only on [`Metadata`], it should be, as this allows the construction //! of the actual `Event`/`Span` to be skipped. //! - For events only (and not spans), [`event_enabled`] is called just before //! processing the event. This gives layers one last chance to say that //! an event should be filtered out, now that the event's fields are known. //! //! ## Per-Layer Filtering //! //! **Note**: per-layer filtering APIs currently require the [`"registry"` crate //! feature flag][feat] to be enabled. //! //! Sometimes, it may be desirable for one `Layer` to record a particular subset //! of spans and events, while a different subset of spans and events are //! recorded by other `Layer`s. For example: //! //! - A layer that records metrics may wish to observe only events including //! particular tracked values, while a logging layer ignores those events. //! - If recording a distributed trace is expensive, it might be desirable to //! only send spans with `INFO` and lower verbosity to the distributed tracing //! system, while logging more verbose spans to a file. //! - Spans and events with a particular target might be recorded differently //! from others, such as by generating an HTTP access log from a span that //! tracks the lifetime of an HTTP request. //! //! The [`Filter`] trait is used to control what spans and events are //! observed by an individual `Layer`, while still allowing other `Layer`s to //! potentially record them. The [`Layer::with_filter`] method combines a //! `Layer` with a [`Filter`], returning a [`Filtered`] layer. //! //! This crate's [`filter`] module provides a number of types which implement //! the [`Filter`] trait, such as [`LevelFilter`], [`Targets`], and //! [`FilterFn`]. These [`Filter`]s provide ready-made implementations of //! common forms of filtering. For custom filtering policies, the [`FilterFn`] //! and [`DynFilterFn`] types allow implementing a [`Filter`] with a closure or //! function pointer. In addition, when more control is required, the [`Filter`] //! trait may also be implemented for user-defined types. //! //! [`Option<Filter>`] also implements [`Filter`], which allows for an optional //! filter. [`None`] filters out _nothing_ (that is, allows everything through). For //! example: //! //! ```rust //! # use tracing_subscriber::{filter::filter_fn, Layer}; //! # use tracing_core::{Metadata, subscriber::Subscriber}; //! # struct MyLayer<S>(std::marker::PhantomData<S>); //! # impl<S> MyLayer<S> { fn new() -> Self { Self(std::marker::PhantomData)} } //! # impl<S: Subscriber> Layer<S> for MyLayer<S> {} //! # fn my_filter(_: &str) -> impl Fn(&Metadata) -> bool { |_| true } //! fn setup_tracing<S: Subscriber>(filter_config: Option<&str>) { //! let layer = MyLayer::<S>::new() //! .with_filter(filter_config.map(|config| filter_fn(my_filter(config)))); //! //... //! } //! ``` //! //! <pre class="compile_fail" style="white-space:normal;font:inherit;"> //! <strong>Warning</strong>: Currently, the <a href="../struct.Registry.html"> //! <code>Registry</code></a> type defined in this crate is the only root //! <code>Subscriber</code> capable of supporting <code>Layer</code>s with //! per-layer filters. In the future, new APIs will be added to allow other //! root <code>Subscriber</code>s to support per-layer filters. //! </pre> //! //! For example, to generate an HTTP access log based on spans with //! the `http_access` target, while logging other spans and events to //! standard out, a [`Filter`] can be added to the access log layer: //! //! ``` //! use tracing_subscriber::{filter, prelude::*}; //! //! // Generates an HTTP access log. //! let access_log = // ... //! # filter::LevelFilter::INFO; //! //! // Add a filter to the access log layer so that it only observes //! // spans and events with the `http_access` target. //! let access_log = access_log.with_filter(filter::filter_fn(|metadata| { //! // Returns `true` if and only if the span or event's target is //! // "http_access". //! metadata.target() == "http_access" //! })); //! //! // A general-purpose logging layer. //! let fmt_layer = tracing_subscriber::fmt::layer(); //! //! // Build a subscriber that combines the access log and stdout log //! // layers. //! tracing_subscriber::registry() //! .with(fmt_layer) //! .with(access_log) //! .init(); //! ``` //! //! Multiple layers can have their own, separate per-layer filters. A span or //! event will be recorded if it is enabled by _any_ per-layer filter, but it //! will be skipped by the layers whose filters did not enable it. Building on //! the previous example: //! //! ``` //! use tracing_subscriber::{filter::{filter_fn, LevelFilter}, prelude::*}; //! //! let access_log = // ... //! # LevelFilter::INFO; //! let fmt_layer = tracing_subscriber::fmt::layer(); //! //! tracing_subscriber::registry() //! // Add the filter for the "http_access" target to the access //! // log layer, like before. //! .with(access_log.with_filter(filter_fn(|metadata| { //! metadata.target() == "http_access" //! }))) //! // Add a filter for spans and events with the INFO level //! // and below to the logging layer. //! .with(fmt_layer.with_filter(LevelFilter::INFO)) //! .init(); //! //! // Neither layer will observe this event //! tracing::debug!(does_anyone_care = false, "a tree fell in the forest"); //! //! // This event will be observed by the logging layer, but not //! // by the access log layer. //! tracing::warn!(dose_roentgen = %3.8, "not great, but not terrible"); //! //! // This event will be observed only by the access log layer. //! tracing::trace!(target: "http_access", "HTTP request started"); //! //! // Both layers will observe this event. //! tracing::error!(target: "http_access", "HTTP request failed with a very bad error!"); //! ``` //! //! A per-layer filter can be applied to multiple [`Layer`]s at a time, by //! combining them into a [`Layered`] layer using [`Layer::and_then`], and then //! calling [`Layer::with_filter`] on the resulting [`Layered`] layer. //! //! Consider the following: //! - `layer_a` and `layer_b`, which should only receive spans and events at //! the [`INFO`] [level] and above. //! - A third layer, `layer_c`, which should receive spans and events at //! the [`DEBUG`] [level] as well. //! //! The layers and filters would be composed thusly: //! //! ``` //! use tracing_subscriber::{filter::LevelFilter, prelude::*}; //! //! let layer_a = // ... //! # LevelFilter::INFO; //! let layer_b = // ... //! # LevelFilter::INFO; //! let layer_c = // ... //! # LevelFilter::INFO; //! //! let info_layers = layer_a //! // Combine `layer_a` and `layer_b` into a `Layered` layer: //! .and_then(layer_b) //! // ...and then add an `INFO` `LevelFilter` to that layer: //! .with_filter(LevelFilter::INFO); //! //! tracing_subscriber::registry() //! // Add `layer_c` with a `DEBUG` filter. //! .with(layer_c.with_filter(LevelFilter::DEBUG)) //! .with(info_layers) //! .init(); //!``` //! //! If a [`Filtered`] [`Layer`] is combined with another [`Layer`] //! [`Layer::and_then`], and a filter is added to the [`Layered`] layer, that //! layer will be filtered by *both* the inner filter and the outer filter. //! Only spans and events that are enabled by *both* filters will be //! observed by that layer. This can be used to implement complex filtering //! trees. //! //! As an example, consider the following constraints: //! - Suppose that a particular [target] is used to indicate events that //! should be counted as part of a metrics system, which should be only //! observed by a layer that collects metrics. //! - A log of high-priority events ([`INFO`] and above) should be logged //! to stdout, while more verbose events should be logged to a debugging log file. //! - Metrics-focused events should *not* be included in either log output. //! //! In that case, it is possible to apply a filter to both logging layers to //! exclude the metrics events, while additionally adding a [`LevelFilter`] //! to the stdout log: //! //! ``` //! # // wrap this in a function so we don't actually create `debug.log` when //! # // running the doctests.. //! # fn docs() -> Result<(), Box<dyn std::error::Error + 'static>> { //! use tracing_subscriber::{filter, prelude::*}; //! use std::{fs::File, sync::Arc}; //! //! // A layer that logs events to stdout using the human-readable "pretty" //! // format. //! let stdout_log = tracing_subscriber::fmt::layer() //! .pretty(); //! //! // A layer that logs events to a file. //! let file = File::create("debug.log")?; //! let debug_log = tracing_subscriber::fmt::layer() //! .with_writer(Arc::new(file)); //! //! // A layer that collects metrics using specific events. //! let metrics_layer = /* ... */ filter::LevelFilter::INFO; //! //! tracing_subscriber::registry() //! .with( //! stdout_log //! // Add an `INFO` filter to the stdout logging layer //! .with_filter(filter::LevelFilter::INFO) //! // Combine the filtered `stdout_log` layer with the //! // `debug_log` layer, producing a new `Layered` layer. //! .and_then(debug_log) //! // Add a filter to *both* layers that rejects spans and //! // events whose targets start with `metrics`. //! .with_filter(filter::filter_fn(|metadata| { //! !metadata.target().starts_with("metrics") //! })) //! ) //! .with( //! // Add a filter to the metrics label that *only* enables //! // events whose targets start with `metrics`. //! metrics_layer.with_filter(filter::filter_fn(|metadata| { //! metadata.target().starts_with("metrics") //! })) //! ) //! .init(); //! //! // This event will *only* be recorded by the metrics layer. //! tracing::info!(target: "metrics::cool_stuff_count", value = 42); //! //! // This event will only be seen by the debug log file layer: //! tracing::debug!("this is a message, and part of a system of messages"); //! //! // This event will be seen by both the stdout log layer *and* //! // the debug log file layer, but not by the metrics layer. //! tracing::warn!("the message is a warning about danger!"); //! # Ok(()) } //! ``` //! //! [`Subscriber`]: tracing_core::subscriber::Subscriber //! [span IDs]: tracing_core::span::Id //! [the current span]: Context::current_span //! [`register_callsite`]: Layer::register_callsite //! [`enabled`]: Layer::enabled //! [`event_enabled`]: Layer::event_enabled //! [`on_enter`]: Layer::on_enter //! [`Layer::register_callsite`]: Layer::register_callsite //! [`Layer::enabled`]: Layer::enabled //! [`Interest::never()`]: tracing_core::subscriber::Interest::never() //! [`Filtered`]: crate::filter::Filtered //! [`filter`]: crate::filter //! [`Targets`]: crate::filter::Targets //! [`FilterFn`]: crate::filter::FilterFn //! [`DynFilterFn`]: crate::filter::DynFilterFn //! [level]: tracing_core::Level //! [`INFO`]: tracing_core::Level::INFO //! [`DEBUG`]: tracing_core::Level::DEBUG //! [target]: tracing_core::Metadata::target //! [`LevelFilter`]: crate::filter::LevelFilter //! [feat]: crate#feature-flags usecrate::filter;
use tracing_core::{
metadata::Metadata,
span,
subscriber::{Interest, Subscriber},
Dispatch, Event, LevelFilter,
};
use core::any::TypeId;
feature! { #![feature = "alloc"] use alloc::boxed::Box; use core::ops::{Deref, DerefMut};
}
mod context; mod layered; pubuseself::{context::*, layered::*};
// The `tests` module is `pub(crate)` because it contains test utilities used by // other modules. #[cfg(test)] pub(crate) mod tests;
/// A composable handler for `tracing` events. /// /// A `Layer` implements a behavior for recording or collecting traces that can /// be composed together with other `Layer`s to build a [`Subscriber`]. See the /// [module-level documentation](crate::layer) for details. /// /// [`Subscriber`]: tracing_core::Subscriber #[cfg_attr(docsrs, doc(notable_trait))] pubtrait Layer<S> where
S: Subscriber, Self: 'static,
{ /// Performs late initialization when installing this layer as a /// [`Subscriber`]. /// /// ## Avoiding Memory Leaks /// /// `Layer`s should not store the [`Dispatch`] pointing to the [`Subscriber`] /// that they are a part of. Because the `Dispatch` owns the `Subscriber`, /// storing the `Dispatch` within the `Subscriber` will create a reference /// count cycle, preventing the `Dispatch` from ever being dropped. /// /// Instead, when it is necessary to store a cyclical reference to the /// `Dispatch` within a `Layer`, use [`Dispatch::downgrade`] to convert a /// `Dispatch` into a [`WeakDispatch`]. This type is analogous to /// [`std::sync::Weak`], and does not create a reference count cycle. A /// [`WeakDispatch`] can be stored within a subscriber without causing a /// memory leak, and can be [upgraded] into a `Dispatch` temporarily when /// the `Dispatch` must be accessed by the subscriber. /// /// [`WeakDispatch`]: tracing_core::dispatcher::WeakDispatch /// [upgraded]: tracing_core::dispatcher::WeakDispatch::upgrade /// [`Subscriber`]: tracing_core::Subscriber fn on_register_dispatch(&self, subscriber: &Dispatch) { let _ = subscriber;
}
/// Performs late initialization when attaching a `Layer` to a /// [`Subscriber`]. /// /// This is a callback that is called when the `Layer` is added to a /// [`Subscriber`] (e.g. in [`Layer::with_subscriber`] and /// [`SubscriberExt::with`]). Since this can only occur before the /// [`Subscriber`] has been set as the default, both the `Layer` and /// [`Subscriber`] are passed to this method _mutably_. This gives the /// `Layer` the opportunity to set any of its own fields with values /// received by method calls on the [`Subscriber`]. /// /// For example, [`Filtered`] layers implement `on_layer` to call the /// [`Subscriber`]'s [`register_filter`] method, and store the returned /// [`FilterId`] as a field. /// /// **Note** In most cases, `Layer` implementations will not need to /// implement this method. However, in cases where a type implementing /// `Layer` wraps one or more other types that implement `Layer`, like the /// [`Layered`] and [`Filtered`] types in this crate, that type MUST ensure /// that the inner `Layer`s' `on_layer` methods are called. Otherwise, /// functionality that relies on `on_layer`, such as [per-layer filtering], /// may not work correctly. /// /// [`Filtered`]: crate::filter::Filtered /// [`register_filter`]: crate::registry::LookupSpan::register_filter /// [per-layer filtering]: #per-layer-filtering /// [`FilterId`]: crate::filter::FilterId fn on_layer(&mutself, subscriber: &mut S) { let _ = subscriber;
}
/// Registers a new callsite with this layer, returning whether or not /// the layer is interested in being notified about the callsite, similarly /// to [`Subscriber::register_callsite`]. /// /// By default, this returns [`Interest::always()`] if [`self.enabled`] returns /// true, or [`Interest::never()`] if it returns false. /// /// <pre class="ignore" style="white-space:normal;font:inherit;"> /// <strong>Note</strong>: This method (and <a href="#method.enabled"> /// <code>Layer::enabled</code></a>) determine whether a span or event is /// globally enabled, <em>not</em> whether the individual layer will be /// notified about that span or event. This is intended to be used /// by layers that implement filtering for the entire stack. Layers which do /// not wish to be notified about certain spans or events but do not wish to /// globally disable them should ignore those spans or events in their /// <a href="#method.on_event"><code>on_event</code></a>, /// <a href="#method.on_enter"><code>on_enter</code></a>, /// <a href="#method.on_exit"><code>on_exit</code></a>, and other notification /// methods. /// </pre> /// /// See [the trait-level documentation] for more information on filtering /// with `Layer`s. /// /// Layers may also implement this method to perform any behaviour that /// should be run once per callsite. If the layer wishes to use /// `register_callsite` for per-callsite behaviour, but does not want to /// globally enable or disable those callsites, it should always return /// [`Interest::always()`]. /// /// [`Interest`]: tracing_core::Interest /// [`Subscriber::register_callsite`]: tracing_core::Subscriber::register_callsite() /// [`Interest::never()`]: tracing_core::subscriber::Interest::never() /// [`Interest::always()`]: tracing_core::subscriber::Interest::always() /// [`self.enabled`]: Layer::enabled() /// [`Layer::enabled`]: Layer::enabled() /// [`on_event`]: Layer::on_event() /// [`on_enter`]: Layer::on_enter() /// [`on_exit`]: Layer::on_exit() /// [the trait-level documentation]: #filtering-with-layers fn register_callsite(&self, metadata: &>'static Metadata<'static>) -> Interest { ifself.enabled(metadata, Context::none()) {
Interest::always()
} else {
Interest::never()
}
}
/// Returns `true` if this layer is interested in a span or event with the /// given `metadata` in the current [`Context`], similarly to /// [`Subscriber::enabled`]. /// /// By default, this always returns `true`, allowing the wrapped subscriber /// to choose to disable the span. /// /// <pre class="ignore" style="white-space:normal;font:inherit;"> /// <strong>Note</strong>: This method (and <a href="#method.register_callsite"> /// <code>Layer::register_callsite</code></a>) determine whether a span or event is /// globally enabled, <em>not</em> whether the individual layer will be /// notified about that span or event. This is intended to be used /// by layers that implement filtering for the entire stack. Layers which do /// not wish to be notified about certain spans or events but do not wish to /// globally disable them should ignore those spans or events in their /// <a href="#method.on_event"><code>on_event</code></a>, /// <a href="#method.on_enter"><code>on_enter</code></a>, /// <a href="#method.on_exit"><code>on_exit</code></a>, and other notification /// methods. /// </pre> /// /// /// See [the trait-level documentation] for more information on filtering /// with `Layer`s. /// /// [`Interest`]: tracing_core::Interest /// [`Subscriber::enabled`]: tracing_core::Subscriber::enabled() /// [`Layer::register_callsite`]: Layer::register_callsite() /// [`on_event`]: Layer::on_event() /// [`on_enter`]: Layer::on_enter() /// [`on_exit`]: Layer::on_exit() /// [the trait-level documentation]: #filtering-with-layers fn enabled(&self, metadata: &Metadata<'_>, ctx: Context<'_, S>) -> bool { let _ = (metadata, ctx); true
}
/// Notifies this layer that a new span was constructed with the given /// `Attributes` and `Id`. fn on_new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, ctx: Context<'_, S>) { let _ = (attrs, id, ctx);
}
// TODO(eliza): do we want this to be a public API? If we end up moving // filtering layers to a separate trait, we may no longer want `Layer`s to // be able to participate in max level hinting... #[doc(hidden)] fn max_level_hint(&self) -> Option<LevelFilter> {
None
}
/// Notifies this layer that a span with the given `Id` recorded the given /// `values`. // Note: it's unclear to me why we'd need the current span in `record` (the // only thing the `Context` type currently provides), but passing it in anyway // seems like a good future-proofing measure as it may grow other methods later... fn on_record(&self, _span: &span::Id, _values: &span::Record<'_>, _ctx: Context<'_, S>) {}
/// Notifies this layer that a span with the ID `span` recorded that it /// follows from the span with the ID `follows`. // Note: it's unclear to me why we'd need the current span in `record` (the // only thing the `Context` type currently provides), but passing it in anyway // seems like a good future-proofing measure as it may grow other methods later... fn on_follows_from(&self, _span: &span::Id, _follows: &span::Id, _ctx: Context<'_, S>) {}
/// Called before [`on_event`], to determine if `on_event` should be called. /// /// <div class="example-wrap" style="display:inline-block"> /// <pre class="ignore" style="white-space:normal;font:inherit;"> /// /// **Note**: This method determines whether an event is globally enabled, /// *not* whether the individual `Layer` will be notified about the /// event. This is intended to be used by `Layer`s that implement /// filtering for the entire stack. `Layer`s which do not wish to be /// notified about certain events but do not wish to globally disable them /// should ignore those events in their [on_event][Self::on_event]. /// /// </pre></div> /// /// See [the trait-level documentation] for more information on filtering /// with `Layer`s. /// /// [`on_event`]: Self::on_event /// [`Interest`]: tracing_core::Interest /// [the trait-level documentation]: #filtering-with-layers #[inline] // collapse this to a constant please mrs optimizer fn event_enabled(&self, _event: &Event<'_>, _ctx: Context<'_, S>) -> bool { true
}
/// Notifies this layer that an event has occurred. fn on_event(&self, _event: &Event<'_>, _ctx: Context<'_, S>) {}
/// Notifies this layer that a span with the given ID was entered. fn on_enter(&self, _id: &span::Id, _ctx: Context<'_, S>) {}
/// Notifies this layer that the span with the given ID was exited. fn on_exit(&self, _id: &span::Id, _ctx: Context<'_, S>) {}
/// Notifies this layer that the span with the given ID has been closed. fn on_close(&self, _id: span::Id, _ctx: Context<'_, S>) {}
/// Notifies this layer that a span ID has been cloned, and that the /// subscriber returned a different ID. fn on_id_change(&self, _old: &span::Id, _new: &span::Id, _ctx: Context<'_, S>) {}
/// Composes this `Layer` with the given [`Subscriber`], returning a /// `Layered` struct that implements [`Subscriber`]. /// /// The returned `Layered` subscriber will call the methods on this `Layer` /// and then those of the wrapped subscriber. /// /// For example: /// ```rust /// # use tracing_subscriber::layer::Layer; /// # use tracing_core::Subscriber; /// pub struct FooLayer { /// // ... /// } /// /// pub struct MySubscriber { /// // ... /// } /// /// impl<S: Subscriber> Layer<S> for FooLayer { /// // ... /// } /// /// # impl FooLayer { /// # fn new() -> Self { Self {} } /// # } /// # impl MySubscriber { /// # fn new() -> Self { Self { }} /// # } /// # use tracing_core::{span::{Id, Attributes, Record}, Metadata}; /// # impl tracing_core::Subscriber for MySubscriber { /// # fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) } /// # fn record(&self, _: &Id, _: &Record) {} /// # fn event(&self, _: &tracing_core::Event) {} /// # fn record_follows_from(&self, _: &Id, _: &Id) {} /// # fn enabled(&self, _: &Metadata) -> bool { false } /// # fn enter(&self, _: &Id) {} /// # fn exit(&self, _: &Id) {} /// # } /// let subscriber = FooLayer::new() /// .with_subscriber(MySubscriber::new()); ///``` /// /// [`Subscriber`]: tracing_core::Subscriber fn with_subscriber(mutself, mut inner: S) -> Layered<Self, S> where Self: Sized,
{ let inner_has_layer_filter = filter::subscriber_has_plf(&inner); self.on_layer(&mut inner);
Layered::new(self, inner, inner_has_layer_filter)
}
/// Combines `self` with a [`Filter`], returning a [`Filtered`] layer. /// /// The [`Filter`] will control which spans and events are enabled for /// this layer. See [the trait-level documentation][plf] for details on /// per-layer filtering. /// /// [`Filtered`]: crate::filter::Filtered /// [plf]: crate::layer#per-layer-filtering #[cfg(all(feature = "registry", feature = "std"))] #[cfg_attr(docsrs, doc(cfg(all(feature = "registry", feature = "std"))))] fn with_filter<F>(self, filter: F) -> filter::Filtered<Self, F, S> where Self: Sized,
F: Filter<S>,
{
filter::Filtered::new(self, filter)
}
/// Erases the type of this [`Layer`], returning a [`Box`]ed `dyn /// Layer` trait object. /// /// This can be used when a function returns a `Layer` which may be of /// one of several types, or when a `Layer` subscriber has a very long type /// signature. /// /// # Examples /// /// The following example will *not* compile, because the value assigned to /// `log_layer` may have one of several different types: /// /// ```compile_fail /// # fn main() -> Result<(), Box<dyn std::error::Error>> { /// use tracing_subscriber::{Layer, filter::LevelFilter, prelude::*}; /// use std::{path::PathBuf, fs::File, io}; /// /// /// Configures whether logs are emitted to a file, to stdout, or to stderr. /// pub enum LogConfig { /// File(PathBuf), /// Stdout, /// Stderr, /// } /// /// let config = // ... /// # LogConfig::Stdout; /// /// // Depending on the config, construct a layer of one of several types. /// let log_layer = match config { /// // If logging to a file, use a maximally-verbose configuration. /// LogConfig::File(path) => { /// let file = File::create(path)?; /// tracing_subscriber::fmt::layer() /// .with_thread_ids(true) /// .with_thread_names(true) /// // Selecting the JSON logging format changes the layer's /// // type. /// .json() /// .with_span_list(true) /// // Setting the writer to use our log file changes the /// // layer's type again. /// .with_writer(file) /// }, /// /// // If logging to stdout, use a pretty, human-readable configuration. /// LogConfig::Stdout => tracing_subscriber::fmt::layer() /// // Selecting the "pretty" logging format changes the /// // layer's type! /// .pretty() /// .with_writer(io::stdout) /// // Add a filter based on the RUST_LOG environment variable; /// // this changes the type too! /// .and_then(tracing_subscriber::EnvFilter::from_default_env()), /// /// // If logging to stdout, only log errors and warnings. /// LogConfig::Stderr => tracing_subscriber::fmt::layer() /// // Changing the writer changes the layer's type /// .with_writer(io::stderr) /// // Only log the `WARN` and `ERROR` levels. Adding a filter /// // changes the layer's type to `Filtered<LevelFilter, ...>`. /// .with_filter(LevelFilter::WARN), /// }; /// /// tracing_subscriber::registry() /// .with(log_layer) /// .init(); /// # Ok(()) } /// ``` /// /// However, adding a call to `.boxed()` after each match arm erases the /// layer's type, so this code *does* compile: /// /// ``` /// # fn main() -> Result<(), Box<dyn std::error::Error>> { /// # use tracing_subscriber::{Layer, filter::LevelFilter, prelude::*}; /// # use std::{path::PathBuf, fs::File, io}; /// # pub enum LogConfig { /// # File(PathBuf), /// # Stdout, /// # Stderr, /// # } /// # let config = LogConfig::Stdout; /// let log_layer = match config { /// LogConfig::File(path) => { /// let file = File::create(path)?; /// tracing_subscriber::fmt::layer() /// .with_thread_ids(true) /// .with_thread_names(true) /// .json() /// .with_span_list(true) /// .with_writer(file) /// // Erase the type by boxing the layer /// .boxed() /// }, /// /// LogConfig::Stdout => tracing_subscriber::fmt::layer() /// .pretty() /// .with_writer(io::stdout) /// .and_then(tracing_subscriber::EnvFilter::from_default_env()) /// // Erase the type by boxing the layer /// .boxed(), /// /// LogConfig::Stderr => tracing_subscriber::fmt::layer() /// .with_writer(io::stderr) /// .with_filter(LevelFilter::WARN) /// // Erase the type by boxing the layer /// .boxed(), /// }; /// /// tracing_subscriber::registry() /// .with(log_layer) /// .init(); /// # Ok(()) } /// ``` #[cfg(any(feature = "alloc", feature = "std"))] #[cfg_attr(docsrs, doc(cfg(any(feature = "alloc", feature = "std"))))] fn boxed(self) -> Box<dyn Layer<S> + Send + Sync + 'static> where Self: Sized, Self: Layer<S> + Send + Sync + 'static,
S: Subscriber,
{ Box::new(self)
}
#[doc(hidden)] unsafefn downcast_raw(&self, id: TypeId) -> Option<*const ()> { if id == TypeId::of::<Self>() {
Some(selfas *const _ as *const ())
} else {
None
}
}
}
/// A per-[`Layer`] filter that determines whether a span or event is enabled /// for an individual layer. /// /// See [the module-level documentation][plf] for details on using [`Filter`]s. /// /// [plf]: crate::layer#per-layer-filtering #[cfg_attr(docsrs, doc(notable_trait))] pubtrait Filter<S> { /// Returns `true` if this layer is interested in a span or event with the /// given [`Metadata`] in the current [`Context`], similarly to /// [`Subscriber::enabled`]. /// /// If this returns `false`, the span or event will be disabled _for the /// wrapped [`Layer`]_. Unlike [`Layer::enabled`], the span or event will /// still be recorded if any _other_ layers choose to enable it. However, /// the layer [filtered] by this filter will skip recording that span or /// event. /// /// If all layers indicate that they do not wish to see this span or event, /// it will be disabled. /// /// [`metadata`]: tracing_core::Metadata /// [`Subscriber::enabled`]: tracing_core::Subscriber::enabled /// [filtered]: crate::filter::Filtered fn enabled(&self, meta: &Metadata<'_>, cx: &Context<'_, S>) -> bool;
/// Returns an [`Interest`] indicating whether this layer will [always], /// [sometimes], or [never] be interested in the given [`Metadata`]. /// /// When a given callsite will [always] or [never] be enabled, the results /// of evaluating the filter may be cached for improved performance. /// Therefore, if a filter is capable of determining that it will always or /// never enable a particular callsite, providing an implementation of this /// function is recommended. /// /// <pre class="ignore" style="white-space:normal;font:inherit;"> /// <strong>Note</strong>: If a <code>Filter</code> will perform /// <em>dynamic filtering</em> that depends on the current context in which /// a span or event was observed (e.g. only enabling an event when it /// occurs within a particular span), it <strong>must</strong> return /// <code>Interest::sometimes()</code> from this method. If it returns /// <code>Interest::always()</code> or <code>Interest::never()</code>, the /// <code>enabled</code> method may not be called when a particular instance /// of that span or event is recorded. /// </pre> /// /// This method is broadly similar to [`Subscriber::register_callsite`]; /// however, since the returned value represents only the interest of /// *this* layer, the resulting behavior is somewhat different. /// /// If a [`Subscriber`] returns [`Interest::always()`][always] or /// [`Interest::never()`][never] for a given [`Metadata`], its [`enabled`] /// method is then *guaranteed* to never be called for that callsite. On the /// other hand, when a `Filter` returns [`Interest::always()`][always] or /// [`Interest::never()`][never] for a callsite, _other_ [`Layer`]s may have /// differing interests in that callsite. If this is the case, the callsite /// will receive [`Interest::sometimes()`][sometimes], and the [`enabled`] /// method will still be called for that callsite when it records a span or /// event. /// /// Returning [`Interest::always()`][always] or [`Interest::never()`][never] from /// `Filter::callsite_enabled` will permanently enable or disable a /// callsite (without requiring subsequent calls to [`enabled`]) if and only /// if the following is true: /// /// - all [`Layer`]s that comprise the subscriber include `Filter`s /// (this includes a tree of [`Layered`] layers that share the same /// `Filter`) /// - all those `Filter`s return the same [`Interest`]. /// /// For example, if a [`Subscriber`] consists of two [`Filtered`] layers, /// and both of those layers return [`Interest::never()`][never], that /// callsite *will* never be enabled, and the [`enabled`] methods of those /// [`Filter`]s will not be called. /// /// ## Default Implementation /// /// The default implementation of this method assumes that the /// `Filter`'s [`enabled`] method _may_ perform dynamic filtering, and /// returns [`Interest::sometimes()`][sometimes], to ensure that [`enabled`] /// is called to determine whether a particular _instance_ of the callsite /// is enabled in the current context. If this is *not* the case, and the /// `Filter`'s [`enabled`] method will always return the same result /// for a particular [`Metadata`], this method can be overridden as /// follows: /// /// ``` /// use tracing_subscriber::layer; /// use tracing_core::{Metadata, subscriber::Interest}; /// /// struct MyFilter { /// // ... /// } /// /// impl MyFilter { /// // The actual logic for determining whether a `Metadata` is enabled /// // must be factored out from the `enabled` method, so that it can be /// // called without a `Context` (which is not provided to the /// // `callsite_enabled` method). /// fn is_enabled(&self, metadata: &Metadata<'_>) -> bool { /// // ... /// # drop(metadata); true /// } /// } /// /// impl<S> layer::Filter<S> for MyFilter { /// fn enabled(&self, metadata: &Metadata<'_>, _: &layer::Context<'_, S>) -> bool { /// // Even though we are implementing `callsite_enabled`, we must still provide a /// // working implementation of `enabled`, as returning `Interest::always()` or /// // `Interest::never()` will *allow* caching, but will not *guarantee* it. /// // Other filters may still return `Interest::sometimes()`, so we may be /// // asked again in `enabled`. /// self.is_enabled(metadata) /// } /// /// fn callsite_enabled(&self, metadata: &'static Metadata<'static>) -> Interest { /// // The result of `self.enabled(metadata, ...)` will always be /// // the same for any given `Metadata`, so we can convert it into /// // an `Interest`: /// if self.is_enabled(metadata) { /// Interest::always() /// } else { /// Interest::never() /// } /// } /// } /// ``` /// /// [`Metadata`]: tracing_core::Metadata /// [`Interest`]: tracing_core::Interest /// [always]: tracing_core::Interest::always /// [sometimes]: tracing_core::Interest::sometimes /// [never]: tracing_core::Interest::never /// [`Subscriber::register_callsite`]: tracing_core::Subscriber::register_callsite /// [`Subscriber`]: tracing_core::Subscriber /// [`enabled`]: Filter::enabled /// [`Filtered`]: crate::filter::Filtered fn callsite_enabled(&self, meta: &'static Metadata<'static>) -> Interest { let _ = meta;
Interest::sometimes()
}
/// Called before the filtered [`Layer]'s [`on_event`], to determine if /// `on_event` should be called. /// /// This gives a chance to filter events based on their fields. Note, /// however, that this *does not* override [`enabled`], and is not even /// called if [`enabled`] returns `false`. /// /// ## Default Implementation /// /// By default, this method returns `true`, indicating that no events are /// filtered out based on their fields. /// /// [`enabled`]: crate::layer::Filter::enabled /// [`on_event`]: crate::layer::Layer::on_event #[inline] // collapse this to a constant please mrs optimizer fn event_enabled(&self, event: &Event<'_>, cx: &Context<'_, S>) -> bool { let _ = (event, cx); true
}
/// Returns an optional hint of the highest [verbosity level][level] that /// this `Filter` will enable. /// /// If this method returns a [`LevelFilter`], it will be used as a hint to /// determine the most verbose level that will be enabled. This will allow /// spans and events which are more verbose than that level to be skipped /// more efficiently. An implementation of this method is optional, but /// strongly encouraged. /// /// If the maximum level the `Filter` will enable can change over the /// course of its lifetime, it is free to return a different value from /// multiple invocations of this method. However, note that changes in the /// maximum level will **only** be reflected after the callsite [`Interest`] /// cache is rebuilt, by calling the /// [`tracing_core::callsite::rebuild_interest_cache`][rebuild] function. /// Therefore, if the `Filter will change the value returned by this /// method, it is responsible for ensuring that /// [`rebuild_interest_cache`][rebuild] is called after the value of the max /// level changes. /// /// ## Default Implementation /// /// By default, this method returns `None`, indicating that the maximum /// level is unknown. /// /// [level]: tracing_core::metadata::Level /// [`LevelFilter`]: crate::filter::LevelFilter /// [`Interest`]: tracing_core::subscriber::Interest /// [rebuild]: tracing_core::callsite::rebuild_interest_cache fn max_level_hint(&self) -> Option<LevelFilter> {
None
}
/// Notifies this filter that a new span was constructed with the given /// `Attributes` and `Id`. /// /// By default, this method does nothing. `Filter` implementations that /// need to be notified when new spans are created can override this /// method. fn on_new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, ctx: Context<'_, S>) { let _ = (attrs, id, ctx);
}
/// Notifies this filter that a span with the given `Id` recorded the given /// `values`. /// /// By default, this method does nothing. `Filter` implementations that /// need to be notified when new spans are created can override this /// method. fn on_record(&self, id: &span::Id, values: &span::Record<'_>, ctx: Context<'_, S>) { let _ = (id, values, ctx);
}
/// Notifies this filter that a span with the given ID was entered. /// /// By default, this method does nothing. `Filter` implementations that /// need to be notified when a span is entered can override this method. fn on_enter(&self, id: &span::Id, ctx: Context<'_, S>) { let _ = (id, ctx);
}
/// Notifies this filter that a span with the given ID was exited. /// /// By default, this method does nothing. `Filter` implementations that /// need to be notified when a span is exited can override this method. fn on_exit(&self, id: &span::Id, ctx: Context<'_, S>) { let _ = (id, ctx);
}
/// Notifies this filter that a span with the given ID has been closed. /// /// By default, this method does nothing. `Filter` implementations that /// need to be notified when a span is closed can override this method. fn on_close(&self, id: span::Id, ctx: Context<'_, S>) { let _ = (id, ctx);
}
}
}
/// Extension trait adding a `with(Layer)` combinator to `Subscriber`s. pubtrait SubscriberExt: Subscriber + crate::sealed::Sealed { /// Wraps `self` with the provided `layer`. fn with<L>(self, layer: L) -> Layered<L, Self> where
L: Layer<Self>, Self: Sized,
{
layer.with_subscriber(self)
}
}
/// A layer that does nothing. #[derive(Clone, Debug, Default)] pubstruct Identity {
_p: (),
}
/// Is a type implementing `Layer` `Option::<_>::None`? pub(crate) fn layer_is_none<L, S>(layer: &L) -> bool where
L: Layer<S>,
S: Subscriber,
{ unsafe { // Safety: we're not actually *doing* anything with this pointer --- // this only care about the `Option`, which is essentially being used // as a bool. We can rely on the pointer being valid, because it is // a crate-private type, and is only returned by the `Layer` impl // for `Option`s. However, even if the layer *does* decide to be // evil and give us an invalid pointer here, that's fine, because we'll // never actually dereference it.
layer.downcast_raw(TypeId::of::<NoneLayerMarker>())
}
.is_some()
}
/// Is a type implementing `Subscriber` `Option::<_>::None`? pub(crate) fn subscriber_is_none<S>(subscriber: &S) -> bool where
S: Subscriber,
{ unsafe { // Safety: we're not actually *doing* anything with this pointer --- // this only care about the `Option`, which is essentially being used // as a bool. We can rely on the pointer being valid, because it is // a crate-private type, and is only returned by the `Layer` impl // for `Option`s. However, even if the subscriber *does* decide to be // evil and give us an invalid pointer here, that's fine, because we'll // never actually dereference it.
subscriber.downcast_raw(TypeId::of::<NoneLayerMarker>())
}
.is_some()
}
#[inline] fn max_level_hint(&self) -> Option<LevelFilter> { matchself {
Some(ref inner) => inner.max_level_hint(),
None => { // There is no inner layer, so this layer will // never enable anything.
Some(LevelFilter::OFF)
}
}
}
fn on_new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, ctx: Context<'_, S>) { for l inself {
l.on_new_span(attrs, id, ctx.clone());
}
}
fn max_level_hint(&self) -> Option<LevelFilter> { // Default to `OFF` if there are no inner layers. letmut max_level = LevelFilter::OFF; for l inself { // NOTE(eliza): this is slightly subtle: if *any* layer // returns `None`, we have to return `None`, assuming there is // no max level hint, since that particular layer cannot // provide a hint. let hint = l.max_level_hint()?;
max_level = core::cmp::max(hint, max_level);
}
Some(max_level)
}
fn on_record(&self, span: &span::Id, values: &span::Record<'_>, ctx: Context<'_, S>) { for l inself {
l.on_record(span, values, ctx.clone())
}
}
fn on_follows_from(&self, span: &span::Id, follows: &span::Id, ctx: Context<'_, S>) { for l inself {
l.on_follows_from(span, follows, ctx.clone());
}
}
fn on_event(&self, event: &Event<'_>, ctx: Context<'_, S>) { for l inself {
l.on_event(event, ctx.clone());
}
}
fn on_enter(&self, id: &span::Id, ctx: Context<'_, S>) { for l inself {
l.on_enter(id, ctx.clone());
}
}
fn on_exit(&self, id: &span::Id, ctx: Context<'_, S>) { for l inself {
l.on_exit(id, ctx.clone());
}
}
fn on_close(&self, id: span::Id, ctx: Context<'_, S>) { for l inself {
l.on_close(id.clone(), ctx.clone());
}
}
#[doc(hidden)] unsafefn downcast_raw(&self, id: TypeId) -> Option<*const ()> { // If downcasting to `Self`, return a pointer to `self`. if id == TypeId::of::<Self>() { return Some(selfas *const _ as *const ());
}
// Someone is looking for per-layer filters. But, this `Vec` // might contain layers with per-layer filters *and* // layers without filters. It should only be treated as a // per-layer-filtered layer if *all* its layers have // per-layer filters. // XXX(eliza): it's a bummer we have to do this linear search every // time. It would be nice if this could be cached, but that would // require replacing the `Vec` impl with an impl for a newtype... if filter::is_plf_downcast_marker(id) && self.iter().any(|s| s.downcast_raw(id).is_none()) { return None;
}
// Otherwise, return the first child of `self` that downcaasts to // the selected type, if any. // XXX(eliza): hope this is reasonable lol self.iter().find_map(|l| l.downcast_raw(id))
}
}
}
// === impl SubscriberExt ===
impl<S: Subscriber> crate::sealed::Sealed for S {} impl<S: Subscriber> SubscriberExt for S {}
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