//! 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 Layer 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 Layer for MyOtherLayer { //! // ... //! } //! //! pub struct MyThirdLayer { //! // ... //! } //! //! impl Layer for MyThirdLayer { //! // ... //! } //! # pub struct MyLayer {} //! # impl Layer 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. //! //! [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. //! //! ### 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. //! //! 
//!     Warning: Currently, the 
//!     Registry type defined in this crate is the only root
//!     Subscriber capable of supporting Layers with
//!     per-layer filters. In the future, new APIs will be added to allow other
//!     root Subscribers to support per-layer filters.
//!
//! //! 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> { //! 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(()) } //! ``` //! //! ## Runtime Configuration With Layers //! //! 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`]s 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> { //! # struct Config { //! # is_prod: bool, //! # path: &'static str, //! # } //! # let cfg = Config { is_prod: false, path: "debug.log" }; //! use std::{fs::File, sync::Arc}; //! 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)); //! subscriber.with(layer) //! } else { //! subscriber //! }; //! //! 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> { //! # struct Config { //! # is_prod: bool, //! # path: &'static str, //! # } //! # let cfg = Config { is_prod: false, path: "debug.log" }; //! use std::{fs::File, sync::Arc}; //! 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(Arc::new(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(()) } //! ``` //! //! [`Subscriber`]: https://docs.rs/tracing-core/latest/tracing_core/subscriber/trait.Subscriber.html //! [span IDs]: https://docs.rs/tracing-core/latest/tracing_core/span/struct.Id.html //! [the current span]: Context::current_span //! [`register_callsite`]: Layer::register_callsite //! [`enabled`]: Layer::enabled //! [`on_enter`]: Layer::on_enter //! [`Layer::register_callsite`]: Layer::register_callsite //! [`Layer::enabled`]: Layer::enabled //! [`Interest::never()`]: https://docs.rs/tracing-core/latest/tracing_core/subscriber/struct.Interest.html#method.never //! [option-impl]: crate::layer::Layer#impl-Layer-for-Option //! [`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 use crate::filter; use tracing_core::{ metadata::Metadata, span, subscriber::{Interest, Subscriber}, Event, LevelFilter, }; use core::any::TypeId; feature! { #![feature = "alloc"] use alloc::boxed::Box; use core::ops::{Deref, DerefMut}; } mod context; mod layered; pub use self::{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))] pub trait Layer where S: Subscriber, Self: 'static, { /// 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 /// recieved 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(&mut self, 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. /// /// 
    /// Note: This method (and 
    /// Layer::enabled) determine whether a span or event is
    /// globally enabled, not 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
    /// on_event,
    /// on_enter,
    /// on_exit, and other notification
    /// methods.
    ///
/// /// 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`]: https://docs.rs/tracing-core/latest/tracing_core/struct.Interest.html /// [`Subscriber::register_callsite`]: https://docs.rs/tracing-core/latest/tracing_core/trait.Subscriber.html#method.register_callsite /// [`Interest::never()`]: https://docs.rs/tracing-core/latest/tracing_core/subscriber/struct.Interest.html#method.never /// [`Interest::always()`]: https://docs.rs/tracing-core/latest/tracing_core/subscriber/struct.Interest.html#method.always /// [`self.enabled`]: #method.enabled /// [`Layer::enabled`]: #method.enabled /// [`on_event`]: #method.on_event /// [`on_enter`]: #method.on_enter /// [`on_exit`]: #method.on_exit /// [the trait-level documentation]: #filtering-with-layers fn register_callsite(&self, metadata: &'static Metadata<'static>) -> Interest { if self.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. /// /// 
    /// Note: This method (and 
    /// Layer::register_callsite) determine whether a span or event is
    /// globally enabled, not 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
    /// on_event,
    /// on_enter,
    /// on_exit, and other notification
    /// methods.
    ///
/// /// /// See [the trait-level documentation] for more information on filtering /// with `Layer`s. /// /// [`Interest`]: https://docs.rs/tracing-core/latest/tracing_core/struct.Interest.html /// [`Context`]: ../struct.Context.html /// [`Subscriber::enabled`]: https://docs.rs/tracing-core/latest/tracing_core/trait.Subscriber.html#method.enabled /// [`Layer::register_callsite`]: #method.register_callsite /// [`on_event`]: #method.on_event /// [`on_enter`]: #method.on_enter /// [`on_exit`]: #method.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 { 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>) {} /// 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 around the given `Layer`, returning a `Layered` /// struct implementing `Layer`. /// /// The returned `Layer` will call the methods on this `Layer` and then /// those of the new `Layer`, before calling the methods on the subscriber /// it wraps. For example: /// /// ```rust /// # use tracing_subscriber::layer::Layer; /// # use tracing_core::Subscriber; /// pub struct FooLayer { /// // ... /// } /// /// pub struct BarLayer { /// // ... /// } /// /// pub struct MySubscriber { /// // ... /// } /// /// impl Layer for FooLayer { /// // ... /// } /// /// impl Layer for BarLayer { /// // ... /// } /// /// # impl FooLayer { /// # fn new() -> Self { Self {} } /// # } /// # impl BarLayer { /// # fn new() -> Self { Self { }} /// # } /// # impl MySubscriber { /// # fn new() -> Self { Self { }} /// # } /// # use tracing_core::{span::{Id, Attributes, Record}, Metadata, Event}; /// # impl tracing_core::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) {} /// # } /// let subscriber = FooLayer::new() /// .and_then(BarLayer::new()) /// .with_subscriber(MySubscriber::new()); /// ``` /// /// Multiple layers may be composed in this manner: /// /// ```rust /// # use tracing_subscriber::layer::Layer; /// # use tracing_core::Subscriber; /// # pub struct FooLayer {} /// # pub struct BarLayer {} /// # pub struct MySubscriber {} /// # impl Layer for FooLayer {} /// # impl Layer for BarLayer {} /// # impl FooLayer { /// # fn new() -> Self { Self {} } /// # } /// # impl BarLayer { /// # fn new() -> Self { Self { }} /// # } /// # impl MySubscriber { /// # fn new() -> Self { Self { }} /// # } /// # use tracing_core::{span::{Id, Attributes, Record}, Metadata, Event}; /// # impl tracing_core::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) {} /// # } /// pub struct BazLayer { /// // ... /// } /// /// impl Layer for BazLayer { /// // ... /// } /// # impl BazLayer { fn new() -> Self { BazLayer {} } } /// /// let subscriber = FooLayer::new() /// .and_then(BarLayer::new()) /// .and_then(BazLayer::new()) /// .with_subscriber(MySubscriber::new()); /// ``` fn and_then(self, layer: L) -> Layered where L: Layer, Self: Sized, { let inner_has_layer_filter = filter::layer_has_plf(&self); Layered::new(layer, self, inner_has_layer_filter) } /// 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 Layer 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`]: https://docs.rs/tracing-core/latest/tracing_core/trait.Subscriber.html fn with_subscriber(mut self, mut inner: S) -> Layered 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]: #per-layer-filtering #[cfg(all(feature = "registry", feature = "std"))] #[cfg_attr(docsrs, doc(cfg(all(feature = "registry", feature = "std"))))] fn with_filter(self, filter: F) -> filter::Filtered where Self: Sized, F: Filter, { filter::Filtered::new(self, filter) } #[doc(hidden)] unsafe fn downcast_raw(&self, id: TypeId) -> Option<*const ()> { if id == TypeId::of::() { Some(self as *const _ as *const ()) } else { None } } } feature! { #![all(feature = "registry", feature = "std")] /// 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))] pub trait Filter { /// 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. /// /// 
        /// Note: If a Filter will perform
        /// dynamic filtering that depends on the current context in which
        /// a span or event was observered (e.g. only enabling an event when it
        /// occurs within a particular span), it must return
        /// Interest::sometimes() from this method. If it returns
        /// Interest::always() or Interest::never(), the
        /// enabled method may not be called when a particular instance
        /// of that span or event is recorded.
        ///
/// /// 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 recieve [`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 layer::Filter 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() } /// 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 { None } } } /// Extension trait adding a `with(Layer)` combinator to `Subscriber`s. pub trait SubscriberExt: Subscriber + crate::sealed::Sealed { /// Wraps `self` with the provided `layer`. fn with(self, layer: L) -> Layered where L: Layer, Self: Sized, { layer.with_subscriber(self) } } /// A layer that does nothing. #[derive(Clone, Debug, Default)] pub struct Identity { _p: (), } // === impl Layer === impl Layer for Option where L: Layer, S: Subscriber, { fn on_layer(&mut self, subscriber: &mut S) { if let Some(ref mut layer) = self { layer.on_layer(subscriber) } } #[inline] fn on_new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, ctx: Context<'_, S>) { if let Some(ref inner) = self { inner.on_new_span(attrs, id, ctx) } } #[inline] fn register_callsite(&self, metadata: &'static Metadata<'static>) -> Interest { match self { Some(ref inner) => inner.register_callsite(metadata), None => Interest::always(), } } #[inline] fn enabled(&self, metadata: &Metadata<'_>, ctx: Context<'_, S>) -> bool { match self { Some(ref inner) => inner.enabled(metadata, ctx), None => true, } } #[inline] fn max_level_hint(&self) -> Option { match self { Some(ref inner) => inner.max_level_hint(), None => None, } } #[inline] fn on_record(&self, span: &span::Id, values: &span::Record<'_>, ctx: Context<'_, S>) { if let Some(ref inner) = self { inner.on_record(span, values, ctx); } } #[inline] fn on_follows_from(&self, span: &span::Id, follows: &span::Id, ctx: Context<'_, S>) { if let Some(ref inner) = self { inner.on_follows_from(span, follows, ctx); } } #[inline] fn on_event(&self, event: &Event<'_>, ctx: Context<'_, S>) { if let Some(ref inner) = self { inner.on_event(event, ctx); } } #[inline] fn on_enter(&self, id: &span::Id, ctx: Context<'_, S>) { if let Some(ref inner) = self { inner.on_enter(id, ctx); } } #[inline] fn on_exit(&self, id: &span::Id, ctx: Context<'_, S>) { if let Some(ref inner) = self { inner.on_exit(id, ctx); } } #[inline] fn on_close(&self, id: span::Id, ctx: Context<'_, S>) { if let Some(ref inner) = self { inner.on_close(id, ctx); } } #[inline] fn on_id_change(&self, old: &span::Id, new: &span::Id, ctx: Context<'_, S>) { if let Some(ref inner) = self { inner.on_id_change(old, new, ctx) } } #[doc(hidden)] #[inline] unsafe fn downcast_raw(&self, id: TypeId) -> Option<*const ()> { if id == TypeId::of::() { Some(self as *const _ as *const ()) } else { self.as_ref().and_then(|inner| inner.downcast_raw(id)) } } } feature! { #![any(feature = "std", feature = "alloc")] macro_rules! layer_impl_body { () => { #[inline] fn on_layer(&mut self, subscriber: &mut S) { self.deref_mut().on_layer(subscriber); } #[inline] fn on_new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, ctx: Context<'_, S>) { self.deref().on_new_span(attrs, id, ctx) } #[inline] fn register_callsite(&self, metadata: &'static Metadata<'static>) -> Interest { self.deref().register_callsite(metadata) } #[inline] fn enabled(&self, metadata: &Metadata<'_>, ctx: Context<'_, S>) -> bool { self.deref().enabled(metadata, ctx) } #[inline] fn max_level_hint(&self) -> Option { self.deref().max_level_hint() } #[inline] fn on_record(&self, span: &span::Id, values: &span::Record<'_>, ctx: Context<'_, S>) { self.deref().on_record(span, values, ctx) } #[inline] fn on_follows_from(&self, span: &span::Id, follows: &span::Id, ctx: Context<'_, S>) { self.deref().on_follows_from(span, follows, ctx) } #[inline] fn on_event(&self, event: &Event<'_>, ctx: Context<'_, S>) { self.deref().on_event(event, ctx) } #[inline] fn on_enter(&self, id: &span::Id, ctx: Context<'_, S>) { self.deref().on_enter(id, ctx) } #[inline] fn on_exit(&self, id: &span::Id, ctx: Context<'_, S>) { self.deref().on_exit(id, ctx) } #[inline] fn on_close(&self, id: span::Id, ctx: Context<'_, S>) { self.deref().on_close(id, ctx) } #[inline] fn on_id_change(&self, old: &span::Id, new: &span::Id, ctx: Context<'_, S>) { self.deref().on_id_change(old, new, ctx) } #[doc(hidden)] #[inline] unsafe fn downcast_raw(&self, id: TypeId) -> Option<*const ()> { self.deref().downcast_raw(id) } }; } impl Layer for Box where L: Layer, S: Subscriber, { layer_impl_body! {} } impl Layer for Box + Send + Sync> where S: Subscriber, { layer_impl_body! {} } } // === impl SubscriberExt === impl crate::sealed::Sealed for S {} impl SubscriberExt for S {} // === impl Identity === impl Layer for Identity {} impl Identity { /// Returns a new `Identity` layer. pub fn new() -> Self { Self { _p: () } } }