subscriber: add Layer impl for Vec<L: Layer> (#2027)

Depends on #2028

## Motivation

In many cases, it is desirable to have a variable number of `Layer`s
at runtime (such as when reading a logging configuration from a config
file). The current approach, where types implementing `Layer` are
composed at the type level into `Layered` layers, doesn't work well
when the number of layers varies at runtime.

## Solution

To solve this, this branch adds a `Layer` implementation for
`Vec<L> where L: Layer`. This allows variable-length lists of
layers to be added to a subscriber. Although the impl for `Vec<L>`
requires all the layers to be the same type, it can also be used in
 onjunction with `Box<dyn Layer<S> + ...>` trait objects to
implement a variable-length list of layers of multiple types.

I also wrote a bunch of docs examples.

## Notes

Alternatively, we could have a separate type defined in
`tracing-subscriber` for a variable-length list of type-erased
`Layer`s. This would have one primary usability benefit, which is
that we could have a `push` operation that takes an `impl Layer`
and boxes it automatically. However, I thought the approach used
here is nicer, since it's based on composing together existing
primitives such as `Vec` and `Box`, rather than adding a whole new API.
Additionally, it allows avoiding the additional `Box`ing in the case
where the list consists of layers that are all the same type.

Closes #1708

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-subscriber/src/layer/mod.rs

@@ -215,11 +215,11 @@
 //! ```
 //!
 //! If a [`Layer`] may be one of several different types, note that [`Box<dyn
-//! Layer<C> + Send + Sync>` implements `Layer`][box-impl].
+//! 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<C> + Send + Sync + 'static>`:
+//! several outputs might return a `Box<dyn Layer<S> + Send + Sync + 'static>`:
 //! ```
 //! use tracing_subscriber::{
 //!     Layer,
@@ -267,6 +267,107 @@
 //! 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
 //! [prelude]: crate::prelude
@@ -1060,6 +1161,7 @@ where
 
 feature! {
     #![all(feature = "registry", feature = "std")]
+
     /// A per-[`Layer`] filter that determines whether a span or event is enabled
     /// for an individual layer.
     ///
@@ -1406,6 +1508,8 @@ where
 
 feature! {
     #![any(feature = "std", feature = "alloc")]
+    #[cfg(not(feature = "std"))]
+    use alloc::vec::Vec;
 
     macro_rules! layer_impl_body {
         () => {
@@ -1491,6 +1595,120 @@ feature! {
     {
         layer_impl_body! {}
     }
+
+
+
+    impl<S, L> Layer<S> for Vec<L>
+    where
+        L: Layer<S>,
+        S: Subscriber,
+    {
+
+        fn on_layer(&mut self, subscriber: &mut S) {
+            for l in self {
+                l.on_layer(subscriber);
+            }
+        }
+
+        fn register_callsite(&self, metadata: &'static Metadata<'static>) -> Interest {
+            // Return highest level of interest.
+            let mut interest = Interest::never();
+            for l in self {
+                let new_interest = l.register_callsite(metadata);
+                if (interest.is_sometimes() && new_interest.is_always())
+                    || (interest.is_never() && !new_interest.is_never())
+                {
+                    interest = new_interest;
+                }
+            }
+
+            interest
+        }
+
+        fn enabled(&self, metadata: &Metadata<'_>, ctx: Context<'_, S>) -> bool {
+            self.iter().all(|l| l.enabled(metadata, ctx.clone()))
+        }
+
+        fn on_new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, ctx: Context<'_, S>) {
+            for l in self {
+                l.on_new_span(attrs, id, ctx.clone());
+            }
+        }
+
+        fn max_level_hint(&self) -> Option<LevelFilter> {
+            let mut max_level = LevelFilter::ERROR;
+            for l in self {
+                // 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 in self {
+                l.on_record(span, values, ctx.clone())
+            }
+        }
+
+        fn on_follows_from(&self, span: &span::Id, follows: &span::Id, ctx: Context<'_, S>) {
+            for l in self {
+                l.on_follows_from(span, follows, ctx.clone());
+            }
+        }
+
+        fn on_event(&self, event: &Event<'_>, ctx: Context<'_, S>) {
+            for l in self {
+                l.on_event(event, ctx.clone());
+            }
+        }
+
+        fn on_enter(&self, id: &span::Id, ctx: Context<'_, S>) {
+            for l in self {
+                l.on_enter(id, ctx.clone());
+            }
+        }
+
+        fn on_exit(&self, id: &span::Id, ctx: Context<'_, S>) {
+            for l in self {
+                l.on_exit(id, ctx.clone());
+            }
+        }
+
+        fn on_close(&self, id: span::Id, ctx: Context<'_, S>) {
+            for l in self {
+                l.on_close(id.clone(), ctx.clone());
+            }
+        }
+
+        #[doc(hidden)]
+        unsafe fn downcast_raw(&self, id: TypeId) -> Option<*const ()> {
+            // If downcasting to `Self`, return a pointer to `self`.
+            if id == TypeId::of::<Self>() {
+                return Some(self as *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 ===

tracing-subscriber/tests/layer_filters/main.rs

@@ -7,9 +7,10 @@ mod downcast_raw;
 mod filter_scopes;
 mod targets;
 mod trees;
+mod vec;
 
 use tracing::{level_filters::LevelFilter, Level};
-use tracing_subscriber::{filter, prelude::*};
+use tracing_subscriber::{filter, prelude::*, Layer};
 
 #[test]
 fn basic_layer_filters() {

tracing-subscriber/tests/layer_filters/vec.rs

@@ -0,0 +1,113 @@
+use super::*;
+use tracing::Subscriber;
+
+#[test]
+fn with_filters_unboxed() {
+    let (trace_layer, trace_handle) = layer::named("trace")
+        .event(event::mock().at_level(Level::TRACE))
+        .event(event::mock().at_level(Level::DEBUG))
+        .event(event::mock().at_level(Level::INFO))
+        .done()
+        .run_with_handle();
+    let trace_layer = trace_layer.with_filter(LevelFilter::TRACE);
+
+    let (debug_layer, debug_handle) = layer::named("debug")
+        .event(event::mock().at_level(Level::DEBUG))
+        .event(event::mock().at_level(Level::INFO))
+        .done()
+        .run_with_handle();
+    let debug_layer = debug_layer.with_filter(LevelFilter::DEBUG);
+
+    let (info_layer, info_handle) = layer::named("info")
+        .event(event::mock().at_level(Level::INFO))
+        .done()
+        .run_with_handle();
+    let info_layer = info_layer.with_filter(LevelFilter::INFO);
+
+    let _subscriber = tracing_subscriber::registry()
+        .with(vec![trace_layer, debug_layer, info_layer])
+        .set_default();
+
+    tracing::trace!("hello trace");
+    tracing::debug!("hello debug");
+    tracing::info!("hello info");
+
+    trace_handle.assert_finished();
+    debug_handle.assert_finished();
+    info_handle.assert_finished();
+}
+
+#[test]
+fn with_filters_boxed() {
+    let (unfiltered_layer, unfiltered_handle) = layer::named("unfiltered")
+        .event(event::mock().at_level(Level::TRACE))
+        .event(event::mock().at_level(Level::DEBUG))
+        .event(event::mock().at_level(Level::INFO))
+        .done()
+        .run_with_handle();
+    let unfiltered_layer = unfiltered_layer.boxed();
+
+    let (debug_layer, debug_handle) = layer::named("debug")
+        .event(event::mock().at_level(Level::DEBUG))
+        .event(event::mock().at_level(Level::INFO))
+        .done()
+        .run_with_handle();
+    let debug_layer = debug_layer.with_filter(LevelFilter::DEBUG).boxed();
+
+    let (target_layer, target_handle) = layer::named("target")
+        .event(event::mock().at_level(Level::INFO))
+        .done()
+        .run_with_handle();
+    let target_layer = target_layer
+        .with_filter(filter::filter_fn(|meta| meta.target() == "my_target"))
+        .boxed();
+
+    let _subscriber = tracing_subscriber::registry()
+        .with(vec![unfiltered_layer, debug_layer, target_layer])
+        .set_default();
+
+    tracing::trace!("hello trace");
+    tracing::debug!("hello debug");
+    tracing::info!(target: "my_target", "hello my target");
+
+    unfiltered_handle.assert_finished();
+    debug_handle.assert_finished();
+    target_handle.assert_finished();
+}
+
+#[test]
+fn mixed_max_level_hint() {
+    let unfiltered = layer::named("unfiltered").run().boxed();
+    let info = layer::named("info")
+        .run()
+        .with_filter(LevelFilter::INFO)
+        .boxed();
+    let debug = layer::named("debug")
+        .run()
+        .with_filter(LevelFilter::DEBUG)
+        .boxed();
+
+    let subscriber = tracing_subscriber::registry().with(vec![unfiltered, info, debug]);
+
+    assert_eq!(subscriber.max_level_hint(), None);
+}
+
+#[test]
+fn all_filtered_max_level_hint() {
+    let warn = layer::named("warn")
+        .run()
+        .with_filter(LevelFilter::WARN)
+        .boxed();
+    let info = layer::named("info")
+        .run()
+        .with_filter(LevelFilter::INFO)
+        .boxed();
+    let debug = layer::named("debug")
+        .run()
+        .with_filter(LevelFilter::DEBUG)
+        .boxed();
+
+    let subscriber = tracing_subscriber::registry().with(vec![warn, info, debug]);
+
+    assert_eq!(subscriber.max_level_hint(), Some(LevelFilter::DEBUG));
+}