subscriber: add combinators for combining filters (#1578)

## Motivation

In some cases, it can be useful to express a more complex filtering
behavior by composing multiple filters. This is especially useful when
re-using the same logic in multiple filters.


## Solution

This branch adds a new `FilterExt` extension trait with combinators for
composing filters. The current set of combinators are `And` (enables
spans/events that are enabled by *both* filters), `Or` (enables any span
or event enabled by *either* filter), and `Not` (negate the output of a
filter).

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

tracing-subscriber/src/filter/layer_filters/combinator.rs

@@ -0,0 +1,380 @@
+//! Filter combinators
+use crate::layer::{Context, Filter};
+use std::{cmp, fmt, marker::PhantomData};
+use tracing_core::{subscriber::Interest, LevelFilter, Metadata};
+
+/// Combines two [`Filter`]s so that spans and events are enabled if and only if
+/// *both* filters return `true`.
+///
+/// This type is typically returned by the [`FilterExt::and`] method. See that
+/// method's documentation for details.
+///
+/// [`Filter`]: crate::layer::Filter
+/// [`FilterExt::and`]: crate::filter::FilterExt::and
+pub struct And<A, B, S> {
+    a: A,
+    b: B,
+    _s: PhantomData<fn(S)>,
+}
+
+/// Combines two [`Filter`]s so that spans and events are enabled if *either* filter
+/// returns `true`.
+///
+/// This type is typically returned by the [`FilterExt::or`] method. See that
+/// method's documentation for details.
+///
+/// [`Filter`]: crate::layer::Filter
+/// [`FilterExt::or`]: crate::filter::FilterExt::or
+pub struct Or<A, B, S> {
+    a: A,
+    b: B,
+    _s: PhantomData<fn(S)>,
+}
+
+/// Inverts the result of a [`Filter`].
+///
+/// If the wrapped filter would enable a span or event, it will be disabled. If
+/// it would disable a span or event, that span or event will be enabled.
+///
+/// This type is typically returned by the [`FilterExt::or`] method. See that
+/// method's documentation for details.
+///
+/// [`Filter`]: crate::layer::Filter
+/// [`FilterExt::or`]: crate::filter::FilterExt::or
+pub struct Not<A, S> {
+    a: A,
+    _s: PhantomData<fn(S)>,
+}
+
+// === impl And ===
+
+impl<A, B, S> And<A, B, S>
+where
+    A: Filter<S>,
+    B: Filter<S>,
+{
+    /// Combines two [`Filter`]s so that spans and events are enabled if and only if
+    /// *both* filters return `true`.
+    ///
+    /// # Examples
+    ///
+    /// Enabling spans or events if they have both a particular target *and* are
+    /// above a certain level:
+    ///
+    /// ```ignore
+    /// use tracing_subscriber::{
+    ///     filter::{filter_fn, LevelFilter, combinator::And},
+    ///     prelude::*,
+    /// };
+    ///
+    /// // Enables spans and events with targets starting with `interesting_target`:
+    /// let target_filter = filter_fn(|meta| {
+    ///     meta.target().starts_with("interesting_target")
+    /// });
+    ///
+    /// // Enables spans and events with levels `INFO` and below:
+    /// let level_filter = LevelFilter::INFO;
+    ///
+    /// // Combine the two filters together so that a span or event is only enabled
+    /// // if *both* filters would enable it:
+    /// let filter = And::new(level_filter, target_filter);
+    ///
+    /// tracing_subscriber::registry()
+    ///     .with(tracing_subscriber::fmt::layer().with_filter(filter))
+    ///     .init();
+    ///
+    /// // This event will *not* be enabled:
+    /// tracing::info!("an event with an uninteresting target");
+    ///
+    /// // This event *will* be enabled:
+    /// tracing::info!(target: "interesting_target", "a very interesting event");
+    ///
+    /// // This event will *not* be enabled:
+    /// tracing::debug!(target: "interesting_target", "interesting debug event...");
+    /// ```
+    ///
+    /// [`Filter`]: crate::layer::Filter
+    pub(crate) fn new(a: A, b: B) -> Self {
+        Self {
+            a,
+            b,
+            _s: PhantomData,
+        }
+    }
+}
+
+impl<A, B, S> Filter<S> for And<A, B, S>
+where
+    A: Filter<S>,
+    B: Filter<S>,
+{
+    #[inline]
+    fn enabled(&self, meta: &Metadata<'_>, cx: &Context<'_, S>) -> bool {
+        self.a.enabled(meta, cx) && self.b.enabled(meta, cx)
+    }
+
+    fn callsite_enabled(&self, meta: &'static Metadata<'static>) -> Interest {
+        let a = self.a.callsite_enabled(meta);
+        if a.is_never() {
+            return a;
+        }
+
+        let b = self.b.callsite_enabled(meta);
+
+        if !b.is_always() {
+            return b;
+        }
+
+        a
+    }
+
+    fn max_level_hint(&self) -> Option<LevelFilter> {
+        // If either hint is `None`, return `None`. Otherwise, return the most restrictive.
+        cmp::min(self.a.max_level_hint(), self.b.max_level_hint())
+    }
+}
+
+impl<A, B, S> Clone for And<A, B, S>
+where
+    A: Clone,
+    B: Clone,
+{
+    fn clone(&self) -> Self {
+        Self {
+            a: self.a.clone(),
+            b: self.b.clone(),
+            _s: PhantomData,
+        }
+    }
+}
+
+impl<A, B, S> fmt::Debug for And<A, B, S>
+where
+    A: fmt::Debug,
+    B: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("And")
+            .field("a", &self.a)
+            .field("b", &self.b)
+            .finish()
+    }
+}
+
+// === impl Or ===
+
+impl<A, B, S> Or<A, B, S>
+where
+    A: Filter<S>,
+    B: Filter<S>,
+{
+    /// Combines two [`Filter`]s so that spans and events are enabled if *either* filter
+    /// returns `true`.
+    ///
+    /// # Examples
+    ///
+    /// Enabling spans and events at the `INFO` level and above, and all spans
+    /// and events with a particular target:
+    ///
+    /// ```ignore
+    /// use tracing_subscriber::{
+    ///     filter::{filter_fn, LevelFilter, combinator::Or},
+    ///     prelude::*,
+    /// };
+    ///
+    /// // Enables spans and events with targets starting with `interesting_target`:
+    /// let target_filter = filter_fn(|meta| {
+    ///     meta.target().starts_with("interesting_target")
+    /// });
+    ///
+    /// // Enables spans and events with levels `INFO` and below:
+    /// let level_filter = LevelFilter::INFO;
+    ///
+    /// // Combine the two filters together so that a span or event is enabled
+    /// // if it is at INFO or lower, or if it has a target starting with
+    /// // `interesting_target`.
+    /// let filter = Or::new(level_filter, target_filter);
+    ///
+    /// tracing_subscriber::registry()
+    ///     .with(tracing_subscriber::fmt::layer().with_filter(filter))
+    ///     .init();
+    ///
+    /// // This event will *not* be enabled:
+    /// tracing::debug!("an uninteresting event");
+    ///
+    /// // This event *will* be enabled:
+    /// tracing::info!("an uninteresting INFO event");
+    ///
+    /// // This event *will* be enabled:
+    /// tracing::info!(target: "interesting_target", "a very interesting event");
+    ///
+    /// // This event *will* be enabled:
+    /// tracing::debug!(target: "interesting_target", "interesting debug event...");
+    /// ```
+    ///
+    /// Enabling a higher level for a particular target by using `Or` in
+    /// conjunction with the [`And`] combinator:
+    ///
+    /// ```ignore
+    /// use tracing_subscriber::{
+    ///     filter::{filter_fn, LevelFilter, combinator},
+    ///     prelude::*,
+    /// };
+    ///
+    /// // This filter will enable spans and events with targets beginning with
+    /// // `my_crate`:
+    /// let my_crate = filter_fn(|meta| {
+    ///     meta.target().starts_with("my_crate")
+    /// });
+    ///
+    /// // Combine the `my_crate` filter with a `LevelFilter` to produce a filter
+    /// // that will enable the `INFO` level and lower for spans and events with
+    /// // `my_crate` targets:
+    /// let filter = combinator::And::new(my_crate, LevelFilter::INFO);
+    ///
+    /// // If a span or event *doesn't* have a target beginning with
+    /// // `my_crate`, enable it if it has the `WARN` level or lower:
+    /// // let filter = combinator::Or::new(filter, LevelFilter::WARN);
+    ///
+    /// tracing_subscriber::registry()
+    ///     .with(tracing_subscriber::fmt::layer().with_filter(filter))
+    ///     .init();
+    /// ```
+    ///
+    /// [`Filter`]: crate::layer::Filter
+    pub(crate) fn new(a: A, b: B) -> Self {
+        Self {
+            a,
+            b,
+            _s: PhantomData,
+        }
+    }
+}
+
+impl<A, B, S> Filter<S> for Or<A, B, S>
+where
+    A: Filter<S>,
+    B: Filter<S>,
+{
+    #[inline]
+    fn enabled(&self, meta: &Metadata<'_>, cx: &Context<'_, S>) -> bool {
+        self.a.enabled(meta, cx) || self.b.enabled(meta, cx)
+    }
+
+    fn callsite_enabled(&self, meta: &'static Metadata<'static>) -> Interest {
+        let a = self.a.callsite_enabled(meta);
+        let b = self.b.callsite_enabled(meta);
+
+        // If either filter will always enable the span or event, return `always`.
+        if a.is_always() || b.is_always() {
+            return Interest::always();
+        }
+
+        // Okay, if either filter will sometimes enable the span or event,
+        // return `sometimes`.
+        if a.is_sometimes() || b.is_sometimes() {
+            return Interest::sometimes();
+        }
+
+        debug_assert!(
+            a.is_never() && b.is_never(),
+            "if neither filter was `always` or `sometimes`, both must be `never` (a={:?}; b={:?})",
+            a,
+            b,
+        );
+        Interest::never()
+    }
+
+    fn max_level_hint(&self) -> Option<LevelFilter> {
+        // If either hint is `None`, return `None`. Otherwise, return the less restrictive.
+        Some(cmp::max(self.a.max_level_hint()?, self.b.max_level_hint()?))
+    }
+}
+
+impl<A, B, S> Clone for Or<A, B, S>
+where
+    A: Clone,
+    B: Clone,
+{
+    fn clone(&self) -> Self {
+        Self {
+            a: self.a.clone(),
+            b: self.b.clone(),
+            _s: PhantomData,
+        }
+    }
+}
+
+impl<A, B, S> fmt::Debug for Or<A, B, S>
+where
+    A: fmt::Debug,
+    B: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Or")
+            .field("a", &self.a)
+            .field("b", &self.b)
+            .finish()
+    }
+}
+
+// === impl Not ===
+
+impl<A, S> Not<A, S>
+where
+    A: Filter<S>,
+{
+    /// Inverts the result of a [`Filter`].
+    ///
+    /// If the wrapped filter would enable a span or event, it will be disabled. If
+    /// it would disable a span or event, that span or event will be enabled.
+    ///
+    /// [`Filter`]: crate::layer::Filter
+    pub(crate) fn new(a: A) -> Self {
+        Self { a, _s: PhantomData }
+    }
+}
+
+impl<A, S> Filter<S> for Not<A, S>
+where
+    A: Filter<S>,
+{
+    #[inline]
+    fn enabled(&self, meta: &Metadata<'_>, cx: &Context<'_, S>) -> bool {
+        !self.a.enabled(meta, cx)
+    }
+
+    fn callsite_enabled(&self, meta: &'static Metadata<'static>) -> Interest {
+        match self.a.callsite_enabled(meta) {
+            i if i.is_always() => Interest::never(),
+            i if i.is_never() => Interest::always(),
+            _ => Interest::sometimes(),
+        }
+    }
+
+    fn max_level_hint(&self) -> Option<LevelFilter> {
+        // TODO(eliza): figure this out???
+        None
+    }
+}
+
+impl<A, S> Clone for Not<A, S>
+where
+    A: Clone,
+{
+    fn clone(&self) -> Self {
+        Self {
+            a: self.a.clone(),
+            _s: PhantomData,
+        }
+    }
+}
+
+impl<A, S> fmt::Debug for Not<A, S>
+where
+    A: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("Not").field(&self.a).finish()
+    }
+}