tracing: add enabled! macro (#1883)

Backports #1821 to `v0.1.x`.

## Motivation

Closes: #1668

My usecase is different than the referenced "avoid doing something
expensive to log": I want to guard turning on `debug` mode for an ffi'd
library, based on some `target` that represents the "module" we care
about.

## Solution

The macro is very similar to `event!`, but adds a few simplistic cases,
and generates ever so slightly different code (to return the correct
value always. It also skips anything to do with `tracing-log`. I
considered (and tried), to share the impl between `event!` and
`enabled!`, but must confess I am not good at macros and got stuck. I
think they are sufficiently different, where copied impls, is easier to
read. We already manage 

Co-authored-by: Eliza Weisman <[email protected]>
Co-authored-by: David Barsky <[email protected]>

tracing-core/src/metadata.rs

@@ -371,6 +371,7 @@ impl<'a> fmt::Debug for Metadata<'a> {
 enum KindInner {
     Event,
     Span,
+    Hint,
 }
 
 impl Kind {
@@ -380,6 +381,11 @@ impl Kind {
     /// `Span` callsite
     pub const SPAN: Kind = Kind(KindInner::Span);
 
+    /// `enabled!` callsite. [`Subscriber`][`crate::subscriber::Subscriber`]s can assume
+    /// this `Kind` means they will never recieve a
+    /// full event with this [`Metadata`].
+    pub const HINT: Kind = Kind(KindInner::Hint);
+
     /// Return true if the callsite kind is `Span`
     pub fn is_span(&self) -> bool {
         matches!(self, Kind(KindInner::Span))
@@ -389,6 +395,11 @@ impl Kind {
     pub fn is_event(&self) -> bool {
         matches!(self, Kind(KindInner::Event))
     }
+
+    /// Return true if the callsite kind is `Hint`
+    pub fn is_hint(&self) -> bool {
+        matches!(self, Kind(KindInner::Hint))
+    }
 }
 
 // ===== impl Level =====

tracing/src/macros.rs

@@ -789,6 +789,138 @@ macro_rules! event {
     );
 }
 
+/// Checks whether a span or event is [enabled] based on the provided [metadata].
+///
+/// [enabled]: crate::Subscriber::enabled
+/// [metadata]: crate::Metadata
+///
+/// This macro is a specialized tool: it is intended to be used prior
+/// to an expensive computation required *just* for that event, but
+/// *cannot* be done as part of an argument to that event, such as
+/// when multiple events are emitted (e.g., iterating over a collection
+/// and emitting an event for each item).
+///
+/// # Usage
+///
+/// [Subscribers] can make filtering decisions based all the data included in a
+/// span or event's [`Metadata`]. This means that it is possible for `enabled!`
+/// to return a _false positive_ (indicating that something would be enabled
+/// when it actually would not be) or a _false negative_ (indicating that
+/// something would be disabled when it would actually be enabled).
+///
+/// [Subscribers]: crate::subscriber::Subscriber
+/// [`Metadata`]: crate::metadata::Metadata
+///
+/// This occurs when a subscriber is using a _more specific_ filter than the
+/// metadata provided to the `enabled!` macro. Some situations that can result
+/// in false positives or false negatives include:
+///
+/// - If a subscriber is using a filter which may enable a span or event based
+/// on field names, but `enabled!` is invoked without listing field names,
+/// `enabled!` may return a false negative if a specific field name would
+/// cause the subscriber to enable something that would otherwise be disabled.
+/// - If a subscriber is using a filter which enables or disables specific events by
+/// file path and line number,  a particular event may be enabled/disabled
+/// even if an `enabled!` invocation with the same level, target, and fields
+/// indicated otherwise.
+/// - The subscriber can choose to enable _only_ spans or _only_ events, which `enabled`
+/// will not reflect.
+///
+/// `enabled!()` requires a [level](crate::Level) argument, an optional `target:`
+/// argument, and an optional set of field names. If the fields are not provided,
+/// they are considered to be unknown. `enabled!` attempts to match the
+/// syntax of `event!()` as closely as possible, which can be seen in the
+/// examples below.
+///
+/// # Examples
+///
+/// If the current subscriber is interested in recording `DEBUG`-level spans and
+/// events in the current file and module path, this will evaluate to true:
+/// ```rust
+/// use tracing::{enabled, Level};
+///
+/// if enabled!(Level::DEBUG) {
+///     // some expensive work...
+/// }
+/// ```
+///
+/// If the current subscriber is interested in recording spans and events
+/// in the current file and module path, with the target "my_crate", and at the
+/// level  `DEBUG`, this will evaluate to true:
+/// ```rust
+/// # use tracing::{enabled, Level};
+/// if enabled!(target: "my_crate", Level::DEBUG) {
+///     // some expensive work...
+/// }
+/// ```
+///
+/// If the current subscriber is interested in recording spans and events
+/// in the current file and module path, with the target "my_crate", at
+/// the level `DEBUG`, and with a field named "hello", this will evaluate
+/// to true:
+///
+/// ```rust
+/// # use tracing::{enabled, Level};
+/// if enabled!(target: "my_crate", Level::DEBUG, hello) {
+///     // some expensive work...
+/// }
+/// ```
+///
+#[macro_export]
+macro_rules! enabled {
+    (target: $target:expr, $lvl:expr, { $($fields:tt)* } )=> ({
+        if $crate::level_enabled!($lvl) {
+            use $crate::__macro_support::Callsite as _;
+            static CALLSITE: $crate::__macro_support::MacroCallsite = $crate::callsite2! {
+                name: concat!(
+                    "enabled ",
+                    file!(),
+                    ":",
+                    line!()
+                ),
+                kind: $crate::metadata::Kind::HINT,
+                target: $target,
+                level: $lvl,
+                fields: $($fields)*
+            };
+            let interest = CALLSITE.interest();
+            if !interest.is_never() && CALLSITE.is_enabled(interest)  {
+                let meta = CALLSITE.metadata();
+                $crate::dispatcher::get_default(|current| current.enabled(meta))
+            } else {
+                false
+            }
+        } else {
+            false
+        }
+    });
+    // Just target and level
+    (target: $target:expr, $lvl:expr ) => (
+        $crate::enabled!(target: $target, $lvl, { })
+    );
+
+    // These two cases handle fields with no values
+    (target: $target:expr, $lvl:expr, $($field:tt)*) => (
+        $crate::enabled!(
+            target: $target,
+            $lvl,
+            { $($field)*}
+        )
+    );
+    ($lvl:expr, $($field:tt)*) => (
+        $crate::enabled!(
+            target: module_path!(),
+            $lvl,
+            { $($field)*}
+        )
+    );
+
+    // Simplest `enabled!` case
+    ( $lvl:expr ) => (
+        $crate::enabled!(target: module_path!(), $lvl, { })
+    );
+}
+
 /// Constructs an event at the trace level.
 ///
 /// This functions similarly to the [`event!`] macro. See [the top-level

tracing/tests/enabled.rs

@@ -0,0 +1,26 @@
+mod support;
+
+use self::support::*;
+use tracing::Level;
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn level_and_target() {
+    let (subscriber, handle) = subscriber::mock()
+        .with_filter(|meta| {
+            if meta.target() == "debug_module" {
+                meta.level() <= &Level::DEBUG
+            } else {
+                meta.level() <= &Level::INFO
+            }
+        })
+        .done()
+        .run_with_handle();
+
+    tracing::subscriber::set_global_default(subscriber).unwrap();
+
+    assert!(tracing::enabled!(target: "debug_module", Level::DEBUG));
+    assert!(tracing::enabled!(Level::ERROR));
+    assert!(!tracing::enabled!(Level::DEBUG));
+    handle.assert_finished();
+}

tracing/tests/macros.rs

@@ -1,6 +1,6 @@
 #![deny(warnings)]
 use tracing::{
-    callsite, debug, debug_span, error, error_span, event, info, info_span, span, trace,
+    callsite, debug, debug_span, enabled, error, error_span, event, info, info_span, span, trace,
     trace_span, warn, warn_span, Level,
 };
 
@@ -334,6 +334,20 @@ fn event() {
     event!(Level::DEBUG, foo);
 }
 
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn enabled() {
+    enabled!(Level::DEBUG, foo, bar.baz, quux,);
+    enabled!(Level::DEBUG, message);
+    enabled!(Level::INFO, foo, bar.baz, quux, message,);
+    enabled!(Level::INFO, foo, bar., message,);
+    enabled!(Level::DEBUG, foo);
+
+    enabled!(Level::DEBUG);
+    enabled!(target: "rando", Level::DEBUG);
+    enabled!(target: "rando", Level::DEBUG, field);
+}
+
 #[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
 #[test]
 fn locals_with_message() {