subscriber: add Targets::would_enable (#1903

## Motivation

As discussed on discord, this API + `Targets` being `: Clone` makes it
easier to solve the original problem I had tried to solve in
#1889.

My plan on how to use this is in
MaterializeInc/materialize#10441 if you are
interested!

## Solution

I considered doing some macro magic to create a `Metadata` with a
callsite and empty fields and everything, to be able to called
`DirectiveSet::enabled`, but it felt cleaner and easier to reason about
the special-case-ness (`Targets` never having field filters) using a new
set of methods that do a similar thing.

For testing I opted for just a doc-test, let me know if thats fine!

tracing-subscriber/src/filter/directive.rs

@@ -4,7 +4,7 @@ use alloc::{
     vec::{self, Vec},
 };
 use core::{cmp::Ordering, fmt, iter::FromIterator, slice, str::FromStr};
-use tracing_core::Metadata;
+use tracing_core::{Level, Metadata};
 /// Indicates that a string could not be parsed as a filtering directive.
 #[derive(Debug)]
 pub struct ParseError {
@@ -141,6 +141,22 @@ impl DirectiveSet<StaticDirective> {
             None => false,
         }
     }
+
+    /// Same as `enabled` above, but skips `Directive`'s with fields.
+    pub(crate) fn target_enabled(&self, target: &str, level: &Level) -> bool {
+        match self.directives_for_target(target).next() {
+            Some(d) => d.level >= *level,
+            None => false,
+        }
+    }
+
+    pub(crate) fn directives_for_target<'a>(
+        &'a self,
+        target: &'a str,
+    ) -> impl Iterator<Item = &'a StaticDirective> + 'a {
+        self.directives()
+            .filter(move |d| d.cares_about_target(target))
+    }
 }
 
 // === impl StaticDirective ===
@@ -157,6 +173,22 @@ impl StaticDirective {
             level,
         }
     }
+
+    pub(in crate::filter) fn cares_about_target(&self, to_check: &str) -> bool {
+        // Does this directive have a target filter, and does it match the
+        // metadata's target?
+        if let Some(ref target) = self.target {
+            if !to_check.starts_with(&target[..]) {
+                return false;
+            }
+        }
+
+        if !self.field_names.is_empty() {
+            return false;
+        }
+
+        true
+    }
 }
 
 impl Ord for StaticDirective {

tracing-subscriber/src/filter/targets.rs

@@ -20,7 +20,7 @@ use core::{
     slice,
     str::FromStr,
 };
-use tracing_core::{Collect, Interest, Metadata};
+use tracing_core::{Collect, Interest, Level, Metadata};
 
 /// A filter that enables or disables spans and events based on their [target]
 /// and [level].
@@ -313,6 +313,35 @@ impl Targets {
             Interest::never()
         }
     }
+
+    /// Returns whether a [target]-[`Level`] pair would be enabled
+    /// by this `Targets`.
+    ///
+    /// This method can be used with [`module_path!`] from `std` as the target
+    /// in order to emulate the behavior of the [`tracing::event!`] and [`tracing::span!`]
+    /// macros.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tracing_subscriber::filter::{Targets, LevelFilter};
+    /// use tracing_core::Level;
+    ///
+    /// let filter = Targets::new()
+    ///     .with_target("my_crate", Level::INFO)
+    ///     .with_target("my_crate::interesting_module", Level::DEBUG);
+    ///
+    /// assert!(filter.would_enable("my_crate", &Level::INFO));
+    /// assert!(!filter.would_enable("my_crate::interesting_module", &Level::TRACE));
+    /// ```
+    ///
+    /// [target]: tracing_core::Metadata::target
+    /// [`module_path!`]: std::module_path!
+    pub fn would_enable(&self, target: &str, level: &Level) -> bool {
+        // "Correct" to call because `Targets` only produces `StaticDirective`'s with NO
+        // fields
+        self.0.target_enabled(target, level)
+    }
 }
 
 impl<T, L> Extend<(T, L)> for Targets