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subscriber: add Targets::would_enable (#1903
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## 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
https://github.com/MaterializeInc/materialize/issues/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!
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guswynn authored and hawkw committed Mar 24, 2022
1 parent eebbe40 commit a1a31ef
Showing 1 changed file with 30 additions and 1 deletion.
31 changes: 30 additions & 1 deletion tracing-subscriber/src/filter/targets.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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].
Expand Down Expand Up @@ -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
Expand Down

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