subscriber: add Targets::targets method to iterate directives (#1574)

There's currently no way to get the registered directives from a
`Targets` instance, which is a barrier to some use-cases such as
combining user-supplied directives with application-supplied defaults.

This commit adds a `Targets::targets` method, which returns an iterator
of `(&str, LevelFilter)` pairs, one for each directive (excluding the
default level, whose `target` is `None`).

Items are yielded as per the underlying `DirectiveSet::directives`,
which would yield itms in specifity order (as constructed by
`DirectiveSet::add`). However, the order has been left explicitly
undefined in the documentation for `Targets::targets` on the basis that
this is an implementation detail that may change.

## Motivation

As hinted in the commit message, my use-case is to have an
application-supplied 'base' `Targets` filter, which can then be
extended/overridden by a user-supplied filter parsed from `RUST_LOG`
(e.g.).

Sadly, there's currently no way of getting the directives out of a
`Targets` instance, nor to re-serialize to the string representation.
Thus, the only way to implement the above would be to manually parse the
user-supplied filters in the application, which is shame since it
duplicates the implementation here and would be prone to drift/subtle
incompatibilities.

## Solution

The proposed solution is to add methods to `Targets` to retrieve the
underlying directives.

```rust
// application-defined defaults, for a useful level of information
let mut filter = Targets::new()
    .with_target("my_crate", LevelFilter::INFO)
    .with_target("important_dependency", LevelFilter::INFO);

if let Ok(overrides) = std::env::var("RUST_LOG") {
    let overrides: Targets = overrides.parse().unwrap();

    // merge user-supplied overrides, which can enable additional tracing
    // or disable default tracing
    filter.extend(overrides.iter());
    //                      ^^^^^^^^^ this is new
}
```

For this PR, I've gone for `fn iter(&self) -> Iter`, e.g. a method
`targets` that returns an iterator over the pairs of `(&str,
LevelFilter)` in the underlying directives. The iterator excludes any
default level(s) where `target` would be `None`. This matches nicely
with the `FromIterator` and `Extend` impls, which use `(impl
Into<String>, impl Into<LevelFilter>)`.

In addition, I've added `IntoIterator` implementations for `&Targets`
and for `Targets`. The by-value `IntoIterator` implementation returns an
`IntoIter` type that moves the targets as `String`s, rather than as
borrowed `&str`s. This makes `extend` more efficient when moving a
second `Targets` by value.

tracing-subscriber/src/filter/directive.rs

@@ -112,6 +112,19 @@ impl<T: Match + Ord> Extend<T> for DirectiveSet<T> {
     }
 }
 
+impl<T> IntoIterator for DirectiveSet<T> {
+    type Item = T;
+
+    #[cfg(feature = "smallvec")]
+    type IntoIter = smallvec::IntoIter<[T; 8]>;
+    #[cfg(not(feature = "smallvec"))]
+    type IntoIter = std::vec::IntoIter<T>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.directives.into_iter()
+    }
+}
+
 // === impl Statics ===
 
 impl DirectiveSet<StaticDirective> {

tracing-subscriber/src/filter/mod.rs

@@ -15,7 +15,7 @@ mod filter_fn;
 mod level;
 #[cfg(feature = "registry")]
 mod subscriber_filters;
-mod targets;
+pub mod targets;
 
 pub use self::directive::ParseError;
 pub use self::filter_fn::*;

tracing-subscriber/src/filter/targets.rs

@@ -1,3 +1,11 @@
+//! A [filter] that enables or disables spans and events based on their [target] and [level].
+//!
+//! See [`Targets`] for details.
+//!
+//! [target]: tracing_core::Metadata::target
+//! [level]: tracing_core::Level
+//! [filter]: crate::layer#filtering-with-layers
+
 use crate::{
     filter::{
         directive::{DirectiveSet, ParseError, StaticDirective},
@@ -6,7 +14,7 @@ use crate::{
     subscribe,
 };
 use std::{
-    iter::{Extend, FromIterator},
+    iter::{Extend, FilterMap, FromIterator},
     str::FromStr,
 };
 use tracing_core::{Collect, Interest, Metadata};
@@ -266,6 +274,34 @@ impl Targets {
         self
     }
 
+    /// Returns an iterator over the [target]-[`LevelFilter`] pairs in this filter.
+    ///
+    /// The order of iteration is undefined.
+    ///
+    /// # 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);
+    ///
+    /// let mut targets: Vec<_> = filter.iter().collect();
+    /// targets.sort();
+    ///
+    /// assert_eq!(targets, vec![
+    ///     ("my_crate", LevelFilter::INFO),
+    ///     ("my_crate::interesting_module", LevelFilter::DEBUG),
+    /// ]);
+    /// ```
+    ///
+    /// [target]: tracing_core::Metadata::target
+    pub fn iter(&self) -> Iter<'_> {
+        self.into_iter()
+    }
+
     #[inline]
     fn interested(&self, metadata: &'static Metadata<'static>) -> Interest {
         if self.0.enabled(metadata) {
@@ -344,20 +380,129 @@ impl<C> subscribe::Filter<C> for Targets {
     }
 }
 
+impl IntoIterator for Targets {
+    type Item = (String, LevelFilter);
+
+    type IntoIter = IntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        IntoIter::new(self)
+    }
+}
+
+impl<'a> IntoIterator for &'a Targets {
+    type Item = (&'a str, LevelFilter);
+
+    type IntoIter = Iter<'a>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        Iter::new(self)
+    }
+}
+
+/// An owning iterator over the [target]-[level] pairs of a `Targets` filter.
+///
+/// This struct is created by the `IntoIterator` trait implementation of [`Targets`].
+///
+/// # Examples
+///
+/// Merge the targets from one `Targets` with another:
+///
+/// ```
+/// use tracing_subscriber::filter::Targets;
+/// use tracing_core::Level;
+///
+/// let mut filter = Targets::new().with_target("my_crate", Level::INFO);
+/// let overrides = Targets::new().with_target("my_crate::interesting_module", Level::DEBUG);
+///
+/// filter.extend(overrides);
+/// # drop(filter);
+/// ```
+///
+/// [target]: tracing_core::Metadata::target
+/// [level]: tracing_core::Level
+#[derive(Debug)]
+pub struct IntoIter(
+    #[allow(clippy::type_complexity)] // alias indirection would probably make this more confusing
+    FilterMap<
+        <DirectiveSet<StaticDirective> as IntoIterator>::IntoIter,
+        fn(StaticDirective) -> Option<(String, LevelFilter)>,
+    >,
+);
+
+impl IntoIter {
+    fn new(targets: Targets) -> Self {
+        Self(targets.0.into_iter().filter_map(|directive| {
+            let level = directive.level;
+            directive.target.map(|target| (target, level))
+        }))
+    }
+}
+
+impl Iterator for IntoIter {
+    type Item = (String, LevelFilter);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.next()
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
+}
+
+/// A borrowing iterator over the [target]-[level] pairs of a `Targets` filter.
+///
+/// This struct is created by [`iter`] method of [`Targets`], or from the `IntoIterator`
+/// implementation for `&Targets`.
+///
+/// [target]: tracing_core::Metadata::target
+/// [level]: tracing_core::Level
+/// [`iter`]: Targets::iter
+#[derive(Debug)]
+pub struct Iter<'a>(
+    FilterMap<
+        std::slice::Iter<'a, StaticDirective>,
+        fn(&'a StaticDirective) -> Option<(&'a str, LevelFilter)>,
+    >,
+);
+
+impl<'a> Iter<'a> {
+    fn new(targets: &'a Targets) -> Self {
+        Self(targets.0.iter().filter_map(|directive| {
+            directive
+                .target
+                .as_deref()
+                .map(|target| (target, directive.level))
+        }))
+    }
+}
+
+impl<'a> Iterator for Iter<'a> {
+    type Item = (&'a str, LevelFilter);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.next()
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::filter::directive::FilterVec;
 
-    fn expect_parse(s: &str) -> FilterVec<StaticDirective> {
+    fn expect_parse(s: &str) -> Targets {
         match dbg!(s).parse::<Targets>() {
             Err(e) => panic!("string {:?} did not parse successfully: {}", s, e),
-            Ok(e) => e.0.into_vec(),
+            Ok(e) => e,
         }
     }
 
     fn expect_parse_ralith(s: &str) {
-        let dirs = expect_parse(s);
+        let dirs = expect_parse(s).0.into_vec();
         assert_eq!(dirs.len(), 2, "\nparsed: {:#?}", dirs);
         assert_eq!(dirs[0].target, Some("server".to_string()));
         assert_eq!(dirs[0].level, LevelFilter::DEBUG);
@@ -369,7 +514,7 @@ mod tests {
     }
 
     fn expect_parse_level_directives(s: &str) {
-        let dirs = expect_parse(s);
+        let dirs = expect_parse(s).0.into_vec();
         assert_eq!(dirs.len(), 6, "\nparsed: {:#?}", dirs);
 
         assert_eq!(dirs[0].target, Some("crate3::mod2::mod1".to_string()));
@@ -414,7 +559,9 @@ mod tests {
 
     #[test]
     fn expect_parse_valid() {
-        let dirs = expect_parse("crate1::mod1=error,crate1::mod2,crate2=debug,crate3=off");
+        let dirs = expect_parse("crate1::mod1=error,crate1::mod2,crate2=debug,crate3=off")
+            .0
+            .into_vec();
         assert_eq!(dirs.len(), 4, "\nparsed: {:#?}", dirs);
         assert_eq!(dirs[0].target, Some("crate1::mod2".to_string()));
         assert_eq!(dirs[0].level, LevelFilter::TRACE);
@@ -457,6 +604,44 @@ mod tests {
         )
     }
 
+    #[test]
+    fn targets_iter() {
+        let filter = expect_parse("crate1::mod1=error,crate1::mod2,crate2=debug,crate3=off")
+            .with_default(LevelFilter::WARN);
+
+        let mut targets: Vec<_> = filter.iter().collect();
+        targets.sort();
+
+        assert_eq!(
+            targets,
+            vec![
+                ("crate1::mod1", LevelFilter::ERROR),
+                ("crate1::mod2", LevelFilter::TRACE),
+                ("crate2", LevelFilter::DEBUG),
+                ("crate3", LevelFilter::OFF),
+            ]
+        );
+    }
+
+    #[test]
+    fn targets_into_iter() {
+        let filter = expect_parse("crate1::mod1=error,crate1::mod2,crate2=debug,crate3=off")
+            .with_default(LevelFilter::WARN);
+
+        let mut targets: Vec<_> = filter.into_iter().collect();
+        targets.sort();
+
+        assert_eq!(
+            targets,
+            vec![
+                ("crate1::mod1".to_string(), LevelFilter::ERROR),
+                ("crate1::mod2".to_string(), LevelFilter::TRACE),
+                ("crate2".to_string(), LevelFilter::DEBUG),
+                ("crate3".to_string(), LevelFilter::OFF),
+            ]
+        );
+    }
+
     #[test]
     fn size_of_filters() {
         fn print_sz(s: &str) {