subscriber: add Targets filter, a lighter-weight EnvFilter (toki…

…o-rs#1550)

This branch adds a new `Targets` filter to `tracing_subscriber`. The
`Targets` filter is very similar to `EnvFilter`, but it _only_ consists
of filtering directives consisting of a target and level. Because it
doesn't support filtering on field names, span contexts, or field
values, the implementation is *much* simpler, and it doesn't require the
`env_filter` feature flag. Also, `Targets` can easily implement the
`Filter` trait for per-layer filtering, while adding a `Filter`
implementation for `EnvFilter` will require additional effort.

Because the `Targets` filter doesn't allow specifiyng span or
field-value filters, the syntax for parsing one from a string is
significantly simpler than `EnvFilter`'s. Therefore, it can have a very
simple handwritten parser implementation that doesn't require the
`regex` crate. This should be useful for users who are concerned about
the number of dependencies required by `EnvFilter`.

The new implementation is quite small, as it mostly uses the same code
as the static filter subset of `EnvFilter`. This code was factored out
into a shared module for use in both `EnvFilter` and `Targets`. The code
required for _dynamic_ filtering with `EnvFilter` (i.e. on fields and
spans) is still in the `filter::env` module and is only enabled by the
`env-filter` feature flag.

I'm open to renaming the new type; I thought `filter::Targets` seemed
good, but would also be willing to go with `TargetFilter` or something.

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

tracing-subscriber/src/filter/env/directive.rs

@@ -161,7 +161,9 @@ impl Directive {
                 .unwrap()
             });
 
-        let caps = DIRECTIVE_RE.captures(from).ok_or_else(ParseError::new)?;
+        let caps = DIRECTIVE_RE
+            .captures(from)
+            .ok_or_else(DirectiveParseError::new)?;
 
         if let Some(level) = caps
             .name("global_level")

tracing-subscriber/src/filter/env/mod.rs

@@ -216,7 +216,7 @@ pub struct FromEnvError {
 
 #[derive(Debug)]
 enum ErrorKind {
-    Parse(ParseError),
+    Parse(DirectiveParseError),
     Env(env::VarError),
 }
 

tracing-subscriber/src/filter/mod.rs

@@ -10,6 +10,7 @@
 //! [`Subscribe`]: crate::subscribe
 mod filter_fn;
 mod level;
+mod targets;
 
 feature! {
     #![all(feature = "env-filter", feature = "std")]

tracing-subscriber/src/layer/mod.rs

@@ -427,6 +427,7 @@ pub(crate) mod tests;
 /// [module-level documentation](crate::layer) for details.
 ///
 /// [`Subscriber`]: tracing_core::Subscriber
+#[cfg_attr(docsrs, doc(notable_trait))]
 pub trait Layer<S>
 where
     S: Subscriber,
@@ -784,7 +785,8 @@ where
 /// A per-[`Layer`] filter that determines whether a span or event is enabled
 /// for an individual layer.
 #[cfg(feature = "registry")]
-#[cfg_attr(docsrs, doc(cfg(feature = "registry"), notable_trait))]
+#[cfg_attr(docsrs, doc(cfg(feature = "registry")))]
+#[cfg_attr(docsrs, doc(notable_trait))]
 pub trait Filter<S> {
     /// Returns `true` if this layer is interested in a span or event with the
     /// given [`Metadata`] in the current [`Context`], similarly to