subscriber: reduce default features (#1647)

This changes `tracing-subscriber` so that the `env-filter`, `json`,
 and `chrono` features are not enabled by default, and instead require
users to opt in. This should significantly reduce the default
dependency footprint.

Of course, this is a breaking change, and therefore will be part of
`tracing-subscriber` v0.3.

Fixes #1258

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

Author: hawkw

examples/Cargo.toml

@@ -15,7 +15,7 @@ tracing-core = { path = "../tracing-core", version = "0.2"}
 tracing-error = { path = "../tracing-error" }
 tracing-flame = { path = "../tracing-flame" }
 tracing-tower = { version = "0.1.0", path = "../tracing-tower" }
-tracing-subscriber = { path = "../tracing-subscriber", version = "0.3", features = ["json"] }
+tracing-subscriber = { path = "../tracing-subscriber", version = "0.3", features = ["json", "env-filter"] }
 tracing-futures = { version = "0.3", path = "../tracing-futures", features = ["futures-01"] }
 tracing-attributes =  { path = "../tracing-attributes", version = "0.2"}
 tracing-log = { path = "../tracing-log", version = "0.2", features = ["env_logger"] }

tracing-subscriber/Cargo.toml

@@ -23,7 +23,7 @@ keywords = ["logging", "tracing", "metrics", "subscriber"]
 
 [features]
 
-default = ["env-filter", "smallvec", "fmt", "ansi", "tracing-log", "json"]
+default = ["smallvec", "fmt", "ansi", "tracing-log"]
 env-filter = ["matchers", "regex", "lazy_static", "tracing"]
 fmt = ["registry"]
 ansi = ["fmt", "ansi_term"]
@@ -36,7 +36,7 @@ local-time = ["time/local-offset"]
 [dependencies]
 tracing-core = { path = "../tracing-core", version = "0.2" }
 
-# only required by the filter feature
+# only required by the `env-filter` feature
 tracing = { optional = true, path = "../tracing", version = "0.2", default-features = false, features = ["std"] }
 matchers = { optional = true, version = "0.1.0" }
 regex = { optional = true, version = "1", default-features = false, features = ["std"] }

tracing-subscriber/benches/reload.rs

@@ -1,3 +1,4 @@
+#![cfg(feature = "env-filter")]
 use criterion::{criterion_group, criterion_main, Criterion};
 use tracing_subscriber::{
     filter::LevelFilter,

tracing-subscriber/src/lib.rs

@@ -23,21 +23,22 @@
 //! ## Feature Flags
 //!
 //! - `env-filter`: Enables the [`EnvFilter`] type, which implements filtering
-//!   similar to the [`env_logger` crate]. Enabled by default.
+//!   similar to the [`env_logger` crate].
 //! - `fmt`: Enables the [`fmt`] module, which provides a subscriber
 //!   implementation for printing formatted representations of trace events.
 //!   Enabled by default.
 //! - `ansi`: Enables `fmt` support for ANSI terminal colors. Enabled by
 //!   default.
 //! - `registry`: enables the [`registry`] module. Enabled by default.
-//! - `json`: Enables `fmt` support for JSON output. In JSON output, the ANSI feature does nothing.
+//! - `json`: Enables `fmt` support for JSON output. In JSON output, the ANSI
+//!   feature does nothing.
 //! - [`local-time`]: Enables local time formatting when using the [`time`
 //!   crate]'s timestamp formatters with the `fmt` subscriber.
 //!
 //! ### Optional Dependencies
 //!
 //! - [`tracing-log`]: Enables better formatting for events emitted by `log`
-//!   macros in the `fmt` subscriber. On by default.
+//!   macros in the `fmt` subscriber. Enabled by default.
 //! - [`time`]: Enables support for using the [`time` crate] for timestamp
 //!   formatting in the `fmt` subscriber.
 //! - [`smallvec`]: Causes the `EnvFilter` type to use the `smallvec` crate (rather