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subscriber: feature flags that require std enable it (#1660)

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## Motivation

Currently, some `tracing-subscriber` dependencies in other crates use
`default-features = false`, to ensure that unneeded default features of
`tracing-subscriber` are not inadvertently enabled. However, some of the
features those crates enable also require the `std` feature flag. The
`default-features = false` config _disables_ the `std` feature, so the
required features are not available.

## Solution

This branch changes `tracing-subscriber`'s `fmt`, `registry`, and
`env-filter` feature flags to also enable the `std` feature flag
automatically.

## Alternatives

As an alternative solution, we could change crates that depend on
`tracing-subscriber` with `default-features = false` to also explicitly
ensure that the `std` feature is enabled (which might be worth doing
regardless). This is what PR #1659 does. However, I think the approach
in this branch is more correct; the feature flags that require standard
library support should automatically enable it. This provides a better
experience for most users, who are simply adding `default-features = false`
in order to avoid enabling un-needed features, not to support
`no_std`, and would like enabling a feature flag to *actually* enable
that feature.

`no_std` users will know not to enable `registry`, `fmt`, and
`env-filter` because the documentation notes that those features require
the standard library.
This commit is contained in:
Eliza Weisman 2021-10-20 12:36:41 -07:00
parent 240d11a7ef
commit 3baf4542ca
2 changed files with 7 additions and 6 deletions
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6
tracing-subscriber/Cargo.toml
View File

@ -26,10 +26,10 @@ keywords = ["logging", "tracing", "metrics", "subscriber"]
default = ["smallvec", "fmt", "ansi", "tracing-log", "std"]
alloc = []
std = ["alloc", "tracing-core/std"]
env-filter = ["matchers", "regex", "lazy_static", "tracing"]
fmt = ["registry"]
env-filter = ["matchers", "regex", "lazy_static", "tracing", "std"]
fmt = ["registry", "std"]
ansi = ["fmt", "ansi_term"]
registry = ["sharded-slab", "thread_local"]
registry = ["sharded-slab", "thread_local", "std"]
json = ["tracing-serde", "serde", "serde_json"]
# Enables support for local time when using the `time` crate timestamp
# formatters.

7
tracing-subscriber/src/lib.rs
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@ -50,15 +50,16 @@
//! (enabled by default).
//! - `alloc`: Depend on [`liballoc`] (enabled by "std").
//! - `env-filter`: Enables the [`EnvFilter`] type, which implements filtering
//! similar to the [`env_logger` crate].
//! similar to the [`env_logger` crate]. **Requires "std"**.
//! - `fmt`: Enables the [`fmt`] module, which provides a subscriber
//! implementation for printing formatted representations of trace events.
//! Enabled by default.
//! Enabled by default. **Requires "std"**.
//! - `ansi`: Enables `fmt` support for ANSI terminal colors. Enabled by
//! default.
//! - `registry`: enables the [`registry`] module. Enabled by default.
//! **Requires "std"**.
//! - `json`: Enables `fmt` support for JSON output. In JSON output, the ANSI
//! feature does nothing.
//! feature does nothing. **Requires "fmt" and "std"**.
//! - [`local-time`]: Enables local time formatting when using the [`time`
//! crate]'s timestamp formatters with the `fmt` subscriber.
//!