serde: allow tracing-serde to work on no_std. (#960)

Allows `tracing-serde` to work on devices without `std`.

*  Enable `#[no_std]` in `tracing-serde` when `cfg(not(feature = std))`
*  No longer have unnecessary dependencies on std in serde/tracing.

No behavior differences or actual code changes other than allowing
for core/alloc fallbacks.

Signed-off-by: Ana Hobden <operator@hoverbear.org>

Author: Hoverbear

tracing-serde/Cargo.toml

@@ -17,9 +17,13 @@ categories = [
 ]
 keywords = ["logging", "tracing", "serialization"]
 
+[features]
+default = ["std"]
+std = ["serde/std", "tracing-core/std"]
+
 [dependencies]
-serde = "1"
-tracing-core = { path = "../tracing-core", version = "0.2"}
+serde = { version = "1", default-features = false, features = ["alloc"] }
+tracing-core = { path = "../tracing-core", version = "0.2", default-features = false }
 
 [dev-dependencies]
 serde_json = "1"

tracing-serde/README.md

@@ -98,6 +98,22 @@ After you implement your `Subscriber`, you can use your `tracing`
 subscriber (`JsonSubscriber` in the above example) to record serialized
 trace data.
 
+##  Crate Feature Flags
+
+The following crate feature flags are available:
+
+* `std`: Depend on the Rust standard library (enabled by default).
+
+  `no_std` users may disable this feature with `default-features = false`:
+
+  ```toml
+  [dependencies]
+  tracing-serde = { version = "0.2", default-features = false }
+  ```
+
+  **Note**:`tracing-serde`'s `no_std` support requires `liballoc`.
+
+
 ## Supported Rust Versions
 
 Tracing is built against the latest stable release. The minimum supported

tracing-serde/src/lib.rs

@@ -109,6 +109,21 @@
 //! subscriber (`JsonSubscriber` in the above example) to record serialized
 //! trace data.
 //!
+//! ##  Crate Feature Flags
+//!
+//! The following crate feature flags are available:
+//!
+//! * `std`: Depend on the Rust standard library (enabled by default).
+//!
+//!   `no_std` users may disable this feature with `default-features = false`:
+//!
+//!   ```toml
+//!   [dependencies]
+//!   tracing-serde = { version = "0.2", default-features = false }
+//!   ```
+//
+//!   **Note**:`tracing-serde`'s `no_std` support requires `liballoc`.
+//!
 //! ## Supported Rust Versions
 //!
 //! Tracing is built against the latest stable release. The minimum supported
@@ -153,7 +168,10 @@
     unused_parens,
     while_true
 )]
-use std::fmt;
+// Support using tracing-serde without the standard library!
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use core::fmt;
 
 use serde::{
     ser::{SerializeMap, SerializeSeq, SerializeStruct, SerializeTupleStruct, Serializer},