docs: document valuable support

examples/examples/valuable.rs

@@ -12,8 +12,6 @@
 //!
 //! Therefore, when `valuable` support is not enabled, this example falls back to using
 //! `fmt::Debug` to record fields that implement `valuable::Valuable`.
-#[cfg(tracing_unstable)]
-use tracing::field::valuable;
 use tracing::{info, info_span};
 use valuable::Valuable;
 
@@ -49,7 +47,7 @@ fn main() {
     // If the `valuable` feature is enabled, record `user` using its'
     // `valuable::Valuable` implementation:
     #[cfg(tracing_unstable)]
-    let span = info_span!("Processing", user = valuable(&user));
+    let span = info_span!("Processing", user = user.as_value());
 
     // Otherwise, record `user` using its `fmt::Debug` implementation:
     #[cfg(not(tracing_unstable))]

netlify.toml

@@ -8,8 +8,10 @@
 [build.environment]
   RUSTDOCFLAGS="""
     -D warnings \
-    --cfg docsrs
+    --cfg docsrs \
+    --cfg tracing_unstable
     """
+  RUSTFLAGS="--cfg tracing_unstable"
 
 [[redirects]]
   from = "/"

tracing-core/Cargo.toml

@@ -8,7 +8,7 @@ name = "tracing-core"
 #   - README.md
 # - Update CHANGELOG.md.
 # - Create "v0.1.x" git tag.
-version = "0.1.21"
+version = "0.1.22"
 authors = ["Tokio Contributors <team@tokio.rs>"]
 license = "MIT"
 readme = "README.md"
@@ -41,4 +41,8 @@ valuable = { version = "0.1.0", optional = true, default_features = false }
 
 [package.metadata.docs.rs]
 all-features = true
-rustdoc-args = ["--cfg", "docsrs"]
+# enable unstable features in the documentation
+rustdoc-args = ["--cfg", "docsrs", "--cfg", "tracing_unstable"]
+# it's necessary to _also_ pass `--cfg tracing_unstable` to rustc, or else
+# dependencies will not be enabled, and the docs build will fail.
+rustc-args = ["--cfg", "tracing_unstable"]

tracing-core/src/field.rs

@@ -1,4 +1,4 @@
-//! Span and `Event` key-value data.
+//! `Span` and `Event` key-value data.
 //!
 //! Spans and events may be annotated with key-value data, referred to as known
 //! as _fields_. These fields consist of a mapping from a key (corresponding to
@@ -24,8 +24,81 @@
 //! recorded as typed data by calling the [`Value::record`] method on these
 //! trait objects with a _visitor_ implementing the [`Visit`] trait. This trait
 //! represents the behavior used to record values of various types. For example,
-//! we might record integers by incrementing counters for their field names,
-//! rather than printing them.
+//! an implementation of `Visit` might record integers by incrementing counters
+//! for their field names rather than printing them.
+//!
+//!
+//! # Using `valuable`
+//!
+//! `tracing`'s [`Value`] trait is intentionally minimalist: it supports only a small
+//! number of Rust primitives as typed values, and only permits recording
+//! user-defined types with their [`fmt::Debug`] or [`fmt::Display`]
+//! implementations. However, there are some cases where it may be useful to record
+//! nested values (such as arrays, `Vec`s, or `HashMap`s containing values), or
+//! user-defined `struct` and `enum` types without having to format them as
+//! unstructured text.
+//!
+//! To address `Value`'s limitations, `tracing` offers experimental support for
+//! the [`valuable`] crate, which provides object-safe inspection of structured
+//! values. User-defined types can implement the [`valuable::Valuable`] trait,
+//! and be recorded as a `tracing` field by calling their [`as_value`] method.
+//! If the [`Subscriber`] also supports the `valuable` crate, it can
+//! then visit those types fields as structured values using `valuable`.
+//!
+//! <pre class="ignore" style="white-space:normal;font:inherit;">
+//!     <strong>Note</strong>: <code>valuable</code> support is an
+//!     <a href = "../index.html#unstable-features">unstable feature</a>. See
+//!     the documentation on unstable features for details on how to enable it.
+//! </pre>
+//!
+//! For example:
+//! ```ignore
+//! // Derive `Valuable` for our types:
+//! use valuable::Valuable;
+//!
+//! #[derive(Clone, Debug, Valuable)]
+//! struct User {
+//!     name: String,
+//!     age: u32,
+//!     address: Address,
+//! }
+//!
+//! #[derive(Clone, Debug, Valuable)]
+//! struct Address {
+//!     country: String,
+//!     city: String,
+//!     street: String,
+//! }
+//!
+//! let user = User {
+//!     name: "Arwen Undomiel".to_string(),
+//!     age: 3000,
+//!     address: Address {
+//!         country: "Middle Earth".to_string(),
+//!         city: "Rivendell".to_string(),
+//!         street: "leafy lane".to_string(),
+//!     },
+//! };
+//!
+//! // Recording `user` as a `valuable::Value` will allow the `tracing` subscriber
+//! // to traverse its fields as a nested, typed structure:
+//! tracing::info!(current_user = user.as_value());
+//! ```
+//!
+//! Alternatively, the [`valuable()`] function may be used to convert a type
+//! implementing [`Valuable`] into a `tracing` field value.
+//!
+//! When the `valuable` feature is enabled, the [`Visit`] trait will include an
+//! optional [`record_value`] method. `Visit` implementations that wish to
+//! record `valuable` values can implement this method with custom behavior.
+//! If a visitor does not implement `record_value`, the [`valuable::Value`] will
+//! be forwarded to the visitor's [`record_debug`] method.
+//!
+//! [`valuable`]: https://crates.io/crates/valuable
+//! [`as_value`]: valuable::Valuable::as_value
+//! [`Subscriber`]: crate::Subscriber
+//! [`record_value`]: Visit::record_value
+//! [`record_debug`]: Visit::record_debug
 //!
 //! [`Value`]: trait.Value.html
 //! [span]: ../span/
@@ -189,7 +262,7 @@ pub trait Visit {
     /// [`valuable`]: https://docs.rs/valuable
     #[cfg(all(tracing_unstable, feature = "valuable"))]
     #[cfg_attr(docsrs, doc(cfg(all(tracing_unstable, feature = "valuable"))))]
-    fn record_value(&mut self, field: &Field, value: &dyn valuable::Valuable) {
+    fn record_value(&mut self, field: &Field, value: &valuable::Value<'_>) {
         self.record_debug(field, &value)
     }
 
@@ -258,14 +331,6 @@ pub struct DisplayValue<T: fmt::Display>(T);
 #[derive(Clone)]
 pub struct DebugValue<T: fmt::Debug>(T);
 
-/// A `Value` which serializes using [`Valuable`].
-///
-/// [`Valuable`]: https://docs.rs/valuable/latest/valuable/trait.Valuable.html
-#[derive(Clone)]
-#[cfg(all(tracing_unstable, feature = "valuable"))]
-#[cfg_attr(docsrs, doc(cfg(all(tracing_unstable, feature = "valuable"))))]
-pub struct ValuableValue<T: valuable::Valuable>(T);
-
 /// Wraps a type implementing `fmt::Display` as a `Value` that can be
 /// recorded using its `Display` implementation.
 pub fn display<T>(t: T) -> DisplayValue<T>
@@ -290,11 +355,11 @@ where
 /// [`Valuable`]: https://docs.rs/valuable/latest/valuable/trait.Valuable.html
 #[cfg(all(tracing_unstable, feature = "valuable"))]
 #[cfg_attr(docsrs, doc(cfg(all(tracing_unstable, feature = "valuable"))))]
-pub fn valuable<T>(t: T) -> ValuableValue<T>
+pub fn valuable<T>(t: &T) -> valuable::Value<'_>
 where
     T: valuable::Valuable,
 {
-    ValuableValue(t)
+    t.as_value()
 }
 
 // ===== impl Visit =====
@@ -572,21 +637,24 @@ impl<T: fmt::Debug> fmt::Debug for DebugValue<T> {
 // ===== impl ValuableValue =====
 
 #[cfg(all(tracing_unstable, feature = "valuable"))]
-impl<T: valuable::Valuable> crate::sealed::Sealed for ValuableValue<T> {}
+impl crate::sealed::Sealed for valuable::Value<'_> {}
 
 #[cfg(all(tracing_unstable, feature = "valuable"))]
 #[cfg_attr(docsrs, doc(cfg(all(tracing_unstable, feature = "valuable"))))]
-impl<T: valuable::Valuable> Value for ValuableValue<T> {
+impl Value for valuable::Value<'_> {
     fn record(&self, key: &Field, visitor: &mut dyn Visit) {
-        visitor.record_value(key, &self.0)
+        visitor.record_value(key, self)
     }
 }
 
+#[cfg(all(tracing_unstable, feature = "valuable"))]
+impl crate::sealed::Sealed for &'_ dyn valuable::Valuable {}
+
 #[cfg(all(tracing_unstable, feature = "valuable"))]
 #[cfg_attr(docsrs, doc(cfg(all(tracing_unstable, feature = "valuable"))))]
-impl<T: valuable::Valuable> fmt::Debug for ValuableValue<T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{:?}", &self.0 as &dyn valuable::Valuable)
+impl Value for &'_ dyn valuable::Valuable {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        visitor.record_value(key, &self.as_value())
     }
 }
 

tracing-core/src/lib.rs

@@ -43,9 +43,9 @@
 //! be used with the `tracing` ecosystem. It includes a collection of
 //! `Subscriber` implementations, as well as utility and adapter crates.
 //!
-//! ### Crate Feature Flags
+//! ## Crate Feature Flags
 //!
-//! The following crate feature flags are available:
+//! The following crate [feature flags] are available:
 //!
 //! * `std`: Depend on the Rust standard library (enabled by default).
 //!
@@ -58,6 +58,37 @@
 //!
 //!   **Note**:`tracing-core`'s `no_std` support requires `liballoc`.
 //!
+//! ### Unstable Features
+//!
+//! These feature flags enable **unstable** features. The public API may break in 0.1.x
+//! releases. To enable these features, the `--cfg tracing_unstable` must be passed to
+//! `rustc` when compiling.
+//!
+//! The following unstable feature flags are currently available:
+//!
+//! * `valuable`: Enables support for recording [field values] using the
+//!   [`valuable`] crate.
+//!
+//! #### Enabling Unstable Features
+//!
+//! The easiest way to set the `tracing_unstable` cfg is to use the `RUSTFLAGS`
+//! env variable when running `cargo` commands:
+//!
+//! ```shell
+//! RUSTFLAGS="--cfg tracing_unstable" cargo build
+//! ```
+//! Alternatively, the following can be added to the `.cargo/config` file in a
+//! project to automatically enable the cfg flag for that project:
+//!
+//! ```toml
+//! [build]
+//! rustflags = ["--cfg", "tracing_unstable"]
+//! ```
+//!
+//! [feature flags]: https://doc.rust-lang.org/cargo/reference/manifest.html#the-features-section
+//! [field values]: crate::field
+//! [`valuable`]: https://crates.io/crates/valuable
+//!
 //! ## Supported Rust Versions
 //!
 //! Tracing is built against the latest stable release. The minimum supported

tracing/Cargo.toml

@@ -28,7 +28,7 @@ edition = "2018"
 rust-version = "1.42.0"
 
 [dependencies]
-tracing-core = { path = "../tracing-core", version = "0.1.21", default-features = false }
+tracing-core = { path = "../tracing-core", version = "0.1.22", default-features = false }
 log = { version = "0.4", optional = true }
 tracing-attributes = { path = "../tracing-attributes", version = "0.1.18", optional = true }
 cfg-if = "1.0.0"
@@ -81,4 +81,8 @@ maintenance = { status = "actively-developed" }
 
 [package.metadata.docs.rs]
 all-features = true
-rustdoc-args = ["--cfg", "docsrs"]
+# enable unstable features in the documentation
+rustdoc-args = ["--cfg", "docsrs", "--cfg", "tracing_unstable"]
+# it's necessary to _also_ pass `--cfg tracing_unstable` to rustc, or else
+# dependencies will not be enabled, and the docs build will fail.
+rustc-args = ["--cfg", "tracing_unstable"]

tracing/src/field.rs

@@ -1,4 +1,116 @@
-//! Structured data associated with `Span`s and `Event`s.
+//! `Span` and `Event` key-value data.
+//!
+//! Spans and events may be annotated with key-value data, referred to as known
+//! as _fields_. These fields consist of a mapping from a key (corresponding to
+//! a `&str` but represented internally as an array index) to a [`Value`].
+//!
+//! # `Value`s and `Subscriber`s
+//!
+//! `Subscriber`s consume `Value`s as fields attached to [span]s or [`Event`]s.
+//! The set of field keys on a given span or is defined on its [`Metadata`].
+//! When a span is created, it provides [`Attributes`] to the `Subscriber`'s
+//! [`new_span`] method, containing any fields whose values were provided when
+//! the span was created; and may call the `Subscriber`'s [`record`] method
+//! with additional [`Record`]s if values are added for more of its fields.
+//! Similarly, the [`Event`] type passed to the subscriber's [`event`] method
+//! will contain any fields attached to each event.
+//!
+//! `tracing` represents values as either one of a set of Rust primitives
+//! (`i64`, `u64`, `f64`, `bool`, and `&str`) or using a `fmt::Display` or
+//! `fmt::Debug` implementation. `Subscriber`s are provided these primitive
+//! value types as `dyn Value` trait objects.
+//!
+//! These trait objects can be formatted using `fmt::Debug`, but may also be
+//! recorded as typed data by calling the [`Value::record`] method on these
+//! trait objects with a _visitor_ implementing the [`Visit`] trait. This trait
+//! represents the behavior used to record values of various types. For example,
+//! an implementation of `Visit` might record integers by incrementing counters
+//! for their field names rather than printing them.
+//!
+//!
+//! # Using `valuable`
+//!
+//! `tracing`'s [`Value`] trait is intentionally minimalist: it supports only a small
+//! number of Rust primitives as typed values, and only permits recording
+//! user-defined types with their [`fmt::Debug`] or [`fmt::Display`]
+//! implementations. However, there are some cases where it may be useful to record
+//! nested values (such as arrays, `Vec`s, or `HashMap`s containing values), or
+//! user-defined `struct` and `enum` types without having to format them as
+//! unstructured text.
+//!
+//! To address `Value`'s limitations, `tracing` offers experimental support for
+//! the [`valuable`] crate, which provides object-safe inspection of structured 
+//! values. User-defined types can implement the [`valuable::Valuable`] trait,
+//! and be recorded as a `tracing` field by calling their [`as_value`] method.
+//! If the [`Subscriber`] also supports the `valuable` crate, it can 
+//! then visit those types fields as structured values using `valuable`.
+//!
+//! <pre class="ignore" style="white-space:normal;font:inherit;">
+//!     <strong>Note</strong>: <code>valuable</code> support is an
+//!     <a href = "../index.html#unstable-features">unstable feature</a>. See
+//!     the documentation on unstable features for details on how to enable it.
+//! </pre>
+//!
+//! For example:
+//! ```ignore
+//! // Derive `Valuable` for our types:
+//! use valuable::Valuable;
+//!
+//! #[derive(Clone, Debug, Valuable)]
+//! struct User {
+//!     name: String,
+//!     age: u32,
+//!     address: Address,
+//! }
+//!
+//! #[derive(Clone, Debug, Valuable)]
+//! struct Address {
+//!     country: String,
+//!     city: String,
+//!     street: String,
+//! }
+//!
+//! let user = User {
+//!     name: "Arwen Undomiel".to_string(),
+//!     age: 3000,
+//!     address: Address {
+//!         country: "Middle Earth".to_string(),
+//!         city: "Rivendell".to_string(),
+//!         street: "leafy lane".to_string(),
+//!     },
+//! };
+//!
+//! // Recording `user` as a `valuable::Value` will allow the `tracing` subscriber
+//! // to traverse its fields as a nested, typed structure:
+//! tracing::info!(current_user = user.as_value());
+//! ```
+//!
+//! Alternatively, the [`valuable()`] function may be used to convert a type
+//! implementing [`Valuable`] into a `tracing` field value.
+//!
+//! When the `valuable` feature is enabled, the [`Visit`] trait will include an
+//! optional [`record_value`] method. `Visit` implementations that wish to
+//! record `valuable` values can implement this method with custom behavior.
+//! If a visitor does not implement `record_value`, the [`valuable::Value`] will
+//! be forwarded to the visitor's [`record_debug`] method.
+//!
+//! [`fmt::Debug`]: std::fmt::Debug
+//! [`fmt::Display`]: std::fmt::Debug
+//! [`valuable`]: https://crates.io/crates/valuable
+//! [`valuable::Valuable`]: https://docs.rs/valuable/latest/valuable/trait.Valuable.html
+//! [`as_value`]: https://docs.rs/valuable/latest/valuable/trait.Valuable.html#tymethod.as_value
+//! [`valuable::Value`]: https://docs.rs/valuable/latest/valuable/enum.Value.html
+//! [`Subscriber`]: crate::Subscriber
+//! [`record_value`]: Visit::record_value
+//! [`record_debug`]: Visit::record_debug
+//! [span]: mod@crate::span
+//! [`Event`]: crate::event::Event
+//! [`Metadata`]: crate::Metadata
+//! [`Attributes`]: crate::span::Attributes
+//! [`Record`]: crate::span::Record
+//! [`new_span`]: crate::Subscriber::new_span
+//! [`record`]: crate::Subscriber::record
+//! [`event`]: crate::Subscriber::event
 pub use tracing_core::field::*;
 
 use crate::Metadata;