Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1219 insertions, 0 deletions

diff --git a/vendor/tracing-core/src/field.rs b/vendor/tracing-core/src/field.rs
new file mode 100644
index 000000000..c9869ce2c
--- /dev/null
+++ b/vendor/tracing-core/src/field.rs
@@ -0,0 +1,1219 @@
+//! `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`, `i128`, `u128`, `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.
+//!
+//! [`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
+//!
+//! [span]: super::span
+//! [`Event`]: super::event::Event
+//! [`Metadata`]: super::metadata::Metadata
+//! [`Attributes`]:  super::span::Attributes
+//! [`Record`]: super::span::Record
+//! [`new_span`]: super::subscriber::Subscriber::new_span
+//! [`record`]: super::subscriber::Subscriber::record
+//! [`event`]:  super::subscriber::Subscriber::event
+//! [`Value::record`]: Value::record
+use crate::callsite;
+use crate::stdlib::{
+    borrow::Borrow,
+    fmt,
+    hash::{Hash, Hasher},
+    num,
+    ops::Range,
+    string::String,
+};
+
+use self::private::ValidLen;
+
+/// An opaque key allowing _O_(1) access to a field in a `Span`'s key-value
+/// data.
+///
+/// As keys are defined by the _metadata_ of a span, rather than by an
+/// individual instance of a span, a key may be used to access the same field
+/// across all instances of a given span with the same metadata. Thus, when a
+/// subscriber observes a new span, it need only access a field by name _once_,
+/// and use the key for that name for all other accesses.
+#[derive(Debug)]
+pub struct Field {
+    i: usize,
+    fields: FieldSet,
+}
+
+/// An empty field.
+///
+/// This can be used to indicate that the value of a field is not currently
+/// present but will be recorded later.
+///
+/// When a field's value is `Empty`. it will not be recorded.
+#[derive(Debug, Eq, PartialEq)]
+pub struct Empty;
+
+/// Describes the fields present on a span.
+pub struct FieldSet {
+    /// The names of each field on the described span.
+    names: &'static [&'static str],
+    /// The callsite where the described span originates.
+    callsite: callsite::Identifier,
+}
+
+/// A set of fields and values for a span.
+pub struct ValueSet<'a> {
+    values: &'a [(&'a Field, Option<&'a (dyn Value + 'a)>)],
+    fields: &'a FieldSet,
+}
+
+/// An iterator over a set of fields.
+#[derive(Debug)]
+pub struct Iter {
+    idxs: Range<usize>,
+    fields: FieldSet,
+}
+
+/// Visits typed values.
+///
+/// An instance of `Visit` ("a visitor") represents the logic necessary to
+/// record field values of various types. When an implementor of [`Value`] is
+/// [recorded], it calls the appropriate method on the provided visitor to
+/// indicate the type that value should be recorded as.
+///
+/// When a [`Subscriber`] implementation [records an `Event`] or a
+/// [set of `Value`s added to a `Span`], it can pass an `&mut Visit` to the
+/// `record` method on the provided [`ValueSet`] or [`Event`]. This visitor
+/// will then be used to record all the field-value pairs present on that
+/// `Event` or `ValueSet`.
+///
+/// # Examples
+///
+/// A simple visitor that writes to a string might be implemented like so:
+/// ```
+/// # extern crate tracing_core as tracing;
+/// use std::fmt::{self, Write};
+/// use tracing::field::{Value, Visit, Field};
+/// pub struct StringVisitor<'a> {
+///     string: &'a mut String,
+/// }
+///
+/// impl<'a> Visit for StringVisitor<'a> {
+///     fn record_debug(&mut self, field: &Field, value: &dyn fmt::Debug) {
+///         write!(self.string, "{} = {:?}; ", field.name(), value).unwrap();
+///     }
+/// }
+/// ```
+/// This visitor will format each recorded value using `fmt::Debug`, and
+/// append the field name and formatted value to the provided string,
+/// regardless of the type of the recorded value. When all the values have
+/// been recorded, the `StringVisitor` may be dropped, allowing the string
+/// to be printed or stored in some other data structure.
+///
+/// The `Visit` trait provides default implementations for `record_i64`,
+/// `record_u64`, `record_bool`, `record_str`, and `record_error`, which simply
+/// forward the recorded value to `record_debug`. Thus, `record_debug` is the
+/// only method which a `Visit` implementation *must* implement. However,
+/// visitors may override the default implementations of these functions in
+/// order to implement type-specific behavior.
+///
+/// Additionally, when a visitor receives a value of a type it does not care
+/// about, it is free to ignore those values completely. For example, a
+/// visitor which only records numeric data might look like this:
+///
+/// ```
+/// # extern crate tracing_core as tracing;
+/// # use std::fmt::{self, Write};
+/// # use tracing::field::{Value, Visit, Field};
+/// pub struct SumVisitor {
+///     sum: i64,
+/// }
+///
+/// impl Visit for SumVisitor {
+///     fn record_i64(&mut self, _field: &Field, value: i64) {
+///        self.sum += value;
+///     }
+///
+///     fn record_u64(&mut self, _field: &Field, value: u64) {
+///         self.sum += value as i64;
+///     }
+///
+///     fn record_debug(&mut self, _field: &Field, _value: &fmt::Debug) {
+///         // Do nothing
+///     }
+/// }
+/// ```
+///
+/// This visitor (which is probably not particularly useful) keeps a running
+/// sum of all the numeric values it records, and ignores all other values. A
+/// more practical example of recording typed values is presented in
+/// `examples/counters.rs`, which demonstrates a very simple metrics system
+/// implemented using `tracing`.
+///
+/// <div class="example-wrap" style="display:inline-block">
+/// <pre class="ignore" style="white-space:normal;font:inherit;">
+/// <strong>Note</strong>: The <code>record_error</code> trait method is only
+/// available when the Rust standard library is present, as it requires the
+/// <code>std::error::Error</code> trait.
+/// </pre></div>
+///
+/// [recorded]: Value::record
+/// [`Subscriber`]: super::subscriber::Subscriber
+/// [records an `Event`]: super::subscriber::Subscriber::event
+/// [set of `Value`s added to a `Span`]: super::subscriber::Subscriber::record
+/// [`Event`]: super::event::Event
+pub trait Visit {
+    /// Visits an arbitrary type implementing the [`valuable`] crate's `Valuable` trait.
+    ///
+    /// [`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: valuable::Value<'_>) {
+        self.record_debug(field, &value)
+    }
+
+    /// Visit a double-precision floating point value.
+    fn record_f64(&mut self, field: &Field, value: f64) {
+        self.record_debug(field, &value)
+    }
+
+    /// Visit a signed 64-bit integer value.
+    fn record_i64(&mut self, field: &Field, value: i64) {
+        self.record_debug(field, &value)
+    }
+
+    /// Visit an unsigned 64-bit integer value.
+    fn record_u64(&mut self, field: &Field, value: u64) {
+        self.record_debug(field, &value)
+    }
+
+    /// Visit a signed 128-bit integer value.
+    fn record_i128(&mut self, field: &Field, value: i128) {
+        self.record_debug(field, &value)
+    }
+
+    /// Visit an unsigned 128-bit integer value.
+    fn record_u128(&mut self, field: &Field, value: u128) {
+        self.record_debug(field, &value)
+    }
+
+    /// Visit a boolean value.
+    fn record_bool(&mut self, field: &Field, value: bool) {
+        self.record_debug(field, &value)
+    }
+
+    /// Visit a string value.
+    fn record_str(&mut self, field: &Field, value: &str) {
+        self.record_debug(field, &value)
+    }
+
+    /// Records a type implementing `Error`.
+    ///
+    /// <div class="example-wrap" style="display:inline-block">
+    /// <pre class="ignore" style="white-space:normal;font:inherit;">
+    /// <strong>Note</strong>: This is only enabled when the Rust standard library is
+    /// present.
+    /// </pre>
+    #[cfg(feature = "std")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+    fn record_error(&mut self, field: &Field, value: &(dyn std::error::Error + 'static)) {
+        self.record_debug(field, &DisplayValue(value))
+    }
+
+    /// Visit a value implementing `fmt::Debug`.
+    fn record_debug(&mut self, field: &Field, value: &dyn fmt::Debug);
+}
+
+/// A field value of an erased type.
+///
+/// Implementors of `Value` may call the appropriate typed recording methods on
+/// the [visitor] passed to their `record` method in order to indicate how
+/// their data should be recorded.
+///
+/// [visitor]: Visit
+pub trait Value: crate::sealed::Sealed {
+    /// Visits this value with the given `Visitor`.
+    fn record(&self, key: &Field, visitor: &mut dyn Visit);
+}
+
+/// A `Value` which serializes using `fmt::Display`.
+///
+/// Uses `record_debug` in the `Value` implementation to
+/// avoid an unnecessary evaluation.
+#[derive(Clone)]
+pub struct DisplayValue<T: fmt::Display>(T);
+
+/// A `Value` which serializes as a string using `fmt::Debug`.
+#[derive(Clone)]
+pub struct DebugValue<T: fmt::Debug>(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>
+where
+    T: fmt::Display,
+{
+    DisplayValue(t)
+}
+
+/// Wraps a type implementing `fmt::Debug` as a `Value` that can be
+/// recorded using its `Debug` implementation.
+pub fn debug<T>(t: T) -> DebugValue<T>
+where
+    T: fmt::Debug,
+{
+    DebugValue(t)
+}
+
+/// Wraps a type implementing [`Valuable`] as a `Value` that
+/// can be recorded using its `Valuable` implementation.
+///
+/// [`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) -> valuable::Value<'_>
+where
+    T: valuable::Valuable,
+{
+    t.as_value()
+}
+
+// ===== impl Visit =====
+
+impl<'a, 'b> Visit for fmt::DebugStruct<'a, 'b> {
+    fn record_debug(&mut self, field: &Field, value: &dyn fmt::Debug) {
+        self.field(field.name(), value);
+    }
+}
+
+impl<'a, 'b> Visit for fmt::DebugMap<'a, 'b> {
+    fn record_debug(&mut self, field: &Field, value: &dyn fmt::Debug) {
+        self.entry(&format_args!("{}", field), value);
+    }
+}
+
+impl<F> Visit for F
+where
+    F: FnMut(&Field, &dyn fmt::Debug),
+{
+    fn record_debug(&mut self, field: &Field, value: &dyn fmt::Debug) {
+        (self)(field, value)
+    }
+}
+
+// ===== impl Value =====
+
+macro_rules! impl_values {
+    ( $( $record:ident( $( $whatever:tt)+ ) ),+ ) => {
+        $(
+            impl_value!{ $record( $( $whatever )+ ) }
+        )+
+    }
+}
+
+macro_rules! ty_to_nonzero {
+    (u8) => {
+        NonZeroU8
+    };
+    (u16) => {
+        NonZeroU16
+    };
+    (u32) => {
+        NonZeroU32
+    };
+    (u64) => {
+        NonZeroU64
+    };
+    (u128) => {
+        NonZeroU128
+    };
+    (usize) => {
+        NonZeroUsize
+    };
+    (i8) => {
+        NonZeroI8
+    };
+    (i16) => {
+        NonZeroI16
+    };
+    (i32) => {
+        NonZeroI32
+    };
+    (i64) => {
+        NonZeroI64
+    };
+    (i128) => {
+        NonZeroI128
+    };
+    (isize) => {
+        NonZeroIsize
+    };
+}
+
+macro_rules! impl_one_value {
+    (f32, $op:expr, $record:ident) => {
+        impl_one_value!(normal, f32, $op, $record);
+    };
+    (f64, $op:expr, $record:ident) => {
+        impl_one_value!(normal, f64, $op, $record);
+    };
+    (bool, $op:expr, $record:ident) => {
+        impl_one_value!(normal, bool, $op, $record);
+    };
+    ($value_ty:tt, $op:expr, $record:ident) => {
+        impl_one_value!(normal, $value_ty, $op, $record);
+        impl_one_value!(nonzero, $value_ty, $op, $record);
+    };
+    (normal, $value_ty:tt, $op:expr, $record:ident) => {
+        impl $crate::sealed::Sealed for $value_ty {}
+        impl $crate::field::Value for $value_ty {
+            fn record(&self, key: &$crate::field::Field, visitor: &mut dyn $crate::field::Visit) {
+                visitor.$record(key, $op(*self))
+            }
+        }
+    };
+    (nonzero, $value_ty:tt, $op:expr, $record:ident) => {
+        // This `use num::*;` is reported as unused because it gets emitted
+        // for every single invocation of this macro, so there are multiple `use`s.
+        // All but the first are useless indeed.
+        // We need this import because we can't write a path where one part is
+        // the `ty_to_nonzero!($value_ty)` invocation.
+        #[allow(clippy::useless_attribute, unused)]
+        use num::*;
+        impl $crate::sealed::Sealed for ty_to_nonzero!($value_ty) {}
+        impl $crate::field::Value for ty_to_nonzero!($value_ty) {
+            fn record(&self, key: &$crate::field::Field, visitor: &mut dyn $crate::field::Visit) {
+                visitor.$record(key, $op(self.get()))
+            }
+        }
+    };
+}
+
+macro_rules! impl_value {
+    ( $record:ident( $( $value_ty:tt ),+ ) ) => {
+        $(
+            impl_one_value!($value_ty, |this: $value_ty| this, $record);
+        )+
+    };
+    ( $record:ident( $( $value_ty:tt ),+ as $as_ty:ty) ) => {
+        $(
+            impl_one_value!($value_ty, |this: $value_ty| this as $as_ty, $record);
+        )+
+    };
+}
+
+// ===== impl Value =====
+
+impl_values! {
+    record_u64(u64),
+    record_u64(usize, u32, u16, u8 as u64),
+    record_i64(i64),
+    record_i64(isize, i32, i16, i8 as i64),
+    record_u128(u128),
+    record_i128(i128),
+    record_bool(bool),
+    record_f64(f64, f32 as f64)
+}
+
+impl<T: crate::sealed::Sealed> crate::sealed::Sealed for Wrapping<T> {}
+impl<T: crate::field::Value> crate::field::Value for Wrapping<T> {
+    fn record(&self, key: &crate::field::Field, visitor: &mut dyn crate::field::Visit) {
+        self.0.record(key, visitor)
+    }
+}
+
+impl crate::sealed::Sealed for str {}
+
+impl Value for str {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        visitor.record_str(key, self)
+    }
+}
+
+#[cfg(feature = "std")]
+impl crate::sealed::Sealed for dyn std::error::Error + 'static {}
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl Value for dyn std::error::Error + 'static {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        visitor.record_error(key, self)
+    }
+}
+
+#[cfg(feature = "std")]
+impl crate::sealed::Sealed for dyn std::error::Error + Send + 'static {}
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl Value for dyn std::error::Error + Send + 'static {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        (self as &dyn std::error::Error).record(key, visitor)
+    }
+}
+
+#[cfg(feature = "std")]
+impl crate::sealed::Sealed for dyn std::error::Error + Sync + 'static {}
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl Value for dyn std::error::Error + Sync + 'static {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        (self as &dyn std::error::Error).record(key, visitor)
+    }
+}
+
+#[cfg(feature = "std")]
+impl crate::sealed::Sealed for dyn std::error::Error + Send + Sync + 'static {}
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl Value for dyn std::error::Error + Send + Sync + 'static {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        (self as &dyn std::error::Error).record(key, visitor)
+    }
+}
+
+impl<'a, T: ?Sized> crate::sealed::Sealed for &'a T where T: Value + crate::sealed::Sealed + 'a {}
+
+impl<'a, T: ?Sized> Value for &'a T
+where
+    T: Value + 'a,
+{
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        (*self).record(key, visitor)
+    }
+}
+
+impl<'a, T: ?Sized> crate::sealed::Sealed for &'a mut T where T: Value + crate::sealed::Sealed + 'a {}
+
+impl<'a, T: ?Sized> Value for &'a mut T
+where
+    T: Value + 'a,
+{
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        // Don't use `(*self).record(key, visitor)`, otherwise would
+        // cause stack overflow due to `unconditional_recursion`.
+        T::record(self, key, visitor)
+    }
+}
+
+impl<'a> crate::sealed::Sealed for fmt::Arguments<'a> {}
+
+impl<'a> Value for fmt::Arguments<'a> {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        visitor.record_debug(key, self)
+    }
+}
+
+impl<T: ?Sized> crate::sealed::Sealed for crate::stdlib::boxed::Box<T> where T: Value {}
+
+impl<T: ?Sized> Value for crate::stdlib::boxed::Box<T>
+where
+    T: Value,
+{
+    #[inline]
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        self.as_ref().record(key, visitor)
+    }
+}
+
+impl crate::sealed::Sealed for String {}
+impl Value for String {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        visitor.record_str(key, self.as_str())
+    }
+}
+
+impl fmt::Debug for dyn Value {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // We are only going to be recording the field value, so we don't
+        // actually care about the field name here.
+        struct NullCallsite;
+        static NULL_CALLSITE: NullCallsite = NullCallsite;
+        impl crate::callsite::Callsite for NullCallsite {
+            fn set_interest(&self, _: crate::subscriber::Interest) {
+                unreachable!("you somehow managed to register the null callsite?")
+            }
+
+            fn metadata(&self) -> &crate::Metadata<'_> {
+                unreachable!("you somehow managed to access the null callsite?")
+            }
+        }
+
+        static FIELD: Field = Field {
+            i: 0,
+            fields: FieldSet::new(&[], crate::identify_callsite!(&NULL_CALLSITE)),
+        };
+
+        let mut res = Ok(());
+        self.record(&FIELD, &mut |_: &Field, val: &dyn fmt::Debug| {
+            res = write!(f, "{:?}", val);
+        });
+        res
+    }
+}
+
+impl fmt::Display for dyn Value {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(self, f)
+    }
+}
+
+// ===== impl DisplayValue =====
+
+impl<T: fmt::Display> crate::sealed::Sealed for DisplayValue<T> {}
+
+impl<T> Value for DisplayValue<T>
+where
+    T: fmt::Display,
+{
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        visitor.record_debug(key, self)
+    }
+}
+
+impl<T: fmt::Display> fmt::Debug for DisplayValue<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self, f)
+    }
+}
+
+impl<T: fmt::Display> fmt::Display for DisplayValue<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+// ===== impl DebugValue =====
+
+impl<T: fmt::Debug> crate::sealed::Sealed for DebugValue<T> {}
+
+impl<T> Value for DebugValue<T>
+where
+    T: fmt::Debug,
+{
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        visitor.record_debug(key, &self.0)
+    }
+}
+
+impl<T: fmt::Debug> fmt::Debug for DebugValue<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+// ===== impl ValuableValue =====
+
+#[cfg(all(tracing_unstable, feature = "valuable"))]
+impl crate::sealed::Sealed for valuable::Value<'_> {}
+
+#[cfg(all(tracing_unstable, feature = "valuable"))]
+#[cfg_attr(docsrs, doc(cfg(all(tracing_unstable, feature = "valuable"))))]
+impl Value for valuable::Value<'_> {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        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 Value for &'_ dyn valuable::Valuable {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        visitor.record_value(key, self.as_value())
+    }
+}
+
+impl crate::sealed::Sealed for Empty {}
+impl Value for Empty {
+    #[inline]
+    fn record(&self, _: &Field, _: &mut dyn Visit) {}
+}
+
+impl<T: Value> crate::sealed::Sealed for Option<T> {}
+
+impl<T: Value> Value for Option<T> {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        if let Some(v) = &self {
+            v.record(key, visitor)
+        }
+    }
+}
+
+// ===== impl Field =====
+
+impl Field {
+    /// Returns an [`Identifier`] that uniquely identifies the [`Callsite`]
+    /// which defines this field.
+    ///
+    /// [`Identifier`]: super::callsite::Identifier
+    /// [`Callsite`]: super::callsite::Callsite
+    #[inline]
+    pub fn callsite(&self) -> callsite::Identifier {
+        self.fields.callsite()
+    }
+
+    /// Returns a string representing the name of the field.
+    pub fn name(&self) -> &'static str {
+        self.fields.names[self.i]
+    }
+}
+
+impl fmt::Display for Field {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad(self.name())
+    }
+}
+
+impl AsRef<str> for Field {
+    fn as_ref(&self) -> &str {
+        self.name()
+    }
+}
+
+impl PartialEq for Field {
+    fn eq(&self, other: &Self) -> bool {
+        self.callsite() == other.callsite() && self.i == other.i
+    }
+}
+
+impl Eq for Field {}
+
+impl Hash for Field {
+    fn hash<H>(&self, state: &mut H)
+    where
+        H: Hasher,
+    {
+        self.callsite().hash(state);
+        self.i.hash(state);
+    }
+}
+
+impl Clone for Field {
+    fn clone(&self) -> Self {
+        Field {
+            i: self.i,
+            fields: FieldSet {
+                names: self.fields.names,
+                callsite: self.fields.callsite(),
+            },
+        }
+    }
+}
+
+// ===== impl FieldSet =====
+
+impl FieldSet {
+    /// Constructs a new `FieldSet` with the given array of field names and callsite.
+    pub const fn new(names: &'static [&'static str], callsite: callsite::Identifier) -> Self {
+        Self { names, callsite }
+    }
+
+    /// Returns an [`Identifier`] that uniquely identifies the [`Callsite`]
+    /// which defines this set of fields..
+    ///
+    /// [`Identifier`]: super::callsite::Identifier
+    /// [`Callsite`]: super::callsite::Callsite
+    pub(crate) fn callsite(&self) -> callsite::Identifier {
+        callsite::Identifier(self.callsite.0)
+    }
+
+    /// Returns the [`Field`] named `name`, or `None` if no such field exists.
+    ///
+    /// [`Field`]: super::Field
+    pub fn field<Q: ?Sized>(&self, name: &Q) -> Option<Field>
+    where
+        Q: Borrow<str>,
+    {
+        let name = &name.borrow();
+        self.names.iter().position(|f| f == name).map(|i| Field {
+            i,
+            fields: FieldSet {
+                names: self.names,
+                callsite: self.callsite(),
+            },
+        })
+    }
+
+    /// Returns `true` if `self` contains the given `field`.
+    ///
+    /// <div class="example-wrap" style="display:inline-block">
+    /// <pre class="ignore" style="white-space:normal;font:inherit;">
+    /// <strong>Note</strong>: If <code>field</code> shares a name with a field
+    /// in this <code>FieldSet</code>, but was created by a <code>FieldSet</code>
+    /// with a different callsite, this <code>FieldSet</code> does <em>not</em>
+    /// contain it. This is so that if two separate span callsites define a field
+    /// named "foo", the <code>Field</code> corresponding to "foo" for each
+    /// of those callsites are not equivalent.
+    /// </pre></div>
+    pub fn contains(&self, field: &Field) -> bool {
+        field.callsite() == self.callsite() && field.i <= self.len()
+    }
+
+    /// Returns an iterator over the `Field`s in this `FieldSet`.
+    pub fn iter(&self) -> Iter {
+        let idxs = 0..self.len();
+        Iter {
+            idxs,
+            fields: FieldSet {
+                names: self.names,
+                callsite: self.callsite(),
+            },
+        }
+    }
+
+    /// Returns a new `ValueSet` with entries for this `FieldSet`'s values.
+    ///
+    /// Note that a `ValueSet` may not be constructed with arrays of over 32
+    /// elements.
+    #[doc(hidden)]
+    pub fn value_set<'v, V>(&'v self, values: &'v V) -> ValueSet<'v>
+    where
+        V: ValidLen<'v>,
+    {
+        ValueSet {
+            fields: self,
+            values: values.borrow(),
+        }
+    }
+
+    /// Returns the number of fields in this `FieldSet`.
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.names.len()
+    }
+
+    /// Returns whether or not this `FieldSet` has fields.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.names.is_empty()
+    }
+}
+
+impl<'a> IntoIterator for &'a FieldSet {
+    type IntoIter = Iter;
+    type Item = Field;
+    #[inline]
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+impl fmt::Debug for FieldSet {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("FieldSet")
+            .field("names", &self.names)
+            .field("callsite", &self.callsite)
+            .finish()
+    }
+}
+
+impl fmt::Display for FieldSet {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_set()
+            .entries(self.names.iter().map(display))
+            .finish()
+    }
+}
+
+// ===== impl Iter =====
+
+impl Iterator for Iter {
+    type Item = Field;
+    fn next(&mut self) -> Option<Field> {
+        let i = self.idxs.next()?;
+        Some(Field {
+            i,
+            fields: FieldSet {
+                names: self.fields.names,
+                callsite: self.fields.callsite(),
+            },
+        })
+    }
+}
+
+// ===== impl ValueSet =====
+
+impl<'a> ValueSet<'a> {
+    /// Returns an [`Identifier`] that uniquely identifies the [`Callsite`]
+    /// defining the fields this `ValueSet` refers to.
+    ///
+    /// [`Identifier`]: super::callsite::Identifier
+    /// [`Callsite`]: super::callsite::Callsite
+    #[inline]
+    pub fn callsite(&self) -> callsite::Identifier {
+        self.fields.callsite()
+    }
+
+    /// Visits all the fields in this `ValueSet` with the provided [visitor].
+    ///
+    /// [visitor]: Visit
+    pub fn record(&self, visitor: &mut dyn Visit) {
+        let my_callsite = self.callsite();
+        for (field, value) in self.values {
+            if field.callsite() != my_callsite {
+                continue;
+            }
+            if let Some(value) = value {
+                value.record(field, visitor);
+            }
+        }
+    }
+
+    /// Returns the number of fields in this `ValueSet` that would be visited
+    /// by a given [visitor] to the [`ValueSet::record()`] method.
+    ///
+    /// [visitor]: Visit
+    /// [`ValueSet::record()`]: ValueSet::record()
+    pub fn len(&self) -> usize {
+        let my_callsite = self.callsite();
+        self.values
+            .iter()
+            .filter(|(field, _)| field.callsite() == my_callsite)
+            .count()
+    }
+
+    /// Returns `true` if this `ValueSet` contains a value for the given `Field`.
+    pub(crate) fn contains(&self, field: &Field) -> bool {
+        field.callsite() == self.callsite()
+            && self
+                .values
+                .iter()
+                .any(|(key, val)| *key == field && val.is_some())
+    }
+
+    /// Returns true if this `ValueSet` contains _no_ values.
+    pub fn is_empty(&self) -> bool {
+        let my_callsite = self.callsite();
+        self.values
+            .iter()
+            .all(|(key, val)| val.is_none() || key.callsite() != my_callsite)
+    }
+
+    pub(crate) fn field_set(&self) -> &FieldSet {
+        self.fields
+    }
+}
+
+impl<'a> fmt::Debug for ValueSet<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.values
+            .iter()
+            .fold(&mut f.debug_struct("ValueSet"), |dbg, (key, v)| {
+                if let Some(val) = v {
+                    val.record(key, dbg);
+                }
+                dbg
+            })
+            .field("callsite", &self.callsite())
+            .finish()
+    }
+}
+
+impl<'a> fmt::Display for ValueSet<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.values
+            .iter()
+            .fold(&mut f.debug_map(), |dbg, (key, v)| {
+                if let Some(val) = v {
+                    val.record(key, dbg);
+                }
+                dbg
+            })
+            .finish()
+    }
+}
+
+// ===== impl ValidLen =====
+
+mod private {
+    use super::*;
+
+    /// Marker trait implemented by arrays which are of valid length to
+    /// construct a `ValueSet`.
+    ///
+    /// `ValueSet`s may only be constructed from arrays containing 32 or fewer
+    /// elements, to ensure the array is small enough to always be allocated on the
+    /// stack. This trait is only implemented by arrays of an appropriate length,
+    /// ensuring that the correct size arrays are used at compile-time.
+    pub trait ValidLen<'a>: Borrow<[(&'a Field, Option<&'a (dyn Value + 'a)>)]> {}
+}
+
+macro_rules! impl_valid_len {
+    ( $( $len:tt ),+ ) => {
+        $(
+            impl<'a> private::ValidLen<'a> for
+                [(&'a Field, Option<&'a (dyn Value + 'a)>); $len] {}
+        )+
+    }
+}
+
+impl_valid_len! {
+    0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
+    21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::metadata::{Kind, Level, Metadata};
+    use crate::stdlib::{borrow::ToOwned, string::String};
+
+    struct TestCallsite1;
+    static TEST_CALLSITE_1: TestCallsite1 = TestCallsite1;
+    static TEST_META_1: Metadata<'static> = metadata! {
+        name: "field_test1",
+        target: module_path!(),
+        level: Level::INFO,
+        fields: &["foo", "bar", "baz"],
+        callsite: &TEST_CALLSITE_1,
+        kind: Kind::SPAN,
+    };
+
+    impl crate::callsite::Callsite for TestCallsite1 {
+        fn set_interest(&self, _: crate::subscriber::Interest) {
+            unimplemented!()
+        }
+
+        fn metadata(&self) -> &Metadata<'_> {
+            &TEST_META_1
+        }
+    }
+
+    struct TestCallsite2;
+    static TEST_CALLSITE_2: TestCallsite2 = TestCallsite2;
+    static TEST_META_2: Metadata<'static> = metadata! {
+        name: "field_test2",
+        target: module_path!(),
+        level: Level::INFO,
+        fields: &["foo", "bar", "baz"],
+        callsite: &TEST_CALLSITE_2,
+        kind: Kind::SPAN,
+    };
+
+    impl crate::callsite::Callsite for TestCallsite2 {
+        fn set_interest(&self, _: crate::subscriber::Interest) {
+            unimplemented!()
+        }
+
+        fn metadata(&self) -> &Metadata<'_> {
+            &TEST_META_2
+        }
+    }
+
+    #[test]
+    fn value_set_with_no_values_is_empty() {
+        let fields = TEST_META_1.fields();
+        let values = &[
+            (&fields.field("foo").unwrap(), None),
+            (&fields.field("bar").unwrap(), None),
+            (&fields.field("baz").unwrap(), None),
+        ];
+        let valueset = fields.value_set(values);
+        assert!(valueset.is_empty());
+    }
+
+    #[test]
+    fn empty_value_set_is_empty() {
+        let fields = TEST_META_1.fields();
+        let valueset = fields.value_set(&[]);
+        assert!(valueset.is_empty());
+    }
+
+    #[test]
+    fn value_sets_with_fields_from_other_callsites_are_empty() {
+        let fields = TEST_META_1.fields();
+        let values = &[
+            (&fields.field("foo").unwrap(), Some(&1 as &dyn Value)),
+            (&fields.field("bar").unwrap(), Some(&2 as &dyn Value)),
+            (&fields.field("baz").unwrap(), Some(&3 as &dyn Value)),
+        ];
+        let valueset = TEST_META_2.fields().value_set(values);
+        assert!(valueset.is_empty())
+    }
+
+    #[test]
+    fn sparse_value_sets_are_not_empty() {
+        let fields = TEST_META_1.fields();
+        let values = &[
+            (&fields.field("foo").unwrap(), None),
+            (&fields.field("bar").unwrap(), Some(&57 as &dyn Value)),
+            (&fields.field("baz").unwrap(), None),
+        ];
+        let valueset = fields.value_set(values);
+        assert!(!valueset.is_empty());
+    }
+
+    #[test]
+    fn fields_from_other_callsets_are_skipped() {
+        let fields = TEST_META_1.fields();
+        let values = &[
+            (&fields.field("foo").unwrap(), None),
+            (
+                &TEST_META_2.fields().field("bar").unwrap(),
+                Some(&57 as &dyn Value),
+            ),
+            (&fields.field("baz").unwrap(), None),
+        ];
+
+        struct MyVisitor;
+        impl Visit for MyVisitor {
+            fn record_debug(&mut self, field: &Field, _: &dyn (crate::stdlib::fmt::Debug)) {
+                assert_eq!(field.callsite(), TEST_META_1.callsite())
+            }
+        }
+        let valueset = fields.value_set(values);
+        valueset.record(&mut MyVisitor);
+    }
+
+    #[test]
+    fn empty_fields_are_skipped() {
+        let fields = TEST_META_1.fields();
+        let values = &[
+            (&fields.field("foo").unwrap(), Some(&Empty as &dyn Value)),
+            (&fields.field("bar").unwrap(), Some(&57 as &dyn Value)),
+            (&fields.field("baz").unwrap(), Some(&Empty as &dyn Value)),
+        ];
+
+        struct MyVisitor;
+        impl Visit for MyVisitor {
+            fn record_debug(&mut self, field: &Field, _: &dyn (crate::stdlib::fmt::Debug)) {
+                assert_eq!(field.name(), "bar")
+            }
+        }
+        let valueset = fields.value_set(values);
+        valueset.record(&mut MyVisitor);
+    }
+
+    #[test]
+    fn record_debug_fn() {
+        let fields = TEST_META_1.fields();
+        let values = &[
+            (&fields.field("foo").unwrap(), Some(&1 as &dyn Value)),
+            (&fields.field("bar").unwrap(), Some(&2 as &dyn Value)),
+            (&fields.field("baz").unwrap(), Some(&3 as &dyn Value)),
+        ];
+        let valueset = fields.value_set(values);
+        let mut result = String::new();
+        valueset.record(&mut |_: &Field, value: &dyn fmt::Debug| {
+            use crate::stdlib::fmt::Write;
+            write!(&mut result, "{:?}", value).unwrap();
+        });
+        assert_eq!(result, "123".to_owned());
+    }
+
+    #[test]
+    #[cfg(feature = "std")]
+    fn record_error() {
+        let fields = TEST_META_1.fields();
+        let err: Box<dyn std::error::Error + Send + Sync + 'static> =
+            std::io::Error::new(std::io::ErrorKind::Other, "lol").into();
+        let values = &[
+            (&fields.field("foo").unwrap(), Some(&err as &dyn Value)),
+            (&fields.field("bar").unwrap(), Some(&Empty as &dyn Value)),
+            (&fields.field("baz").unwrap(), Some(&Empty as &dyn Value)),
+        ];
+        let valueset = fields.value_set(values);
+        let mut result = String::new();
+        valueset.record(&mut |_: &Field, value: &dyn fmt::Debug| {
+            use core::fmt::Write;
+            write!(&mut result, "{:?}", value).unwrap();
+        });
+        assert_eq!(result, format!("{}", err));
+    }
+}