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    //! 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`, `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,
    //! we might record integers by incrementing counters for their field names,
    //! rather than printing them.
    //!
    //! [`Value`]: trait.Value.html
    //! [span]: ../span/
    //! [`Event`]: ../event/struct.Event.html
    //! [`Metadata`]: ../metadata/struct.Metadata.html
    //! [`Attributes`]:  ../span/struct.Attributes.html
    //! [`Record`]: ../span/struct.Record.html
    //! [`new_span`]: ../subscriber/trait.Subscriber.html#method.new_span
    //! [`record`]: ../subscriber/trait.Subscriber.html#method.record
    //! [`event`]:  ../subscriber/trait.Subscriber.html#method.event
    //! [`Value::record`]: trait.Value.html#method.record
    //! [`Visit`]: trait.Visit.html
    use crate::callsite;
    use crate::stdlib::{
        borrow::Borrow,
        fmt,
        hash::{Hash, Hasher},
        num,
        ops::Range,
    };

    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: &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="information">
    ///     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
    /// </div>
    /// <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>
    ///
    /// [`Value`]: trait.Value.html
    /// [recorded]: trait.Value.html#method.record
    /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
    /// [records an `Event`]: ../subscriber/trait.Subscriber.html#method.event
    /// [set of `Value`s added to a `Span`]: ../subscriber/trait.Subscriber.html#method.record
    /// [`Event`]: ../event/struct.Event.html
    /// [`ValueSet`]: struct.ValueSet.html
    pub trait Visit {
        /// 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 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="information">
        ///     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
        /// </div>
        /// <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, &format_args!("{}", 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]: trait.Visit.html
    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)
    }

    // ===== 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 {
        (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_bool(bool)
    }

    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)
        }
    }

    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> 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 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, &format_args!("{}", self.0))
        }
    }

    impl<T: fmt::Display> fmt::Debug for DisplayValue<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "{}", self.0)
        }
    }

    impl<T: fmt::Display> fmt::Display for DisplayValue<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt(&self.0, f)
        }
    }

    // ===== impl DebugValue =====

    impl<T: fmt::Debug> crate::sealed::Sealed for DebugValue<T> {}

    impl<T: fmt::Debug> 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 {
            write!(f, "{:?}", self.0)
        }
    }

    impl crate::sealed::Sealed for Empty {}
    impl Value for Empty {
        #[inline]
        fn record(&self, _: &Field, _: &mut dyn Visit) {}
    }

    // ===== impl Field =====

    impl Field {
        /// Returns an [`Identifier`] that uniquely identifies the [`Callsite`]
        /// which defines this field.
        ///
        /// [`Identifier`]: ../callsite/struct.Identifier.html
        /// [`Callsite`]: ../callsite/trait.Callsite.html
        #[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`]: ../callsite/struct.Identifier.html
        /// [`Callsite`]: ../callsite/trait.Callsite.html
        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`]: ../struct.Field.html
        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="information">
        ///     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
        /// </div>
        /// <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`]: ../callsite/struct.Identifier.html
        /// [`Callsite`]: ../callsite/trait.Callsite.html
        #[inline]
        pub fn callsite(&self) -> callsite::Identifier {
            self.fields.callsite()
        }

        /// Visits all the fields in this `ValueSet` with the provided [visitor].
        ///
        /// [visitor]: ../trait.Visit.html
        pub(crate) 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 `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(crate) 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());
        }
    }