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    //! Spans represent periods of time in the execution of a program.
    use crate::parent::Parent;
    use crate::stdlib::num::NonZeroU64;
    use crate::{field, Metadata};

    /// Identifies a span within the context of a subscriber.
    ///
    /// They are generated by [`Subscriber`]s for each span as it is created, by
    /// the [`new_span`] trait method. See the documentation for that method for
    /// more information on span ID generation.
    ///
    /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
    /// [`new_span`]: ../subscriber/trait.Subscriber.html#method.new_span
    #[derive(Clone, Debug, PartialEq, Eq, Hash)]
    pub struct Id(NonZeroU64);

    /// Attributes provided to a `Subscriber` describing a new span when it is
    /// created.
    #[derive(Debug)]
    pub struct Attributes<'a> {
        metadata: &'static Metadata<'static>,
        values: &'a field::ValueSet<'a>,
        parent: Parent,
    }

    /// A set of fields recorded by a span.
    #[derive(Debug)]
    pub struct Record<'a> {
        values: &'a field::ValueSet<'a>,
    }

    /// Indicates what [the `Subscriber` considers] the "current" span.
    ///
    /// As subscribers may not track a notion of a current span, this has three
    /// possible states:
    /// - "unknown", indicating that the subscriber does not track a current span,
    /// - "none", indicating that the current context is known to not be in a span,
    /// - "some", with the current span's [`Id`] and [`Metadata`].
    ///
    /// [the `Subscriber` considers]: ../subscriber/trait.Subscriber.html#method.current_span
    /// [`Id`]: struct.Id.html
    /// [`Metadata`]: ../metadata/struct.Metadata.html
    #[derive(Debug)]
    pub struct Current {
        inner: CurrentInner,
    }

    #[derive(Debug)]
    enum CurrentInner {
        Current {
            id: Id,
            metadata: &'static Metadata<'static>,
        },
        None,
        Unknown,
    }

    // ===== impl Span =====

    impl Id {
        /// Constructs a new span ID from the given `u64`.
        ///
        /// <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>: Span IDs must be greater than zero.</pre></div>
        ///
        /// # Panics
        /// - If the provided `u64` is 0
        pub fn from_u64(u: u64) -> Self {
            Id(NonZeroU64::new(u).expect("span IDs must be > 0"))
        }

        /// Constructs a new span ID from the given `NonZeroU64`.
        ///
        /// Unlike [`Id::from_u64`](#method.from_u64), this will never panic.
        #[inline]
        pub const fn from_non_zero_u64(id: NonZeroU64) -> Self {
            Id(id)
        }

        // Allow `into` by-ref since we don't want to impl Copy for Id
        #[allow(clippy::wrong_self_convention)]
        /// Returns the span's ID as a `u64`.
        pub fn into_u64(&self) -> u64 {
            self.0.get()
        }

        // Allow `into` by-ref since we don't want to impl Copy for Id
        #[allow(clippy::wrong_self_convention)]
        /// Returns the span's ID as a `NonZeroU64`.
        #[inline]
        pub const fn into_non_zero_u64(&self) -> NonZeroU64 {
            self.0
        }
    }

    impl<'a> Into<Option<Id>> for &'a Id {
        fn into(self) -> Option<Id> {
            Some(self.clone())
        }
    }

    // ===== impl Attributes =====

    impl<'a> Attributes<'a> {
        /// Returns `Attributes` describing a new child span of the current span,
        /// with the provided metadata and values.
        pub fn new(metadata: &'static Metadata<'static>, values: &'a field::ValueSet<'a>) -> Self {
            Attributes {
                metadata,
                values,
                parent: Parent::Current,
            }
        }

        /// Returns `Attributes` describing a new span at the root of its own trace
        /// tree, with the provided metadata and values.
        pub fn new_root(metadata: &'static Metadata<'static>, values: &'a field::ValueSet<'a>) -> Self {
            Attributes {
                metadata,
                values,
                parent: Parent::Root,
            }
        }

        /// Returns `Attributes` describing a new child span of the specified
        /// parent span, with the provided metadata and values.
        pub fn child_of(
            parent: Id,
            metadata: &'static Metadata<'static>,
            values: &'a field::ValueSet<'a>,
        ) -> Self {
            Attributes {
                metadata,
                values,
                parent: Parent::Explicit(parent),
            }
        }

        /// Returns a reference to the new span's metadata.
        pub fn metadata(&self) -> &'static Metadata<'static> {
            self.metadata
        }

        /// Returns a reference to a `ValueSet` containing any values the new span
        /// was created with.
        pub fn values(&self) -> &field::ValueSet<'a> {
            self.values
        }

        /// Returns true if the new span should be a root.
        pub fn is_root(&self) -> bool {
            match self.parent {
                Parent::Root => true,
                _ => false,
            }
        }

        /// Returns true if the new span's parent should be determined based on the
        /// current context.
        ///
        /// If this is true and the current thread is currently inside a span, then
        /// that span should be the new span's parent. Otherwise, if the current
        /// thread is _not_ inside a span, then the new span will be the root of its
        /// own trace tree.
        pub fn is_contextual(&self) -> bool {
            match self.parent {
                Parent::Current => true,
                _ => false,
            }
        }

        /// Returns the new span's explicitly-specified parent, if there is one.
        ///
        /// Otherwise (if the new span is a root or is a child of the current span),
        /// returns false.
        pub fn parent(&self) -> Option<&Id> {
            match self.parent {
                Parent::Explicit(ref p) => Some(p),
                _ => None,
            }
        }

        /// Records all the fields in this set of `Attributes` with the provided
        /// [Visitor].
        ///
        /// [visitor]: ../field/trait.Visit.html
        pub fn record(&self, visitor: &mut dyn field::Visit) {
            self.values.record(visitor)
        }

        /// Returns `true` if this set of `Attributes` contains a value for the
        /// given `Field`.
        pub fn contains(&self, field: &field::Field) -> bool {
            self.values.contains(field)
        }

        /// Returns true if this set of `Attributes` contains _no_ values.
        pub fn is_empty(&self) -> bool {
            self.values.is_empty()
        }
    }

    // ===== impl Record =====

    impl<'a> Record<'a> {
        /// Constructs a new `Record` from a `ValueSet`.
        pub fn new(values: &'a field::ValueSet<'a>) -> Self {
            Self { values }
        }

        /// Records all the fields in this `Record` with the provided [Visitor].
        ///
        /// [visitor]: ../field/trait.Visit.html
        pub fn record(&self, visitor: &mut dyn field::Visit) {
            self.values.record(visitor)
        }

        /// Returns `true` if this `Record` contains a value for the given `Field`.
        pub fn contains(&self, field: &field::Field) -> bool {
            self.values.contains(field)
        }

        /// Returns true if this `Record` contains _no_ values.
        pub fn is_empty(&self) -> bool {
            self.values.is_empty()
        }
    }

    // ===== impl Current =====

    impl Current {
        /// Constructs a new `Current` that indicates the current context is a span
        /// with the given `metadata` and `metadata`.
        pub fn new(id: Id, metadata: &'static Metadata<'static>) -> Self {
            Self {
                inner: CurrentInner::Current { id, metadata },
            }
        }

        /// Constructs a new `Current` that indicates the current context is *not*
        /// in a span.
        pub fn none() -> Self {
            Self {
                inner: CurrentInner::None,
            }
        }

        /// Constructs a new `Current` that indicates the `Subscriber` does not
        /// track a current span.
        pub(crate) fn unknown() -> Self {
            Self {
                inner: CurrentInner::Unknown,
            }
        }

        /// Returns `true` if the `Subscriber` that constructed this `Current` tracks a
        /// current span.
        ///
        /// If this returns `true` and [`id`], [`metadata`], or [`into_inner`]
        /// return `None`, that indicates that we are currently known to *not* be
        /// inside a span. If this returns `false`, those methods will also return
        /// `None`, but in this case, that is because the subscriber does not keep
        /// track of the currently-entered span.
        ///
        /// [`id`]: #method.id
        /// [`metadata`]: #method.metadata
        /// [`into_inner`]: #method.into_inner
        pub fn is_known(&self) -> bool {
            match self.inner {
                CurrentInner::Unknown => false,
                _ => true,
            }
        }

        /// Consumes `self` and returns the span `Id` and `Metadata` of the current
        /// span, if one exists and is known.
        pub fn into_inner(self) -> Option<(Id, &'static Metadata<'static>)> {
            match self.inner {
                CurrentInner::Current { id, metadata } => Some((id, metadata)),
                _ => None,
            }
        }

        /// Borrows the `Id` of the current span, if one exists and is known.
        pub fn id(&self) -> Option<&Id> {
            match self.inner {
                CurrentInner::Current { ref id, .. } => Some(id),
                _ => None,
            }
        }

        /// Borrows the `Metadata` of the current span, if one exists and is known.
        pub fn metadata(&self) -> Option<&'static Metadata<'static>> {
            match self.inner {
                CurrentInner::Current { ref metadata, .. } => Some(*metadata),
                _ => None,
            }
        }
    }

    impl<'a> Into<Option<&'a Id>> for &'a Current {
        fn into(self) -> Option<&'a Id> {
            self.id()
        }
    }

    impl<'a> Into<Option<Id>> for &'a Current {
        fn into(self) -> Option<Id> {
            self.id().cloned()
        }
    }

    impl Into<Option<Id>> for Current {
        fn into(self) -> Option<Id> {
            self.id().cloned()
        }
    }

    impl<'a> Into<Option<&'static Metadata<'static>>> for &'a Current {
        fn into(self) -> Option<&'static Metadata<'static>> {
            self.metadata()
        }
    }