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    //! Metadata describing trace data.
    use super::{callsite, field};
    use crate::stdlib::{
        cmp, fmt,
        str::FromStr,
        sync::atomic::{AtomicUsize, Ordering},
    };

    /// Metadata describing a [span] or [event].
    ///
    /// All spans and events have the following metadata:
    /// - A [name], represented as a static string.
    /// - A [target], a string that categorizes part of the system where the span
    ///   or event occurred. The `tracing` macros default to using the module
    ///   path where the span or event originated as the target, but it may be
    ///   overridden.
    /// - A [verbosity level].
    /// - The names of the [fields] defined by the span or event.
    /// - Whether the metadata corresponds to a span or event.
    ///
    /// In addition, the following optional metadata describing the source code
    /// location where the span or event originated _may_ be provided:
    /// - The [file name]
    /// - The [line number]
    /// - The [module path]
    ///
    /// Metadata is used by [`Subscriber`]s when filtering spans and events, and it
    /// may also be used as part of their data payload.
    ///
    /// When created by the `event!` or `span!` macro, the metadata describing a
    /// particular event or span is constructed statically and exists as a single
    /// static instance. Thus, the overhead of creating the metadata is
    /// _significantly_ lower than that of creating the actual span. Therefore,
    /// filtering is based on metadata, rather than on the constructed span.
    ///
    /// <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>: Although instances of <code>Metadata</code> cannot
    /// be compared directly, they provide a method <a href="struct.Metadata.html#method.id">
    /// <code>id</code></a>, returning an opaque <a href="../callsite/struct.Identifier.html">
    /// callsite identifier</a>  which uniquely identifies the callsite where the metadata
    /// originated. This can be used to determine if two <code>Metadata</code> correspond to
    /// the same callsite.
    /// </pre></div>
    ///
    /// [span]: ../span/index.html
    /// [event]: ../event/index.html
    /// [name]: #method.name
    /// [target]: #method.target
    /// [fields]: #method.fields
    /// [verbosity level]: #method.level
    /// [file name]: #method.file
    /// [line number]: #method.line
    /// [module path]: #method.module
    /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
    /// [`id`]: struct.Metadata.html#method.id
    /// [callsite identifier]: ../callsite/struct.Identifier.html
    pub struct Metadata<'a> {
        /// The name of the span described by this metadata.
        name: &'static str,

        /// The part of the system that the span that this metadata describes
        /// occurred in.
        target: &'a str,

        /// The level of verbosity of the described span.
        level: Level,

        /// The name of the Rust module where the span occurred, or `None` if this
        /// could not be determined.
        module_path: Option<&'a str>,

        /// The name of the source code file where the span occurred, or `None` if
        /// this could not be determined.
        file: Option<&'a str>,

        /// The line number in the source code file where the span occurred, or
        /// `None` if this could not be determined.
        line: Option<u32>,

        /// The names of the key-value fields attached to the described span or
        /// event.
        fields: field::FieldSet,

        /// The kind of the callsite.
        kind: Kind,
    }

    /// Indicates whether the callsite is a span or event.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub struct Kind(KindInner);

    /// Describes the level of verbosity of a span or event.
    #[derive(Copy, Clone, Debug, PartialEq, Eq)]
    pub struct Level(LevelInner);

    /// A filter comparable to a verbosity `Level`.
    ///
    /// If a `Level` is considered less than a `LevelFilter`, it should be
    /// considered disabled; if greater than or equal to the `LevelFilter`, that
    /// level is enabled.
    ///
    /// Note that this is essentially identical to the `Level` type, but with the
    /// addition of an `OFF` level that completely disables all trace
    /// instrumentation.
    #[repr(transparent)]
    #[derive(Copy, Clone, Eq, PartialEq)]
    pub struct LevelFilter(Option<Level>);

    /// Indicates that a string could not be parsed to a valid level.
    #[derive(Clone, Debug)]
    pub struct ParseLevelFilterError(());

    static MAX_LEVEL: AtomicUsize = AtomicUsize::new(LevelFilter::OFF_USIZE);

    // ===== impl Metadata =====

    impl<'a> Metadata<'a> {
        /// Construct new metadata for a span or event, with a name, target, level, field
        /// names, and optional source code location.
        pub const fn new(
            name: &'static str,
            target: &'a str,
            level: Level,
            file: Option<&'a str>,
            line: Option<u32>,
            module_path: Option<&'a str>,
            fields: field::FieldSet,
            kind: Kind,
        ) -> Self {
            Metadata {
                name,
                target,
                level,
                module_path,
                file,
                line,
                fields,
                kind,
            }
        }

        /// Returns the names of the fields on the described span or event.
        pub fn fields(&self) -> &field::FieldSet {
            &self.fields
        }

        /// Returns the level of verbosity of the described span or event.
        pub fn level(&self) -> &Level {
            &self.level
        }

        /// Returns the name of the span.
        pub fn name(&self) -> &'static str {
            self.name
        }

        /// Returns a string describing the part of the system where the span or
        /// event that this metadata describes occurred.
        ///
        /// Typically, this is the module path, but alternate targets may be set
        /// when spans or events are constructed.
        pub fn target(&self) -> &'a str {
            self.target
        }

        /// Returns the path to the Rust module where the span occurred, or
        /// `None` if the module path is unknown.
        pub fn module_path(&self) -> Option<&'a str> {
            self.module_path
        }

        /// Returns the name of the source code file where the span
        /// occurred, or `None` if the file is unknown
        pub fn file(&self) -> Option<&'a str> {
            self.file
        }

        /// Returns the line number in the source code file where the span
        /// occurred, or `None` if the line number is unknown.
        pub fn line(&self) -> Option<u32> {
            self.line
        }

        /// Returns an opaque `Identifier` that uniquely identifies the callsite
        /// this `Metadata` originated from.
        #[inline]
        pub fn callsite(&self) -> callsite::Identifier {
            self.fields.callsite()
        }

        /// Returns true if the callsite kind is `Event`.
        pub fn is_event(&self) -> bool {
            self.kind.is_event()
        }

        /// Return true if the callsite kind is `Span`.
        pub fn is_span(&self) -> bool {
            self.kind.is_span()
        }
    }

    impl<'a> fmt::Debug for Metadata<'a> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            let mut meta = f.debug_struct("Metadata");
            meta.field("name", &self.name)
                .field("target", &self.target)
                .field("level", &self.level);

            if let Some(path) = self.module_path() {
                meta.field("module_path", &path);
            }

            match (self.file(), self.line()) {
                (Some(file), Some(line)) => {
                    meta.field("location", &format_args!("{}:{}", file, line));
                }
                (Some(file), None) => {
                    meta.field("file", &format_args!("{}", file));
                }

                // Note: a line num with no file is a kind of weird case that _probably_ never occurs...
                (None, Some(line)) => {
                    meta.field("line", &line);
                }
                (None, None) => {}
            };

            meta.field("fields", &format_args!("{}", self.fields))
                .field("callsite", &self.callsite())
                .field("kind", &self.kind)
                .finish()
        }
    }

    #[derive(Clone, Debug, Eq, PartialEq)]
    enum KindInner {
        Event,
        Span,
    }

    impl Kind {
        /// `Event` callsite
        pub const EVENT: Kind = Kind(KindInner::Event);

        /// `Span` callsite
        pub const SPAN: Kind = Kind(KindInner::Span);

        /// Return true if the callsite kind is `Span`
        pub fn is_span(&self) -> bool {
            match self {
                Kind(KindInner::Span) => true,
                _ => false,
            }
        }

        /// Return true if the callsite kind is `Event`
        pub fn is_event(&self) -> bool {
            match self {
                Kind(KindInner::Event) => true,
                _ => false,
            }
        }
    }

    // ===== impl Level =====

    impl Level {
        /// The "error" level.
        ///
        /// Designates very serious errors.
        pub const ERROR: Level = Level(LevelInner::Error);
        /// The "warn" level.
        ///
        /// Designates hazardous situations.
        pub const WARN: Level = Level(LevelInner::Warn);
        /// The "info" level.
        ///
        /// Designates useful information.
        pub const INFO: Level = Level(LevelInner::Info);
        /// The "debug" level.
        ///
        /// Designates lower priority information.
        pub const DEBUG: Level = Level(LevelInner::Debug);
        /// The "trace" level.
        ///
        /// Designates very low priority, often extremely verbose, information.
        pub const TRACE: Level = Level(LevelInner::Trace);
    }

    impl fmt::Display for Level {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {
                Level::TRACE => f.pad("TRACE"),
                Level::DEBUG => f.pad("DEBUG"),
                Level::INFO => f.pad("INFO"),
                Level::WARN => f.pad("WARN"),
                Level::ERROR => f.pad("ERROR"),
            }
        }
    }

    #[cfg(feature = "std")]
    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
    impl crate::stdlib::error::Error for ParseLevelError {}

    impl FromStr for Level {
        type Err = ParseLevelError;
        fn from_str(s: &str) -> Result<Self, ParseLevelError> {
            s.parse::<usize>()
                .map_err(|_| ParseLevelError { _p: () })
                .and_then(|num| match num {
                    1 => Ok(Level::ERROR),
                    2 => Ok(Level::WARN),
                    3 => Ok(Level::INFO),
                    4 => Ok(Level::DEBUG),
                    5 => Ok(Level::TRACE),
                    _ => Err(ParseLevelError { _p: () }),
                })
                .or_else(|_| match s {
                    s if s.eq_ignore_ascii_case("error") => Ok(Level::ERROR),
                    s if s.eq_ignore_ascii_case("warn") => Ok(Level::WARN),
                    s if s.eq_ignore_ascii_case("info") => Ok(Level::INFO),
                    s if s.eq_ignore_ascii_case("debug") => Ok(Level::DEBUG),
                    s if s.eq_ignore_ascii_case("trace") => Ok(Level::TRACE),
                    _ => Err(ParseLevelError { _p: () }),
                })
        }
    }

    #[repr(usize)]
    #[derive(Copy, Clone, Debug, Hash, Eq, PartialEq)]
    enum LevelInner {
        /// The "trace" level.
        ///
        /// Designates very low priority, often extremely verbose, information.
        Trace = 0,
        /// The "debug" level.
        ///
        /// Designates lower priority information.
        Debug = 1,
        /// The "info" level.
        ///
        /// Designates useful information.
        Info = 2,
        /// The "warn" level.
        ///
        /// Designates hazardous situations.
        Warn = 3,
        /// The "error" level.
        ///
        /// Designates very serious errors.
        Error = 4,
    }

    // === impl LevelFilter ===

    impl From<Level> for LevelFilter {
        #[inline]
        fn from(level: Level) -> Self {
            Self::from_level(level)
        }
    }

    impl From<Option<Level>> for LevelFilter {
        #[inline]
        fn from(level: Option<Level>) -> Self {
            Self(level)
        }
    }

    impl Into<Option<Level>> for LevelFilter {
        #[inline]
        fn into(self) -> Option<Level> {
            self.into_level()
        }
    }

    impl LevelFilter {
        /// The "off" level.
        ///
        /// Designates that trace instrumentation should be completely disabled.
        pub const OFF: LevelFilter = LevelFilter(None);
        /// The "error" level.
        ///
        /// Designates very serious errors.
        pub const ERROR: LevelFilter = LevelFilter::from_level(Level::ERROR);
        /// The "warn" level.
        ///
        /// Designates hazardous situations.
        pub const WARN: LevelFilter = LevelFilter::from_level(Level::WARN);
        /// The "info" level.
        ///
        /// Designates useful information.
        pub const INFO: LevelFilter = LevelFilter::from_level(Level::INFO);
        /// The "debug" level.
        ///
        /// Designates lower priority information.
        pub const DEBUG: LevelFilter = LevelFilter::from_level(Level::DEBUG);
        /// The "trace" level.
        ///
        /// Designates very low priority, often extremely verbose, information.
        pub const TRACE: LevelFilter = LevelFilter(Some(Level::TRACE));

        /// Returns a `LevelFilter` that enables spans and events with verbosity up
        /// to and including `level`.
        pub const fn from_level(level: Level) -> Self {
            Self(Some(level))
        }

        /// Returns the most verbose [`Level`] that this filter accepts, or `None`
        /// if it is [`OFF`].
        ///
        /// [`Level`]: ../struct.Level.html
        /// [`OFF`]: #associatedconstant.OFF
        pub const fn into_level(self) -> Option<Level> {
            self.0
        }

        // These consts are necessary because `as` casts are not allowed as
        // match patterns.
        const ERROR_USIZE: usize = LevelInner::Error as usize;
        const WARN_USIZE: usize = LevelInner::Warn as usize;
        const INFO_USIZE: usize = LevelInner::Info as usize;
        const DEBUG_USIZE: usize = LevelInner::Debug as usize;
        const TRACE_USIZE: usize = LevelInner::Trace as usize;
        // Using the value of the last variant + 1 ensures that we match the value
        // for `Option::None` as selected by the niche optimization for
        // `LevelFilter`. If this is the case, converting a `usize` value into a
        // `LevelFilter` (in `LevelFilter::current`) will be an identity conversion,
        // rather than generating a lookup table.
        const OFF_USIZE: usize = LevelInner::Error as usize + 1;

        /// Returns a `LevelFilter` that matches the most verbose [`Level`] that any
        /// currently active [`Subscriber`] will enable.
        ///
        /// User code should treat this as a *hint*. If a given span or event has a
        /// level *higher* than the returned `LevelFilter`, it will not be enabled.
        /// However, if the level is less than or equal to this value, the span or
        /// event is *not* guaranteed to be enabled; the subscriber will still
        /// filter each callsite individually.
        ///
        /// Therefore, comparing a given span or event's level to the returned
        /// `LevelFilter` **can** be used for determining if something is
        /// *disabled*, but **should not** be used for determining if something is
        /// *enabled*.`
        ///
        /// [`Level`]: ../struct.Level.html
        /// [`Subscriber`]: ../../trait.Subscriber.html
        #[inline(always)]
        pub fn current() -> Self {
            match MAX_LEVEL.load(Ordering::Relaxed) {
                Self::ERROR_USIZE => Self::ERROR,
                Self::WARN_USIZE => Self::WARN,
                Self::INFO_USIZE => Self::INFO,
                Self::DEBUG_USIZE => Self::DEBUG,
                Self::TRACE_USIZE => Self::TRACE,
                Self::OFF_USIZE => Self::OFF,
                #[cfg(debug_assertions)]
                unknown => unreachable!(
                    "/!\\ `LevelFilter` representation seems to have changed! /!\\ \n\
                    This is a bug (and it's pretty bad). Please contact the `tracing` \
                    maintainers. Thank you and I'm sorry.\n \
                    The offending repr was: {:?}",
                    unknown,
                ),
                #[cfg(not(debug_assertions))]
                _ => unsafe {
                    // Using `unreachable_unchecked` here (rather than
                    // `unreachable!()`) is necessary to ensure that rustc generates
                    // an identity conversion from integer -> discriminant, rather
                    // than generating a lookup table. We want to ensure this
                    // function is a single `mov` instruction (on x86) if at all
                    // possible, because it is called *every* time a span/event
                    // callsite is hit; and it is (potentially) the only code in the
                    // hottest path for skipping a majority of callsites when level
                    // filtering is in use.
                    //
                    // safety: This branch is only truly unreachable if we guarantee
                    // that no values other than the possible enum discriminants
                    // will *ever* be present. The `AtomicUsize` is initialized to
                    // the `OFF` value. It is only set by the `set_max` function,
                    // which takes a `LevelFilter` as a parameter. This restricts
                    // the inputs to `set_max` to the set of valid discriminants.
                    // Therefore, **as long as `MAX_VALUE` is only ever set by
                    // `set_max`**, this is safe.
                    crate::stdlib::hint::unreachable_unchecked()
                },
            }
        }

        pub(crate) fn set_max(LevelFilter(level): LevelFilter) {
            let val = match level {
                Some(Level(level)) => level as usize,
                None => Self::OFF_USIZE,
            };

            // using an AcqRel swap ensures an ordered relationship of writes to the
            // max level.
            MAX_LEVEL.swap(val, Ordering::AcqRel);
        }
    }

    impl fmt::Display for LevelFilter {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {
                LevelFilter::OFF => f.pad("off"),
                LevelFilter::ERROR => f.pad("error"),
                LevelFilter::WARN => f.pad("warn"),
                LevelFilter::INFO => f.pad("info"),
                LevelFilter::DEBUG => f.pad("debug"),
                LevelFilter::TRACE => f.pad("trace"),
            }
        }
    }

    impl fmt::Debug for LevelFilter {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {
                LevelFilter::OFF => f.pad("LevelFilter::OFF"),
                LevelFilter::ERROR => f.pad("LevelFilter::ERROR"),
                LevelFilter::WARN => f.pad("LevelFilter::WARN"),
                LevelFilter::INFO => f.pad("LevelFilter::INFO"),
                LevelFilter::DEBUG => f.pad("LevelFilter::DEBUG"),
                LevelFilter::TRACE => f.pad("LevelFilter::TRACE"),
            }
        }
    }

    impl FromStr for LevelFilter {
        type Err = ParseLevelFilterError;
        fn from_str(from: &str) -> Result<Self, Self::Err> {
            from.parse::<usize>()
                .ok()
                .and_then(|num| match num {
                    0 => Some(LevelFilter::OFF),
                    1 => Some(LevelFilter::ERROR),
                    2 => Some(LevelFilter::WARN),
                    3 => Some(LevelFilter::INFO),
                    4 => Some(LevelFilter::DEBUG),
                    5 => Some(LevelFilter::TRACE),
                    _ => None,
                })
                .or_else(|| match from {
                    "" => Some(LevelFilter::ERROR),
                    s if s.eq_ignore_ascii_case("error") => Some(LevelFilter::ERROR),
                    s if s.eq_ignore_ascii_case("warn") => Some(LevelFilter::WARN),
                    s if s.eq_ignore_ascii_case("info") => Some(LevelFilter::INFO),
                    s if s.eq_ignore_ascii_case("debug") => Some(LevelFilter::DEBUG),
                    s if s.eq_ignore_ascii_case("trace") => Some(LevelFilter::TRACE),
                    s if s.eq_ignore_ascii_case("off") => Some(LevelFilter::OFF),
                    _ => None,
                })
                .ok_or_else(|| ParseLevelFilterError(()))
        }
    }

    /// Returned if parsing a `Level` fails.
    #[derive(Debug)]
    pub struct ParseLevelError {
        _p: (),
    }

    impl fmt::Display for ParseLevelError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad(
                "error parsing level: expected one of \"error\", \"warn\", \
                 \"info\", \"debug\", \"trace\", or a number 1-5",
            )
        }
    }

    impl fmt::Display for ParseLevelFilterError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad(
                "error parsing level filter: expected one of \"off\", \"error\", \
                \"warn\", \"info\", \"debug\", \"trace\", or a number 0-5",
            )
        }
    }

    #[cfg(feature = "std")]
    impl std::error::Error for ParseLevelFilterError {}

    // ==== Level and LevelFilter comparisons ====

    // /!\ BIG, IMPORTANT WARNING /!\
    // Do NOT mess with these implementations! They are hand-written for a reason!
    //
    // Since comparing `Level`s and `LevelFilter`s happens in a *very* hot path
    // (potentially, every time a span or event macro is hit, regardless of whether
    // or not is enabled), we *need* to ensure that these comparisons are as fast as
    // possible. Therefore, we have some requirements:
    //
    // 1. We want to do our best to ensure that rustc will generate integer-integer
    //    comparisons wherever possible.
    //
    //    The derived `Ord`/`PartialOrd` impls for `LevelFilter` will not do this,
    //    because `LevelFilter`s are represented by `Option<Level>`, rather than as
    //    a separate `#[repr(usize)]` enum. This was (unfortunately) necessary for
    //    backwards-compatibility reasons, as the  `tracing` crate's original
    //    version of `LevelFilter` defined `const fn` conversions between `Level`s
    //    and `LevelFilter`, so we're stuck with the `Option<Level>` repr.
    //    Therefore, we need hand-written `PartialOrd` impls that cast both sides of
    //    the comparison to `usize`s, to force the compiler to generate integer
    //    compares.
    //
    // 2. The hottest `Level`/`LevelFilter` comparison, the one that happens every
    //    time a callsite is hit, occurs *within the `tracing` crate's macros*.
    //    This means that the comparison is happening *inside* a crate that
    //    *depends* on `tracing-core`, not in `tracing-core` itself. The compiler
    //    will only inline function calls across crate boundaries if the called
    //    function is annotated with an `#[inline]` attribute, and we *definitely*
    //    want the comparison functions to be inlined: as previously mentioned, they
    //    should compile down to a single integer comparison on release builds, and
    //    it seems really sad to push an entire stack frame to call a function
    //    consisting of one `cmp` instruction!
    //
    //    Therefore, we need to ensure that all the comparison methods have
    //    `#[inline]` or `#[inline(always)]` attributes. It's not sufficient to just
    //    add the attribute to `partial_cmp` in a manual implementation of the
    //    trait, since it's the comparison operators (`lt`, `le`, `gt`, and `ge`)
    //    that will actually be *used*, and the default implementation of *those*
    //    methods, which calls `partial_cmp`, does not have an inline annotation.
    //
    // 3. We need the comparisons to be inverted. The discriminants for the
    //    `LevelInner` enum are assigned in "backwards" order, with `TRACE` having
    //    the *lowest* value. However, we want `TRACE` to compare greater-than all
    //    other levels.
    //
    //    Why are the numeric values inverted? In order to ensure that `LevelFilter`
    //    (which, as previously mentioned, *has* to be internally represented by an
    //    `Option<Level>`) compiles down to a single integer value. This is
    //    necessary for storing the global max in an `AtomicUsize`, and for ensuring
    //    that we use fast integer-integer comparisons, as mentioned previously. In
    //    order to ensure this, we exploit the niche optimization. The niche
    //    optimization for `Option<{enum with a numeric repr}>` will choose
    //    `(HIGHEST_DISCRIMINANT_VALUE + 1)` as the representation for `None`.
    //    Therefore, the integer representation of `LevelFilter::OFF` (which is
    //    `None`) will be the number 5. `OFF` must compare higher than every other
    //    level in order for it to filter as expected. Since we want to use a single
    //    `cmp` instruction, we can't special-case the integer value of `OFF` to
    //    compare higher, as that will generate more code. Instead, we need it to be
    //    on one end of the enum, with `ERROR` on the opposite end, so we assign the
    //    value 0 to `ERROR`.
    //
    //    This *does* mean that when parsing `LevelFilter`s or `Level`s from
    //    `String`s, the integer values are inverted, but that doesn't happen in a
    //    hot path.
    //
    //    Note that we manually invert the comparisons by swapping the left-hand and
    //    right-hand side. Using `Ordering::reverse` generates significantly worse
    //    code (per Matt Godbolt's Compiler Explorer).
    //
    // Anyway, that's a brief history of why this code is the way it is. Don't
    // change it unless you know what you're doing.

    impl PartialEq<LevelFilter> for Level {
        #[inline(always)]
        fn eq(&self, other: &LevelFilter) -> bool {
            self.0 as usize == filter_as_usize(&other.0)
        }
    }

    impl PartialOrd for Level {
        #[inline(always)]
        fn partial_cmp(&self, other: &Level) -> Option<cmp::Ordering> {
            Some(self.cmp(other))
        }

        #[inline(always)]
        fn lt(&self, other: &Level) -> bool {
            (other.0 as usize) < (self.0 as usize)
        }

        #[inline(always)]
        fn le(&self, other: &Level) -> bool {
            (other.0 as usize) <= (self.0 as usize)
        }

        #[inline(always)]
        fn gt(&self, other: &Level) -> bool {
            (other.0 as usize) > (self.0 as usize)
        }

        #[inline(always)]
        fn ge(&self, other: &Level) -> bool {
            (other.0 as usize) >= (self.0 as usize)
        }
    }

    impl Ord for Level {
        #[inline(always)]
        fn cmp(&self, other: &Self) -> cmp::Ordering {
            (other.0 as usize).cmp(&(self.0 as usize))
        }
    }

    impl PartialOrd<LevelFilter> for Level {
        #[inline(always)]
        fn partial_cmp(&self, other: &LevelFilter) -> Option<cmp::Ordering> {
            Some(filter_as_usize(&other.0).cmp(&(self.0 as usize)))
        }

        #[inline(always)]
        fn lt(&self, other: &LevelFilter) -> bool {
            filter_as_usize(&other.0) < (self.0 as usize)
        }

        #[inline(always)]
        fn le(&self, other: &LevelFilter) -> bool {
            filter_as_usize(&other.0) <= (self.0 as usize)
        }

        #[inline(always)]
        fn gt(&self, other: &LevelFilter) -> bool {
            filter_as_usize(&other.0) > (self.0 as usize)
        }

        #[inline(always)]
        fn ge(&self, other: &LevelFilter) -> bool {
            filter_as_usize(&other.0) >= (self.0 as usize)
        }
    }

    #[inline(always)]
    fn filter_as_usize(x: &Option<Level>) -> usize {
        match x {
            Some(Level(f)) => *f as usize,
            None => LevelFilter::OFF_USIZE,
        }
    }

    impl PartialEq<Level> for LevelFilter {
        #[inline(always)]
        fn eq(&self, other: &Level) -> bool {
            filter_as_usize(&self.0) == other.0 as usize
        }
    }

    impl PartialOrd for LevelFilter {
        #[inline(always)]
        fn partial_cmp(&self, other: &LevelFilter) -> Option<cmp::Ordering> {
            Some(self.cmp(other))
        }

        #[inline(always)]
        fn lt(&self, other: &LevelFilter) -> bool {
            filter_as_usize(&other.0) < filter_as_usize(&self.0)
        }

        #[inline(always)]
        fn le(&self, other: &LevelFilter) -> bool {
            filter_as_usize(&other.0) <= filter_as_usize(&self.0)
        }

        #[inline(always)]
        fn gt(&self, other: &LevelFilter) -> bool {
            filter_as_usize(&other.0) > filter_as_usize(&self.0)
        }

        #[inline(always)]
        fn ge(&self, other: &LevelFilter) -> bool {
            filter_as_usize(&other.0) >= filter_as_usize(&self.0)
        }
    }

    impl Ord for LevelFilter {
        #[inline(always)]
        fn cmp(&self, other: &Self) -> cmp::Ordering {
            filter_as_usize(&other.0).cmp(&filter_as_usize(&self.0))
        }
    }

    impl PartialOrd<Level> for LevelFilter {
        #[inline(always)]
        fn partial_cmp(&self, other: &Level) -> Option<cmp::Ordering> {
            Some((other.0 as usize).cmp(&filter_as_usize(&self.0)))
        }

        #[inline(always)]
        fn lt(&self, other: &Level) -> bool {
            (other.0 as usize) < filter_as_usize(&self.0)
        }

        #[inline(always)]
        fn le(&self, other: &Level) -> bool {
            (other.0 as usize) <= filter_as_usize(&self.0)
        }

        #[inline(always)]
        fn gt(&self, other: &Level) -> bool {
            (other.0 as usize) > filter_as_usize(&self.0)
        }

        #[inline(always)]
        fn ge(&self, other: &Level) -> bool {
            (other.0 as usize) >= filter_as_usize(&self.0)
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;
        use crate::stdlib::mem;

        #[test]
        fn level_from_str() {
            assert_eq!("error".parse::<Level>().unwrap(), Level::ERROR);
            assert_eq!("4".parse::<Level>().unwrap(), Level::DEBUG);
            assert!("0".parse::<Level>().is_err())
        }

        #[test]
        fn filter_level_conversion() {
            let mapping = [
                (LevelFilter::OFF, None),
                (LevelFilter::ERROR, Some(Level::ERROR)),
                (LevelFilter::WARN, Some(Level::WARN)),
                (LevelFilter::INFO, Some(Level::INFO)),
                (LevelFilter::DEBUG, Some(Level::DEBUG)),
                (LevelFilter::TRACE, Some(Level::TRACE)),
            ];
            for (filter, level) in mapping.iter() {
                assert_eq!(filter.clone().into_level(), *level);
                match level {
                    Some(level) => {
                        let actual: LevelFilter = level.clone().into();
                        assert_eq!(actual, *filter);
                    }
                    None => {
                        let actual: LevelFilter = None.into();
                        assert_eq!(actual, *filter);
                    }
                }
            }
        }

        #[test]
        fn level_filter_is_usize_sized() {
            assert_eq!(
                mem::size_of::<LevelFilter>(),
                mem::size_of::<usize>(),
                "`LevelFilter` is no longer `usize`-sized! global MAX_LEVEL may now be invalid!"
            )
        }

        #[test]
        fn level_filter_reprs() {
            let mapping = [
                (LevelFilter::OFF, LevelInner::Error as usize + 1),
                (LevelFilter::ERROR, LevelInner::Error as usize),
                (LevelFilter::WARN, LevelInner::Warn as usize),
                (LevelFilter::INFO, LevelInner::Info as usize),
                (LevelFilter::DEBUG, LevelInner::Debug as usize),
                (LevelFilter::TRACE, LevelInner::Trace as usize),
            ];
            for &(filter, expected) in &mapping {
                let repr = unsafe {
                    // safety: The entire purpose of this test is to assert that the
                    // actual repr matches what we expect it to be --- we're testing
                    // that *other* unsafe code is sound using the transmuted value.
                    // We're not going to do anything with it that might be unsound.
                    mem::transmute::<LevelFilter, usize>(filter)
                };
                assert_eq!(expected, repr, "repr changed for {:?}", filter)
            }
        }
    }