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    use Span;

    /// An enum representing a diagnostic level.
    #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
    #[derive(Copy, Clone, Debug)]
    #[non_exhaustive]
    pub enum Level {
        /// An error.
        Error,
        /// A warning.
        Warning,
        /// A note.
        Note,
        /// A help message.
        Help,
    }

    /// Trait implemented by types that can be converted into a set of `Span`s.
    #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
    pub trait MultiSpan {
        /// Converts `self` into a `Vec<Span>`.
        fn into_spans(self) -> Vec<Span>;
    }

    #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
    impl MultiSpan for Span {
        fn into_spans(self) -> Vec<Span> {
            vec![self]
        }
    }

    #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
    impl MultiSpan for Vec<Span> {
        fn into_spans(self) -> Vec<Span> {
            self
        }
    }

    #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
    impl<'a> MultiSpan for &'a [Span] {
        fn into_spans(self) -> Vec<Span> {
            self.to_vec()
        }
    }

    /// A structure representing a diagnostic message and associated children
    /// messages.
    #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
    #[derive(Clone, Debug)]
    pub struct Diagnostic {
        level: Level,
        message: String,
        spans: Vec<Span>,
        children: Vec<Diagnostic>
    }

    macro_rules! diagnostic_child_methods {
        ($spanned:ident, $regular:ident, $level:expr) => (
            /// Add a new child diagnostic message to `self` with the level
            /// identified by this method's name with the given `spans` and
            /// `message`.
            #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
            pub fn $spanned<S, T>(mut self, spans: S, message: T) -> Diagnostic
                where S: MultiSpan, T: Into<String>
            {
                self.children.push(Diagnostic::spanned(spans, $level, message));
                self
            }

            /// Add a new child diagnostic message to `self` with the level
            /// identified by this method's name with the given `message`.
            #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
            pub fn $regular<T: Into<String>>(mut self, message: T) -> Diagnostic {
                self.children.push(Diagnostic::new($level, message));
                self
            }
        )
    }

    /// Iterator over the children diagnostics of a `Diagnostic`.
    #[derive(Debug, Clone)]
    #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
    pub struct Children<'a>(::std::slice::Iter<'a, Diagnostic>);

    #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
    impl<'a> Iterator for Children<'a> {
        type Item = &'a Diagnostic;

        fn next(&mut self) -> Option<Self::Item> {
            self.0.next()
        }
    }

    #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
    impl Diagnostic {
        /// Create a new diagnostic with the given `level` and `message`.
        #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
        pub fn new<T: Into<String>>(level: Level, message: T) -> Diagnostic {
            Diagnostic {
                level: level,
                message: message.into(),
                spans: vec![],
                children: vec![]
            }
        }

        /// Create a new diagnostic with the given `level` and `message` pointing to
        /// the given set of `spans`.
        #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
        pub fn spanned<S, T>(spans: S, level: Level, message: T) -> Diagnostic
            where S: MultiSpan, T: Into<String>
        {
            Diagnostic {
                level: level,
                message: message.into(),
                spans: spans.into_spans(),
                children: vec![]
            }
        }

        diagnostic_child_methods!(span_error, error, Level::Error);
        diagnostic_child_methods!(span_warning, warning, Level::Warning);
        diagnostic_child_methods!(span_note, note, Level::Note);
        diagnostic_child_methods!(span_help, help, Level::Help);

        /// Returns the diagnostic `level` for `self`.
        #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
        pub fn level(&self) -> Level {
            self.level
        }

        /// Sets the level in `self` to `level`.
        #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
        pub fn set_level(&mut self, level: Level) {
            self.level = level;
        }

        /// Returns the message in `self`.
        #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
        pub fn message(&self) -> &str {
            &self.message
        }

        /// Sets the message in `self` to `message`.
        #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
        pub fn set_message<T: Into<String>>(&mut self, message: T) {
            self.message = message.into();
        }

        /// Returns the `Span`s in `self`.
        #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
        pub fn spans(&self) -> &[Span] {
            &self.spans
        }

        /// Sets the `Span`s in `self` to `spans`.
        #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
        pub fn set_spans<S: MultiSpan>(&mut self, spans: S) {
            self.spans = spans.into_spans();
        }

        /// Returns an iterator over the children diagnostics of `self`.
        #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
        pub fn children(&self) -> Children {
            Children(self.children.iter())
        }

        /// Emit the diagnostic.
        #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
        pub fn emit(self) {
            fn to_internal(spans: Vec<Span>) -> ::bridge::client::MultiSpan {
                let mut multi_span = ::bridge::client::MultiSpan::new();
                for span in spans {
                    multi_span.push(span.0);
                }
                multi_span
            }

            let mut diag = ::bridge::client::Diagnostic::new(
                self.level,
                &self.message[..],
                to_internal(self.spans),
            );
            for c in self.children {
                diag.sub(c.level, &c.message[..], to_internal(c.spans));
            }
            diag.emit();
        }
    }

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