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    //! A trait that can provide the `Span` of the complete contents of a syntax
    //! tree node.
    //!
    //! *This module is available if Syn is built with both the `"parsing"` and
    //! `"printing"` features.*
    //!
    //! # Example
    //!
    //! Suppose in a procedural macro we have a [`Type`] that we want to assert
    //! implements the [`Sync`] trait. Maybe this is the type of one of the fields
    //! of a struct for which we are deriving a trait implementation, and we need to
    //! be able to pass a reference to one of those fields across threads.
    //!
    //! [`Type`]: ../enum.Type.html
    //! [`Sync`]: https://doc.rust-lang.org/std/marker/trait.Sync.html
    //!
    //! If the field type does *not* implement `Sync` as required, we want the
    //! compiler to report an error pointing out exactly which type it was.
    //!
    //! The following macro code takes a variable `ty` of type `Type` and produces a
    //! static assertion that `Sync` is implemented for that type.
    //!
    //! ```edition2018
    //! # extern crate proc_macro;
    //! #
    //! use proc_macro::TokenStream;
    //! use proc_macro2::Span;
    //! use quote::quote_spanned;
    //! use syn::Type;
    //! use syn::spanned::Spanned;
    //!
    //! # const IGNORE_TOKENS: &str = stringify! {
    //! #[proc_macro_derive(MyMacro)]
    //! # };
    //! pub fn my_macro(input: TokenStream) -> TokenStream {
    //!     # let ty = get_a_type();
    //!     /* ... */
    //!
    //!     let assert_sync = quote_spanned! {ty.span()=>
    //!         struct _AssertSync where #ty: Sync;
    //!     };
    //!
    //!     /* ... */
    //!     # input
    //! }
    //! #
    //! # fn get_a_type() -> Type {
    //! #     unimplemented!()
    //! # }
    //! ```
    //!
    //! By inserting this `assert_sync` fragment into the output code generated by
    //! our macro, the user's code will fail to compile if `ty` does not implement
    //! `Sync`. The errors they would see look like the following.
    //!
    //! ```text
    //! error[E0277]: the trait bound `*const i32: std::marker::Sync` is not satisfied
    //!   --> src/main.rs:10:21
    //!    |
    //! 10 |     bad_field: *const i32,
    //!    |                ^^^^^^^^^^ `*const i32` cannot be shared between threads safely
    //! ```
    //!
    //! In this technique, using the `Type`'s span for the error message makes the
    //! error appear in the correct place underlining the right type.

    use proc_macro2::{Span, TokenStream};
    use quote::ToTokens;

    /// A trait that can provide the `Span` of the complete contents of a syntax
    /// tree node.
    ///
    /// This trait is automatically implemented for all types that implement
    /// [`ToTokens`] from the `quote` crate. It is sealed and cannot be implemented
    /// outside of the Syn crate other than by implementing `ToTokens`.
    ///
    /// [`ToTokens`]: https://docs.rs/quote/0.6/quote/trait.ToTokens.html
    ///
    /// See the [module documentation] for an example.
    ///
    /// [module documentation]: index.html
    ///
    /// *This trait is available if Syn is built with both the `"parsing"` and
    /// `"printing"` features.*
    pub trait Spanned: private::Sealed {
        /// Returns a `Span` covering the complete contents of this syntax tree
        /// node, or [`Span::call_site()`] if this node is empty.
        ///
        /// [`Span::call_site()`]: https://docs.rs/proc-macro2/0.4/proc_macro2/struct.Span.html#method.call_site
        fn span(&self) -> Span;
    }

    mod private {
        use quote::ToTokens;
        pub trait Sealed {}
        impl<T: ToTokens> Sealed for T {}
    }

    impl<T> Spanned for T
    where
        T: ToTokens,
    {
        fn span(&self) -> Span {
            join_spans(self.into_token_stream())
        }
    }

    fn join_spans(tokens: TokenStream) -> Span {
        let mut iter = tokens.into_iter().filter_map(|tt| {
            // FIXME: This shouldn't be required, since optimally spans should
            // never be invalid. This filter_map can probably be removed when
            // https://github.com/rust-lang/rust/issues/43081 is resolved.
            let span = tt.span();
            let debug = format!("{:?}", span);
            if debug.ends_with("bytes(0..0)") {
                None
            } else {
                Some(span)
            }
        });

        let mut joined = match iter.next() {
            Some(span) => span,
            None => return Span::call_site(),
        };

        #[cfg(procmacro2_semver_exempt)]
        {
            for next in iter {
                if let Some(span) = joined.join(next) {
                    joined = span;
                }
            }
        }

        #[cfg(not(procmacro2_semver_exempt))]
        {
            // We can't join spans without procmacro2_semver_exempt so just grab the
            // first one.
            joined = joined;
        }

        joined
    }

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