# Module syn::spanned

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Expand description

A trait that can provide the `Span` of the complete contents of a syntax tree node.

This module is available only 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.

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.

    use proc_macro::TokenStream;
    use proc_macro2::Span;
    use quote::quote_spanned;
    use syn::Type;
    use syn::spanned::Spanned;

    #[proc_macro_derive(MyMacro)]
    pub fn my_macro(input: TokenStream) -> TokenStream {
        /* ... */

        let assert_sync = quote_spanned! {ty.span()=>
            struct _AssertSync where #ty: Sync;
        };

        /* ... */
    }

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.

    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.

## Limitations

The underlying `proc_macro::Span::join` method is nightly-only. When called from within a procedural macro in a nightly compiler, `Spanned` will use `join` to produce the intended span. When not using a nightly compiler, only the span of the first token of the syntax tree node is returned.

In the common case of wanting to use the joined span as the span of a `syn::Error`, consider instead using `syn::Error::new_spanned` which is able to span the error correctly under the complete syntax tree node without needing the unstable `join`.

## Traits

Spanned

A trait that can provide the `Span` of the complete contents of a syntax tree node.