Module syn::spanned

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.

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.