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.

#[macro_use]
extern crate quote;

extern crate syn;
extern crate proc_macro;
extern crate proc_macro2;

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

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

    let def_site = Span::def_site();
    let ty_span = ty.span().resolved_at(def_site);
    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. But it is incredibly important that the span for the assertion is resolved at the procedural macro definition site rather than at the Type's span. This way we guarantee that it refers to the Sync trait that we expect. If the assertion were resolved at the same place that ty is resolved, the user could circumvent the check by defining their own Sync trait that is implemented for their type.

Traits

Spanned

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