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