Span is bytes(0..0) in many error cases #482
Description
Assume we have this trivial proc-macro-attribute:
extern crate proc_macro;
extern crate syn;
use proc_macro::TokenStream;
use syn::ImplItemMethod;
use syn::parse::Error as PError;
#[proc_macro_attribute]
pub fn dummy_attr(_: TokenStream, input: TokenStream) -> TokenStream {
match check_if_impl_item_method(input.clone()) {
Ok(()) => input,
Err(e) => e.to_compile_error().into(),
}
}
fn check_if_impl_item_method(input: TokenStream) -> Result<(), PError> {
let err_msg = "dummy_attr only applies to methods in an impl block";
match syn::parse::<ImplItemMethod>(input) {
Ok(_) => Ok(()),
Err(e) => {
println!("span = {:?}", e.span());
Err(PError::new(e.span(), err_msg))
},
}
}If we try it on a struct, we get a useful span:
extern crate dummy_attr;
pub mod simple {
use dummy_attr::dummy_attr;
#[dummy_attr]
pub struct Simple {
num: i32,
}
}Compile error:
span = #0 bytes(102..108)
error: dummy_attr only applies to methods in an impl block
--> src/lib.rs:7:9
|
7 | pub struct Simple {
| ^^^^^^
error: aborting due to previous error
But if we try it in other contexts like a module, an impl block, or a statement, the span is bytes(0..0).
For example:
extern crate dummy_attr;
pub mod simple {
use dummy_attr::dummy_attr;
pub struct Simple {
num: i32,
}
#[dummy_attr]
impl Simple {
pub fn new(n: i32) -> Self {
Self { num: n }
}
}
}The error does not have a useful span:
span = #0 bytes(0..0)
error: dummy_attr only applies to methods in an impl block
error: aborting due to previous error