Spans lost with feature(proc_macro_hygiene)

[HMPerson1]

(demo: https://github.com/HMPerson1/rustc-macros-span-bug)

If we have a proc_macro like this:

#[proc_macro_attribute]
pub fn reforest(_attr: TokenStream, item: TokenStream) -> TokenStream {
    item.into_iter().collect()
}

And we invoke it like this:

#![feature(proc_macro_hygiene)]
#[reforest]
fn foo() {
    macro_rules! m {()=>()}
}

All the tokens processed by reforest have their spans set to 0.

You can see this in the warning messages:

warning: unused macro definition                                                                                                                                            
  |                                                                                                                                                                         
  = note: #[warn(unused_macros)] on by default                                                                                                                              
                                                                                                                                                                                                                                                                                                                                     
warning: function is never used: `foo`                                                                                                                                      
  |                                                                                                                                                                         
  = note: #[warn(dead_code)] on by default                                                                                                                                  

Note that everything is fine we do any of the following:

1. Change the proc_macro body to item
2. Change the proc_macro body to TokenStream::from_str(&item.to_string())
3. Comment out/delete macro_rules! m {()=>()}

[HMPerson1]

Possibly related to #54727?

[dtolnay]

Closing as a consequence of #43081. For now you only get spans if the input is formatted in the way that rustc's ugly-printer would format it. In the case of macro_rules invocations that would be:

#[reforest]
fn foo() {
    let a = 0;
    macro_rules! m (
        () => {}
    );
}