odd span with unused import generated by macro

[durka]

I noticed this in a much more complex macro (it was recursive), but I minimized it:

macro_rules! m { ($($i:ident)*) => { use $($i)::*; } }

m!(std cmp);

warning: unused import, #[warn(unused_imports)] on by default
 --> /Users/alex/.multirust/toolchains/beta/cargo/.cargo/script-cache/expr-8aedeaeb93034069/expr.rs:5:64
  |
5 | {macro_rules! m { ($($i:ident)*) => { use $($i)::*; } } m!(std cmp);}
  |                                                                ^^^

I can't say the span is wrong... but it also doesn't seem right. cc @jonathandturner

[sophiajt]

I see something a little different, granted I tweaked your example to be:

macro_rules! m { ($($i:ident)*) => { use $($i)::*; } }
fn main() {
  m!(std cmp);
}

But I get...

warning: unused import, #[warn(unused_imports)] on by default
 --> durka.rs:3:10
  |
3 |   m!(std cmp);
  |   -------^^^--
  |   |
  |   in this macro invocation

That's not great either (primary span needs a label), but it's different

[durka]

Yeah, it is only pointing to part of the path for some reason, but I don't know where it should point -- it might not even be a contiguous span since the path is being assembled by a macro.

[sophiajt]

In the future, ideally, we'll have a macro debugger that should be a much better experience than debugging macros like this with the compiler errors.

cc @nrc - who might have some ideas.

[steveklabnik]

Triage: now is

warning: unused import: `$i`
 --> src/lib.rs:1:44
  |
1 | macro_rules! m { ($($i:ident)*) => { use $($i)::*; } }
  |                                            ^^
2 | 
3 | m!(std cmp);
  | ------------ in this macro invocation
  |
  = note: `#[warn(unused_imports)]` on by default
  = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)