rust emits a warning at line 0 when compiling an empty proc-macro crate

[Eijebong]

Steps to reproduce:

• Create a new crate
• Set it as proc-macro = true
• Empty the src/lib.rs file
• Compile
• Profit

warning: unused variable: `registrar`
--> src/lib.rs:0:1
 |
 |
 = note: #[warn(unused_variables)] on by default
 = note: to avoid this warning, consider using `_registrar` instead

[etaoins]

This was fixed in #47655

[estebank]

Closing as per the above comment.