Display derive with doc comments fails on unit structs

[dr-orlovsky]

Checklist
Put x in the boxes below:

• I am using rust compiler version no less than 1.41.1
• I am using the latest code (git checkout master && git pull origin)
• I did cargo update
• I recompiler adter removing target directory

Bug type

Put x in the boxes below:

• Build issue
• Test failing
• Runtime panic
• Incorrect results
• Unexpected/undocumented behavior

Problem description

Display derive with doc comments fails on unit structs

Reproducing

Steps to reproduce the behavior:

1. Operating system: MacOS
2. Rust compiler version: nightly
3. Code snippet:

   /// Address type do not support ONION address format and can be used only with
   /// IPv4 or IPv6 addresses
   #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, Display, Error)]
   #[display(doc_comments)]
   pub struct NoOnionSupportError;

Logs

Please copy and paste content of cargo test --all --all-features -v in a block
below right after "console" line:

error[E0308]: mismatched types
  --> src/internet.rs:30:68
   |
30 | #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, Display, Error)]
   |                                                                    ^^^^^^^
   |                                                                    |
   |                                                                    expected enum `std::result::Result`, found `()`
   |                                                                    implicitly returns `()` as its body has no tail or `return` expression
   |
   = note:   expected enum `std::result::Result<(), std::fmt::Error>`
           found unit type `()`
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)