write!() -> writeln!() invalid when just printing a newline

[ChrisJefferson]

This is visible on both stable and nightly. When advising changing write! to writeln!, the clippy outputs no string but still prints the comma. There has to either be no comma after out_file.

warning: using `write!()` with a format string that ends in a single newline
  --> src/gap_chat.rs:80:9
   |
80 |         write!(gap_channel.out_file, "\n").unwrap();
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(clippy::write_with_newline)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#write_with_newline
help: use `writeln!()` instead
   |
80 |         writeln!(gap_channel.out_file, ).unwrap();
   |         ^^^^^^^                       --

warning: 1 warning emitted

[giraffate]

Oh, it seems to work even if there is nothing after comma in writeln!. I didn't know that.
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9d3870a528373142c2b00ca7f8c7bc7b

[ChrisJefferson]

Yes, at first I thought the bug was no empty string, then when I put an empty string in clippy said (correctly) "Hey, you can remove that empty string!".

[th1000s]

Fixed in #7183 - however by manually constructing a Span which also removes the comma, not by making check_tts also return the comma as that function is already complex enough.

[GuillaumeGomez]

@flip1995 Can be closed.