Bad lint suggestion: `'_` in type definition

[mark-i-m]

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=af7140af40052d9c78faf8897ce19525

#![deny(rust_2018_idioms)]

struct Bar<'a> { x: &'a u8 }

struct Foo(Bar);

error: hidden lifetime parameters in types are deprecated
 --> src/lib.rs:6:12
  |
6 | struct Foo(Bar);
  |            ^^^- help: indicate the anonymous lifetime: `<'_>`
  |

Except that using '_ is not allowed in the type definition...