diagnostics: fp single_use_lifetimes: only used in #[derive(Trait)]

[matthiaskrgr]

cargo.toml

cargo-features = ["edition"]
[package]
name = "warn"
version = "0.1.0"
authors = ["me"]
edition = "2018"
[dependencies]

src/main.rs

#![warn(single_use_lifetimes)]
mod a;
fn main() {}

src/a.rs

#[warn(single_use_lifetimes)]
#[derive(Debug)]
pub(crate) struct pathThing {
    pub(crate) path: std::path::Path,
}
#[derive(Debug, Clone)]
pub(crate) struct DirSizes<'a> {
    pub(crate) b: &'a pathThing,
}
impl<'a> DirSizes<'a> {
    pub(crate) fn new(a: &'a pathThing) -> Self {
        let b = &a;
        Self { b }
    }
}
impl<'a> std::fmt::Display for DirSizes<'a> {
    fn fmt(&self, f: &'_ mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", "hello")?;
        Ok(())
    }
}

when building, I get

warning: lifetime parameter `'a` only used once
  --> src/a.rs:19:6
   |
19 | impl<'a> std::fmt::Display for DirSizes<'a> {
   |      ^^ this lifetime...                -- ...is used only here
   |
note: lint level defined here
  --> src/main.rs:1:9
   |
1  | #![warn(single_use_lifetimes)]
   |         ^^^^^^^^^^^^^^^^^^^^
warning: lifetime parameter `'a` only used once
 --> src/a.rs:8:28
  |
7 | #[derive(Debug, Clone)]
  |          ----- ...is used only here
8 | pub(crate) struct DirSizes<'a> {
  |                            ^^ this lifetime...
warning: method is never used: `new`
  --> src/a.rs:13:5
   |
13 |     pub(crate) fn new(a: &'a pathThing) -> Self {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(dead_code)] on by default
warning: type `pathThing` should have a camel case name such as `Paththing`
 --> src/a.rs:3:1
  |
3 | / pub(crate) struct pathThing {
4 | |     pub(crate) path: std::path::Path,
5 | | }
  | |_^
  |
  = note: #[warn(non_camel_case_types)] on by default
    Finished dev [unoptimized + debuginfo] target(s) in 0.41

the

warning: lifetime parameter `'a` only used once
 --> src/a.rs:8:28
  |
7 | #[derive(Debug, Clone)]
  |          ----- ...is used only here
8 | pub(crate) struct DirSizes<'a> {
  |                            ^^ this lifetime...

is what looks sorta wrong.
rustc 1.30.0-nightly (0198a1ea4 2018-09-08)