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)