Rocket liniting issues

Recently I started to use Clippy extensively and also applied it to my Rocket-based projects.

On this page I am going to collected the linting issues I've encountered and hope to also provide answers.

examples/rocket/lint/Cargo.toml

[package]
name = "lint"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
rocket = "0.5"

examples/rocket/lint/src/main.rs

#[macro_use]
extern crate rocket;

#[get("/")]
fn hello_world() -> &'static str {
    "Hello, world!"
}

#[launch]
fn rocket() -> _ {
    rocket::build()
        .mount(
            "/",
            routes![
                hello_world
            ],
        )
}

// cargo clippy -- -Dclippy::no_effect_underscore_binding

cargo clippy -- -Dclippy::no_effect_underscore_binding

complains:

    Checking lint v0.1.0 (/home/gabor/work/rust.code-maven.com/lint)
error: binding to `_` prefixed variable with no side-effect
  --> src/main.rs:15:17
   |
15 |                 hello_world
   |                 ^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#no_effect_underscore_binding
   = note: requested on the command line with `-D clippy::no-effect-underscore-binding`

error: binding to `_` prefixed variable with no side-effect
  --> src/main.rs:15:17
   |
15 |                 hello_world
   |                 ^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#no_effect_underscore_binding

error: binding to `_` prefixed variable with no side-effect
  --> src/main.rs:10:16
   |
10 | fn rocket() -> _ {
   |                ^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#no_effect_underscore_binding

error: could not compile `lint` (bin "lint") due to 3 previous errors

Reported as no_effect_underscore_binding false positive for Rocket routes