new lint to remove redundant method calls that have no effect

What it does

Detect useless method calls like the second .trim() in string.trim().trim()

Advantage

• Improve readability
• (Possibly) improve performance

Drawbacks

No response

Example

I think covering all the examples below would be excessive, but I’ll list as many as I can think of anyway.

use std::cmp::Ordering;
use std::future::IntoFuture;
use std::net::{IpAddr, Ipv6Addr, TcpStream};
use std::process::Termination;
use std::thread::Builder;
use proc_macro::Span;

let mut vec = vec!['a', 'b', 'c'];

let _ = 1_i32.abs().abs();
let _ = 3.14_f32.floor().floor();
let _ = "Hello".trim().trim();
let _ = "Hello".to_lowercase().to_lowercase();
let _ = b"Hello".to_vec().to_vec();
let _ = (&0 as *const i32).cast::<u32>().cast::<u32>();
let _ = IpAddr::V6(Ipv6Addr::LOCALHOST).to_canonical().to_canonical();

// with arguments (be cautious of side effects)
let _ = 1_i32.max(0).max(0);
let _ = 1_i32.clamp(-1, 1).clamp(-1, 1);
let _ = vec.split_off(1).split_off(1);
let _ = Some(1).and(Some(1)).and(Some(1));
let _ = Ordering::Equal.then(Ordering::Less).then(Ordering::Less);

// rewriting might cause compilation errors (Wrap<Wrap<T>> -> Wrap<T>)
let _ = std::iter::once(1).cycle().cycle();
let _ = std::iter::once(Some(1)).fuse().fuse();
let _ = (0..).take(3).take(3);
let _ = (0..).peekable().peekable();

// some more weird examples for what it’s worth
let _ = Builder::new().stack_size(4096).stack_size(4096);
let _ = Err::<(), _>("oops").report().report();
let _ = Span::call_site().start().start();
let _ = TcpStream::connect("127.0.0.1:8080").as_socket().as_socket(); // Windows only
let _ = async {}.into_future().into_future();

Could be written as:

use std::cmp::Ordering;
use std::future::IntoFuture;
use std::net::{IpAddr, Ipv6Addr, TcpStream};
use std::process::Termination;
use std::thread::Builder;
use proc_macro::Span;

let mut vec = vec!['a', 'b', 'c'];

let _ = 1_i32.abs();
let _ = 3.14_f32.floor();
let _ = "Hello".trim();
let _ = "Hello".to_lowercase();
let _ = b"Hello".to_vec();
let _ = (&0 as *const i32).cast::<u32>();
let _ = IpAddr::V6(Ipv6Addr::LOCALHOST).to_canonical();

// with arguments (be cautious of side effects)
let _ = 1_i32.max(0);
let _ = 1_i32.clamp(-1, 1);
let _ = vec.split_off(1);
let _ = Some(1).and(Some(1));
let _ = Ordering::Equal.then(Ordering::Less);

// rewriting might cause compilation errors (Wrap<Wrap<T>> -> Wrap<T>)
let _ = std::iter::once(1).cycle();
let _ = std::iter::once(Some(1)).fuse();
let _ = (0..).take(3);
let _ = (0..).peekable();

// some more weird examples for what it’s worth
let _ = Builder::new().stack_size(4096);
let _ = Err::<(), _>("oops").report();
let _ = Span::call_site().start();
let _ = TcpStream::connect("127.0.0.1:8080").as_socket(); // Windows only
let _ = async {}.into_future();

Comparison with existing lints

redundant_clone catches useless .to_owned(), .to_string(), and .to_os_string()

#![warn(clippy::nursery)]

use std::ffi::OsStr;

fn main() {
    let _ = "Hello".to_owned().to_owned();
    let _ = 0.to_string().to_string();
    let _ = OsStr::new("foo").to_os_string().to_os_string();
}

warning: redundant clone
 --> src/main.rs:6:31
  |
6 |     let _ = "Hello".to_owned().to_owned();
  |                               ^^^^^^^^^^^ help: remove this
  |
note: this value is dropped without further use
 --> src/main.rs:6:13
  |
6 |     let _ = "Hello".to_owned().to_owned();
  |             ^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_clone
note: the lint level is defined here
 --> src/main.rs:1:9
  |
1 | #![warn(clippy::nursery)]
  |         ^^^^^^^^^^^^^^^
  = note: `#[warn(clippy::redundant_clone)]` implied by `#[warn(clippy::nursery)]`

warning: redundant clone
 --> src/main.rs:7:26
  |
7 |     let _ = 0.to_string().to_string();
  |                          ^^^^^^^^^^^^ help: remove this
  |
note: this value is dropped without further use
 --> src/main.rs:7:13
  |
7 |     let _ = 0.to_string().to_string();
  |             ^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_clone

warning: redundant clone
 --> src/main.rs:8:45
  |
8 |     let _ = OsStr::new("foo").to_os_string().to_os_string();
  |                                             ^^^^^^^^^^^^^^^ help: remove this
  |
note: this value is dropped without further use
 --> src/main.rs:8:13
  |
8 |     let _ = OsStr::new("foo").to_os_string().to_os_string();
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_clone

useless_conversion catches useless .into_iter() and From::from()

fn main() {
    let _ = Vec::<()>::new().into_iter().into_iter();
    let _ = String::from(String::from("Hello"));
}

warning: useless conversion to the same type: `std::vec::IntoIter<()>`
 --> src/main.rs:2:13
  |
2 |     let _ = Vec::<()>::new().into_iter().into_iter();
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `Vec::<()>::new().into_iter()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion
  = note: `#[warn(clippy::useless_conversion)]` on by default

warning: useless conversion to the same type: `std::string::String`
 --> src/main.rs:3:13
  |
3 |     let _ = String::from(String::from("Hello"));
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `String::from()`: `String::from("Hello")`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion

Additional Context

No response