unnecessary_operation false positive on std::ops impl with side effects

[m4rch3n1ng]

Summary

implementing Shl with side effects (like calling println!) causes clippy to still throw out an "unnecessary_operation" warning, but only if the right hand side value comes from a function call

if you implement an empty drop it also lints for the value that does not come from a function call

this is also true for other std::ops traits like Add

Lint Name

unnecessary_operation

Reproducer

(i know the code is a bit fucked up)

use std::{fmt::Display, ops::Shl};

#[allow(non_camel_case_types)]
struct cout;

impl<T> Shl<T> for cout
where
	T: Display,
{
	type Output = Self;
	fn shl(self, rhs: T) -> Self::Output {
		println!("{}", rhs);
		self
	}
}

fn n() -> i32 {
	42
}

#[allow(unused_must_use)]
#[allow(clippy::no_effect)]
fn main() {
	cout << 142; // <= no unnecessary_operation warning
	cout << n(); // <= unnecessary_operation warning
}

the output for cargo clippy:

$ cargo clippy
warning: unnecessary operation
  --> src/main.rs:25:2
   |
25 |     cout << n(); // <= unnecessary_operation warning
   |     ^^^^^^^^^^^^ help: statement can be reduced to: `cout;n();`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_operation
   = note: `#[warn(clippy::unnecessary_operation)]` on by default

warning: `stuff` (bin "stuff") generated 1 warning (run `cargo clippy --fix --bin "stuff"` to apply 1 suggestion)
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s 

the lint only triggers on the second line, and if i remove the suggestion only the 420 gets printed

$ cargo run
420
42

$ cargo clippy --fix
$ cargo run
420

if you implement Drop for cout, the lint triggers on both lines

Version

rustc 1.75.0-beta.1 (782883f60 2023-11-12)
binary: rustc
commit-hash: 782883f609713fe9617ba64d90086742ec62d374
commit-date: 2023-11-12
host: x86_64-unknown-linux-gnu
release: 1.75.0-beta.1
LLVM version: 17.0.4

Additional Labels

No response

[cocodery]

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