suspicious_map ignores side effects

[zvavybir]

suspicious_map ignores possible side effects of the map. That was already mentioned #5253 and fixed, but only if it mutates something and not other kinds of side effects.

Lint name:
suspicious_map

I tried this code:

use std::sync::mpsc::Sender;

fn send_iter_and_get_len<I: Iterator>(iter: I, tx: Sender<I::Item>) -> usize
{
    iter.map(|x| tx.send(x)).count()
}

I expected to see this happen: No clippy warnings.

Instead, this happened: I got the following warning and wrong suggestions:

warning: this call to `map()` won't have an effect on the call to `count()`
 --> src/main.rs:5:5
  |
5 |     iter.map(|x| tx.send(x)).count()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(clippy::suspicious_map)]` on by default
  = help: make sure you did not confuse `map` with `filter` or `for_each`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#suspicious_map

Meta

Rust version (rustc -Vv):

rustc 1.57.0-nightly (9dbb26efe 2021-10-03)
binary: rustc
commit-hash: 9dbb26efe8a886f7bba94c2b996c9535ce07f917
commit-date: 2021-10-03
host: x86_64-unknown-linux-gnu
release: 1.57.0-nightly
LLVM version: 13.0.0

[xFrednet]

I sadly don't see an easy way to fix this without creating a bunch of false negatives. I would also consider this lint to be correct here. Is there a reason why you used map? If you're only trying to interact/inspect each element, then it might be better to use inspect instead. It might be good to reference inspect() in the lint description. 🙃

[zvavybir]

I already thought that I might be missing a more appropriate method, but I figured it's still an issue since it should be mentioned. So I agree that it can be just an documentation edit. I'll open a pull request, or?

[xFrednet]

It would be awesome if you could create a PR. 🙃 The description can be found here:

rust-clippy/clippy_lints/src/methods/mod.rs

Lines 1281 to 1297 in abe551e

	declare_clippy_lint! {
	/// ### What it does
	/// Checks for calls to `map` followed by a `count`.
	///
	/// ### Why is this bad?
	/// It looks suspicious. Maybe `map` was confused with `filter`.
	/// If the `map` call is intentional, this should be rewritten. Or, if you intend to
	/// drive the iterator to completion, you can just use `for_each` instead.
	///
	/// ### Example
	/// ```rust
	/// let _ = (0..3).map(|x| x + 2).count();
	/// ```
	pub SUSPICIOUS_MAP,
	suspicious,
	"suspicious usage of map"
	}