Suggestion: lint against side-effectful map

What it does

Lint against side-effects in map, or at least println!

Categories (optional)

• Kind: Correctness or Style

rustc already lints against unused iterators, but sometimes people "fix" this by marking the iterator as discarded. Clippy should be more suspicious: It is not very likely an experienced Rustacean really meant to use map and not for_each here.

Drawbacks

Occasionally, sometimes this will be on purpose.

Example

fn main() {
    let values = vec!["a", "b", "c"];
    let _results = values.into_iter()
        .map(|val| {
            println!("Value: {}", val);
        });
}

Could be written as:

fn main() {
    let values = vec!["a", "b", "c"];
    let _results = values.into_iter()
        .for_each(|val| {
            println!("Value: {}", val);
        });
}

Start by reading Clippy's existing iterator-related lints and the issue's map and for_each examples. Determine how side effects such as println! should be identified and how intentional uses can avoid noisy warnings. Done means the lint detects the described pattern, gives a useful recommendation, and covers both the warning and acceptable cases with tests.