New Lint: push same item to Vec (#4078)

[EthanTheMaster]

This PR has an implementation for the lint suggested in issue #4078. Pushing the same item into a Vec can be done with vec![item; SIZE] or with Vec::resize(). I did try to reduce false positives by not giving lints for for loops with conditional branching, loops, or multiple pushes.

I noticed how problematic these things can be when I saw insidesrc/ptr.rs where the branching makes the loop terminate early. As a result, these kinds of loops are not considered for linting.

for (_, ref mutbl, ref argspan) in decl
    .inputs
    .iter()
    .filter_map(|ty| get_rptr_lm(ty))
    .filter(|&(lt, _, _)| lt.name == out.name)
{
    if *mutbl == MutMutable {
        return;
    }
    immutables.push(*argspan);
}

Similarly, src/booleans.rs has multiple pushes which may occur in an iteration depending on some condition.

for simple in Bool::Not(Box::new(expr)).simplify() {
    match simple {
        Bool::Not(_) | Bool::True | Bool::False => {},
        _ => simplified.push(Bool::Not(Box::new(simple.clone()))),
    }
    let simple_negated = simple_negate(simple);
    if simplified.iter().any(|s| *s == simple_negated) {
        continue;
    }
    simplified.push(simple_negated);
}

changelog: New lint: push_same_item

[bors]

☔ The latest upstream changes (presumably #4560) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #4683) made this pull request unmergeable. Please resolve the merge conflicts.

[jplatte]

@EthanTheMaster you have merge conflict markers in your latest commit (in README.md)

[bors]

☔ The latest upstream changes (presumably #4650) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #4788) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #4839) made this pull request unmergeable. Please resolve the merge conflicts.

[flip1995]

Thanks for contributing to Clippy! Sadly this PR was not updated in quite some time. If you waited on input from a reviewer, we're sorry that this fell under the radar. If you want to continue to work on this, just reopen the PR and/or ping a reviewer.

[jonboh]

@rustbot label -S-inactive-closed