Suggest using find rather than implementing it using a for loop

What it does

Suggests replacing a for loop that searches for an item in an iterable, with the find iterator method.

Categories (optional)

• Kind: correctness

It's easier to make mistakes in a manual implementation. Using find is simpler and easier to maintain, focusing the attention on the predicate.

Drawbacks

Suggestions might not work in complex edge cases.

If the lookup does a mapping, it might be harder to create a good suggestion. If the lookup finds an index, we should suggest Iterator::index instead of find.

Example

const ARRAY: &[u32; 5] = &[2, 7, 1, 9, 3];

fn lookup(n: u32) -> Option<u32> {
    for &v in ARRAY {
        if v == n {
            return Some(v);
        }
    }
    None
}

Could be written as:

const ARRAY: &[u32; 5] = &[2, 7, 1, 9, 3];

fn lookup(n: u32) -> Option<u32> {
    ARRAY.iter().find(|&&v| v == n).cloned()
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=80518a6a20d1a57658a6c72bd7967a44

Start from the issue's Rust examples and compare the manual lookup with Iterator::find, including the noted mapping and index cases. Determine which loop shapes are safe to suggest and how the lint should avoid complex edge cases; done means the supported patterns produce correct suggestions without changing behavior.