Suggest `collect`ing a range instead of pushing `i` onto vector in a `for` loop

What it does

This lint finds for loops (with incrementing i) used only to populate vectors with the value of i on each execution of the loop. The lint suggest the better way of populating a vector, that is using collect on a range. Please look in the code example to see a demonstration. I also invite you to run the benchmarks yourself to see how much faster the collect method is; I have written some test and published them in this gist.

I have encountered this issue in the wild. In someone's code a vector was being filled in a for loop with consecutive numbers. To my great surprie clippy did not find this worrisome. That's why I am submitting this issue.

Lint Name

needless_for_loop

Category

perf

Advantage

• Massive performance increase in comparison to a for loop.
• More compact, idiomatic code.

Drawbacks

Populating a vector with a for loop guarantees that the vector is mutable. Populating a vector with range collection does not make such a guarantee. However if the lint is implemented correctly then it should not care about mutability nor should it change mutability if --fix is on.

Example

let mut v: Vec<i32> = Vec::new();
for i in 0..50000 {
    v.push(i);
}

Could be written as:

let mut v = (0..50000).collect::<Vec<i32>>();

Start from the needless_for_loop lint description and its Rust example. Review the benchmark gist to compare the loop and range-collect approaches. Done means the lint recognizes this vector-population pattern, suggests range collection, and does not incorrectly alter mutability when --fix is used.