Improve suggestions for same_item_push lint

[djc]

The suggestion to use resize() is not optimal because it needs calculation of the difference between current size of the collection and the desired size. The suggestion to use vec![] is not great because it will generally require an extra allocation. Instead, I've landed upon this solution. Maybe it could be suggested:

-        for _ in 0..forward {
-            self.buffer.push(255);
-        }
+        self.buffer.extend(std::iter::repeat(255).take(forward));

[ebroto]

The suggestion to use vec![] is not great because it will generally require an extra allocation

That suggestion is intended to replace the statement that creates the vector, so the number of allocations should be potentially smaller because the size is known beforehand, as opposed to creating it with e.g. Vec::new() and reallocating as we push.

The suggestion to use resize() is not optimal because it needs calculation of the difference between current size of the collection and the desired size

You can use the vector to get its current size, so the new size can be calculated using vec.len() + x, which I think is more concise than using repeat + take. Also, if the start of the range is different than 0, the argument of take would need to be forward - range_start, which also requires some calculation :) Note that the lint currently only knows there is a for loop; the argument is not taken into account, as it can be an arbitrary expression so not always a range.

---

I think we can improve the suggestions, though. The lint should be able to differentiate between the first loop+push (vec![] case) and the rest (resize case), and be specific in the suggestion.

Do you think that would be an improvement?

[djc]

Maybe self.buffer.resize(self.buffer.len() + forward, 255) might be better in this case. The other case I ran into was this:

     #[test]
     fn test_bin_overlap_enforced() {
-        let mut bytes = Vec::with_capacity(512);
-        let n = 31;
+        let mut bytes: Vec<u8> = Vec::with_capacity(512);
+        let n: u8 = 31;
         for _ in 0..=5 {
-            for _ in 0..=n {
-                bytes.push(n);
-            }
+            bytes.extend(iter::repeat(n).take(n as usize));
         }
         bytes.push(n + 1);
         for b in 0..n {

Which is maybe a bit more complex.

But yeah, making the resize() suggestion fully specific would help quite a bit.