`needless_vec`: `Vec::append` or `Vec::extend` using `vec!`.

[jakubdabek]

What it does

Detects code that uses Vec::append or Vec::extend with a newly-created vec!.

Advantage

• Removes unnecessary allocations
• Clearer intent with push instead of append/extend

Drawbacks

Should there be a separate lint for some_vec.extend([single_elem]) to use push instead? This lint could create code that would require linting again.

Example

some_vec.append(&mut vec![elem]); // 1
some_vec.extend(vec![elem]); // 2
some_vec.append(&mut vec![elem1, elem2]); // 3
some_vec.extend(vec![elem1, elem2]); // 4
// 5:
let mut v = vec![elem1, elem2];
some_vec.append(&mut v);

Could be written as:

some_vec.push(elem); // 1 and 2
some_vec.extend([elem1, elem2]); // 3 and 4
// 5
let v = [elem1, elem2];
some_vec.extend(v);

[lolbinarycat]

don't forget extend_from_slice.

[ada4a]

This could arguably be 3 separate lints:

• turn .append(&mut v) into .extend(v) (has a lint request already: #12277)
• turn .extend(vec) into extend(arr) (would be an enhancement on useless_vec)
• turn extend([elem]) into .push(elem) (would be its own lint)

This lint could create code that would require linting again.

That's fine. cargo clippy --fix will repeatedly run Clippy and apply the fixes, so the end result will be what you were looking for