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

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);

Start by reviewing the proposed Vec::append and Vec::extend patterns in the issue examples and determine the intended lint scope, including whether single-element arrays need separate handling. Resolve the question about lint interactions before implementing; done means the accepted cases and exclusions are clearly defined and covered by appropriate Clippy tests.