add lint for empty vec! invocation

[andrewbanchich]

What it does

instead of calling vec![] to create an empty vec, use Vec::new(). the vec! macro's purpose is for adding any number of items to a vec in an ergonomic and optimized way, so there is no advantage to creating an empty vec with a macro.

vec![] will expand to Vec::new() anyway, so just write Vec::new().

Advantage

• remove unnecessary macro call for times you just need an empty vec
  -- not sure if this counts as a micro/nano-optimization since there no more macro expansion
  -- at the very least, i think it's good to reserve macros for only when they provide some value

Drawbacks

No response

Example

let nums: Vec<usize> = vec![];

Could be written as:

let nums: Vec<usize> = Vec::new();

[matthiaskrgr]

Note that there have been cases where the macro actually generated better code than vec::new()

rust-lang/rust#54475

[Noratrieb]

I don't think this should be more than a restriction lint. There is nothing clearer and more readable about new imo, especially when there are several other uses of the vec macro with values in the same function, it's more consistent.