New lint: unnecessary_option

[jubnzv]

What it does

This lint would warn against using the Option type for local variables when the None value is unused.

Advantage

Remove the unnecessary Options makes the source code more readable and effective.

Drawbacks

• To make the lint more effective and simple, the implementation of the lint should not be exhaustive. It makes sense to iterate over HIR and mark an Option variable as "used" if it is used as an argument of a function call, i.e. don't propagate these values across functions.
• The lint will warn only at the definition point of the local variable. Therefore, the user has to fix all the cases when the Some value is used by himself.

Example

fn test() {
  let a = Some(5);
  // Any statements here, but the `None` value of `a` is unused, e.g.:
  // if a.is_some() { /* ... */ }
  // if let Some(_) = a { /* ... */ }
}

Could be written as:

fn test() {
  let a = 5;
}