Warning for local variable shadowing

[aidan-waite]

New Issue Checklist

• I've Updated SwiftLint to the latest version.
• I've searched for existing GitHub issues.

Feature or Enhancement Proposal

Variable shadowing is an anti-pattern because it makes code harder to understand. It is a compiler error in many languages. I think it would be great to warn against it in SwiftLint. Here is an example:

var b: String? = "thank"

test(a:"you")

func test(a: String?) {
    if let b = a {
        print("b:\(b)")
        let b = "wow"
        print("b:\(b)")
    }
}

• The line if let b = a { shadows the above variable b
• The line let b = "wow" redeclares b as a new variable

In my opinion both of these should be covered by SwiftLint.