Two options for converting character ranges into arrays

CharacterArray.swift

// We can't use `Character` or `String` ranges directly because they aren't countable
// Create a countable range of ASCII values instead
let range = UInt8(ascii: "a")...UInt8(ascii: "z") // CountableClosedRange<UInt8>

// Convert ASCII codes into Character values
range.map { Character(UnicodeScalar($0)) } // Array<Character>
// → ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]

StringArray.swift

// Create a sequence of UnicodeScalar values from "a" to "z"
let seq = sequence(first: UnicodeScalar("a")!) { scalar in
    guard scalar < UnicodeScalar("z")! else { return nil }
    return UnicodeScalar(scalar.value + 1)!
}

// Convert UnicodeScalars to strings
seq.map { String($0) } // Array<String>
// → ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]

Also check out @natecook1000's solution: making UnicodeScalar conform to Strideable.