%> cat v.swift
import Foundation

func f(_ data: Data) -> Void {
   for var x in data {
      print("\(x)")
   }
}
%> swift v.swift
v.swift:4:12: warning: variable 'x' was never mutated; consider changing to 'let' constant
   for var x in data {
       ~~~~^

%> cat l.swift
import Foundation

func f(_ data: Data) -> Void {
   for let x in data {
      print("\(x)")
   }
}
%> swift l.swift
l.swift:4:8: error: 'let' pattern cannot appear nested in an already immutable context
   for let x in data {
       ^