~Copyable struct error: cannot assign to property: 'self' is immutable

Using Swift
1

Hello!
Could, somebody, please explain why this code

struct TestNonCopyable: ~Copyable { 
     private var counter: Int 
      
     public init(counter: Int) { 
         self.counter = counter 
     } 
      
     deinit { 
         self.counter = 0 
     } 
}

leads to such error?

repl.swift:9:14: error: cannot assign to property: 'self' is immutable
        self.counter = 0
        ~~~~ ^

Swift version 5.9-dev (LLVM 677d82daa6d87d2, Swift 291fe21d5020234)
Target: x86_64-unknown-linux-gnu

2

Currently self is immutable in deinits.

3

thank you!