Replace unsafe code with safer variant

[cynecx]

I've been digging in some of the internals of shred with emphasis on unsafe code and I've noticed this particular method:

shred/src/cell.rs

Lines 91 to 112 in c1bfcda

 pub fn map<U, F>(self, f: F) -> Ref<'a, U> 
 where 
     F: FnOnce(&T) -> &U, 
     U: ?Sized, 
 { 
     // Extract the values from the `Ref` through a pointer so that we do not run 
     // `Drop`. Because the returned `Ref` has the same lifetime `'a` as the 
     // given `Ref`, the lifetime we created through turning the pointer into 
     // a ref is valid. 
     let flag = unsafe { &*(self.flag as *const _) }; 
     let value = unsafe { &*(self.value as *const _) }; 
  
     // We have to forget self so that we do not run `Drop`. Further it's safe 
     // because we are creating a new `Ref`, with the same flag, which will 
     // run the cleanup when it's dropped. 
     std::mem::forget(self); 
  
     Ref { 
         flag, 
         value: f(value), 
     } 
 } 

I can't see why one would use unsafe here, if I am not missing anything crucial here, this could be replaced with a completely safe alternative:

    pub fn map<U, F>(self, f: F) -> Ref<'a, U>
    where
        F: FnOnce(&T) -> &U,
        U: ?Sized,
    {
        let val = Ref {
            flag: self.flag,
            value: f(self.value),
        };

        std::mem::forget(self);

        val
    }

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