Reference with supertype lifetime can't be used for subtype lifetime












1















I'm struggling to understand how subtyping of lifetimes works. The name subtype suggests to me that if 'b is a subtype of 'a, then things of type 'a can be used anywhere something of type 'b will be used. And practically, in the context of lifetimes, I don't see what could go wrong from allowing that. However, the following code



fn test<'a, 'b: 'a>(first: &'a mut str, second: &'b mut str) -> &'b str {

    // do something to choose between the two arguments,

    // eventually pick first on some branch

    first

}


doesn't work, because "these two types are declared with different lifetimes... but data from first flows into second here."



So what would go wrong if this was allowed?






rust







share|improve this question





























    1















    I'm struggling to understand how subtyping of lifetimes works. The name subtype suggests to me that if 'b is a subtype of 'a, then things of type 'a can be used anywhere something of type 'b will be used. And practically, in the context of lifetimes, I don't see what could go wrong from allowing that. However, the following code



    fn test<'a, 'b: 'a>(first: &'a mut str, second: &'b mut str) -> &'b str {
    
    // do something to choose between the two arguments,
    
    // eventually pick first on some branch
    
    first
    
}


    doesn't work, because "these two types are declared with different lifetimes... but data from first flows into second here."



    So what would go wrong if this was allowed?






    rust







    share|improve this question



























      1












      1








      1








      I'm struggling to understand how subtyping of lifetimes works. The name subtype suggests to me that if 'b is a subtype of 'a, then things of type 'a can be used anywhere something of type 'b will be used. And practically, in the context of lifetimes, I don't see what could go wrong from allowing that. However, the following code



      fn test<'a, 'b: 'a>(first: &'a mut str, second: &'b mut str) -> &'b str {
      
    // do something to choose between the two arguments,
      
    // eventually pick first on some branch
      
    first
      
}


      doesn't work, because "these two types are declared with different lifetimes... but data from first flows into second here."



      So what would go wrong if this was allowed?






      rust







      share|improve this question
















      I'm struggling to understand how subtyping of lifetimes works. The name subtype suggests to me that if 'b is a subtype of 'a, then things of type 'a can be used anywhere something of type 'b will be used. And practically, in the context of lifetimes, I don't see what could go wrong from allowing that. However, the following code



      fn test<'a, 'b: 'a>(first: &'a mut str, second: &'b mut str) -> &'b str {
      
    // do something to choose between the two arguments,
      
    // eventually pick first on some branch
      
    first
      
}


      doesn't work, because "these two types are declared with different lifetimes... but data from first flows into second here."



      So what would go wrong if this was allowed?






      rust




      rust






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited Dec 29 '18 at 16:38









      E_net4 wishes happy holidays

      12.1k63568




      12.1k63568










      asked Dec 29 '18 at 16:09









      Ben PiousBen Pious

      3,85011626




      3,85011626
























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          So what would go wrong if this was allowed?




          Your reasoning was inverted in this example: a constraint 'b: 'a reads as "'b lives as long as 'a". Since the output of test needs to live for at least as long as the lifetime 'b, 'a still represents a possibly incompatible lifetime, and first might actually not live long enough.



          If you flip the lifetimes around, the code will then compile.



          fn test<'a, 'b: 'a>(first: &'b mut str, second: &'a mut str) -> &'a str {
          
    first
          
}





          share|improve this answer























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            So what would go wrong if this was allowed?




            Your reasoning was inverted in this example: a constraint 'b: 'a reads as "'b lives as long as 'a". Since the output of test needs to live for at least as long as the lifetime 'b, 'a still represents a possibly incompatible lifetime, and first might actually not live long enough.



            If you flip the lifetimes around, the code will then compile.



            fn test<'a, 'b: 'a>(first: &'b mut str, second: &'a mut str) -> &'a str {
            
    first
            
}





            share|improve this answer




























              1















              So what would go wrong if this was allowed?




              Your reasoning was inverted in this example: a constraint 'b: 'a reads as "'b lives as long as 'a". Since the output of test needs to live for at least as long as the lifetime 'b, 'a still represents a possibly incompatible lifetime, and first might actually not live long enough.



              If you flip the lifetimes around, the code will then compile.



              fn test<'a, 'b: 'a>(first: &'b mut str, second: &'a mut str) -> &'a str {
              
    first
              
}





              share|improve this answer


























                1












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                So what would go wrong if this was allowed?




                Your reasoning was inverted in this example: a constraint 'b: 'a reads as "'b lives as long as 'a". Since the output of test needs to live for at least as long as the lifetime 'b, 'a still represents a possibly incompatible lifetime, and first might actually not live long enough.



                If you flip the lifetimes around, the code will then compile.



                fn test<'a, 'b: 'a>(first: &'b mut str, second: &'a mut str) -> &'a str {
                
    first
                
}





                share|improve this answer














                So what would go wrong if this was allowed?




                Your reasoning was inverted in this example: a constraint 'b: 'a reads as "'b lives as long as 'a". Since the output of test needs to live for at least as long as the lifetime 'b, 'a still represents a possibly incompatible lifetime, and first might actually not live long enough.



                If you flip the lifetimes around, the code will then compile.



                fn test<'a, 'b: 'a>(first: &'b mut str, second: &'a mut str) -> &'a str {
                
    first
                
}






                share|improve this answer












                share|improve this answer



                share|improve this answer










                answered Dec 29 '18 at 16:44









                E_net4 wishes happy holidaysE_net4 wishes happy holidays

                12.1k63568




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