What does 'let x = x' do in Rust?












13















I saw this code in the wild:



fields.sort_by_key(|&(_, ref field)| field.tags().into_iter().min().unwrap());

let fields = fields;


What does the let fields = fields; line do? Why is it there?






rust







share|improve this question





























    13















    I saw this code in the wild:



    fields.sort_by_key(|&(_, ref field)| field.tags().into_iter().min().unwrap());
    
let fields = fields;


    What does the let fields = fields; line do? Why is it there?






    rust







    share|improve this question



























      13












      13








      13








      I saw this code in the wild:



      fields.sort_by_key(|&(_, ref field)| field.tags().into_iter().min().unwrap());
      
let fields = fields;


      What does the let fields = fields; line do? Why is it there?






      rust







      share|improve this question
















      I saw this code in the wild:



      fields.sort_by_key(|&(_, ref field)| field.tags().into_iter().min().unwrap());
      
let fields = fields;


      What does the let fields = fields; line do? Why is it there?






      rust




      rust






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited 3 hours ago









      Shepmaster

      153k14301439




      153k14301439










      asked 3 hours ago









      timtheliontimthelion

      663919




      663919
























          2 Answers
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          active

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          23














          It makes fields immutable again.



          fields was previously defined as mutable (let mut fields = …;), to be used with sort_by_key which sorts in-place and requires the target to be mutable. The author has chosen here to explicitly prevent further mutability.



          "Downgrading" a mutable binding to immutable is quite common in Rust.



          Another common way to do this is to use a block expression:



          let fields = {
          
    let mut fields = …;
          
    fields.sort_by_key(…);
          
    fields
          
};





          share|improve this answer

































            6














            The statement let var = var; makes var immutable and bound to its current value. fields was declared as mut earlier.






            share|improve this answer























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              2 Answers
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              2 Answers
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              active

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              active

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              23














              It makes fields immutable again.



              fields was previously defined as mutable (let mut fields = …;), to be used with sort_by_key which sorts in-place and requires the target to be mutable. The author has chosen here to explicitly prevent further mutability.



              "Downgrading" a mutable binding to immutable is quite common in Rust.



              Another common way to do this is to use a block expression:



              let fields = {
              
    let mut fields = …;
              
    fields.sort_by_key(…);
              
    fields
              
};





              share|improve this answer






























                23














                It makes fields immutable again.



                fields was previously defined as mutable (let mut fields = …;), to be used with sort_by_key which sorts in-place and requires the target to be mutable. The author has chosen here to explicitly prevent further mutability.



                "Downgrading" a mutable binding to immutable is quite common in Rust.



                Another common way to do this is to use a block expression:



                let fields = {
                
    let mut fields = …;
                
    fields.sort_by_key(…);
                
    fields
                
};





                share|improve this answer




























                  23












                  23








                  23







                  It makes fields immutable again.



                  fields was previously defined as mutable (let mut fields = …;), to be used with sort_by_key which sorts in-place and requires the target to be mutable. The author has chosen here to explicitly prevent further mutability.



                  "Downgrading" a mutable binding to immutable is quite common in Rust.



                  Another common way to do this is to use a block expression:



                  let fields = {
                  
    let mut fields = …;
                  
    fields.sort_by_key(…);
                  
    fields
                  
};





                  share|improve this answer















                  It makes fields immutable again.



                  fields was previously defined as mutable (let mut fields = …;), to be used with sort_by_key which sorts in-place and requires the target to be mutable. The author has chosen here to explicitly prevent further mutability.



                  "Downgrading" a mutable binding to immutable is quite common in Rust.



                  Another common way to do this is to use a block expression:



                  let fields = {
                  
    let mut fields = …;
                  
    fields.sort_by_key(…);
                  
    fields
                  
};






                  share|improve this answer














                  share|improve this answer



                  share|improve this answer








                  edited 3 hours ago

























                  answered 3 hours ago









                  mcartonmcarton

                  6,43112435




                  6,43112435

























                      6














                      The statement let var = var; makes var immutable and bound to its current value. fields was declared as mut earlier.






                      share|improve this answer




























                        6














                        The statement let var = var; makes var immutable and bound to its current value. fields was declared as mut earlier.






                        share|improve this answer


























                          6












                          6








                          6







                          The statement let var = var; makes var immutable and bound to its current value. fields was declared as mut earlier.






                          share|improve this answer













                          The statement let var = var; makes var immutable and bound to its current value. fields was declared as mut earlier.







                          share|improve this answer












                          share|improve this answer



                          share|improve this answer










                          answered 3 hours ago









                          Govind ParmarGovind Parmar

                          9,54553257




                          9,54553257






























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