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












24















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

























  • This line feel so nice! "Let fields be fields!"

    – val
    1 hour ago
















24















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

























  • This line feel so nice! "Let fields be fields!"

    – val
    1 hour ago














24












24








24








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 7 hours ago









Shepmaster

153k14301441




153k14301441










asked 8 hours ago









timtheliontimthelion

718920




718920













  • This line feel so nice! "Let fields be fields!"

    – val
    1 hour ago



















  • This line feel so nice! "Let fields be fields!"

    – val
    1 hour ago

















This line feel so nice! "Let fields be fields!"

– val
1 hour ago





This line feel so nice! "Let fields be fields!"

– val
1 hour ago












2 Answers
2






active

oldest

votes


















35














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
      2






      active

      oldest

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      2 Answers
      2






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      35














      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






























        35














        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




























          35












          35








          35







          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 7 hours ago

























          answered 8 hours ago









          mcartonmcarton

          6,43112535




          6,43112535

























              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 8 hours ago









                  Govind ParmarGovind Parmar

                  9,54553258




                  9,54553258






























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