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cyrgani opened this issue Apr 22, 2025 · 2 comments · May be fixed by #140671
Open

Fn(a: u8) should get a clear diagnostic #140169

cyrgani opened this issue Apr 22, 2025 · 2 comments · May be fixed by #140671
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@xizheyin
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A-diagnostics Area: Messages for errors, warnings, and lints A-parser Area: The lexing & parsing of Rust source code to an AST D-terse Diagnostics: An error or lint that doesn't give enough information about the problem at hand. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@cyrgani
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cyrgani commented Apr 22, 2025 •
edited by rustbot
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Code

fn f(_: impl Fn(a: u8)) {}

Current output

error: expected one of `!`, `(`, `)`, `+`, `,`, `::`, or `<`, found `:`
 --> src/lib.rs:1:18
  |
1 | fn f(_: impl Fn(a: u8)) {}
  |                  ^ expected one of 7 possible tokens

Desired output

error: `Fn`-trait syntax does not allow named arguments

Rationale and extra context

fn f(_: fn(a: u8)) {} is valid syntax, so the difference should get better diagnostics

Other cases



Rust Version


rustc 1.88.0-nightly (d6c1e454a 2025-04-21)
binary: rustc
commit-hash: d6c1e454aa8af5e7e59fbf5c4e7d3128d2f99582
commit-date: 2025-04-21
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2


Anything else?


No response






      		
    

      
  





        



            
            

              
            

        

      


      
    








      

    



      

  
            

    

      
    

    


      

          @cyrgani
cyrgani




            added
      

  A-diagnostics

  Area: Messages for errors, warnings, and lints

      

  T-compiler

  Relevant to the compiler team, which will review and decide on the PR/issue.

  labels


      Apr 22, 2025

    

  








      

  
      



  

  @petrochenkov





  


    

      

  

    

        
    
    

  


      
          
  
      
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There's an RFC issue for actually making this legal (rust-lang/rfcs#2812), but implementing parsing recovery for this would be a good first step.

      		
    

  





        


            

            
            

              
            

        

      


      

            
  

    
  
    
    

  

  


    












      

  
            

    

      
    

    


      

          @fmease
fmease




            added
      

  A-parser

  Area: The lexing & parsing of Rust source code to an AST

      

  D-terse

  Diagnostics: An error or lint that doesn't give enough information about the problem at hand.

  labels


      Apr 22, 2025

    

  








      

  
      



  

  @xizheyin





  


    

      

  

    

        
    
    

  


      
          
  
      
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            xizheyin
  

      

      

      commented


        May 3, 2025


      
    


  





      


        


  
    

      
          
@rustbot claim

      		
    

  





        


            

            
            

              
            

        

      


      

            
  

    
  
    
    

  

  


    












      

  
            


  

  

    
  


  

      

    @xizheyin
xizheyin



    linked a pull request
    
      May 5, 2025
    

    that will
    close
    this issue





  


  

      
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    Labels
  



    

      

    A-diagnostics

  Area: Messages for errors, warnings, and lints
  

    A-parser

  Area: The lexing & parsing of Rust source code to an AST
  

    D-terse

  Diagnostics: An error or lint that doesn't give enough information about the problem at hand.
  

    T-compiler

  Relevant to the compiler team, which will review and decide on the PR/issue.








      

  

    

        

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Successfully merging a pull request may close this issue.





    


  
          
      Parser: Recover error from named params while parse_path

    
      xizheyin/rust
    





      
    

  




      

    

  
      

  

    

      4 participants
    

    

        
          @petrochenkov 
        
          @fmease 
        
          @xizheyin 
        
          @cyrgani