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13705 :: Forcing the `=dup` definition?

• darkestpigeon (orginal) [2026-02-09T12:13:55+01:00] view original

  As of

  
  Nim Compiler Version 2.3.1 [Linux: amd64]
  Compiled at 2026-02-06
  Copyright (c) 2006-2026 by Andreas Rumpf
  
  git hash: 12a2333817ad8864cbb2e36367701c908631c787
  active boot switches: -d:release
  Run

  this example

  
  type Foo = distinct int
  
  proc `=copy`(a: var Foo, b: Foo) =
    if cast[int](a) == cast[int](b):
      return
    
    copyMem(addr a, addr b, sizeof(Foo))
    echo "copy!"
  
  type Bar = object
    val: Foo
  
  
  let x = Bar(val: Foo(13))
  Run

  does not compile (example.nim(14, 12) Error: '=dup' is not provided while a custom '=copy' is defined for type 'Foo'). The analogous example with object instead of distinct throws a deprecation warning instead.

  If my understanding is correct, a custom =dup allows to skip the self-assignment check. In overwhelming number of cases, the performance impact is trivial. Am I missing something? If not, why force an additional hook and break a non-trivial amount of code?

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• Araq (orginal) [2026-02-09T12:22:23+01:00] view original

  I noticed the amount of warnings recently too. :-) I had no idea how common this is even in our own stdlib, we gotta do something about that...

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