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13158 :: Hint: implicit copy

• zack (orginal) [2025-07-01T13:02:19+02:00] view original

  I added this --hint:Performance:on to the compiler and this message is displayed :

  Hint: passing 'e.msg' to a sink parameter introduces an implicit copy; if possible, rearrange your program's control flow to prevent it [Performance]

  
  try:
    let opts = mycommand()
  except ValueError as e:
    raise newException(ValueError, e.msg)
  Run

  I don't understand, what should I do in my code to eliminate this warning?

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• planetis (orginal) [2025-07-01T13:54:32+02:00] view original

  Well it's a hint not a warning, so you don't have to do anything :) But it informs you that the constructor of ValueError, hidden behind the newException template, takes a copy of e.msg, since it's not the last use of e.msg. I don't think there's much to do here.

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• Araq (orginal) [2025-07-01T14:07:04+02:00] view original

  You can try:

  
  
  raise newException(ValueError, move e.msg)
  Run

  But I doubt it compiles as e is readonly. Or maybe not, I don't know. :-)

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• arnetheduck (orginal) [2025-07-01T16:16:53+02:00] view original

  e is a ref so it's probably not readonly (unfortunately) so it probably can be moved from - enforceMove can probably tell - we should really rename move to moveIfPossibleAndNotBuggy and enforceMove to move ...

  Since this is the last use of e, msg could be moved from it automatically as well - this is a similar issue as https://github.com/nim-lang/Nim/issues/23395.

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• zack (orginal) [2025-07-02T08:53:25+02:00] view original

  Thank you all for your comments.

  @Araq, with move the hint has disappeared.

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