Dafny, triggers in forall assignment

Question

in my method that converts a sequence to an array, I get a recommendation by debugger of dafny for VSCode that I can not understand what it is.

    method toArrayConvert(s:seq<int>) returns(res:array<int>)
    requires |s|>0;
    ensures |s| == res.Length;
    ensures forall i::0<=i<res.Length ==> s[i] == res[i];
    {
      res :=new int[|s|];
      forall i|0<=i && i<|s| {res[i]:=s[i];}  /*on this line I get the following*/
     // rewrite: forall i#inv: int {:trigger res[i#inv]} | 0 <= i#inv && i#inv < |s| { res[i#inv] := s[i#inv]; }
    //Not generating triggers for "res[i#inv] == s[i#inv]".
      return res;
    }

score 4 · Accepted Answer · answered May 22 '18 at 18:19

This is not a warning or error, but just a diagnostic message from Dafny telling you how it plans to encode the forall assignment. You can safely ignore it.

I agree that the message is a little bit confusing, since it contains the string "Not generating triggers", when, in fact, it has already generated a trigger. This message is due to some internal technical details of how Dafny handles forall statements. I will file an issue to look at it.

Dafny, triggers in forall assignment

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