Fix S4158 FP: rule raised where there is no guarantees of emptiness

[rjgotten]

Description

S4158 regarding iteration of known empty collections may be raised in situations where there is no guarantee that the collection is empty.

Repro steps

private static void Demo(IEnumerable<int> input)
{
  var list = new List<int>();
  var flag = false;

  foreach(var item in input)
  {
    if (item > 10)
    {
      flag = true;
      continue;
    }

    list.Add(item);
  }

  if (flag)
  {
    foreach(var item in list)
    {
      // do something with items that were <= 10
      // only when items > 10 were found.
    }
  }
}

Expected behavior

S4158 is not raised, because there is no guarantee that list will be empty when flag is true.

Actual behavior

S4158 is raised.

Known workarounds

Suppress the rule.

Related information

• C#/VB.NET Plugins version : C# 9.4.0.72892 (SonarLint 7.0.0.74072)
• Visual Studio version: 17.6.4
• MSBuild / dotnet version: dotnet 7.0.304
• Operating System: Windows 11

[Tim-Pohlmann]

Thanks @rjgotten, for reporting this. I confirm it as an FP.

[pavel-mikula-sonarsource]

It is due to the fact that we don't visit the instruction more than twice.
1st run learns flag=true and keeps list=empty
2nd run keeps flag=false and learns list=NotEmpty
So if(flag) is true only when list=empty and that's why we raise.

Finetuning these limits will be done later.

There's a possible high performance price for these changes that we need to evaluate.