IDE0008 (Use explicit type) Fix All operation is too broad

[sharwell]

Version Used: Visual Studio 16.4 Preview 1

Steps to Reproduce:

csharp_style_var_elsewhere = false:silent
csharp_style_var_for_built_in_types = false:warning
csharp_style_var_when_type_is_apparent = false:silent

class ClassName
{
  void Method()
  {
    var x = new object();
    var y = 0;
  }
}

Apply Fix All in Solution for the warning reported on var y.

Expected Behavior:

class ClassName
{
  void Method()
  {
    var x = new object();
    int y = 0;
  }
}

Actual Behavior:

class ClassName
{
  void Method()
  {
    object x = new object();
    int y = 0;
  }
}

[mavasani]

IMO, quite a few FixAll operations need customized semantics to adjust the scope, so this issue is not specific to this particular fixer. I think we should have each of such customizable code fixes be configurable, so that the defaults match the most common use case, but users can specify editorconfig options to configure the semantics of individual code fixes or FixAll operation.

[jasonmalinowski]

Decision from the design meeting was to update this fixer to use an equivalence key based on the rule being violated so fix all only fixes that set. A UI affordance was also suggested (which we would implement if not too expensive, but isn't considered blocking) to show in the Fix All UI a little piece of text that says "this will fix all violations of csharp_style_var_for_built_in_types" so it's clear we're only fixing a subset of them, and perhaps it'll also make it more clear which underlying rule is also causing issues.