SA1313: Adding features and tests.

[azarasi3]

#2974 SA1313: Parameter '__' must begin with lower-case letter.

Support for parameter names containing only '_'

The issue with lambda expression parameter names can be resolved by apply the following three options:

• Allow
  1. All _

  System.Action<int, int, int> action = (_, _, _) => { };
  

  2. Increasing from _ by one character

  System.Action<int, int, int> action = (_, __, ___) => { };
  

• Not allow
  1. Single __ is not allowed

  System.Action<int> action = __ => { };
  

A compromise between IDE0060 and SA1313

Although it was decided not to fix it in #2599, since there is an exception rule for _ in SA1313, we think that it can be resolved by allowing parameters starting with _ as an exception when they are unused, in response to IDE0060 in SA1313.

• Exception rule:
  • Since _ in method parameter names can be used as variables, the naming convention prioritizes unused intent and allows unused parameter names starting with _.
    However, to prioritize simple code, explicit discarding is not allowed.
  • In the case where parameter names start with _ and increase by one character at a time.
• Examples
  • Allowed

    void Handler(object sender, EventArgs e) { _ = sender; _ = e; } // Explicit discards

  • Allow

    void Handler(object _, EventArg __) { }
    void MethodName(int _, int a, int __) { }

  • Not allow

    void Handler(object _1, EventArg _2) { _ = _1; _ = _2; } // Explicit discards
    void MethodName(ref int _) { ++_; }

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.45%. Comparing base (f5843ae) to head (8bd213f).
⚠️ Report is 291 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #3892    +/-   ##
========================================
  Coverage   97.44%   97.45%            
========================================
  Files         926      932     +6     
  Lines      110283   110571   +288     
  Branches     3319     3328     +9     
========================================
+ Hits       107467   107758   +291     
+ Misses       1847     1845     -2     
+ Partials      969      968     -1     

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