DisallowShadowing,Tests: ignore vars starting with _

In DisallowShadowing rule, ignore variables that start with
underscore (`_`). Added test for this case.

Author: webwarrior-ws

src/FSharpLint.Core/Rules/Conventions/DisallowShadowing.fs

@@ -26,7 +26,7 @@ let rec private extractIdentifiersFromSimplePats (simplePats: SynSimplePats) : L
 let private checkIdentifier (args: AstNodeRuleParams) (identifier: Ident) : array<WarningDetails> =
     let name = identifier.idText
     match args.CheckInfo with
-    | Some checkResults -> 
+    | Some checkResults when not (name.StartsWith '_') -> 
         let allUsages = checkResults.GetAllUsesOfAllSymbolsInFile()
         let definitionsWithSameName = 
             allUsages 
@@ -115,7 +115,7 @@ let private checkIdentifier (args: AstNodeRuleParams) (identifier: Ident) : arra
         else
             Array.empty
 
-    | None -> Array.empty
+    | _ -> Array.empty
 
 let runner (args: AstNodeRuleParams) =
     match args.AstNode with

tests/FSharpLint.Core.Tests/Rules/Conventions/DisallowShadowing.fs

@@ -136,3 +136,13 @@ module Foo =
 let foo = 1"""
 
         Assert.IsTrue this.NoErrorsExist
+
+    [<Test>]
+    member this.``Should not produce error when variable name starts with underscore``() =
+        this.Parse """
+let _foo = 0
+
+module Foo =
+    let _foo = 1"""
+
+        Assert.IsTrue this.NoErrorsExist