Fix PH2030: Avoid variables named exactly "_"

Co-authored-by: bcollamore <57269455+bcollamore@users.noreply.github.com>

Author: Copilot

Documentation/Diagnostics/PH2030.md

@@ -14,9 +14,12 @@
 
 Fields look like _foo. Locals look like foo. (This analyzer does not respect IntelliSense settings in the .editorconfig. It assumes this is your naming convention.)
 
+Variables named exactly `_` (a single underscore) are not allowed as they can be confused with discards.
+
 ## How to solve
 
 Rename fields to start with an underscore.
+Rename variables that are exactly `_` to a meaningful name.
 
 ## Example
 
@@ -25,6 +28,11 @@ Code that triggers a diagnostic:
 class BadExample
 {
     private int number;
+    
+    private void ProcessData()
+    {
+        byte[] _ = ExtractData(reader);  // Variable named exactly "_"
+    }
 }
 
 ```
@@ -34,6 +42,11 @@ And the replacement code:
 class GoodExample
 {
     private int _number;
+    
+    private void ProcessData()
+    {
+        byte[] data = ExtractData(reader);  // Use a meaningful name
+    }
 }
 
 ```

Philips.CodeAnalysis.MaintainabilityAnalyzers/Naming/VariableNamingConventionAnalyzer.cs

@@ -134,6 +134,12 @@ private bool IsNameValid(Regex validator, SyntaxToken identifier)
 		{
 			var name = identifier.ValueText;
 
+			// Variables named exactly "_" should be avoided as they can be confused with discards
+			if (name == "_")
+			{
+				return false;
+			}
+
 			return validator.IsMatch(name);
 		}
 	}

Philips.CodeAnalysis.Test/Maintainability/Naming/VariableNamingConventionAnalyzerTest.cs

@@ -416,6 +416,35 @@ public async Task GeneratedCodeFilesShouldIgnoreLocalVariable()
 			await VerifySuccessfulCompilation(givenText, "GlobalSuppressions").ConfigureAwait(false);
 		}
 
+		[TestMethod]
+		[TestCategory(TestDefinitions.UnitTests)]
+		public async Task VariableNamedExactlyUnderscoreShouldFail()
+		{
+			var givenText = @"class Foo 
+{{
+    private void Bar()
+    {{
+        byte[] _ = ExtractData(reader);
+    }}
+
+    private byte[] ExtractData(object reader) => new byte[0];
+}}
+";
+			await VerifyDiagnostic(givenText, DiagnosticId.VariableNamingConventions).ConfigureAwait(false);
+		}
+
+		[TestMethod]
+		[TestCategory(TestDefinitions.UnitTests)]
+		public async Task FieldNamedExactlyUnderscoreShouldFail()
+		{
+			var givenText = @"class Foo 
+{{
+    private byte[] _ = new byte[0];
+}}
+";
+			await VerifyDiagnostic(givenText, DiagnosticId.VariableNamingConventions).ConfigureAwait(false);
+		}
+
 		#endregion
 	}
 }