C#: Remove duplicated pattern extraction logic

Author: tamasvajk

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Patterns/Pattern.cs

@@ -50,6 +50,8 @@ internal static Expression Create(Context cx, PatternSyntax syntax, IExpressionP
                             }
 
                             throw new InternalError(varPattern, "Unable to get the declared symbol of the var pattern designation.");
+                        case DiscardDesignationSyntax discard:
+                            return new Expressions.Discard(cx, discard, parent, child);
                         default:
                             throw new InternalError("var pattern designation is unhandled");
                     }

csharp/extractor/Semmle.Extraction.CSharp/Entities/Statements/Case.cs

@@ -68,53 +68,9 @@ internal class CasePattern : Case<CasePatternSwitchLabelSyntax>
         private CasePattern(Context cx, CasePatternSwitchLabelSyntax node, Switch parent, int child)
             : base(cx, node, parent, child) { }
 
-        private void PopulatePattern(PatternSyntax pattern, TypeSyntax optionalType, VariableDesignationSyntax designation)
-        {
-            var isVar = optionalType is null;
-            switch (designation)
-            {
-                case SingleVariableDesignationSyntax _:
-                    if (cx.GetModel(pattern).GetDeclaredSymbol(designation) is ILocalSymbol symbol)
-                    {
-                        var type = Type.Create(cx, symbol.GetAnnotatedType());
-                        Expressions.VariableDeclaration.Create(cx, symbol, type, optionalType, cx.Create(pattern.GetLocation()), isVar, this, 0);
-                    }
-                    break;
-                case DiscardDesignationSyntax discard:
-                    if (isVar)
-                        new Expressions.Discard(cx, discard, this, 0);
-                    else
-                        Expressions.TypeAccess.Create(cx, optionalType, this, 0);
-                    break;
-                case null:
-                    break;
-                case ParenthesizedVariableDesignationSyntax paren:
-                    Expressions.VariableDeclaration.CreateParenthesized(cx, (VarPatternSyntax)pattern, paren, this, 0);
-                    break;
-                default:
-                    throw new InternalError(pattern, "Unhandled designation in case statement");
-            }
-        }
-
         protected override void PopulateStatement(TextWriter trapFile)
         {
-            switch (Stmt.Pattern)
-            {
-                case VarPatternSyntax varPattern:
-                    PopulatePattern(varPattern, null, varPattern.Designation);
-                    break;
-                case DeclarationPatternSyntax declarationPattern:
-                    PopulatePattern(declarationPattern, declarationPattern.Type, declarationPattern.Designation);
-                    break;
-                case ConstantPatternSyntax pattern:
-                    Expression.Create(cx, pattern.Expression, this, 0);
-                    break;
-                case RecursivePatternSyntax recPattern:
-                    new Expressions.RecursivePattern(cx, recPattern, this, 0);
-                    break;
-                default:
-                    throw new InternalError(Stmt, "Case pattern not handled");
-            }
+            Expressions.Pattern.Create(cx, Stmt.Pattern, this, 0);
 
             if (Stmt.WhenClause != null)
             {