Allow same CodeWriterDeclaration to be written twice (#3854)

This came up when testing out the plugin extensibility where a visitor
would write out a method body statement. The code declaration would be
declared twice - once when actually writing the library, and the second
time when writing the method body statement from the visitor.

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/src/Writers/CodeWriter.cs

@@ -666,7 +666,11 @@ namespace Microsoft.Generator.CSharp
                 throw new InvalidOperationException("Can't declare variables inside documentation.");
             }
 
-            declaration.SetActualName(GetTemporaryVariable(declaration.RequestedName));
+            if (!declaration.HasBeenDeclared)
+            {
+                declaration.SetActualName(GetTemporaryVariable(declaration.RequestedName));
+            }
+
             return WriteDeclaration(declaration.ActualName);
         }
 

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/test/Writers/CodeWriterDeclarationTests.cs

@@ -0,0 +1,59 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using System;
+using Microsoft.Generator.CSharp.Expressions;
+using Microsoft.Generator.CSharp.Primitives;
+using Microsoft.Generator.CSharp.Providers;
+using Microsoft.Generator.CSharp.Statements;
+using NUnit.Framework;
+using static Microsoft.Generator.CSharp.Snippets.Snippet;
+
+namespace Microsoft.Generator.CSharp.Tests.Writers
+{
+    public class CodeWriterDeclarationTests
+    {
+        [Test]
+        public void DeclarationTest()
+        {
+            var expr = Declare(new VariableExpression(typeof(int), "foo"), Literal(5));
+            var ifStatement = new IfStatement(Literal(true)) { expr };
+            var codeWriter = new CodeWriter();
+            ifStatement.Write(codeWriter);
+            Assert.AreEqual(Helpers.GetExpectedFromFile(), codeWriter.ToString(false));
+        }
+
+        [Test]
+        public void DeclarationCanBeUsedTwice()
+        {
+            var declaration = new CodeWriterDeclaration("foo");
+            var localFunctionStatement = new DeclareLocalFunctionStatement(declaration, Array.Empty<ParameterProvider>(), typeof(int), Literal(5));
+
+            var codeWriter = new CodeWriter();
+            localFunctionStatement.Write(codeWriter);
+            codeWriter = new CodeWriter();
+            localFunctionStatement = new DeclareLocalFunctionStatement(declaration, Array.Empty<ParameterProvider>(), typeof(int), Literal(5));
+            localFunctionStatement.Write(codeWriter);
+            Assert.AreEqual(Helpers.GetExpectedFromFile(), codeWriter.ToString(false));
+        }
+
+        [Test]
+        [Ignore("https://github.com/microsoft/typespec/issues/3853")]
+        public void AlreadyDeclaredVariableIsRenamedInScopeWithConflictingName()
+        {
+            var expr = Declare(new VariableExpression(typeof(int), "foo"), Literal(5));
+            var usingExpr = new IfStatement(Literal(true)) { expr };
+            var codeWriter = new CodeWriter();
+            usingExpr.Write(codeWriter);
+
+            var ifStatement = new IfStatement(Literal(true))
+            {
+                Declare(new VariableExpression(typeof(string), "foo"), Literal("bar")),
+                expr
+            };
+            codeWriter = new CodeWriter();
+            ifStatement.Write(codeWriter);
+            Assert.AreEqual(Helpers.GetExpectedFromFile(), codeWriter.ToString(false));
+        }
+    }
+}

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/test/Writers/TestData/CodeWriterDeclarationTests/AlreadyDeclaredVariableIsRenamedInScopeWithConflictingName.cs

@@ -0,0 +1,5 @@
+if (true)
+{
+    string foo = "bar";
+    int foo0 = 5;
+}

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/test/Writers/TestData/CodeWriterDeclarationTests/DeclarationCanBeUsedTwice.cs

@@ -0,0 +1 @@
+int foo() => 5;

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/test/Writers/TestData/CodeWriterDeclarationTests/DeclarationTest.cs

@@ -0,0 +1,4 @@
+if (true)
+{
+    int foo = 5;
+}