Enable and fix IDE0007 violations (Prefer "var")

.editorconfig

@@ -300,4 +300,10 @@ dotnet_diagnostic.IDE0051.severity = warning
 dotnet_diagnostic.IDE0052.severity = warning
 
 # IDE0059: Unnecessary assignment to a value
-dotnet_diagnostic.IDE0059.severity = warning
\ No newline at end of file
+dotnet_diagnostic.IDE0059.severity = warning
+
+# Prefer "var" everywhere
+dotnet_diagnostic.IDE0007.severity = warning
+csharp_style_var_for_built_in_types = true:warning
+csharp_style_var_when_type_is_apparent = true:warning
+csharp_style_var_elsewhere = true:warning
\ No newline at end of file

src/EditorFeatures/CSharp.Wpf/Interactive/CSharpSendToInteractiveSubmissionProvider.cs

@@ -28,14 +28,14 @@ public CSharpSendToInteractiveSubmissionProvider()
         protected override bool CanParseSubmission(string code)
         {
             ParseOptions options = CSharpParseOptions.Default.WithKind(SourceCodeKind.Script);
-            SyntaxTree tree = SyntaxFactory.ParseSyntaxTree(code, options);
+            var tree = SyntaxFactory.ParseSyntaxTree(code, options);
             return tree.HasCompilationUnitRoot &&
                 !tree.GetDiagnostics().Any(diagnostic => diagnostic.Severity == DiagnosticSeverity.Error);
         }
 
         protected override IEnumerable<TextSpan> GetExecutableSyntaxTreeNodeSelection(TextSpan selectionSpan, SyntaxNode root)
         {
-            SyntaxNode expandedNode = GetSyntaxNodeForSubmission(selectionSpan, root);
+            var expandedNode = GetSyntaxNodeForSubmission(selectionSpan, root);
             return expandedNode != null
                 ? new TextSpan[] { expandedNode.Span }
                 : Array.Empty<TextSpan>();

src/EditorFeatures/CSharpTest/CodeActions/ConvertLinq/ConvertLinqQueryToForEachTests.cs

@@ -4190,7 +4190,7 @@ IEnumerable<int> M(IEnumerable<int> nums)
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsConvertQueryToForEach)]
         public async Task EnumerableFunctionDoesNotUseLocalFunctionName()
         {
-            string source = @"
+            var source = @"
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -4204,7 +4204,7 @@ public static void Main(string[] args)
         void enumerable() { }
     }
 }";
-            string output = @"
+            var output = @"
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -4232,7 +4232,7 @@ IEnumerable<int> enumerable1()
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsConvertQueryToForEach)]
         public async Task EnumerableFunctionCanUseLocalFunctionParameterName()
         {
-            string source = @"
+            var source = @"
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -4246,7 +4246,7 @@ public static void Main(string[] args)
         void M(IEnumerable<int> enumerable) { }
     }
 }";
-            string output = @"
+            var output = @"
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -4274,7 +4274,7 @@ IEnumerable<int> enumerable()
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsConvertQueryToForEach)]
         public async Task EnumerableFunctionDoesNotUseLambdaParameterNameWithCSharpLessThan8()
         {
-            string source = @"
+            var source = @"
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -4288,7 +4288,7 @@ public static void Main(string[] args)
         Action<int> myLambda = enumerable => { };
     }
 }";
-            string output = @"
+            var output = @"
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -4316,7 +4316,7 @@ IEnumerable<int> enumerable1()
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsConvertQueryToForEach)]
         public async Task EnumerableFunctionCanUseLambdaParameterNameInCSharp8()
         {
-            string source = @"
+            var source = @"
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -4330,7 +4330,7 @@ public static void Main(string[] args)
         Action<int> myLambda = enumerable => { };
     }
 }";
-            string output = @"
+            var output = @"
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -4362,7 +4362,7 @@ IEnumerable<int> enumerable()
         [WorkItem(35180, "https://github.com/dotnet/roslyn/issues/35180")]
         public async Task DeclarationSelection()
         {
-            string source = @"
+            var source = @"
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -4374,7 +4374,7 @@ public static void Main(string[] args)
         var r = [|from i in c select i+1;|]
     }
 }";
-            string output = @"
+            var output = @"
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -4401,7 +4401,7 @@ IEnumerable<int> enumerable()
         [WorkItem(35180, "https://github.com/dotnet/roslyn/issues/35180")]
         public async Task LocalAssignmentSelection()
         {
-            string source = @"
+            var source = @"
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -4414,7 +4414,7 @@ public static void Main(string[] args)
         [|r = from i in c select i+1;|]
     }
 }";
-            string output = @"
+            var output = @"
 using System;
 using System.Collections.Generic;
 using System.Linq;

src/EditorFeatures/CSharpTest/Completion/CompletionProviders/TypeImportCompletionProviderTests.cs

@@ -1020,7 +1020,7 @@ namespace Test
     class Program { }
 }";
 
-            string expectedCodeAfterCommit = @"
+            var expectedCodeAfterCommit = @"
 using Foo;
 
 namespace Test
@@ -1082,7 +1082,7 @@ class Program
     }
 }";
 
-            string expectedCodeAfterCommit = @"
+            var expectedCodeAfterCommit = @"
 using Foo;
 
 namespace Test
@@ -1141,7 +1141,7 @@ namespace Test
     class Program { }
 }";
 
-            string expectedCodeAfterCommit = @"
+            var expectedCodeAfterCommit = @"
 using Foo;
 
 namespace Test
@@ -1203,7 +1203,7 @@ class Program
     }
 }";
 
-            string expectedCodeAfterCommit = @"
+            var expectedCodeAfterCommit = @"
 using Foo;
 
 namespace Test

src/EditorFeatures/CSharpTest/Diagnostics/DiagnosticAnalyzerDriver/DiagnosticAnalyzerDriverTests.cs

@@ -624,7 +624,7 @@ public async Task TestNuGetAndVsixAnalyzer_MultipleNuGetAnalyzersCollectivelyRep
 
             diagnostics.Verify(expectedDiagnostics.Select(d => d.diagnostic).ToArray());
 
-            int index = 0;
+            var index = 0;
             foreach (var (_, expectedMessage) in expectedDiagnostics)
             {
                 Assert.Equal(expectedMessage, diagnostics[index].GetMessage());

src/EditorFeatures/CSharpTest/EditAndContinue/ActiveStatementTests.cs

@@ -4087,7 +4087,7 @@ static unsafe void Main(string[] args)
         [Fact]
         public void ForEach_Update_Nullable()
         {
-            string src1 = @"
+            var src1 = @"
 class C
 {
     static void F()
@@ -4100,7 +4100,7 @@ static void F()
     }
 }
 ";
-            string src2 = @"
+            var src2 = @"
 class C
 {
     static void F()
@@ -4122,7 +4122,7 @@ static void F()
         [Fact]
         public void ForEach_DeleteBody()
         {
-            string src1 = @"
+            var src1 = @"
 class C
 {
     static void F()
@@ -4131,7 +4131,7 @@ static void F()
     }
 }
 ";
-            string src2 = @"
+            var src2 = @"
 class C
 {
     static void F()
@@ -4149,7 +4149,7 @@ static void F()
         [Fact]
         public void ForEachVariable_DeleteBody()
         {
-            string src1 = @"
+            var src1 = @"
 class C
 {
     static void F()
@@ -4158,7 +4158,7 @@ static void F()
     }
 }
 ";
-            string src2 = @"
+            var src2 = @"
 class C
 {
     static void F()
@@ -5442,7 +5442,7 @@ public static void Main()
         [Fact]
         public void DoWhileBody_Delete()
         {
-            string src1 = @"
+            var src1 = @"
 class C
 {
     static void F()
@@ -5451,7 +5451,7 @@ static void F()
     }
 }
 ";
-            string src2 = @"
+            var src2 = @"
 class C
 {
     static void F()
@@ -10437,7 +10437,7 @@ static void F(object o1, object o2)
         [Fact]
         public void ChangeLocalNullableToNonNullable()
         {
-            string src1 = @"
+            var src1 = @"
 class C
 {
     static void F()
@@ -10446,7 +10446,7 @@ static void F()
     }
 }
 ";
-            string src2 = @"
+            var src2 = @"
 class C
 {
     static void F()
@@ -10464,7 +10464,7 @@ static void F()
         [Fact]
         public void ChangeLocalNonNullableToNullable()
         {
-            string src1 = @"
+            var src1 = @"
 class C
 {
     static void F()
@@ -10473,7 +10473,7 @@ static void F()
     }
 }
 ";
-            string src2 = @"
+            var src2 = @"
 class C
 {
     static void F()
@@ -10615,7 +10615,7 @@ public static void F()
         [Fact]
         public void Block_Delete()
         {
-            string src1 = @"
+            var src1 = @"
 class C
 {
     public static void F()
@@ -10626,7 +10626,7 @@ public static void F()
     }
 }
 ";
-            string src2 = @"
+            var src2 = @"
 class C
 {
     public static void F()

src/EditorFeatures/CSharpTest/EditAndContinue/BreakpointSpansTests.cs

@@ -74,7 +74,7 @@ public static IEnumerable<TextSpan> GetBreakpointSequence(SyntaxNode root, int p
         {
             TextSpan lastSpan = default;
             var endPosition = root.Span.End;
-            for (int p = position; p < endPosition; p++)
+            for (var p = position; p < endPosition; p++)
             {
                 if (BreakpointSpans.TryGetClosestBreakpointSpan(root, p, out var span) && span.Start > lastSpan.Start)
                 {

src/EditorFeatures/CSharpTest/EditAndContinue/TopLevelEditingTests.cs

@@ -2847,14 +2847,14 @@ static void Main(string[] b)
         [Fact]
         public void MethodUpdate_UpdateParameterToNullable()
         {
-            string src1 = @"
+            var src1 = @"
 class C
 {
     static void M(string s)
     {
     }
 }";
-            string src2 = @"
+            var src2 = @"
 class C
 {
     static void M(string? s)
@@ -2873,7 +2873,7 @@ static void M(string? s)
         [Fact]
         public void MethodUpdate_UpdateParameterToNonNullable()
         {
-            string src1 = @"
+            var src1 = @"
 class C
 {
     static void M(string? s)
@@ -2881,7 +2881,7 @@ static void M(string? s)
         
     }
 }";
-            string src2 = @"
+            var src2 = @"
 class C
 {
     static void M(string s)

src/EditorFeatures/CSharpTest/TypeInferrer/TypeInferrerTests.cs

@@ -33,7 +33,7 @@ protected override async Task TestWorkerAsync(Document document, TextSpan textSp
 
             if (mode == TestMode.Position)
             {
-                int position = node?.SpanStart ?? textSpan.Start;
+                var position = node?.SpanStart ?? textSpan.Start;
                 inferredType = typeInference.InferType(await document.ReuseExistingSpeculativeModelAsync(position, CancellationToken.None), position, objectAsDefault: true, cancellationToken: CancellationToken.None);
             }
             else

src/EditorFeatures/Core/Implementation/CodeFixes/CodeFixService.cs

@@ -205,11 +205,11 @@ public async Task<ImmutableArray<CodeFixCollection>> GetFixesAsync(Document docu
             if (result.Count > 0 && _fixerPriorityMap.TryGetValue(document.Project.Language, out var fixersForLanguage))
             {
                 // sort the result to the order defined by the fixers
-                ImmutableDictionary<CodeFixProvider, int> priorityMap = fixersForLanguage.Value;
+                var priorityMap = fixersForLanguage.Value;
                 result.Sort((d1, d2) => GetValue(d1).CompareTo(GetValue(d2)));
 
                 int GetValue(CodeFixCollection c)
-                    => priorityMap.TryGetValue((CodeFixProvider)c.Provider, out var value) ? value : int.MaxValue;
+                    => priorityMap!.TryGetValue((CodeFixProvider)c.Provider, out var value) ? value : int.MaxValue;
             }
 
             // TODO (https://github.com/dotnet/roslyn/issues/4932): Don't restrict CodeFixes in Interactive

src/EditorFeatures/Core/Implementation/EditAndContinue/ActiveStatementTrackingService.cs

@@ -206,7 +206,7 @@ private async Task TrackActiveSpansAsync(CancellationToken cancellationToken)
 
                     lock (_trackingSpans)
                     {
-                        for (int i = 0; i < baseActiveStatementSpans.Length; i++)
+                        for (var i = 0; i < baseActiveStatementSpans.Length; i++)
                         {
                             var document = currentSolution.GetDocument(openDocumentIds[i]);
                             if (document == null)
@@ -327,7 +327,7 @@ internal async Task<ImmutableArray<ActiveStatementTrackingSpan>> GetLatestSpansA
 
                     lock (_trackingSpans)
                     {
-                        bool hasExistingSpans = _trackingSpans.TryGetValue(document.Id, out var oldSpans);
+                        var hasExistingSpans = _trackingSpans.TryGetValue(document.Id, out var oldSpans);
 
                         if (activeStatementSpans.IsDefault)
                         {

src/EditorFeatures/Core/Shared/Extensions/InferredIndentationExtensions.cs

@@ -27,7 +27,7 @@ internal static class InferredIndentationOptions
 
             if (snapshot != null)
             {
-                indentationManagerService.GetIndentation(snapshot.TextBuffer, explicitFormat, out bool convertTabsToSpaces, out int tabSize, out int indentSize);
+                indentationManagerService.GetIndentation(snapshot.TextBuffer, explicitFormat, out var convertTabsToSpaces, out var tabSize, out var indentSize);
 
                 options = options.WithChangedOption(FormattingOptions.UseTabs, !convertTabsToSpaces)
                                  .WithChangedOption(FormattingOptions.IndentationSize, indentSize)

src/EditorFeatures/Test/Diagnostics/IDEDiagnosticIDConfigurationTests.cs

@@ -76,7 +76,7 @@ public class IDEDiagnosticIDConfigurationTests
             expectedLines = expected.Split(new[] { Environment.NewLine }, StringSplitOptions.RemoveEmptyEntries);
             Assert.True(expectedLines.Length % 2 == 0);
             var expectedMap = new Dictionary<string, string>();
-            for (int i = 0; i < expectedLines.Length; i += 2)
+            for (var i = 0; i < expectedLines.Length; i += 2)
             {
                 expectedMap.Add(expectedLines[i].Trim(), expectedLines[i + 1].Trim());
             }

src/EditorFeatures/Test/EditAndContinue/Helpers/SymReaderTestHelpers.cs

@@ -51,7 +51,7 @@ public static ISymUnmanagedReader5 OpenDummySymReader(ImmutableArray<byte> pdbIm
             var pdbStreamCom = SymUnmanagedStreamFactory.CreateStream(pdbStream);
             if (pdbImage.Length > 4 && pdbImage[0] == 'B' && pdbImage[1] == 'S' && pdbImage[2] == 'J' && pdbImage[3] == 'B')
             {
-                int hr = symBinder.GetReaderFromPdbStream(metadataImportProvider, pdbStreamCom, out var symReader);
+                var hr = symBinder.GetReaderFromPdbStream(metadataImportProvider, pdbStreamCom, out var symReader);
                 Assert.Equal(0, hr);
                 return (ISymUnmanagedReader5)symReader;
             }

src/EditorFeatures/TestUtilities/Workspaces/TestWorkspace_XmlConsumption.cs

@@ -765,7 +765,7 @@ private static CompilationOptions CreateCompilationOptions(TestWorkspace workspa
             HostLanguageServices languageServiceProvider,
             ImmutableArray<string> roles)
         {
-            string markupCode = documentElement.NormalizedValue();
+            var markupCode = documentElement.NormalizedValue();
 
             var folders = GetFolders(documentElement);
             var optionsElement = documentElement.Element(ParseOptionsElementName);