Initial work to support converting a for-statement to a foreach.

src/Features/CSharp/Portable/CSharpFeaturesResources.Designer.cs

@@ -287,6 +287,15 @@ internal class CSharpFeaturesResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Convert &apos;for&apos; to &apos;foreach&apos;.
+        /// </summary>
+        internal static string Convert_for_to_foreach {
+            get {
+                return ResourceManager.GetString("Convert_for_to_foreach", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Convert &apos;if&apos; to &apos;switch&apos;.
         /// </summary>
@@ -467,15 +476,6 @@ internal class CSharpFeaturesResources {
             }
         }
         
-        /// <summary>
-        ///   Looks up a localized string similar to &lt;implicit array creation&gt;.
-        /// </summary>
-        internal static string implicit_array_creation {
-            get {
-                return ResourceManager.GetString("implicit_array_creation", resourceCulture);
-            }
-        }
-        
         /// <summary>
         ///   Looks up a localized string similar to indexer.
         /// </summary>

src/Features/CSharp/Portable/CSharpFeaturesResources.resx

@@ -521,4 +521,7 @@
   <data name="Add_parentheses_around_conditional_expression_in_interpolated_string" xml:space="preserve">
     <value>Add parentheses</value>
   </data>
+  <data name="Convert_for_to_foreach" xml:space="preserve">
+    <value>Convert 'for' to 'foreach'</value>
+  </data>
 </root>
\ No newline at end of file

src/Features/CSharp/Portable/ConvertForToForEach/CSharpConvertForToForEachCodeRefactoringProvider.cs

+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Composition;
+using System.Text;
+using System.Threading;
+using Microsoft.CodeAnalysis.CodeRefactorings;
+using Microsoft.CodeAnalysis.ConvertForToForEach;
+using Microsoft.CodeAnalysis.CSharp.CodeGeneration;
+using Microsoft.CodeAnalysis.CSharp.CodeStyle.TypeStyle;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Options;
+
+namespace Microsoft.CodeAnalysis.CSharp.ConvertForToForEach
+{
+    [ExportCodeRefactoringProvider(LanguageNames.CSharp, Name = nameof(CSharpConvertForToForEachCodeRefactoringProvider)), Shared]
+    internal class CSharpConvertForToForEachCodeRefactoringProvider :
+        AbstractConvertForToForEachCodeRefactoringProvider<
+            StatementSyntax, 
+            ForStatementSyntax, 
+            ExpressionSyntax,
+            MemberAccessExpressionSyntax,
+            TypeSyntax,
+            VariableDeclaratorSyntax>
+    {
+        protected override string GetTitle()
+            => CSharpFeaturesResources.Convert_for_to_foreach;
+
+        protected override SyntaxList<StatementSyntax> GetBodyStatements(ForStatementSyntax forStatement)
+        {
+            if (forStatement.Statement is BlockSyntax block)
+            {
+                return block.Statements;
+            }
+
+            return SyntaxFactory.SingletonList(forStatement.Statement);
+        }
+        protected override bool TryGetForStatementComponents(
+            ForStatementSyntax forStatement,
+            out SyntaxToken iterationVariable, out ExpressionSyntax initializer,
+            out MemberAccessExpressionSyntax memberAccess, out ExpressionSyntax stepValue,
+            CancellationToken cancellationToken)
+        {
+            if (forStatement.Declaration != null &&
+                forStatement.Condition is BinaryExpressionSyntax binaryExpression &&
+                forStatement.Incrementors.Count == 1)
+            {
+                var declaration = forStatement.Declaration;
+                if (declaration.Variables.Count == 1)
+                {
+                    var declarator = declaration.Variables[0];
+                    if (declarator.Initializer != null)
+                    {
+                        iterationVariable = declarator.Identifier;
+                        initializer = declarator.Initializer.Value;
+
+                        // Look for:  i < expr.Length
+                        if (binaryExpression.Kind() == SyntaxKind.LessThanExpression &&
+                            binaryExpression.Left is IdentifierNameSyntax identifierName &&
+                            identifierName.Identifier.ValueText == iterationVariable.ValueText &&
+                            binaryExpression.Right is MemberAccessExpressionSyntax)
+                        {
+                            memberAccess = (MemberAccessExpressionSyntax)binaryExpression.Right;
+
+                            var incrementor = forStatement.Incrementors[0];
+
+                            if (TryGetStepValue(iterationVariable, incrementor, out stepValue, cancellationToken))
+                            {
+                                return true;
+                            }
+                        }
+                    }
+                }
+            }
+
+            iterationVariable = default;
+            memberAccess = default;
+            initializer = default;
+            stepValue = default;
+            return false;
+        }
+
+        private static bool TryGetStepValue(
+            SyntaxToken iterationVariable, ExpressionSyntax incrementor,
+            out ExpressionSyntax stepValue, CancellationToken cancellationToken)
+        {
+            // support
+            //  x++
+            //  ++x
+            //  x += constant_1
+
+            ExpressionSyntax operand;
+            if (incrementor.Kind() == SyntaxKind.PostIncrementExpression)
+            {
+                operand = ((PostfixUnaryExpressionSyntax)incrementor).Operand;
+                stepValue = default;
+            }
+            else if (incrementor.Kind() == SyntaxKind.PreIncrementExpression)
+            {
+                operand = ((PrefixUnaryExpressionSyntax)incrementor).Operand;
+                stepValue = default;
+            }
+            else if (incrementor.Kind() == SyntaxKind.AddAssignmentExpression)
+            {
+                var assignment = (AssignmentExpressionSyntax)incrementor;
+                operand = assignment.Left;
+                stepValue = assignment.Right;
+            }
+            else
+            {
+                stepValue = default;
+                return false;
+            }
+
+            return operand is IdentifierNameSyntax identifierName &&
+                identifierName.Identifier.ValueText == iterationVariable.ValueText;
+        }
+
+
+        protected override SyntaxNode ConvertForNode(
+            ForStatementSyntax forStatement, TypeSyntax typeNode, 
+            SyntaxToken foreachIdentifier, ExpressionSyntax collectionExpression,
+            ITypeSymbol iterationVariableType, OptionSet optionSet)
+        {
+            if (typeNode == null)
+            {
+                // types are not apperant in foreach statements.
+                var isBuiltInTypeContext = TypeStyleHelper.IsBuiltInType(iterationVariableType);
+                if (TypeStyleHelper.IsImplicitStylePreferred(
+                        optionSet, isBuiltInTypeContext, isTypeApparentContext: false))
+                {
+                    typeNode = SyntaxFactory.IdentifierName("var");
+                }
+                else
+                {
+                    typeNode = (TypeSyntax)CSharpSyntaxGenerator.Instance.TypeExpression(iterationVariableType);
+                }
+            }
+
+            return SyntaxFactory.ForEachStatement(
+                SyntaxFactory.Token(SyntaxKind.ForEachKeyword).WithTriviaFrom(forStatement.ForKeyword),
+                forStatement.OpenParenToken,
+                typeNode,
+                foreachIdentifier,
+                SyntaxFactory.Token(SyntaxKind.InKeyword),
+                collectionExpression,
+                forStatement.CloseParenToken,
+                forStatement.Statement);
+        }
+
+        protected override bool IsValidVariableDeclarator(VariableDeclaratorSyntax firstVariable)
+            => true;
+    }
+}

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.cs.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.de.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.es.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.fr.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.it.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.ja.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.ko.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.pl.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.pt-BR.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.ru.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.tr.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.zh-Hans.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/CSharp/Portable/xlf/CSharpFeaturesResources.zh-Hant.xlf

@@ -632,6 +632,11 @@
         <target state="new">Add parentheses</target>
         <note />
       </trans-unit>
+      <trans-unit id="Convert_for_to_foreach">
+        <source>Convert 'for' to 'foreach'</source>
+        <target state="new">Convert 'for' to 'foreach'</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/ConvertForToForEach/AbstractConvertForToForEachCodeRefactoringProvider.cs

+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using System;
+using System.Collections;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.CodeActions;
+using Microsoft.CodeAnalysis.CodeRefactorings;
+using Microsoft.CodeAnalysis.Editing;
+using Microsoft.CodeAnalysis.LanguageServices;
+using Microsoft.CodeAnalysis.Options;
+using Microsoft.CodeAnalysis.Shared.Extensions;
+
+namespace Microsoft.CodeAnalysis.ConvertForToForEach
+{
+    internal abstract class AbstractConvertForToForEachCodeRefactoringProvider<
+        TStatementSyntax,
+        TForStatementSyntax,
+        TExpressionSyntax,
+        TMemberAccessExpressionSyntax,
+        TTypeNode,
+        TVariableDeclaratorSyntax> : CodeRefactoringProvider
+        where TStatementSyntax : SyntaxNode
+        where TForStatementSyntax : TStatementSyntax
+        where TExpressionSyntax : SyntaxNode
+        where TMemberAccessExpressionSyntax : SyntaxNode
+        where TTypeNode : SyntaxNode
+        where TVariableDeclaratorSyntax : SyntaxNode
+    {
+        protected abstract string GetTitle();
+
+        protected abstract SyntaxNode ConvertForNode(
+            TForStatementSyntax currentFor, TTypeNode typeNode, SyntaxToken foreachIdentifier, 
+            TExpressionSyntax collectionExpression, ITypeSymbol iterationVariableType, OptionSet options);
+        protected abstract SyntaxList<TStatementSyntax> GetBodyStatements(TForStatementSyntax forStatement);
+        protected abstract bool IsValidVariableDeclarator(TVariableDeclaratorSyntax firstVariable);
+        protected abstract bool TryGetForStatementComponents(
+            TForStatementSyntax forStatement,
+            out SyntaxToken iterationVariable, out TExpressionSyntax initializer,
+            out TMemberAccessExpressionSyntax memberAccess, out TExpressionSyntax stepValue,
+            CancellationToken cancellationToken);
+
+        public override async Task ComputeRefactoringsAsync(CodeRefactoringContext context)
+        {
+            var cancellationToken = context.CancellationToken;
+            var document = context.Document;
+
+            var root = await document.GetSyntaxRootAsync(cancellationToken).ConfigureAwait(false);
+            var token = root.FindToken(context.Span.Start);
+
+            // position has to be inside the 'for' span, or if there is a selection, it must
+            // match the 'for' span exactly.
+            if (context.Span.IsEmpty && !token.Span.IntersectsWith(context.Span.Start))
+            {
+                return;
+            }
+
+            if (!context.Span.IsEmpty && context.Span != token.Span)
+            {
+                return;
+            }
+
+            var forStatement = token.Parent as TForStatementSyntax;
+            if (forStatement == null)
+            {
+                return;
+            }
+
+            if (!TryGetForStatementComponents(forStatement,
+                    out var iterationVariable, out var initializer,
+                    out var memberAccess, out var stepValueOpt, cancellationToken))
+            {
+                return;
+            }
+
+            var syntaxFacts = document.GetLanguageService<ISyntaxFactsService>();
+            syntaxFacts.GetPartsOfMemberAccessExpression(memberAccess,
+                out var collectionExpressionNode, out var memberAccessNameNode);
+
+            var collectionExpression = (TExpressionSyntax)collectionExpressionNode;
+            syntaxFacts.GetNameAndArityOfSimpleName(memberAccessNameNode, out var memberAccessName, out _);
+            if (memberAccessName != nameof(Array.Length) && memberAccessName != nameof(IList.Count))
+            {
+                return;
+            }
+
+            var semanticModel = await document.GetSemanticModelAsync(cancellationToken).ConfigureAwait(false);
+
+            // Make sure we're starting at 0.
+            var initializerValue = semanticModel.GetConstantValue(initializer, cancellationToken);
+            if (!(initializerValue.HasValue && initializerValue.Value is 0))
+            {
+                return;
+            }
+
+            // Make sure we're incrementing by 1.
+            if (stepValueOpt != null)
+            {
+                var stepValue = semanticModel.GetConstantValue(stepValueOpt);
+                if (!(stepValue.HasValue && stepValue.Value is 1))
+                {
+                    return;
+                }
+            }
+
+            var collectionType = semanticModel.GetTypeInfo(collectionExpression, cancellationToken);
+            if (collectionType.Type == null && collectionType.Type.TypeKind == TypeKind.Error)
+            {
+                return;
+            }
+
+            var containingType = semanticModel.GetEnclosingNamedType(context.Span.Start, cancellationToken);
+            if (containingType == null)
+            {
+                return;
+            }
+
+            var ienumerableType = semanticModel.Compilation.GetSpecialType(SpecialType.System_Collections_Generic_IEnumerable_T);
+            var ienumeratorType = semanticModel.Compilation.GetSpecialType(SpecialType.System_Collections_Generic_IEnumerator_T);
+
+            // make sure the collection can be iterated.
+            if (TryGetIterationElementType(
+                    containingType, collectionType.Type,
+                    ienumerableType, ienumeratorType,
+                    out var iterationType))
+            {
+                return;
+            }
+
+            // If the user uses the iteration variable for any other reason, we can't convert this.
+            var bodyStatements = GetBodyStatements(forStatement);
+            foreach (var statement in bodyStatements)
+            {
+                if (IterationVariableIsUsedForMoreThanCollectionIndex(statement))
+                {
+                    return;
+                }
+            }
+
+            // Looks good.  We can conver this.
+            context.RegisterRefactoring(new MyCodeAction(GetTitle(),
+                c => ConvertForToForEachAsync(
+                    document, forStatement, iterationVariable, 
+                    collectionExpression, iterationType, c)));
+
+            // local functions
+            bool IterationVariableIsUsedForMoreThanCollectionIndex(SyntaxNode current)
+            {
+                if (syntaxFacts.IsIdentifierName(current))
+                {
+                    syntaxFacts.GetNameAndArityOfSimpleName(current, out var name, out _);
+                    if (name == iterationVariable.ValueText)
+                    {
+                        // found a reference.  make sure it's only used inside something like
+                        // list[i]
+
+                        if (!syntaxFacts.IsElementAccessExpression(current.Parent))
+                        {
+                            // used in something other than accessing into a collection.
+                            // can't convert this for-loop.
+                            return true;
+                        }
+
+                        if (!syntaxFacts.AreEquivalent(current.Parent, collectionExpression))
+                        {
+                            // was indexing into something other than the collection.
+                            // can't convert this for-loop.
+                            return true;
+                        }
+
+                        // this usage of the for-variable is fine.
+                    }
+                }
+
+                foreach (var child in current.ChildNodesAndTokens())
+                {
+                    if (child.IsNode)
+                    {
+                        if (IterationVariableIsUsedForMoreThanCollectionIndex(child.AsNode()))
+                        {
+                            return true;
+                        }
+                    }
+                }
+
+                return false;
+            }
+        }
+
+        private bool TryGetIterationElementType(
+            INamedTypeSymbol containingType, ITypeSymbol collectionType, 
+            INamedTypeSymbol ienumerableType, INamedTypeSymbol ienumeratorType,
+            out ITypeSymbol iterationType)
+        {
+            // Check in the class/struct hierarchy first.
+            var methods = collectionType.GetAccessibleMembersInThisAndBaseTypes<IMethodSymbol>(containingType);
+            var getEnumeratorMethod = methods.FirstOrDefault(m => m.Name == nameof(IEnumerable.GetEnumerator));
+            if (getEnumeratorMethod != null)
+            {
+                return TryGetIterationElementTypeFromGetEnumerator(
+                    containingType, getEnumeratorMethod, ienumeratorType, out iterationType);
+            }
+
+            // couldn't find .GetEnumerator on the class/struct.  Check the interface hierarchy.
+            var instantiatedIEnumerableType = collectionType.GetAllInterfacesIncludingThis().FirstOrDefault(
+                t => Equals(t.OriginalDefinition, ienumerableType));
+
+            if (instantiatedIEnumerableType != null)
+            {
+                iterationType = instantiatedIEnumerableType.TypeArguments[0];
+                return true;
+            }
+
+            iterationType = default;
+            return false;
+        }
+
+        private bool TryGetIterationElementTypeFromGetEnumerator(
+            INamedTypeSymbol containingType, IMethodSymbol getEnumeratorMethod, 
+            INamedTypeSymbol ienumeratorType, out ITypeSymbol iterationType)
+        {
+            var getEnumeratorReturnType = getEnumeratorMethod.ReturnType;
+
+            // Check in the class/struct hierarchy first.
+            var properties = getEnumeratorReturnType.GetAccessibleMembersInThisAndBaseTypes<IPropertySymbol>(containingType);
+            var currentProperty = properties.FirstOrDefault(m => m.Name == nameof(IEnumerator.Current));
+            if (currentProperty != null)
+            {
+                iterationType = currentProperty.Type;
+                return true;
+            }
+
+            // couldn't find .Current on the class/struct.  Check the interface hierarchy.
+            var instantiatedIEnumeratorType = getEnumeratorReturnType.GetAllInterfacesIncludingThis().FirstOrDefault(
+                t => Equals(t.OriginalDefinition, ienumeratorType));
+
+            if (instantiatedIEnumeratorType != null)
+            {
+                iterationType = instantiatedIEnumeratorType.TypeArguments[0];
+                return true;
+            }
+
+            iterationType = default;
+            return false;
+        }
+
+        private async Task<Document> ConvertForToForEachAsync(
+            Document document, TForStatementSyntax forStatement,
+            SyntaxToken iterationVariable, TExpressionSyntax collectionExpression,
+            ITypeSymbol iterationType, CancellationToken cancellationToken)
+        {
+            var syntaxFacts = document.GetLanguageService<ISyntaxFactsService>();
+            var semanticFacts = document.GetLanguageService<ISemanticFactsService>();
+            var generator = SyntaxGenerator.GetGenerator(document);
+
+            var semanticModel = await document.GetSemanticModelAsync(cancellationToken).ConfigureAwait(false);
+            var root = await document.GetSyntaxRootAsync(cancellationToken).ConfigureAwait(false);
+            var editor = new SyntaxEditor(root, generator);
+
+            // create a dummy "list[i]" expression.  We'll use this to find all places to replace
+            // in the current for statement.
+            var indexExpression = generator.ElementAccessExpression(
+                collectionExpression, generator.IdentifierName(iterationVariable));
+
+            // See if the first statement in the for loop is of the form:
+            //      var x = list[i]   or
+            //
+            // If so, we'll use those as the iteration variables for the new foreach statement.
+            var bodyStatements = GetBodyStatements(forStatement);
+
+            TTypeNode typeNode = default;
+            SyntaxToken foreachIdentifier = default;
+            SyntaxNode ignoreStatement = default;
+            if (bodyStatements.Count >= 1)
+            {
+                var firstStatement = bodyStatements[0];
+                if (syntaxFacts.IsLocalDeclarationStatement(firstStatement))
+                {
+                    var variables = syntaxFacts.GetVariablesOfLocalDeclarationStatement(firstStatement);
+                    if (variables.Count == 1)
+                    {
+                        var firstVariable = (TVariableDeclaratorSyntax)variables[0];
+                        if (IsValidVariableDeclarator(firstVariable))
+                        {
+                            var firstVariableInitializer = syntaxFacts.GetInitializerOfVariableDeclarator(firstVariable);
+                            if (syntaxFacts.AreEquivalent(firstVariableInitializer, indexExpression))
+                            {
+                                typeNode = (TTypeNode)syntaxFacts.GetTypeOfVariableDeclarator(firstVariable);
+                                foreachIdentifier = syntaxFacts.GetIdentifierOfVariableDeclarator(firstVariable);
+                                editor.RemoveNode(firstStatement);
+                                ignoreStatement = firstStatement;
+                            }
+                        }
+                    }
+                }
+            }
+
+            if (foreachIdentifier.RawKind == 0)
+            {
+                foreachIdentifier = generator.Identifier("v");
+            }
+
+            var foreachIdentifierName = generator.IdentifierName(foreachIdentifier).WithoutTrivia();
+            foreachIdentifier = foreachIdentifier.WithAdditionalAnnotations(RenameAnnotation.Create());
+
+            // Walk the for statement, replacing any matches we find.
+            recurse(forStatement);
+
+            var options = await document.GetOptionsAsync(cancellationToken).ConfigureAwait(false);
+            editor.ReplaceNode(
+                forStatement,
+                (currentFor, _) => this.ConvertForNode(
+                    (TForStatementSyntax)currentFor, typeNode, foreachIdentifier,
+                    collectionExpression, iterationType, options));
+
+            return document.WithSyntaxRoot(editor.GetChangedRoot());
+
+            // local functions
+            void recurse(SyntaxNode current)
+            {
+                // Do not replace in the first statement if we're just going to remove it
+                // anyways.
+                if (current == ignoreStatement)
+                {
+                    return;
+                }
+
+                if (syntaxFacts.AreEquivalent(current, indexExpression))
+                {
+                    // Found a match.  replace with iteration variable.
+                    if (semanticFacts.IsWrittenTo(semanticModel, current, cancellationToken))
+                    {
+                        editor.ReplaceNode(current, foreachIdentifierName.WithAdditionalAnnotations(
+                            WarningAnnotation.Create(FeaturesResources.Warning_colon_Collection_was_modified_during_iteration)));
+                    }
+                    else
+                    {
+                        editor.ReplaceNode(current, foreachIdentifierName);
+                    }
+                }
+
+                foreach (var child in current.ChildNodesAndTokens())
+                {
+                    if (child.IsNode)
+                    {
+                        recurse(child.AsNode());
+                    }
+                }
+            }
+        }
+
+        private class MyCodeAction : CodeAction.DocumentChangeAction
+        {
+            public MyCodeAction(string title, Func<CancellationToken, Task<Document>> createChangedDocument) 
+                : base(title, createChangedDocument, title)
+            {
+            }
+        }
+    }
+}

src/Features/Core/Portable/FeaturesResources.Designer.cs

@@ -428,6 +428,15 @@ internal class FeaturesResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Alias ambiguous type &apos;{0}&apos;.
+        /// </summary>
+        internal static string Alias_ambiguous_type_0 {
+            get {
+                return ResourceManager.GetString("Alias_ambiguous_type_0", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to All lowercase.
         /// </summary>
@@ -3635,15 +3644,6 @@ internal class FeaturesResources {
             }
         }
         
-        /// <summary>
-        ///   Looks up a localized string similar to Variable declaration can be deconstructed.
-        /// </summary>
-        internal static string Variable_declaration_can_be_deconstructed {
-            get {
-                return ResourceManager.GetString("Variable_declaration_can_be_deconstructed", resourceCulture);
-            }
-        }
-        
         /// <summary>
         ///   Looks up a localized string similar to Using readonly references will prevent the debug session from continuing..
         /// </summary>
@@ -3671,6 +3671,15 @@ internal class FeaturesResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Variable declaration can be deconstructed.
+        /// </summary>
+        internal static string Variable_declaration_can_be_deconstructed {
+            get {
+                return ResourceManager.GetString("Variable_declaration_can_be_deconstructed", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Variable declaration can be inlined.
         /// </summary>
@@ -3689,6 +3698,15 @@ internal class FeaturesResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Warning: Collection was modified during iteration..
+        /// </summary>
+        internal static string Warning_colon_Collection_was_modified_during_iteration {
+            get {
+                return ResourceManager.GetString("Warning_colon_Collection_was_modified_during_iteration", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Warning: Declaration changes scope and may change meaning..
         /// </summary>

src/Features/Core/Portable/FeaturesResources.resx

@@ -1334,4 +1334,10 @@ This version used in: {2}</value>
   <data name="indexer_" xml:space="preserve">
     <value>indexer</value>
   </data>
+  <data name="Alias_ambiguous_type_0" xml:space="preserve">
+    <value>Alias ambiguous type '{0}'</value>
+  </data>
+  <data name="Warning_colon_Collection_was_modified_during_iteration" xml:space="preserve">
+    <value>Warning: Collection was modified during iteration.</value>
+  </data>
 </root>

src/Features/Core/Portable/xlf/FeaturesResources.cs.xlf

@@ -1985,6 +1985,16 @@ Tato verze se používá zde: {2}.</target>
         <target state="translated">indexer</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/xlf/FeaturesResources.de.xlf

@@ -1985,6 +1985,16 @@ Diese Version wird verwendet in: {2}</target>
         <target state="translated">Indexer</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/xlf/FeaturesResources.es.xlf

@@ -1985,6 +1985,16 @@ Esta versión se utiliza en: {2}</target>
         <target state="translated">indizador</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/xlf/FeaturesResources.fr.xlf

@@ -1985,6 +1985,16 @@ Version utilisée dans : {2}</target>
         <target state="translated">indexeur</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/xlf/FeaturesResources.it.xlf

@@ -1985,6 +1985,16 @@ Questa versione è usata {2}</target>
         <target state="translated">indicizzatore</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/xlf/FeaturesResources.ja.xlf

@@ -1985,6 +1985,16 @@ This version used in: {2}</source>
         <target state="translated">インデクサー</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/xlf/FeaturesResources.ko.xlf

@@ -1985,6 +1985,16 @@ This version used in: {2}</source>
         <target state="translated">인덱서</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/xlf/FeaturesResources.pl.xlf

@@ -1985,6 +1985,16 @@ Ta wersja jest używana wersja: {2}</target>
         <target state="translated">indeksator</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/xlf/FeaturesResources.pt-BR.xlf

@@ -1985,6 +1985,16 @@ Essa versão é usada no: {2}</target>
         <target state="translated">indexador</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/xlf/FeaturesResources.ru.xlf

@@ -1985,6 +1985,16 @@ This version used in: {2}</source>
         <target state="translated">индексатор</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/xlf/FeaturesResources.tr.xlf

@@ -1985,6 +1985,16 @@ Bu sürüm şurada kullanılır: {2}</target>
         <target state="translated">dizin oluşturucu</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/xlf/FeaturesResources.zh-Hans.xlf

@@ -1985,6 +1985,16 @@ This version used in: {2}</source>
         <target state="translated">索引器</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Features/Core/Portable/xlf/FeaturesResources.zh-Hant.xlf

@@ -1985,6 +1985,16 @@ This version used in: {2}</source>
         <target state="translated">索引子</target>
         <note />
       </trans-unit>
+      <trans-unit id="Alias_ambiguous_type_0">
+        <source>Alias ambiguous type '{0}'</source>
+        <target state="new">Alias ambiguous type '{0}'</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="Warning_colon_Collection_was_modified_during_iteration">
+        <source>Warning: Collection was modified during iteration.</source>
+        <target state="new">Warning: Collection was modified during iteration.</target>
+        <note />
+      </trans-unit>
     </body>
   </file>
 </xliff>
\ No newline at end of file

src/Workspaces/CSharp/Portable/CodeStyle/TypeStyle/TypeStyleHelper.cs

@@ -46,13 +46,26 @@ internal static class TypeStyleHelper
             ExpressionSyntax initializerExpression,
             SemanticModel semanticModel,
             CancellationToken cancellationToken) =>
-                semanticModel.GetTypeInfo(initializerExpression, cancellationToken).Type?.IsSpecialType() == true;
+                IsBuiltInType(semanticModel.GetTypeInfo(initializerExpression, cancellationToken).Type);
 
-        private static bool IsImplicitStylePreferred(TypeStylePreference stylePreferences,
+        public static bool IsBuiltInType(ITypeSymbol type)
+            => type?.IsSpecialType() == true;
+
+        public static bool IsImplicitStylePreferred(
+            OptionSet optionSet,
             bool isBuiltInTypeContext,
             bool isTypeApparentContext)
         {
+            return IsImplicitStylePreferred(
+                GetCurrentTypeStylePreferences(optionSet),
+                isBuiltInTypeContext,
+                isTypeApparentContext);
+        }
 
+        private static bool IsImplicitStylePreferred(TypeStylePreference stylePreferences,
+            bool isBuiltInTypeContext,
+            bool isTypeApparentContext)
+        {
             return isBuiltInTypeContext
                     ? stylePreferences.HasFlag(TypeStylePreference.ImplicitTypeForIntrinsicTypes)
                     : isTypeApparentContext

src/Workspaces/CSharp/Portable/LanguageServices/CSharpSyntaxFactsService.cs

@@ -1528,14 +1528,10 @@ public void GetPartsOfMemberAccessExpression(SyntaxNode node, out SyntaxNode exp
         }
 
         public SyntaxToken GetIdentifierOfSimpleName(SyntaxNode node)
-        {
-            return ((SimpleNameSyntax)node).Identifier;
-        }
+            => ((SimpleNameSyntax)node).Identifier;
 
         public SyntaxToken GetIdentifierOfVariableDeclarator(SyntaxNode node)
-        {
-            return ((VariableDeclaratorSyntax)node).Identifier;
-        }
+            => ((VariableDeclaratorSyntax)node).Identifier;
 
         public bool IsIdentifierName(SyntaxNode node)
             => node.IsKind(SyntaxKind.IdentifierName);
@@ -1808,6 +1804,9 @@ public SeparatedSyntaxList<SyntaxNode> GetVariablesOfLocalDeclarationStatement(S
         public SyntaxNode GetInitializerOfVariableDeclarator(SyntaxNode node)
             => ((VariableDeclaratorSyntax)node).Initializer;
 
+        public SyntaxNode GetTypeOfVariableDeclarator(SyntaxNode node)
+            => ((VariableDeclarationSyntax)((VariableDeclaratorSyntax)node).Parent).Type;
+
         public SyntaxNode GetValueOfEqualsValueClause(SyntaxNode node)
             => ((EqualsValueClauseSyntax)node).Value;
 

src/Workspaces/Core/Portable/LanguageServices/SyntaxFactsService/ISyntaxFactsService.cs

@@ -159,6 +159,7 @@ internal interface ISyntaxFactsService : ILanguageService
         SyntaxToken GetIdentifierOfGenericName(SyntaxNode node);
         SyntaxToken GetIdentifierOfSimpleName(SyntaxNode node);
         SyntaxToken GetIdentifierOfVariableDeclarator(SyntaxNode node);
+        SyntaxNode GetTypeOfVariableDeclarator(SyntaxNode node);
 
         /// <summary>
         /// True if this is an argument with just an expression and nothing else (i.e. no ref/out,

src/Workspaces/VisualBasic/Portable/LanguageServices/VisualBasicSyntaxFactsService.vb

@@ -1715,6 +1715,13 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
             Return DirectCast(node, VariableDeclaratorSyntax).Initializer
         End Function
 
+        Public Function GetTypeOfVariableDeclarator(node As SyntaxNode) As SyntaxNode Implements ISyntaxFactsService.GetTypeOfVariableDeclarator
+            Dim declarator = DirectCast(node, VariableDeclaratorSyntax)
+            Return If(TypeOf declarator.AsClause Is SimpleAsClauseSyntax,
+                      DirectCast(declarator.AsClause, SimpleAsClauseSyntax).Type,
+                      Nothing)
+        End Function
+
         Public Function GetValueOfEqualsValueClause(node As SyntaxNode) As SyntaxNode Implements ISyntaxFactsService.GetValueOfEqualsValueClause
             Return DirectCast(node, EqualsValueSyntax).Value
         End Function