Add structure provider for interpolated strings

src/EditorFeatures/CSharpTest/Structure/InterpolatedStringExpressionStructureTests.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.Threading.Tasks;
+using Microsoft.CodeAnalysis.CSharp.Structure;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Structure;
+using Microsoft.CodeAnalysis.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.Structure
+{
+    public class InterpolatedStringExpressionStructureTests : AbstractCSharpSyntaxNodeStructureTests<InterpolatedStringExpressionSyntax>
+    {
+        internal override AbstractSyntaxStructureProvider CreateProvider()
+            => new InterpolatedStringExpressionStructureProvider();
+
+        [Fact, Trait(Traits.Feature, Traits.Features.Outlining)]
+        public async Task TestMultiLineStringLiteral()
+        {
+            await VerifyBlockSpansAsync(
+@"
+class C
+{
+    void M()
+    {
+        var v =
+{|hint:{|textspan:$$$@""
+{123}
+""|}|};
+    }
+}",
+                Region("textspan", "hint", CSharpStructureHelpers.Ellipsis, autoCollapse: true));
+        }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.Outlining)]
+        public async Task TestMissingOnIncompleteStringLiteral()
+        {
+            await VerifyNoBlockSpansAsync(
+@"
+class C
+{
+    void M()
+    {
+        var v = $$$"";
+    }
+}");
+        }
+    }
+}

src/Features/CSharp/Portable/Structure/Providers/InterpolatedStringExpressionStructureProvider.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.Threading;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Options;
+using Microsoft.CodeAnalysis.PooledObjects;
+using Microsoft.CodeAnalysis.Structure;
+
+namespace Microsoft.CodeAnalysis.CSharp.Structure
+{
+    internal sealed class InterpolatedStringExpressionStructureProvider : AbstractSyntaxNodeStructureProvider<InterpolatedStringExpressionSyntax>
+    {
+        protected override void CollectBlockSpans(InterpolatedStringExpressionSyntax node, ArrayBuilder<BlockSpan> spans, OptionSet options, CancellationToken cancellationToken)
+        {
+            if (node.StringStartToken.IsMissing ||
+                node.StringEndToken.IsMissing)
+            {
+                return;
+            }
+
+            spans.Add(new BlockSpan(
+                isCollapsible: true,
+                textSpan: node.Span,
+                hintSpan: node.Span,
+                type: BlockTypes.Expression,
+                autoCollapse: true,
+                isDefaultCollapsed: false));
+        }
+    }
+}