Fix out of bounds when creating structure guides for types without bodies (#79543)

Author: CyrusNajmabadi

src/EditorFeatures/CSharpTest/Structure/TypeDeclarationStructureTests.cs

@@ -7,6 +7,7 @@
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.Structure;
 using Microsoft.CodeAnalysis.Test.Utilities;
+using Roslyn.Test.Utilities;
 using Xunit;
 
 namespace Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.Structure;
@@ -231,4 +232,21 @@ public Task TestStructWithNestedComments()
                 """,
             Region("textspan1", "hint1", CSharpStructureHelpers.Ellipsis, autoCollapse: false),
             Region("span2", "// Goo ...", autoCollapse: true));
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/62506")]
+    public Task TestTypeDeclarationNoBraces1()
+        => VerifyBlockSpansAsync("""
+                {|comment:// comment|}
+                $$struct S
+                """,
+            Region("comment", "// comment ...", autoCollapse: true));
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/62506")]
+    public Task TestTypeDeclarationNoBraces2()
+        => VerifyBlockSpansAsync("""
+                {|comment:// comment|}
+                {|hint1:$$struct S{|textspan1:;|}|}
+                """,
+            Region("comment", "// comment ...", autoCollapse: true),
+            Region("textspan1", "hint1", CSharpStructureHelpers.Ellipsis, autoCollapse: false));
 }

src/Features/CSharp/Portable/Structure/CSharpStructureHelpers.cs

@@ -286,8 +286,12 @@ static SyntaxToken GetEndToken(SyntaxNode node)
         static SyntaxToken GetHintTextEndToken(SyntaxNode node)
             => node switch
             {
-                EnumDeclarationSyntax enumDeclaration => enumDeclaration.OpenBraceToken.GetPreviousToken(),
-                TypeDeclarationSyntax typeDeclaration => typeDeclaration.OpenBraceToken.GetPreviousToken(),
+                EnumDeclarationSyntax enumDeclaration
+                    => enumDeclaration.OpenBraceToken.GetPreviousToken(),
+                TypeDeclarationSyntax typeDeclaration
+                    => typeDeclaration.OpenBraceToken != default
+                        ? typeDeclaration.OpenBraceToken.GetPreviousToken()
+                        : typeDeclaration.SemicolonToken.GetPreviousToken(),
                 _ => node.GetLastToken()
             };
     }