Rename: Don't restrict rename project search set for explicit interface implementations

Fixes #8139

We were previously asserting that renaming a private member meant that the rename operation would occur entirely in that member's declaring project, but rename affect other projects if the private member is an explicit implementation of a public interface member.

src/EditorFeatures/Test2/Rename/RenameEngineTests.vb

@@ -3153,8 +3153,48 @@ namespace X
                         </Document>
                     </Project>
                 </Workspace>, renameTo:="IFooEx")
+            End Using
+        End Sub
+
+        <WorkItem(8139, "https://github.com/dotnet/roslyn/issues/8139")>
+        <Fact, Trait(Traits.Feature, Traits.Features.Rename)>
+        Public Sub RenameExplicitInterfaceImplementationFromDifferentProject()
+            Using result = RenameEngineResult.Create(
+                <Workspace>
+
+                    <Project Language="C#" AssemblyName="Project1" CommonReferences="true">
+                        <ProjectReference>Project2</ProjectReference>
+                        <Document>
+class Program
+{
+    static void Main(string[] args)
+    {
+        Test(new Class1());
+    }
+
+    static void Test(IInterface i)
+    {
+        i.[|M|]();
+    }
+}
+                        </Document>
+                    </Project>
 
+                    <Project Language="C#" AssemblyName="Project2" CommonReferences="true">
+                        <Document>
+public class Class1 : IInterface
+{
+    void IInterface.[|$$M|]() { }  // Rename 'M' from here
+}
 
+public interface IInterface
+{
+    void [|M|]();
+}
+                        </Document>
+                    </Project>
+
+                </Workspace>, renameTo:="Foo")
             End Using
         End Sub
 

src/Workspaces/Core/Portable/Rename/RenameUtilities.cs

@@ -79,9 +79,8 @@ internal static IEnumerable<Document> GetDocumentsAffectedByRename(ISymbol symbo
                     .Concat(documentsOfRenameSymbolDeclaration.First().Id)
                     .Select(d => d.ProjectId).Distinct();
 
-                if (symbol.DeclaredAccessibility == Accessibility.Private)
+                if (ShouldRenameOnlyAffectDeclaringProject(symbol))
                 {
-                    // private members or classes cannot be used outside of the project they are declared in
                     var isSubset = renameLocations.Select(l => l.DocumentId.ProjectId).Distinct().Except(projectIdsOfRenameSymbolDeclaration).IsEmpty();
                     Contract.ThrowIfFalse(isSubset);
                     return projectIdsOfRenameSymbolDeclaration.SelectMany(p => solution.GetProject(p).Documents);
@@ -97,6 +96,17 @@ internal static IEnumerable<Document> GetDocumentsAffectedByRename(ISymbol symbo
             }
         }
 
+        /// <summary>
+        /// Renaming a private symbol typically confines the set of references and potential
+        /// conflicts to that symbols declaring project. However, rename may cascade to
+        /// non-public symbols which may then require other projects be considered.
+        /// </summary>
+        private static bool ShouldRenameOnlyAffectDeclaringProject(ISymbol symbol)
+        {
+            // Explicit interface implementations can cascade to other projects
+            return symbol.DeclaredAccessibility == Accessibility.Private && !symbol.ExplicitInterfaceImplementations().Any();
+        }
+
         internal static TokenRenameInfo GetTokenRenameInfo(
             ISemanticFactsService semanticFacts,
             SemanticModel semanticModel,