Keep compilation alive in GetPartialSemanticModelAsync

src/Workspaces/Core/Portable/Shared/Extensions/DocumentExtensions.cs

@@ -181,7 +181,11 @@ public static async Task<SemanticModel> GetPartialSemanticModelAsync(this Docume
             if (document.Project.TryGetCompilation(out var compilation))
             {
                 // We already have a compilation, so at this point it's fastest to just get a SemanticModel
-                return await document.GetSemanticModelAsync(cancellationToken).ConfigureAwait(false);
+                var semanticModel = await document.GetSemanticModelAsync(cancellationToken).ConfigureAwait(false);
+
+                // Make sure the compilation is kept alive so that GetSemanticModelAsync() doesn't become expensive
+                GC.KeepAlive(compilation);
+                return semanticModel;
             }
             else
             {