method
TryGetSubjectBufferSpan
Microsoft.VisualStudio.LanguageServices.Snippets.ISnippetExpansionLanguageHelper.TryGetSubjectBufferSpan(Microsoft.VisualStudio.Text.Editor.ITextView, Microsoft.VisualStudio.Text.ITextBuffer, Microsoft.VisualStudio.TextManager.Interop.TextSpan, out Microsoft.VisualStudio.Text.SnapshotSpan)