Attached is a binary/pdb created by the Roslyn C# compiler. This type of debug info is not new, but because of a subtle issue (bug) with the way previous C# compilers wrote these metadata records, other pdb consumers were not reading them.
static void Main()
var host = new DefaultWindowsRuntimeHost();
var reader = new PdbReader(new FileStream("containsdynamiclocals.pdb", FileMode.Open), host);
Unhandled Exception: Microsoft.Cci.Pdb.PdbDebugException: Unknown custom metadata item kind: 5