Returning cstring from a DLL

[bobd]

Adventures in calling Nim DLLs from Delphi again... If I use the following code 
I can get the string on the Delphi side using PAnsiChar as expected:
    
    
    proc GetString*(): cstring {.cdecl, exportc, dynlib.} =
      var cs: cstring
      cs = "String1"
      return cs
    
    
    Run

Generated C code looks like:
    
    
    N_LIB_EXPORT N_CDECL(NCSTRING, GetStringList)(void) {
            NCSTRING result;
            NCSTRING cs;
    {   result = (NCSTRING)0;
            cs = (NCSTRING)0;
            cs = "String1";
            result = cs;
            goto BeforeRet_;
            }BeforeRet_: ;
            return result;
    
    
    Run

However if I use the following I get an exception:
    
    
    proc GetStringList*(): cstring {.cdecl, exportc, dynlib.} =
      var s: string
      s = "String1"
      return cstring(s)
    
    
    Run

Not surprisingly the C code is different: 
    
    
    STRING_LITERAL(TM_OCIjdUBZYh7z3CKkm9az9a8Q_2, "String1", 7);
    
    N_LIB_EXPORT N_CDECL(NCSTRING, GetStringList)(void) {
            NCSTRING result;
            NimStringDesc* s;
    {   result = (NCSTRING)0;
            s = (NimStringDesc*)0;
            s = copyString(((NimStringDesc*) &TM_OCIjdUBZYh7z3CKkm9az9a8Q_2));
            result = nimToCStringConv(s);
            goto BeforeRet_;
            }BeforeRet_: ;
            return result;
    }
    
    
    Run

Anyone got any idea why these two cstrings appear to have a different 
structure? I'm using 0.19 and am aware of the possible need to use GC_ref 
(although that's not the issue here). Thanks.