How to test if a string is pointing into read-only memory?

[jfondren]

std.string.toStringz always allocates a new string, but it has 
this note:

```d
/+ Unfortunately, this isn't reliable.
  We could make this work if string literals are put
  in read-only memory and we test if s[] is pointing into
  that.

  /* Peek past end of s[], if it's 0, no conversion necessary.
  * Note that the compiler will put a 0 past the end of static
  * strings, and the storage allocator will put a 0 past the end
  * of newly allocated char[]'s.
  */
  char* p = &s[0] + s.length;
  if (*p == 0)
  return s;
  +/
```

and string literals weren't reliably in read-only memory as 
recently as early 2017: 
https://github.com/dlang/dmd/pull/6546#issuecomment-280612721

What's a reliable test that could be used in a toStringz that 
skips allocation when given a string in read-only memory?

As for whether it's a necessarily a good idea to patch toStringz, 
I'd worry that

1. someone will slice a string literal and pass the test while 
not having NUL where it's expected

2. people are probably relying by now on toStringz always 
allocating, to e.g. safely cast immutable off the result.