This patch implements mozilla::NotNull, which is similar but not identicial to
gsl::not_null.
The current draft(?) implementation of gsl::not_null is at
https://github.com/Microsoft/GSL/blob/master/include/gsl.h.
The main difference is that not_null allows implicit conversion from T to
not_null<T>. In contrast, NotNull only allows explicit conversion from T to
NotNull<T> via WrapNotNull().
The rationale for this is that when converting from a less-constrained type to
a more constrained type, implicit conversion is undesirable. For example, if I
changed a function f from this:
f(int* aPtr);
to this:
f(gsl::not_null<int*> aPtr);
no call sites would have to be modified. But if I changed it to this:
f(mozilla::NotNull<int*> aPtr);
call sites *would* need to be modified. This is a good thing! It forces the
author to audit the call sites for non-nullness, and encourages them to
back-propagate NotNull throughout the code.
The other difference between not_null and NotNull is that not_null disables
pointer arithmetic, which means it cannot be used with array pointers. I have
not implemented this restriction for NotNull because it seems arbitrary and
unnecessary.
