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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.