The Nim Manual says:
> Nim distinguishes between traced and untraced references. Untraced references
are also called pointers. Traced references point to objects of a
garbage-collected heap, untraced references point to manually allocated objects
or objects somewhere else in memory. Thus, untraced references are unsafe.
However, for certain low-level operations (accessing the hardware) untraced
references are unavoidable.
>
> Traced references are declared with the ref keyword, untraced references are
declared with the ptr keyword. In general, a ptr T is implicitly convertible to
the pointer type.

An empty subscript [] notation can be used to de-refer a reference, the addr
procedure returns the address of an item. An address is always an untraced
reference. Thus, the usage of addr is an unsafe feature.