I suspect it would be a terrible performance hit.
It would be nice to have some numbers backing this up.
This the approach taken by Active Oberon and Modula-3.
Pointers are GC by default, but can be declared as untraced
pointers in code considered @system like in D.
Do they have concurrent gc and emit barriers for each write to a
default pointer? Do they have precise scanning and don't scan the
untraced pointers? Are the meaningful performance comparisons
between the two pointer types that would enable us to estimate
how costly emitting those barriers in D would be?
