Why are immutable array literals heap allocated?

[Nick Treleaven]

immutable(int[]) f() @nogc {
     return [1,2];
}

onlineapp.d(2): Error: array literal in `@nogc` function 
`onlineapp.f` may cause a GC allocation

This makes dynamic array literals unusable with @nogc, and adds 
to GC pressure for no reason. What code would break if dmd used 
only static data for [1,2]?