The correct use @_implementationOnly from a non-resilient module is not
enforced by the compiler at this time. Using that configuration can
cause memory corruption at runtime if it affects the memory layout of
types, and it can lead to compiler crashes at compilation in general.
Some modules rely on this configuration with a very limited use for it.
We can grandfather them in its use and silence the warning on specific
import edges.
@_implementationOnly was designed for use from resilient modules only,
using it from non-resilient modules in unsupported. This change adds a
warning about it.
If anyone hits this warning, they should either enable library-evolution
or consider adopting the new `internal import` when it is available as
it handles this scenario properly.
What leads to a crash: an @_implementationOnly import fully hides the
dependency from indirect clients. This can lead to the compiler being
unaware of some internal details of a non-resilient module when building
a client against it. In turn this may lead to run time crashes from
miscompilation.
In general one could still use @_implementationOnly in a non-resilient
modules as long as it's referenced only from function bodies. However,
references from even non-public properties does lead to important memory
layout information being hidden from clients of the module.
rdar://78129903
