Alexis Laferrière fabb345624 [Sema] Warn on uses of @_implementationOnly imports without library-evolution ...

@_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

2023-07-05 16:20:42 -07:00

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