Explanation: There was an inconsistency between non-const and const FRT
pointers. The former used Direct_Unowned the latter used Indirect
calling convention. We want to use Direct_Unowned for both cases. The
crash was the result of a calling convention mismatch between the
SILFunctionType of a Swift closure and the SILFunctionType of the C++
function's formal parameter that is taking a function pointer. The
compiler tried to insert a conversion between the two function types
that does not exist and caused an assertion in debug compilers and
miscompilation in production compilers.
Issue: rdar://149398905
Risk: Low, the fix is targeted and we change to a well-tested behavior
with non-const FRT pointers.
Testing: Regression test added.
Original PR: #81070
Reviewer: @j-hui
This corresponds to the parameter-passing convention of the Itanium C++
ABI, in which the argument is passed indirectly and possibly modified,
but not destroyed, by the callee.
@in_cxx is handled the same way as @in in callers and @in_guaranteed in
callees. OwnershipModelEliminator emits the call to destroy_addr that is
needed to destroy the argument in the caller.
rdar://122707697
