Guard the lifetime of StringObject.nativeStorage in mutating methods. #42623
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Plug a hole in the semantics of deinitialization barriers. Adds strong_copy_(unowned|unmanaged)_value to mayLoadWeakOrUnowned. Deinitialization barriers includes loads from weak references. Converting an unowned or unmanaged reference to a strong reference is the moral equivalent.
Use an Unmanaged reference when reinterpreting a strong reference to any unmanaged (trivial) representation. Mutating StringGuts methods, such as appendInPlace, are currently miscompiled just by reordering the optimization pipeline. The fact that 'self' is guaranteed within a method scope only applies to nonmutating methods. In mutating methods, the contents of 'self' are conceptually destroyed at the beginning of the method. Futhermore, the compiler cannot provide any lifetime guarantees in the presence of a bitcasting from an integer to a strong reference. As a basic rule, Swift code should never directly bitcast an integer to a reference. There's no reasonable way for the compiler to support this. Generating a strong reference out of thin air requires an explicit conversion from an unmanaged reference. This rule will need to be clearly documented. To start with, it will be explained in an upcoming proposal for object lifetimes.
StringGuts has mutating methods that access StringObject.nativeStorage while modifying self as such: self = _StringGuts(self._object.nativeStorage) `self._object` can be destroyed immediately after its last use. We can't rely on `self` having a lexical lifetime within a mutating method. This follows from the fact that `self` is effectively `inout` within this scope, and thus conceptually destroyed on function entry. Since the nativeStorage property generates a new reference out of thin air (from an integer bitcast), `object` may be destroyed before the new reference is retained. Fix this by creating a separate copy of object with its own lexical scope that covers the use of `nativeStorage`: var object = self._object self = _StringGuts(object.nativeStorage)
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This is a placeholder to demonstrate a fix for reference. The two fixes are
The other commits are for temporary testing purposes.