53e8e7b1d8
Their definition is fully visible to clients to be copied into them, and there isn't necessarily a symbol for the property descriptor in the defining module, so it isn't necessary or desirable to try to use a property descriptor with them. Trying to reference the descriptor leads to missing symbol errors at load time when trying to use keypaths with older versions of the defining dylib, which goes against the purpose of `@_alwaysEmitIntoClient` meaning "no ABI liabilities for the defining module". Fixes rdar://94049160.
Resilient Library Evolution Tests
This directory tests for the correctness of resilience, which is a
broad term for Swift maximizing binary compatibility of a dependency
while maintaining the freedom to do things that would normally break
clients in other languages, such as changing the layout of nominal
types. The detailed explanation of resilience is out of scope of this
little readme and can be found in docs/LibraryEvolution.rst.
Each main test file should compile against "before" and "after" versions of the corresponding library file. The old and new versions are selected via the BEFORE or AFTER preprocessor variables.
In the library, going from BEFORE to AFTER must be an ABI-compatible change, as documented in the library evolution specification.
In the main program, going from BEFORE to AFTER does not have to be backward compatible.
In the main file, use your test library's getVersion helper function
to know which version of the library to expect at runtime.
There are four valid combinations for each test:
- Main file compiled against old library, linked against old library a.k.a. "before before"
- Main file compiled against old library, linked against new library, a.k.a. "before after"
- Main file compiled against new library, linked against old library, a.k.a. "after before"
- Main file compiled against new library, linked against new library, a.k.a. "after after"
The version of the library available at compile time determines which serialized declarations and transparent function bodies are visible.
When adding a new test, see the boilerplate at the top of each file for
the general pattern for this kind of test. Use the StdlibUnittest
library for assertions.