When computing linkage, the compiler would treat unavailable declarations as if
they were "always available" when they lack an `introduced:` version:
```
// Library
@available(macOS, unavailable)
public func foo() {
// …
}
// Client
import Library
@available(macOS, unavailable)
func bar() {
// Even though foo() and bar() are unavalable on macOS the compiler still
// strongly links foo().
foo()
}
```
This created an unnecessary dependency between libraries and their clients and
also can interfere with back deployment, since unavailable declarations may not
be present in a library on all OS versions. Developers could work around these
issues by conditionally compiling the code that references an unavailable
declaration, but they shouldn't have to given that unavailable code is meant to
be provably unreachable at runtime. Additionally, it could improve library code
size if we allowed the compiler to strip unavailable declarations from a binary
completely.
Resolves rdar://106673713
When computing linkage, the compiler would treat unavailable declarations as if
they were "always available" when they lack an `introduced:` version:
```
// Library
@available(macOS, unavailable)
public func foo() {
// …
}
// Client
import Library
@available(macOS, unavailable)
func bar() {
// Even though foo() and bar() are unavalable on macOS the compiler still
// strongly links foo().
foo()
}
```
This created an unnecessary dependency between libraries and their clients and
also can interfere with back deployment, since unavailable declarations may not
be present in a library on all OS versions. Developers could work around these
issues by conditionally compiling the code that references an unavailable
declaration, but they shouldn't have to given that unavailable code is meant to
be provably unreachable at runtime. Additionally, it could improve library code
size if we allowed the compiler to strip unavailable declarations from a binary
completely.
Resolves rdar://106673713
The main point of this change is to make sure that a shared function always has a body: both, in the optimizer pipeline and in the swiftmodule file.
This is important because the compiler always needs to emit code for a shared function. Shared functions cannot be referenced from outside the module.
In several corner cases we missed to maintain this invariant which resulted in unresolved-symbol linker errors.
As side-effect of this change we can drop the shared_external SIL linkage and the IsSerializable flag, which simplifies the serialization and linkage concept.
