Reflection: API to distinguish types imported from C/C++/ObjC

[grynspan]

Motivation
swift-testing needs to distinguish types imported from C, in particular C enumerations, so that we can correctly display instances of those types when a test fails. For example:

typedef NS_ENUM(int, Vegetable) {
  VegetableLettuce, VegetableCarrot, VegetableParsnip
}

let vegetable = ...
#expect(vegetable == .Carrot)

If the value of vegetable does not equal .Carrot, swift-testing diagnoses the issue with a message like:

Expectation failed: (vegetable -> ACTUAL_VALUE) == (.Carrot -> EXPECTED_VALUE)

Where ACTUAL_VALUE and EXPECTED_VALUE equal String(describing: vegetable) and String(describing: Vegetable.Carrot) respectively.

Solution

We'd like to be able to refine this further for C types which currently describe themselves as just Vegetable. Case names aren't available, which is understandable, but being able to distinguish C types means we can special-case these types and present the raw values of the cases (e.g. (.Carrot -> Vegetable(rawValue: 1)).) This improves the diagnostic output from swift-testing.

Alternatives considered

• Leaving our output as-is. We get sub-optimal output for imported C types as a result.
• Using the unsupported public function _mangledTypeName() to get the mangled name and looking for evidence of the __C module. We're doing this today as a temporary workaround, but would prefer to use supported API.
• Spelunking in ABI for this information. That's even worse.