[Review] ST-0022 Custom reflection during testing

Evolution Proposal Reviews
1

Hello Swift Community!

The review of ST-0022 " Custom reflection during testing" starts now and runs through Tuesday, April 14th, 2026. The proposal is available here: swift-evolution/proposals/testing/0022-customtestreflectable.md at main · swiftlang/swift-evolution · GitHub

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...
dependencies: [
  ...
  .package(url: "https://github.com/swiftlang/swift-testing.git", branch: "main"),
]

Then, add a target dependency to your test target:

.testTarget(
  ...
  dependencies: [
    ...
    .product(name: "Testing", package: "swift-testing"),
  ]
)

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Maarten Engels

Review Manager

4 Likes
[Accepted] ST-0022: Custom reflection during testing
Testing Workgroup meeting April 20, 2026
2

I am curious how we expect adoption of this feature to work out in practice. I assume most types that want to conform to this protocol live in targets or executableTargets. Those targets cannot easily import Testing since Testing is only available during test execution. We have run into this problem previously where packages are defining "test helper" targets that are only used when running tests; however, in this case I expect that the Testing usage here would proliferate outside of "test helper" targets. Do we expect developers to conditionally #if canImport(Testing) in their targets to add these conformances?

3

As with CustomTestStringConvertible, the conformances would be expected to live in a test target. If they rely on implementation details that are not visible in the test target (via @testable import or otherwise), developers can write something like:

extension MyType {
  package var customTestMirror: Mirror { ... }
}

And then just add the formal conformance in an (empty) extension in the test target.

4

And just for clarity, it doesn't have to be a .testTarget, right? A regular .target is fine if it's using the "test support target" pattern where you have a library shared by multiple test targets. The only thing that matters is that you don't put it into a dependency that gets linked into any production code.

5

Could we reflect that guidance in the proposal and the protocol documentation somewhere?

2 Likes
6

Yep, I'll make sure to discuss this approach when I write the prose documentation. We can add a note about it to the proposal text as well post-review.

2 Likes
7

Correct. I'm using "test target" in my above comments imprecisely to refer to both.

Right-o!

1 Like
8

ST-0022 has been accepted! Thanks everyone for participating!

1 Like