[ST-NNNN] Targeted Interoperability between Swift Testing and XCTest #2982

jerryjrchen · 2025-10-02T20:48:59Z

Draft proposal for interop between Swift Testing and XCTest.

Preview it here: https://github.com/jerryjrchen/swift-evolution/blob/swt-xct-interop/proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

stmontgomery · 2025-10-02T21:12:55Z

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

+  empowered to choose Swift Testing when writing new tests or test helpers, as
+  it will work properly in both types of tests.
+
+We don't propose supporting interoperability for APIs without risk of data loss,


Suggested change

We don't propose supporting interoperability for APIs without risk of data loss,

We don't propose supporting interoperability for APIs which are not lossy without interop,

Don't not hate the double negative here, but can't think of better wording at the moment :)

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

grynspan · 2025-10-03T14:44:03Z

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

+For the remainder of this proposal, we’ll describe test APIs which exhibit this
+problem as **lossy without interop**.
+
+This problem risks regressing test coverage for projects which migrate to Swift


"This problem" is a bit vague in this context since the problem would actually be the regressed test coverage. Consider rephrasing so that it's clear that the lack of interop is what leads to symptoms/problems X, Y, and Z.

Good point. I reworded this whole section for better clarity

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

grynspan · 2025-10-03T14:46:37Z

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

+  from XCTest to Swift Testing. For issue details unique to XCTest, we will
+  include them as a comment when constructing the Swift Testing issue.
+
+Note that no changes are proposed for the `XCTSkip` API, because they already


Can we think of other XCTest features that are explicitly not supported and should be called out here?

XCTestAssertion and XCTWaiter, I guess!

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

grynspan · 2025-10-03T14:48:16Z

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

+We propose supporting the following Swift Testing APIs in XCTest:
+
+- `#expect` and `#require`
+  - Includes [exit testing][]


Suggested change

- Includes [exit testing][]

- Includes `#expect(throws:)`

- Includes [exit testing][]

Assuming #expect(throws:) will work! If it won't work, we need to call that out too.

Based on my understanding of that API, we're catching thrown errors and recording issues if not thrown. Since we'd need to add interop support for Swift Testing Issue as part of the default #expect, it seems to me the check throws variant should work too

grynspan · 2025-10-03T14:49:56Z

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

+  usage occurs when running Swift Testing tests by turning those warnings into a
+  `fatalError`.
+
+Configure the interoperability mode when running tests using the


This environment variable affects the behaviour of XCTest API, right? Then I think we want to put it in the XCTEST_ namespace. 🤔

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

grynspan · 2025-10-03T20:32:19Z

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

+Many projects want to migrate from XCTest to Swift Testing, and may be in an
+intermediate state where test helpers written using XCTest API are called from
+Swift Testing. Today, the Swift Testing and XCTest libraries stand mostly
+independently, which means an [`XCTAssert`][XCTest assertions] failure in a


Suggested change

independently, which means an [`XCTAssert`][XCTest assertions] failure in a

independently, which means an [`XCTAssert()`][XCTest assertions] failure in a

This and the next suggestion are completely nitpickish.

grynspan · 2025-10-03T20:32:27Z

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

+intermediate state where test helpers written using XCTest API are called from
+Swift Testing. Today, the Swift Testing and XCTest libraries stand mostly
+independently, which means an [`XCTAssert`][XCTest assertions] failure in a
+Swift Testing test or an [`#expect`][Swift Testing expectations] failure in an


Suggested change

Swift Testing test or an [`#expect`][Swift Testing expectations] failure in an

Swift Testing test or an [`#expect()`][Swift Testing expectations] failure in an

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

grynspan · 2025-10-03T20:35:08Z

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

+- [Expected failures][], such as `XCTExpectFailure`: marking a Swift Testing
+  issue in this way will generate a runtime warning issue.
+- [`XCTAttachment`][XCTest attachments]
+- [Issue handling traits][]: we will make our best effort to translate issues


Is this point about traits or about issue handling in general? (Since we're not translating traits.)

Well, we're not proposing that you could attach a Swift Testing trait to an XCTest test function. This is about the issue handling trait in a Swift Testing test, and what happens when it's asked to handle a XCTAssert failure.

proposals/testing/NNNN-targeted-interoperability-swift-testing-and-xctest.md

jerryjrchen added 5 commits October 2, 2025 10:42

Interoperability between Swift Testing and XCTest

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Minor changes

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Update naming of default interop mode -> permissive

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Update interop support philosophy

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Rename doc

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stmontgomery reviewed Oct 2, 2025

View reviewed changes

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