[TWG] Exit test value capturing proposal

Add the initial draft of ST-NNNN Capturing values in exit tests.

Author: grynspan

proposals/testing/NNNN-exit-test-value-capturing.md

@@ -0,0 +1,264 @@
+# Capturing values in exit tests
+
+* Proposal: [ST-NNNN](NNNN-filename.md)
+* Authors: [Jonathan Grynspan](https://github.com/grynspan)
+* Review Manager: TBD
+* Status: **Awaiting review**
+* Bug: [swiftlang/swift-testing#1157](https://github.com/swiftlang/swift-testing/issues/1157)
+* Implementation: [swiftlang/swift-testing#1040](https://github.com/swiftlang/swift-testing/pull/1040) _et al._
+* Review: ([pitch](https://forums.swift.org/t/pitch-capturing-values-in-exit-tests/80494))
+
+## Introduction
+
+In Swift 6.2, we introduced the concept of an _exit test_: a section of code in
+a test function that would run in an independent process and allow test authors
+to test code that terminates the process. For example:
+
+```swift
+enum Fruit: Equatable {
+  case apple, orange, olive, tomato
+  var isSweet: Bool { get }
+
+  consuming func feed(to bat: FruitBat) {
+    precondition(self.isSweet, "Fruit bats don't like savory fruits!")
+    ...
+  }
+}
+
+@Test func `Fruit bats don't eat savory fruits`() async {
+  await #expect(processExitsWith: .failure) {
+    let fruit = Fruit.olive
+    let bat = FruitBat(named: "Chauncey")
+    fruit.feed(to: bat) // should trigger a precondition failure and process termination
+  }
+}
+```
+
+This proposal extends exit tests to support capturing state from the enclosing
+context (subject to several practical constraints.)
+
+## Motivation
+
+Exit tests in their current form are useful, but there is no reliable way to
+pass non-constant information from the parent process to the child process,
+which makes them difficult to use with parameterized tests. Consider:
+
+```swift
+@Test(arguments: [Fruit.olive, .tomato])
+func `Fruit bats don't eat savory fruits`(_ fruit: Fruit) async {
+  await #expect(processExitsWith: .failure) {
+    let bat = FruitBat(named: "Chauncey")
+    fruit.feed(to: bat) // 🛑 can't capture 'fruit' from enclosing scope
+  }
+}
+```
+
+In the above example, the test function's argument cannot be passed into the
+exit test. In a trivial example like this one, it wouldn't be difficult to write
+two tests that differ only in the case of `Fruit` they use in their exit test
+bodies, but this approach doesn't scale very far and is generally an
+anti-pattern when using Swift Testing.
+
+## Proposed solution
+
+We propose allowing the capture of values in an exit test when they are
+specified in a closure capture list on the exit test's body.
+
+## Detailed design
+
+The signatures of the exit test macros `expect(processExitsWith:)` and
+`require(processExitsWith:)` are unchanged. A test author may now add a closure
+capture list to the body of an exit test:
+
+```swift
+@Test(arguments: [Fruit.olive, .tomato])
+func `Fruit bats don't eat savory fruits`(_ fruit: Fruit) async {
+  await #expect(processExitsWith: .failure) { [fruit] in
+    let bat = FruitBat(named: "Chauncey")
+    fruit.feed(to: bat)
+  }
+}
+```
+
+This feature has some necessary basic constraints:
+
+### Captured values must be explicitly listed in a closure capture list
+
+Swift Testing needs to know what values need to be encoded, sent to the child
+process, and decoded. Swift macros including `#expect(processExitsWith:)` must
+rely solely on syntax—that is, the code typed by a test author. An implicit
+capture within an exit test body is indistinguishable from any other identifier
+or symbol name.
+
+Hence, only values listed in the closure's capture list will be captured.
+Implicitly captured values will produce a compile-time diagnostic as they do
+today.
+
+### Captured values must conform to Sendable and Codable
+
+Captured values will be sent across process boundaries and, in order to support
+that operation, must conform to `Codable`. As well, captured values need to make
+their way through the various internal mechanisms of Swift Testing and its host
+infrastructure, and so must conform to `Sendable`. Conformance to `Copyable` and
+`Escapable` is implied.
+
+If a value that does _not_ conform to the above protocols is specified in an
+exit test body's capture list, a diagnostic is emitted:
+
+```swift
+let bat: FruitBat = ...
+await #expect(processExitsWith: .failure) { [bat] in
+  // 🛑 Type of captured value 'bat' must conform to 'Sendable' and 'Codable'
+  ...
+}
+```
+
+### Captured values' types must be visible to the exit test macro
+
+In order for us to successfully _decode_ captured values in the child process,
+we must know their Swift types. Type information is not readily available during
+macro expansion and we must, in general, rely on the parsed syntax tree for it.
+
+The type of `self` and the types of arguments to the calling function are,
+generally, known and can be inferred from context[^shadows]. The types of other
+values, including local variables and global state, are not visible in the
+syntax tree and must be specified explicitly in the capture list using an `as`
+expression:
+
+```swift
+await #expect(processExitsWith: .failure) { [fruit = fruit as Fruit] in
+  ...
+}
+```
+
+Finally, the types of captured literals (e.g. `[x = 123]`) are known at compile
+time and can always be inferred as `IntegerLiteralType` etc., although we don't
+anticipate this will be particularly useful in practice.
+
+If the type of a captured value cannot be resolved from context, the test author
+will see an error at compile time:
+
+```swift
+await #expect(processExitsWith: .failure) { [fruit] in
+  // 🛑 Type of captured value 'fruit' is ambiguous
+  //     Fix-It: Add '= fruit as T'
+  ...
+}
+```
+
+See the **Future directions** section of this proposal for more information on
+how we hope to lift this constraint. If we are able to lift this constraint in
+the future, we expect it will not require (no pun intended) a second Swift
+Evolution proposal.
+
+[^shadows]: If a local variable is declared that shadows `self` or a function
+  argument, we may incorrectly infer the type of that value when captured. When
+  this occurs, Swift Testing emits a diagnostic of the form "🛑 Type of captured
+  value 'foo' is ambiguous".
+
+## Source compatibility
+
+This change is additive and relies on syntax that would previously be rejected
+at compile time.
+
+## Integration with supporting tools
+
+Xcode, Swift Package Manager, and the Swift VS Code plugin _already_ support
+captured values in exit tests as they use Swift Testing's built-in exit test
+handling logic.
+
+Tools that implement their own exit test handling logic will need to account for
+captured values. The `ExitTest` type now has a new SPI property:
+
+```swift
+extension ExitTest {
+  /// The set of values captured in the parent process before the exit test is
+  /// called.
+  ///
+  /// This property is automatically set by the testing library when using the
+  /// built-in exit test handler and entry point functions. Do not modify the
+  /// value of this property unless you are implementing a custom exit test
+  /// handler or entry point function.
+  ///
+  /// The order of values in this array must be the same between the parent and
+  /// child processes.
+  @_spi(ForToolsIntegrationOnly)
+  public var capturedValues: [CapturedValue] { get set }
+}
+```
+
+In the parent process (that is, for an instance of `ExitTest` passed to
+`Configuration.exitTestHandler`), this property represents the values captured
+at runtime by the exit test. In the child process (that is, for an instance of
+`ExitTest` returned from `ExitTest.find(identifiedBy:)`), the elements in this
+array do not have values associated with them until the hosting tool provides
+them.
+
+## Future directions
+
+- Supporting captured values without requiring type information
+
+  We need the types of captured values in order to successfully decode them, but
+  we are constrained by macros being syntax-only. In the future, the compiler
+  may gain a language feature similar to `decltype()` in C++ or `typeof()` in
+  C23, in which case we should be able to use it and avoid the need for explicit
+  types in the capture list. ([rdar://153389205](rdar://153389205))
+
+- Supporting capturing values that do not conform to `Codable`
+
+  Alternatives to `Codable` exist or have been proposed, such as
+  [`NSSecureCoding`](https://developer.apple.com/documentation/foundation/nssecurecoding)
+  or [`JSONCodable`](https://forums.swift.org/t/the-future-of-serialization-deserialization-apis/78585).
+  In the future, we may want to extend support for values that conform to these
+  protocols instead of `Codable`.
+
+## Alternatives considered
+
+- Doing nothing. There is sufficient motivation to support capturing values in
+  exit tests and it is within our technical capabilities.
+
+- Passing captured values as arguments to `#expect(processExitsWith:)` and its
+  body closure. For example:
+
+  ```swift
+  await #expect(
+    processExitsWith: .failure,
+    arguments: [fruit, bat]
+  ) { fruit, bat in
+    ...
+  }
+  ```
+
+  This is technically feasible, but:
+
+  - It requires that the caller state the capture list twice;
+  - Type information still isn't available for captured values, so you'd still
+    need to _actually_ write `{ (fruit: Fruit, bat: Bat) in ... }` (or otherwise
+    specify the types somewhere in the macro invocation); and
+  - The language already has a dedicated syntax for specifying lists of values
+    that should be captured in a closure.
+
+- Supporting non-`Sendable` or non-`Codable` captured values. Since exit tests'
+  bodies are, by definition, in separate isolation domains from the caller, and
+  since they, by nature, run in separate processes, conformance to these
+  protocols is fundamentally necessary.
+
+- Implicitly capturing `self`. This would require us to statically detect during
+  macro expansion whether `self` conformed to the necessary protocols _and_
+  would preclude capturing any state from static or free test functions.
+
+- Forking the exit test process such that all captured values are implicitly
+  copied by the kernel into the new process. Forking, in the UNIX fashion, is
+  fundamentally incompatible with the Swift runtime and the Swift thread pool.
+  On Darwin, you [cannot fork a process that links to Core Foundation without
+  immediately calling `exec()`](https://duckduckgo.com/?q=__THE_PROCESS_HAS_FORKED_AND_YOU_CANNOT_USE_THIS_COREFOUNDATION_FUNCTIONALITY___YOU_MUST_EXEC__),
+  and `fork()` isn't even present on Windows.
+
+## Acknowledgments
+
+Thanks to @rintaro for assistance investigating swift-syntax diagnostic support
+and to @xedin for humouring my questions about `decltype()`.
+
+Thanks to the Swift Testing team and the Testing Workgroup as always. And thanks
+to those individuals, who shall remain unnamed, who nerd-sniped me into building
+this feature.