Add a (internal-only) CustomIssueRepresentable protocol. (swiftlang#945)

This PR adds a (for now internal-only) protocol that lets us transform
arbitrary errors to arbitrary issues (rather than always emitting them
as `.errorCaught`). This protocol replaces the special-casing we were
doing for `SystemError` and `APIMisuseError`.

This change also causes us to emit an API misuse issue if a developer
passes an error of type `ExpectationFailedError` to `Issue.record()` as
we can reasonably assume that such an error was already recorded
correctly as `.expectationFailed`.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

---------

Co-authored-by: Stuart Montgomery <smontgomery@apple.com>

Author: grynspan

Sources/Testing/CMakeLists.txt

@@ -73,14 +73,14 @@ add_library(Testing
   Support/Additions/WinSDKAdditions.swift
   Support/CartesianProduct.swift
   Support/CError.swift
+  Support/CustomIssueRepresentable.swift
   Support/Environment.swift
   Support/FileHandle.swift
   Support/GetSymbol.swift
   Support/Graph.swift
   Support/JSON.swift
   Support/Locked.swift
   Support/Locked+Platform.swift
-  Support/SystemError.swift
   Support/Versions.swift
   Discovery.swift
   Discovery+Platform.swift

Sources/Testing/Issues/Issue+Recording.swift

@@ -27,19 +27,11 @@ extension Issue {
   /// - Returns: The issue that was recorded (`self` or a modified copy of it.)
   @discardableResult
   func record(configuration: Configuration? = nil) -> Self {
-    // If this issue is a caught error of kind SystemError, reinterpret it as a
-    // testing system issue instead (per the documentation for SystemError.)
+    // If this issue is a caught error that has a custom issue representation,
+    // perform that customization now.
     if case let .errorCaught(error) = kind {
-      // TODO: consider factoring this logic out into a protocol
-      if let error = error as? SystemError {
-        var selfCopy = self
-        selfCopy.kind = .system
-        selfCopy.comments.append(Comment(rawValue: String(describingForTest: error)))
-        return selfCopy.record(configuration: configuration)
-      } else if let error = error as? APIMisuseError {
-        var selfCopy = self
-        selfCopy.kind = .apiMisused
-        selfCopy.comments.append(Comment(rawValue: String(describingForTest: error)))
+      if let error = error as? any CustomIssueRepresentable {
+        let selfCopy = error.customize(self)
         return selfCopy.record(configuration: configuration)
       }
     }

Sources/Testing/Support/CustomIssueRepresentable.swift

@@ -0,0 +1,86 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024–2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+//
+
+/// A protocol that provides instances of conforming types with the ability to
+/// record themselves as test issues.
+///
+/// When a type conforms to this protocol, values of that type can be passed to
+/// ``Issue/record(_:_:)``. The testing library then calls the
+/// ``customize(_:)`` function and passes it an instance of ``Issue`` that will
+/// be used to represent the value. The function can then reconfigure or replace
+/// the issue as needed.
+///
+/// This protocol may become part of the testing library's public interface in
+/// the future. There's not really anything _requiring_ it to conform to `Error`
+/// but all our current use cases are error types. If we want to allow other
+/// types to be represented as issues, we will need to add an overload of
+/// `Issue.record()` that takes `some CustomIssueRepresentable`.
+protocol CustomIssueRepresentable: Error {
+  /// Customize the issue that will represent this value.
+  ///
+  /// - Parameters:
+  ///   - issue: The issue to customize. The function consumes this value.
+  ///
+  /// - Returns: A customized copy of `issue`.
+  func customize(_ issue: consuming Issue) -> Issue
+}
+
+// MARK: - Internal error types
+
+/// A type representing an error in the testing library or its underlying
+/// infrastructure.
+///
+/// When an error of this type is thrown and caught by the testing library, it
+/// is recorded as an issue of kind ``Issue/Kind/system`` rather than
+/// ``Issue/Kind/errorCaught(_:)``.
+///
+/// This type is not part of the public interface of the testing library.
+/// External callers should generally record issues by throwing their own errors
+/// or by calling ``Issue/record(_:sourceLocation:)``.
+struct SystemError: Error, CustomStringConvertible, CustomIssueRepresentable {
+  var description: String
+
+  func customize(_ issue: consuming Issue) -> Issue {
+    issue.kind = .system
+    issue.comments.append("\(self)")
+    return issue
+  }
+}
+
+/// A type representing misuse of testing library API.
+///
+/// When an error of this type is thrown and caught by the testing library, it
+/// is recorded as an issue of kind ``Issue/Kind/apiMisused`` rather than
+/// ``Issue/Kind/errorCaught(_:)``.
+///
+/// This type is not part of the public interface of the testing library.
+/// External callers should generally record issues by throwing their own errors
+/// or by calling ``Issue/record(_:sourceLocation:)``.
+struct APIMisuseError: Error, CustomStringConvertible, CustomIssueRepresentable {
+  var description: String
+
+  func customize(_ issue: consuming Issue) -> Issue {
+    issue.kind = .apiMisused
+    issue.comments.append("\(self)")
+    return issue
+  }
+}
+
+extension ExpectationFailedError: CustomIssueRepresentable {
+  func customize(_ issue: consuming Issue) -> Issue {
+    // Instances of this error type are thrown by `#require()` after an issue of
+    // kind `.expectationFailed` has already been recorded. Code that rethrows
+    // this error does not generate a new issue, but code that passes this error
+    // to Issue.record() is misbehaving.
+    issue.kind = .apiMisused
+    issue.comments.append("Recorded an error of type \(Self.self) representing an expectation that failed and was already recorded: \(expectation)")
+    return issue
+  }
+}

Sources/Testing/Support/SystemError.swift

@@ -1515,6 +1515,84 @@ final class IssueTests: XCTestCase {
 
     await fulfillment(of: [expectationFailed], timeout: 0.0)
   }
+
+  func testThrowing(_ error: some Error, producesIssueMatching issueMatcher: @escaping @Sendable (Issue) -> Bool) async {
+    let issueRecorded = expectation(description: "Issue recorded")
+
+    var configuration = Configuration()
+    configuration.eventHandler = { event, _ in
+      guard case let .issueRecorded(issue) = event.kind else {
+        return
+      }
+      if issueMatcher(issue) {
+        issueRecorded.fulfill()
+        let description = String(describing: error)
+        #expect(issue.comments.map(String.init(describing:)).contains(description))
+      } else {
+        Issue.record("Unexpected issue \(issue)")
+      }
+    }
+
+    await Test {
+      throw error
+    }.run(configuration: configuration)
+
+    await fulfillment(of: [issueRecorded], timeout: 0.0)
+  }
+
+  func testThrowingSystemErrorProducesSystemIssue() async {
+    await testThrowing(
+      SystemError(description: "flinging excuses"),
+      producesIssueMatching: { issue in
+        if case .system = issue.kind {
+          return true
+        }
+        return false
+      }
+    )
+  }
+
+  func testThrowingAPIMisuseErrorProducesAPIMisuseIssue() async {
+    await testThrowing(
+      APIMisuseError(description: "you did it wrong"),
+      producesIssueMatching: { issue in
+        if case .apiMisused = issue.kind {
+          return true
+        }
+        return false
+      }
+    )
+  }
+
+  func testRethrowingExpectationFailedErrorCausesAPIMisuseError() async {
+    let expectationFailed = expectation(description: "Expectation failed (issue recorded)")
+    let apiMisused = expectation(description: "API misused (issue recorded)")
+
+    var configuration = Configuration()
+    configuration.eventHandler = { event, _ in
+      guard case let .issueRecorded(issue) = event.kind else {
+        return
+      }
+      switch issue.kind {
+      case .expectationFailed:
+        expectationFailed.fulfill()
+      case .apiMisused:
+        apiMisused.fulfill()
+      default:
+        Issue.record("Unexpected issue \(issue)")
+      }
+    }
+
+    await Test {
+      do {
+        try #require(Bool(false))
+      } catch {
+        Issue.record(error)
+      }
+    }.run(configuration: configuration)
+
+    await fulfillment(of: [expectationFailed, apiMisused], timeout: 0.0)
+  }
 }
 #endif