Proposal for testing support in SwiftPM

Author: mxcl

proposals/0008-package-manager-testing.md

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+# Swift Testing
+
+* Authors:
+  [Max Howell](https://github.com/mxcl),
+  [Daniel Dunbar](https://github.com/ddunbar),
+  [Mattt Thompson](https://github.com/mattt)
+* Status: **Review**
+* Review Manager: Rick Ballard
+
+## Introduction
+
+Testing is an essential part of modern software development.
+Tight integration of testing into the Swift Package Manager
+will help ensure a stable and reliable packaging ecosystem.
+
+## Proposed Solution
+
+We propose to extend our conventional package directory layout
+to accomodate test modules.
+Any subdirectory of the package root directory named "Tests"
+or any subdirectory of an existing module directory named "Tests"
+will comprise a test module.
+For example:
+
+    Package
+    ├── Sources
+    │   └── Foo
+    │       └──Foo.swift
+    └── Tests
+        └── Foo
+            └── Test.swift
+
+Or:
+
+    Package
+    └── Sources
+        ├── Foo.swift
+        └── Tests
+            └── Test.swift
+
+Or, for simpler projects:
+
+    Package
+    ├── Sources
+    │   └── Foo.swift
+    └── Tests
+        └── TestFoo.swift
+        
+The filename: `TestFoo.swift` is arbituary.
+
+In the examples above
+a test case is created for the module `Foo`
+based on the sources in the relevant subdirectories.
+
+Additionally we will support directories called `FooTests`.
+This layout style is prevalent in existing open source projects
+and supporting it will minimize vexation for their authors.
+However in the interest of consistency and the corresponding 
+reduction of cognitive-load when examining new Swift packages
+we will not recommend this layout. For example:
+
+    Package
+    └── Sources
+    │   └── Foo.swift
+    └── FooTests
+        └── Test.swift
+
+Additionally, we propose that building a module
+also builds that module's corresponding tests.
+Although this would result in slightly increased build times,
+we believe that tests are important enough to justify this
+(one might even consider slow building tests to be a code smell).
+We would prefer to go even further by executing the tests
+each time a module is built as well,
+but we understand that this would impede debug cycles.
+
+As an exception, when packages are built in release mode we will
+not build tests because for release builds we should not enable
+testability. However, considering the need for release-mode testing
+this will be a future direction.
+
+### Command-Line Interface
+
+We propose the following syntax to execute tests:
+
+    $ swift build --test
+
+Or:
+
+    $ swift build -t
+
+In the future, we may choose to promote the `--test` option
+to be a subcommand of the `swift` command itself:
+
+    $ swift test
+
+However, any such decision would warrant extensive design consideration,
+so as to avoid polluting or crowding the command-line interface.
+Should there be sufficient demand and justification for it, though,
+it would be straightforward to add this functionality.
+
+### Command Output
+
+Executing a test from the terminal will produce user-readable output.
+This should incorporate colorization and other formatting
+similar to other testing tools
+to indicate the success and failure of different tests.
+For example:
+
+    $ swift test --output module
+    Running tests for PackageX (x/100)
+    .........x.....x...................
+
+    Completed
+    Elapsed time: 0.2s
+
+    98 Success
+     2 Failure
+     1 Warning
+
+    FAILURE: Tests/TestsA.swift:24 testFoo()
+    XCTAssertTrue expected true, got false
+
+    FAILURE: Tests/TestsB.swift:10 testBar()
+    XCTAssertEqual
+
+    WARNING: Tests/TestsC.swift:1
+    "Some Warning"
+
+An additional option may be passed to the testing command
+to output JUnit-style XML or other formats that can be integrated
+with continuous integration (CI) and other systems.
+
+### Backwards Compatibility
+
+In order to accomodate existing packages,
+we will allow test module targets and their targets
+to be overridden in the `Package.swift` manifest file.
+However, this functionality will likely not be implemented
+in the initial release of this feature,
+and instead be added at a later point in time.
+
+### Automatic Dependency Determination
+
+Testing is important and it is important to make the barrier to testing
+as minimal as possible. Thus, by analyzing the names of test targets,
+we will automatically determine the most likely dependency of that test
+and accomodate accordingly.
+For example,
+a test for "Foo" will depend on compilation of the library target `Foo`.
+Any additional dependencies or dependencies that could not be automatically determined
+would need to be specified in a package manifest separately.
+
+### Debug / Release Configuration
+
+Although tests built in debug configuration
+are generally run against modules also build in debug configuration,
+it is sometimes necessary to specify the build configuration for tests separately.
+It is also sometimes necessary to explicitly specify this information for every build,
+such as when building in a release configuration to execute performance tests.
+We would like to eventually support these use cases,
+however this will not be present in the initial implementation of this feature.
+
+### Testability
+
+Swift can build modules with "testability",
+which allows tests to access entities with `internal` access control.
+Because it would be tedious for users to specify this requirement for tests,
+we intend to build debug builds with testability by default.
+
+### Test Frameworks
+
+Initially,
+the Swift Package Manager will use `XCTest` as its underlying test framework.
+
+However, testing is an evolving artform,
+so we'd like to support other approaches
+that might allow frameworks other than XCTest
+to be supported by the package manager.
+We expect that such an implementation would take the form of
+a Swift protocol that the package manager defines,
+which other testing frameworks can adopt.
+
+## Impact On Existing Code
+
+Current releases of the package manager already exclude directories named
+"Tests" from target-determination. Directories named `FooTests` are not
+excluded, but as it stands this is a cause of compile failure, so in fact
+these changes will positively impact existing code.
+
+## Alternatives Considered
+
+Because this is a relatively broad proposal,
+no complete alternatives were considered.