Adding tests

The principle of testing a macro is to test a code block before and after the macro expansion.

As part of our Swift Macros package, we have a test target (Figure 10.10):

Figure 10.10: A testing target for StructInitMacro

Each Swift package comes with a testing target, and in this case, we already have one test with the stringify macro that comes when we create a new Swift Macros package.

Let’s clear the test file and start our own test.

To test a macro, we need to create the XCTestCase subclass and create a new method called testMacro. Remember that test functions in XCTest always start with the phrase “test” followed by the test name.

To test a macro expansion, we will use a particular SwiftSyntax function called assertMacroExpansion. The most important function parameters are as follows:

• _originalSource: The original code before the expansion, including the macro attribute itself
• expandedSource...