How to unit test throwing functions in Swift?

EDIT: Updated the code for Swift 4.1 (still valid with Swift 5.2)

Here's the latest Swift version of Fyodor Volchyok's answer who used XCTAssertThrowsError:

    enum MyError: Error {
        case someExpectedError
        case someUnexpectedError
    }

    func functionThatThrows() throws {
        throw MyError.someExpectedError
    }

    func testFunctionThatThrows() {
        XCTAssertThrowsError(try functionThatThrows()) { error in
            XCTAssertEqual(error as! MyError, MyError.someExpectedError)
        }
    }

If your Error enum has associated values, you can either have your Error enum conform to Equatable, or use the if case statement:

    enum MyError: Error, Equatable {
        case someExpectedError
        case someUnexpectedError
        case associatedValueError(value: Int)
    }

    func functionThatThrows() throws {
        throw MyError.associatedValueError(value: 10)
    }

    // Equatable pattern: simplest solution if you have a simple associated value that can be tested inside 1 XCTAssertEqual
    func testFunctionThatThrows() {
        XCTAssertThrowsError(try functionThatThrows()) { error in
            XCTAssertEqual(error as! MyError, MyError.associatedValueError(value: 10))
        }
    }

    // if case pattern: useful if you have one or more associated values more or less complex (struct, classes...)
    func testFunctionThatThrows() {
        XCTAssertThrowsError(try functionThatThrows()) { error in
            guard case MyError.associatedValueError(let value) = error else {
                return XCTFail()
            }

            XCTAssertEqual(value, 10)
            // if you have several values or if they require more complex tests, you can do it here
        }
    }

At least of Xcode 7.3 (maybe earlier) you could use built-in XCTAssertThrowsError():

XCTAssertThrowsError(try methodThatThrows())

If nothing is thrown during test you'll see something like this:

If you want to check if thrown error is of some concrete type, you could use errorHandler parameter of XCTAssertThrowsError():

enum Error: ErrorType {
    case SomeExpectedError
    case SomeUnexpectedError
}

func functionThatThrows() throws {
    throw Error.SomeExpectedError
}

XCTAssertThrowsError(try functionThatThrows(), "some message") { (error) in
    XCTAssertEqual(error as? Error, Error.SomeExpectedError)
}