PassOrFail.swift

import XCTest

@resultBuilder
struct AssertionBuilder {
    static func buildArray(_ components: [Bool]) -> Bool {
        components.reduce(true) { $0 && $1 }
    }

    static func buildBlock(_ components: Bool...) -> Bool {
        components.reduce(true) { $0 && $1 }
    }

    static func buildEither(first component: Bool) -> Bool {
        component
    }

    static func buildEither(second component: Bool) -> Bool {
        component
    }

    static func buildOptional(_ component: Bool?) -> Bool {
        component ?? false
    }

    static func buildExpression<T>(_ expression: Optional<T>) -> Bool {
        switch expression {
        case .some: return true
        case .none: return false
        }
    }
}

func pass(file: StaticString = #file, line: UInt = #line) {
    XCTAssertTrue(true)
}

func pass(@AssertionBuilder _ builder: () throws -> Bool, file: StaticString = #file, line: UInt = #line) rethrows {
    XCTAssertTrue(try builder(), file: file, line: line)
}

func unwrap<T>(_ expression: Optional<T>, file: StaticString = #file, line: UInt = #line) throws -> T {
    try XCTUnwrap(expression, file: file, line: line)
}

func unwrap<T>(_ expression: Optional<T>, @AssertionBuilder _ builder: (T) -> Bool, file: StaticString = #file, line: UInt = #line) throws {
    XCTAssertTrue(builder(try unwrap(expression, file: file, line: line)), file: file, line: line)
}

struct DidNotExplode: Error {}
func fail<T>(_ expression: @autoclosure () throws -> T, file: StaticString = #file, line: UInt = #line) throws {
    do {
        _ = try expression()
        throw DidNotExplode()
    } catch {
        pass(file: file, line: line)
    }
}

func fail(_ message: String = "", file: StaticString = #file, line: UInt = #line) {
    XCTFail(message, file: file, line: line)
}

// Tests:

final class PassOrFailTests: XCTestCase {
    func testEquals() {
        let a = 1, b = 1
        pass { a == b }
    }

    func testUnwrap() {
        let a: String? = ""
        pass { a }
    }

    func testTryUnwrap() throws {
        let a: String? = ""
        try unwrap(a) { $0.isEmpty }
    }

    func testTryUnwrapInPass() throws {
        let a: String? = ""
        let value = { try unwrap(a) }
        try pass { try value().isEmpty }
    }

    func testFail() throws {
        var a: String? = nil
        try fail { try unwrap(a) }

        a = ""
        do {
            try fail { try unwrap(a) }
            pass()
        } catch {
            fail()
        }
    }
}