src/ch11-00-testing.md - rust-lang/book - Git at Google

 # Writing Automated Tests

 In his 1972 essay “The Humble Programmer,” Edsger W. Dijkstra said that “Program
 testing can be a very effective way to show the presence of bugs, but it is
 hopelessly inadequate for showing their absence.” That doesn’t mean we shouldn’t
 try to test as much as we can!

 Correctness in our programs is the extent to which our code does what we intend
 it to do. Rust is designed with a high degree of concern about the correctness
 of programs, but correctness is complex and not easy to prove. Rust’s type
 system shoulders a huge part of this burden, but the type system cannot catch
 everything. As such, Rust includes support for writing automated software tests.

 Say we write a function `add_two` that adds 2 to whatever number is passed to
 it. This function’s signature accepts an integer as a parameter and returns an
 integer as a result. When we implement and compile that function, Rust does all
 the type checking and borrow checking that you’ve learned so far to ensure that,
 for instance, we aren’t passing a `String` value or an invalid reference to this
 function. But Rust _can’t_ check that this function will do precisely what we
 intend, which is return the parameter plus 2 rather than, say, the parameter
 plus 10 or the parameter minus 50! That’s where tests come in.

 We can write tests that assert, for example, that when we pass `3` to the
 `add_two` function, the returned value is `5`. We can run these tests whenever
 we make changes to our code to make sure any existing correct behavior has not
 changed.

 Testing is a complex skill: although we can’t cover in one chapter every detail
 about how to write good tests, in this chapter we will discuss the mechanics of
 Rust’s testing facilities. We’ll talk about the annotations and macros available
 to you when writing your tests, the default behavior and options provided for
 running your tests, and how to organize tests into unit tests and integration
 tests.
	# Writing Automated Tests

	In his 1972 essay “The Humble Programmer,” Edsger W. Dijkstra said that “Program
	testing can be a very effective way to show the presence of bugs, but it is
	hopelessly inadequate for showing their absence.” That doesn’t mean we shouldn’t
	try to test as much as we can!

	Correctness in our programs is the extent to which our code does what we intend
	it to do. Rust is designed with a high degree of concern about the correctness
	of programs, but correctness is complex and not easy to prove. Rust’s type
	system shoulders a huge part of this burden, but the type system cannot catch
	everything. As such, Rust includes support for writing automated software tests.

	Say we write a function `add_two` that adds 2 to whatever number is passed to
	it. This function’s signature accepts an integer as a parameter and returns an
	integer as a result. When we implement and compile that function, Rust does all
	the type checking and borrow checking that you’ve learned so far to ensure that,
	for instance, we aren’t passing a `String` value or an invalid reference to this
	function. But Rust _can’t_ check that this function will do precisely what we
	intend, which is return the parameter plus 2 rather than, say, the parameter
	plus 10 or the parameter minus 50! That’s where tests come in.

	We can write tests that assert, for example, that when we pass `3` to the
	`add_two` function, the returned value is `5`. We can run these tests whenever
	we make changes to our code to make sure any existing correct behavior has not
	changed.

	Testing is a complex skill: although we can’t cover in one chapter every detail
	about how to write good tests, in this chapter we will discuss the mechanics of
	Rust’s testing facilities. We’ll talk about the annotations and macros available
	to you when writing your tests, the default behavior and options provided for
	running your tests, and how to organize tests into unit tests and integration
	tests.