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+% The Rust Testing Guide
+
+> Program testing can be a very effective way to show the presence of bugs, but
+> it is hopelessly inadequate for showing their absence. 
+>
+> Edsger W. Dijkstra, "The Humble Programmer" (1972)
+
+Let's talk about how to test Rust code. What we will not be talking about is
+the right way to test Rust code. There are many schools of thought regarding
+the right and wrong way to write tests. All of these approaches use the same
+basic tools, and so we'll show you the syntax for using them.
+
+# The `test` attribute
+
+At its simplest, a test in Rust is a function that's annotated with the `test`
+attribute. Let's make a new project with Cargo called `adder`:
+
+```bash
+$ cargo new adder
+$ cd adder
+```
+
+Cargo will automatically generate a simple test when you make a new project.
+Here's the contents of `src/lib.rs`:
+
+```rust
+#[test]
+fn it_works() {
+}
+```
+
+Note the `#[test]`. This attribute indicates that this is a test function. It
+currently has no body. That's good enough to pass! We can run the tests with
+`cargo test`:
+
+```bash
+$ cargo test
+   Compiling adder v0.0.1 (file:///home/you/projects/adder)
+     Running target/adder-91b3e234d4ed382a
+
+running 1 test
+test it_works ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
+
+   Doc-tests adder
+
+running 0 tests
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
+```
+
+Cargo compiled and ran our tests. There are two sets of output here: one
+for the test we wrote, and another for documentation tests. We'll talk about
+those later. For now, see this line:
+
+```text
+test it_works ... ok
+```
+
+Note the `it_works`. This comes from the name of our function:
+
+```rust
+fn it_works() {
+# }
+```
+
+We also get a summary line:
+
+```text
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
+```
+
+So why does our do-nothing test pass? Any test which doesn't `panic!` passes,
+and any test that does `panic!` fails. Let's make our test fail:
+
+```rust
+#[test]
+fn it_works() {
+    assert!(false);
+}
+```
+
+`assert!` is a macro provided by Rust which takes one argument: if the argument
+is `true`, nothing happens. If the argument is false, it `panic!`s. Let's run
+our tests again:
+
+```bash
+$ cargo test
+   Compiling adder v0.0.1 (file:///home/you/projects/adder)
+     Running target/adder-91b3e234d4ed382a
+
+running 1 test
+test it_works ... FAILED
+
+failures:
+
+---- it_works stdout ----
+        task 'it_works' panicked at 'assertion failed: false', /home/steve/tmp/adder/src/lib.rs:3
+
+
+
+failures:
+    it_works
+
+test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
+
+task '<main>' panicked at 'Some tests failed', /home/steve/src/rust/src/libtest/lib.rs:247
+```
+
+Rust indicates that our test failed:
+
+```text
+test it_works ... FAILED
+```
+
+And that's reflected in the summary line:
+
+```text
+test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
+```
+
+We also get a non-zero status code:
+
+```bash
+$ echo $?
+101
+```
+
+This is useful if you want to integrate `cargo test` into other tooling.
+
+We can invert our test's failure with another attribute: `should_fail`:
+
+```rust
+#[test]
+#[should_fail]
+fn it_works() {
+    assert!(false);
+}
+```
+
+This test will now succeed if we `panic!` and fail if we complete. Let's try it:
+
+```bash
+$ cargo test
+   Compiling adder v0.0.1 (file:///home/you/projects/adder)
+     Running target/adder-91b3e234d4ed382a
+
+running 1 test
+test it_works ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
+
+   Doc-tests adder
+
+running 0 tests
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
+```
+
+Rust provides another macro, `assert_eq!`, that compares two arguments for
+equality:
+
+```rust
+#[test]
+#[should_fail]
+fn it_works() {
+    assert_eq!("Hello", "world");
+}
+```
+
+Does this test pass or fail? Because of the `should_fail` attribute, it
+passes:
+
+```bash
+$ cargo test
+   Compiling adder v0.0.1 (file:///home/you/projects/adder)
+     Running target/adder-91b3e234d4ed382a
+
+running 1 test
+test it_works ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
+
+   Doc-tests adder
+
+running 0 tests
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
+```
+
+`should_fail` tests can be fragile, as it's hard to guarantee that the test
+didn't fail for an unexpected reason. To help with this, an optional `expected`
+parameter can be added to the `should_fail` attribute. The test harness will
+make sure that the failure message contains the provided text. A safer version
+of the example above would be:
+
+```
+#[test]
+#[should_fail(expected = "assertion failed")]
+fn it_works() {
+    assert_eq!("Hello", "world");
+}
+```
+
+That's all there is to the basics! Let's write one 'real' test:
+
+```{rust,ignore}
+pub fn add_two(a: i32) -> i32 {
+    a + 2
+}
+
+#[test]
+fn it_works() {
+    assert_eq!(4, add_two(2));
+}
+```
+
+This is a very common use of `assert_eq!`: call some function with
+some known arguments and compare it to the expected output.
+
+# The `test` module
+
+There is one way in which our existing example is not idiomatic: it's
+missing the test module. The idiomatic way of writing our example
+looks like this:
+
+```{rust,ignore}
+pub fn add_two(a: i32) -> i32 {
+    a + 2
+}
+
+#[cfg(test)]
+mod tests {
+    use super::add_two;
+
+    #[test]
+    fn it_works() {
+        assert_eq!(4, add_two(2));
+    }
+}
+```
+
+There's a few changes here. The first is the introduction of a `mod tests` with
+a `cfg` attribute. The module allows us to group all of our tests together, and
+to also define helper functions if needed, that don't become a part of the rest
+of our crate. The `cfg` attribute only compiles our test code if we're
+currently trying to run the tests. This can save compile time, and also ensures
+that our tests are entirely left out of a normal build.
+
+The second change is the `use` declaration. Because we're in an inner module,
+we need to bring our test function into scope. This can be annoying if you have
+a large module, and so this is a common use of the `glob` feature. Let's change
+our `src/lib.rs` to make use of it:
+
+```{rust,ignore}
+#![feature(globs)]
+
+pub fn add_two(a: i32) -> i32 {
+    a + 2
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        assert_eq!(4, add_two(2));
+    }
+}
+```
+
+Note the `feature` attribute, as well as the different `use` line. Now we run
+our tests:
+
+```bash
+$ cargo test
+    Updating registry `https://github.com/rust-lang/crates.io-index`
+   Compiling adder v0.0.1 (file:///home/you/projects/adder)
+     Running target/adder-91b3e234d4ed382a
+
+running 1 test
+test test::it_works ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
+
+   Doc-tests adder
+
+running 0 tests
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
+```
+
+It works!
+
+The current convention is to use the `test` module to hold your "unit"-style
+tests. Anything that just tests one small bit of functionality makes sense to
+go here. But what about "integration"-style tests instead? For that, we have
+the `tests` directory
+
+# The `tests` directory
+
+To write an integration test, let's make a `tests` directory, and
+put a `tests/lib.rs` file inside, with this as its contents:
+
+```{rust,ignore}
+extern crate adder;
+
+#[test]
+fn it_works() {
+    assert_eq(4, adder::add_two(2));
+}   
+```
+
+This looks similar to our previous tests, but slightly different. We now have
+an `extern crate adder` at the top. This is because the tests in the `tests`
+directory are an entirely separate crate, and so we need to import our library.
+This is also why `tests` is a suitable place to write integration-style tests:
+they use the library like any other consumer of it would.
+
+Let's run them:
+
+```bash
+$ cargo test
+   Compiling adder v0.0.1 (file:///home/you/projects/adder)
+     Running target/adder-91b3e234d4ed382a
+
+running 1 test
+test test::it_works ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
+
+     Running target/lib-c18e7d3494509e74
+
+running 1 test
+test it_works ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
+
+   Doc-tests adder
+
+running 0 tests
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
+```
+
+Now we have three sections: our previous test is also run, as well as our new
+one.
+
+That's all there is to the `tests` directory. The `test` module isn't needed
+here, since the whole thing is focused on tests.
+
+Let's finally check out that third section: documentation tests.
+
+# Documentation tests
+
+Nothing is better than documentation with examples. Nothing is worse than
+examples that don't actually work, because the code has changed since the
+documentation has been written. To this end, Rust supports automatically
+running examples in your documentation. Here's a fleshed-out `src/lib.rs`
+with examples:
+
+```{rust,ignore}
+//! The `adder` crate provides functions that add numbers to other numbers.
+//!
+//! # Examples
+//!
+//! ```
+//! assert_eq!(4, adder::add_two(2));
+//! ```
+
+#![feature(globs)]
+
+/// This function adds two to its argument.
+///
+/// # Examples
+///
+/// ```
+/// use adder::add_two;
+///
+/// assert_eq!(4, add_two(2));
+/// ```
+pub fn add_two(a: i32) -> i32 {
+    a + 2
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        assert_eq!(4, add_two(2));
+    }
+}
+```
+
+Note the module-level documentation with `//!` and the function-level
+documentation with `///`. Rust's documentation supports Markdown in comments,
+and so triple graves mark code blocks. It is conventional to include the
+`# Examples` section, exactly like that, with examples following.
+
+Let's run the tests again:
+
+```bash
+$ cargo test
+   Compiling adder v0.0.1 (file:///home/steve/tmp/adder)
+     Running target/adder-91b3e234d4ed382a
+
+running 1 test
+test test::it_works ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
+
+     Running target/lib-c18e7d3494509e74
+
+running 1 test
+test it_works ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
+
+   Doc-tests adder
+
+running 2 tests
+test add_two_0 ... ok
+test _0 ... ok
+
+test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured
+```
+
+Now we have all three kinds of tests running! Note the names of the
+documentation tests: the `_0` is generated for the module test, and `add_two_0`
+for the function test. These will auto increment with names like `add_two_1` as
+you add more examples.
+
+# Benchmark tests
+
+Rust also supports benchmark tests, which can test the performance of your
+code. Let's make our `src/lib.rs` look like this (comments elided):
+
+```{rust,ignore}
+#![feature(globs)]
+
+extern crate test;
+
+pub fn add_two(a: i32) -> i32 {
+    a + 2
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use test::Bencher;
+
+    #[test]
+    fn it_works() {
+        assert_eq!(4, add_two(2));
+    }
+
+    #[bench]
+    fn bench_add_two(b: &mut Bencher) {
+        b.iter(|| add_two(2));
+    }
+}
+```
+
+We've imported the `test` crate, which contains our benchmarking support.
+We have a new function as well, with the `bench` attribute. Unlike regular
+tests, which take no arguments, benchmark tests take a `&mut Bencher`. This
+`Bencher` provides an `iter` method, which takes a closure. This closure
+contains the code we'd like to benchmark.
+
+We can run benchmark tests with `cargo bench`:
+
+```bash
+$ cargo bench
+   Compiling adder v0.0.1 (file:///home/steve/tmp/adder)
+     Running target/release/adder-91b3e234d4ed382a
+
+running 2 tests
+test tests::it_works ... ignored
+test tests::bench_add_two ... bench:         1 ns/iter (+/- 0)
+
+test result: ok. 0 passed; 0 failed; 1 ignored; 1 measured
+```
+
+Our non-benchmark test was ignored. You may have noticed that `cargo bench`
+takes a bit longer than `cargo test`. This is because Rust runs our benchmark
+a number of times, and then takes the average. Because we're doing so little
+work in this example, we have a `1 ns/iter (+/- 0)`, but this would show
+the variance if there was one.
+
+Advice on writing benchmarks:
+
+
+* Move setup code outside the `iter` loop; only put the part you want to measure inside
+* Make the code do "the same thing" on each iteration; do not accumulate or change state
+* Make the outer function idempotent too; the benchmark runner is likely to run
+  it many times
+*  Make the inner `iter` loop short and fast so benchmark runs are fast and the
+   calibrator can adjust the run-length at fine resolution
+* Make the code in the `iter` loop do something simple, to assist in pinpointing
+  performance improvements (or regressions)
+
+## Gotcha: optimizations
+
+There's another tricky part to writing benchmarks: benchmarks compiled with
+optimizations activated can be dramatically changed by the optimizer so that
+the benchmark is no longer benchmarking what one expects. For example, the
+compiler might recognize that some calculation has no external effects and
+remove it entirely.
+
+```{rust,ignore}
+extern crate test;
+use test::Bencher;
+
+#[bench]
+fn bench_xor_1000_ints(b: &mut Bencher) {
+    b.iter(|| {
+        range(0u, 1000).fold(0, |old, new| old ^ new);
+    });
+}
+```
+
+gives the following results
+
+```text
+running 1 test
+test bench_xor_1000_ints ... bench:         0 ns/iter (+/- 0)
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured
+```
+
+The benchmarking runner offers two ways to avoid this. Either, the closure that
+the `iter` method receives can return an arbitrary value which forces the
+optimizer to consider the result used and ensures it cannot remove the
+computation entirely. This could be done for the example above by adjusting the
+`b.iter` call to
+
+```rust
+# struct X;
+# impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
+b.iter(|| {
+    // note lack of `;` (could also use an explicit `return`).
+    range(0u, 1000).fold(0, |old, new| old ^ new)
+});
+```
+
+Or, the other option is to call the generic `test::black_box` function, which
+is an opaque "black box" to the optimizer and so forces it to consider any
+argument as used.
+
+```rust
+extern crate test;
+
+# fn main() {
+# struct X;
+# impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
+b.iter(|| {
+    let mut n = 1000_u32;
+
+    test::black_box(&mut n); // pretend to modify `n`
+
+    range(0, n).fold(0, |a, b| a ^ b)
+})
+# }
+```
+
+Neither of these read or modify the value, and are very cheap for small values.
+Larger values can be passed indirectly to reduce overhead (e.g.
+`black_box(&huge_struct)`).
+
+Performing either of the above changes gives the following benchmarking results
+
+```text
+running 1 test
+test bench_xor_1000_ints ... bench:       1 ns/iter (+/- 0)
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured
+```
+
+However, the optimizer can still modify a testcase in an undesirable manner
+even when using either of the above.