Add testing chapter

src/SUMMARY.md

@@ -17,6 +17,7 @@
     - [Incremental compilation](./incremental-compilation.md)
     - [Debugging and Testing](./incrcomp-debugging.md)
 - [The parser](./the-parser.md)
+- [Testing](./testing.md)
 - [Macro expansion](./macro-expansion.md)
 - [Name resolution](./name-resolution.md)
 - [The HIR (High-level IR)](./hir.md)

src/testing.md

@@ -0,0 +1,145 @@
+### The `#[test]` attribute
+Today, rust programmers rely on a built in attribute called `#[test]`.
+All you have to do is mark a function as a test and include some asserts like so:
+
+```rust,ignore
+#[test]
+fn my_test() {
+  assert!(2+2 == 4);
+}
+```
+
+When this program is compiled using `rustc --test` or `cargo test`, it will
+produce an executable that can run this, and any other test function. This
+method of testing allows tests to live alongside code in an organic way. You
+can even put tests inside private modules:
+
+```rust,ignore
+mod my_priv_mod {
+  fn my_priv_func() -> bool {}
+
+  #[test]
+  fn test_priv_func() {
+    assert!(my_priv_func());
+  }
+}
+```
+Private items can thus be easily tested without worrying about how to expose
+the them to any sort of external testing apparatus. This is key to the
+ergonomics of testing in Rust. Semantically, however, it's rather odd.
+How does any sort of `main` function invoke these tests if they're not visible?
+What exactly is `rustc --test` doing?
+
+`#[test]` is implemented as a syntactic transformation inside the compiler's
+[`libsyntax` crate][libsyntax]. Essentially, it's a fancy macro, that
+rewrites the crate in 3 steps:
+
+#### Step 1: Re-Exporting
+
+As mentioned earlier, tests can exist inside private modules, so we need a way of
+exposing them to the main function, without breaking any existing code. To that end,
+`libsyntax` will create local modules called `__test_reexports` that recursively reexport tests.
+This expansion translates the above example into:
+
+```rust,ignore
+mod my_priv_mod {
+  fn my_priv_func() -> bool {}
+
+  pub fn test_priv_func() {
+    assert!(my_priv_func());
+  }
+
+  pub mod __test_reexports {
+    pub use super::test_priv_func;
+  }
+}
+```
+
+Now, our test can be accessed as
+`my_priv_mod::__test_reexports::test_priv_func`. For deeper module
+structures, `__test_reexports` will reexport modules that contain tests, so a
+test at `a::b::my_test` becomes
+`a::__test_reexports::b::__test_reexports::my_test`. While this process seems
+pretty safe, what happens if there is an existing `__test_reexports` module?
+The answer: nothing.
+
+To explain, we need to understand [how the AST represents
+identifiers][Ident]. The name of every function, variable, module, etc. is
+not stored as a string, but rather as an opaque [Symbol][Symbol] which is
+essentially an ID number for each identifier. The compiler keeps a separate
+hashtable that allows us to recover the human-readable name of a Symbol when
+necessary (such as when printing a syntax error). When the compiler generates
+the `__test_reexports` module, it generates a new Symbol for the identifier,
+so while the compiler-generated `__test_reexports` may share a name with your
+hand-written one, it will not share a Symbol. This technique prevents name
+collision during code generation and is the foundation of Rust's macro
+hygiene.
+
+#### Step 2: Harness Generation
+Now that our tests are accessible from the root of our crate, we need to do something with them.
+`libsyntax` generates a module like so:
+
+```rust,ignore
+pub mod __test {
+  extern crate test;
+  const TESTS: &'static [self::test::TestDescAndFn] = &[/*...*/];
+
+  #[main]
+  pub fn main() {
+    self::test::test_static_main(TESTS);
+  }
+}
+```
+
+While this transformation is simple, it gives us a lot of insight into how tests are actually run.
+The tests are aggregated into an array and passed to a test runner called `test_static_main`.
+We'll come back to exactly what `TestDescAndFn` is, but for now, the key takeaway is that there is a crate
+called [`test`][test] that is part of Rust core, that implements all of the runtime for testing. `test`'s interface is unstable,
+so the only stable way to interact with it is through the `#[test]` macro.
+
+#### Step 3: Test Object Generation
+If you've written tests in Rust before, you may be familiar with some of the optional attributes available on test functions.
+For example, a test can be annotated with `#[should_panic]` if we expect the test to cause a panic. It looks something like this:
+
+```rust,ignore
+#[test]
+#[should_panic]
+fn foo() {
+  panic!("intentional");
+}
+```
+
+This means our tests are more than just simple functions, they have configuration information as well. `test` encodes this configuration
+data into a struct called [`TestDesc`][TestDesc]. For each test function in a crate, `libsyntax` will parse its attributes and generate a `TestDesc` instance.
+It then combines the `TestDesc` and test function into the predictably named `TestDescAndFn` struct, that `test_static_main` operates on.
+For a given test, the generated `TestDescAndFn` instance looks like so:
+
+```rust,ignore
+self::test::TestDescAndFn{
+  desc: self::test::TestDesc{
+    name: self::test::StaticTestName("foo"),
+    ignore: false,
+    should_panic: self::test::ShouldPanic::Yes,
+    allow_fail: false,
+  },
+  testfn: self::test::StaticTestFn(||
+    self::test::assert_test_result(::crate::__test_reexports::foo())),
+}
+```
+
+Once we've constructed an array of these test objects, they're passed to the
+test runner via the harness generated in step 2.
+
+### Inspecting the generated code
+On nightly rust, there's an unstable flag called `unpretty` that you can use to print out the module source after macro expansion:
+
+```bash
+$ rustc my_mod.rs -Z unpretty=hir
+```
+
+[test]: https://doc.rust-lang.org/test/index.html
+[TestDesc]: https://doc.rust-lang.org/test/struct.TestDesc.html
+[Symbol]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/struct.Ident.html
+[Ident]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/struct.Ident.html
+[eRFC]: https://github.com/rust-lang/rfcs/blob/master/text/2318-custom-test-frameworks.md
+[libsyntax]: https://github.com/rust-lang/rust/tree/master/src/libsyntax