Update test-implementation.md (#1937)

* Update test-implementation.md

* Update test-implementation.md

src/test-implementation.md

@@ -2,8 +2,8 @@
 
 <!-- toc -->
 
-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:
+Many Rust programmers rely on a built-in attribute called `#[test]`. All
+you have to do is mark a function and include some asserts like so:
 
 ```rust,ignore
 #[test]
@@ -35,14 +35,14 @@ 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
-[`rustc_ast` crate][rustc_ast]. Essentially, it's a fancy macro, that
+[`rustc_ast`][rustc_ast]. 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, `rustc_ast` will create local modules called
+code. To that end, [`rustc_ast`][rustc_ast] will create local modules called
 `__test_reexports` that recursively reexport tests. This expansion translates
 the above example into:
 
@@ -68,22 +68,22 @@ test at `a::b::my_test` becomes
 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
+To explain, we need to understand how Rust's [Abstract Syntax Tree][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.
+the `__test_reexports` module, it generates a new [Symbol][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][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. `rustc_ast` generates a module like so:
+something with them using [`rustc_ast`][ast] generates a module like so:
 
 ```rust,ignore
 #[main]
@@ -93,14 +93,14 @@ pub fn main() {
 }
 ```
 
-where `path::to::test1` is a constant of type `test::TestDescAndFn`.
+Here `path::to::test1` is a constant of type [`test::TestDescAndFn`][tdaf].
 
 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_main_static`. We'll come back to exactly what
-`TestDescAndFn` is, but for now, the key takeaway is that there is a crate
+[`TestDescAndFn`][tdaf] 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
+runtime for testing. [`test`][test]'s interface is unstable, so the only stable way
 to interact with it is through the `#[test]` macro.
 
 ## Step 3: Test Object Generation
@@ -119,12 +119,13 @@ fn foo() {
 ```
 
 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, `rustc_ast` 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_main_static` operates
-on. For a given test, the generated `TestDescAndFn` instance looks like so:
+configuration information as well. `test` encodes this configuration data into
+a `struct` called [`TestDesc`]. For each test function in a crate,
+[`rustc_ast`][rustc_ast] will parse its attributes and generate a [`TestDesc`]
+instance. It then combines the [`TestDesc`] and test function into the
+predictably named [`TestDescAndFn`][tdaf] `struct`, that [`test_main_static`]
+operates on.
+For a given test, the generated [`TestDescAndFn`][tdaf] instance looks like so:
 
 ```rust,ignore
 self::test::TestDescAndFn{
@@ -140,19 +141,23 @@ self::test::TestDescAndFn{
 ```
 
 Once we've constructed an array of these test objects, they're passed to the
-test runner via the harness generated in step 2.
+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:
+On `nightly` `rustc`, 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/rustc_span/symbol/struct.Symbol.html
+[`macro`]: ./macro-expansion.md
+[`TestDesc`]: https://doc.rust-lang.org/test/struct.TestDesc.html
+[ast]: ./ast-validation.md
 [Ident]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html
 [rustc_ast]: https://github.com/rust-lang/rust/tree/master/compiler/rustc_ast
+[Symbol]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Symbol.html
+[test]: https://doc.rust-lang.org/test/index.html
+[tdaf]: https://doc.rust-lang.org/test/struct.TestDescAndFn.html
+[`test_main_static`]: https://doc.rust-lang.org/test/fn.test_main_static.html