Auto merge of #43812 - steveklabnik:rustdoc-doc-tests, r=QuietMisdreavus

rustdoc doc tests

This is mostly adapted from the book v1's chapter.

cc #42322

r? @rust-lang/docs

Author: bors

src/doc/rustdoc/src/documentation-tests.md

@@ -1,3 +1,239 @@
 # Documentation tests
 
-Coming soon!
+`rustdoc` supports executing your documentation examples as tests. This makes sure
+that your tests are up to date and working.
+
+The basic idea is this:
+
+```rust,ignore
+/// # Examples
+///
+/// ```
+/// let x = 5;
+/// ```
+```
+
+The triple backticks start and end code blocks. If this were in a file named `foo.rs`,
+running `rustdoc --test foo.rs` will extract this example, and then run it as a test.
+
+There's some subtlety though! Read on for more details.
+
+## Pre-processing examples
+
+In the example above, you'll note something strange: there's no `main`
+function! Forcing you to write `main` for every example, no matter how small,
+adds friction. So `rustdoc` processes your examples slightly before
+running them. Here's the full algorithm rustdoc uses to preprocess examples:
+
+1. Any leading `#![foo]` attributes are left intact as crate attributes.
+2. Some common `allow` attributes are inserted, including
+   `unused_variables`, `unused_assignments`, `unused_mut`,
+   `unused_attributes`, and `dead_code`. Small examples often trigger
+   these lints.
+3. If the example does not contain `extern crate`, then `extern crate
+   <mycrate>;` is inserted (note the lack of `#[macro_use]`).
+4. Finally, if the example does not contain `fn main`, the remainder of the
+   text is wrapped in `fn main() { your_code }`.
+
+For more about that caveat in rule 3, see "Documeting Macros" below.
+
+## Hiding portions of the example
+
+Sometimes, you need some setup code, or other things that would distract
+from your example, but are important to make the tests work. Consider
+an example block that looks like this:
+
+```text
+/// Some documentation.
+# fn foo() {}
+```
+
+It will render like this:
+
+```rust
+/// Some documentation.
+# fn foo() {}
+```
+
+Yes, that's right: you can add lines that start with `# `, and they will
+be hidden from the output, but will be used when compiling your code. You
+can use this to your advantage. In this case, documentation comments need
+to apply to some kind of function, so if I want to show you just a
+documentation comment, I need to add a little function definition below
+it. At the same time, it's only there to satisfy the compiler, so hiding
+it makes the example more clear. You can use this technique to explain
+longer examples in detail, while still preserving the testability of your
+documentation.
+
+For example, imagine that we wanted to document this code:
+
+```rust
+let x = 5;
+let y = 6;
+println!("{}", x + y);
+```
+
+We might want the documentation to end up looking like this:
+
+> First, we set `x` to five:
+>
+> ```rust
+> let x = 5;
+> # let y = 6;
+> # println!("{}", x + y);
+> ```
+>
+> Next, we set `y` to six:
+>
+> ```rust
+> # let x = 5;
+> let y = 6;
+> # println!("{}", x + y);
+> ```
+>
+> Finally, we print the sum of `x` and `y`:
+>
+> ```rust
+> # let x = 5;
+> # let y = 6;
+> println!("{}", x + y);
+> ```
+
+To keep each code block testable, we want the whole program in each block, but
+we don't want the reader to see every line every time.  Here's what we put in
+our source code:
+
+```text
+    First, we set `x` to five:
+
+    ```rust
+    let x = 5;
+    # let y = 6;
+    # println!("{}", x + y);
+    ```
+
+    Next, we set `y` to six:
+
+    ```rust
+    # let x = 5;
+    let y = 6;
+    # println!("{}", x + y);
+    ```
+
+    Finally, we print the sum of `x` and `y`:
+
+    ```rust
+    # let x = 5;
+    # let y = 6;
+    println!("{}", x + y);
+    ```
+```
+
+By repeating all parts of the example, you can ensure that your example still
+compiles, while only showing the parts that are relevant to that part of your
+explanation.
+
+Another case where the use of `#` is handy is when you want to ignore
+error handling. Lets say you want the following,
+
+```rust,ignore
+/// use std::io;
+/// let mut input = String::new();
+/// io::stdin().read_line(&mut input)?;
+```
+
+The problem is that `?` returns a `Result<T, E>` and test functions
+don't return anything so this will give a mismatched types error.
+
+```rust,ignore
+/// A doc test using ?
+///
+/// ```
+/// use std::io;
+/// # fn foo() -> io::Result<()> {
+/// let mut input = String::new();
+/// io::stdin().read_line(&mut input)?;
+/// # Ok(())
+/// # }
+/// ```
+# fn foo() {}
+```
+
+You can get around this by wrapping the code in a function. This catches
+and swallows the `Result<T, E>` when running tests on the docs. This
+pattern appears regularly in the standard library.
+
+### Documenting macros
+
+Here’s an example of documenting a macro:
+
+```rust
+/// Panic with a given message unless an expression evaluates to true.
+///
+/// # Examples
+///
+/// ```
+/// # #[macro_use] extern crate foo;
+/// # fn main() {
+/// panic_unless!(1 + 1 == 2, “Math is broken.”);
+/// # }
+/// ```
+///
+/// ```rust,should_panic
+/// # #[macro_use] extern crate foo;
+/// # fn main() {
+/// panic_unless!(true == false, “I’m broken.”);
+/// # }
+/// ```
+#[macro_export]
+macro_rules! panic_unless {
+    ($condition:expr, $($rest:expr),+) => ({ if ! $condition { panic!($($rest),+); } });
+}
+# fn main() {}
+```
+
+You’ll note three things: we need to add our own `extern crate` line, so that
+we can add the `#[macro_use]` attribute. Second, we’ll need to add our own
+`main()` as well (for reasons discussed above). Finally, a judicious use of
+`#` to comment out those two things, so they don’t show up in the output.
+
+## Attributes
+
+There are a few annotations that are useful to help `rustdoc` do the right
+thing when testing your code:
+
+```rust
+/// ```ignore
+/// fn foo() {
+/// ```
+# fn foo() {}
+```
+
+The `ignore` directive tells Rust to ignore your code. This is almost never
+what you want, as it's the most generic. Instead, consider annotating it
+with `text` if it's not code, or using `#`s to get a working example that
+only shows the part you care about.
+
+```rust
+/// ```should_panic
+/// assert!(false);
+/// ```
+# fn foo() {}
+```
+
+`should_panic` tells `rustdoc` that the code should compile correctly, but
+not actually pass as a test.
+
+```rust
+/// ```no_run
+/// loop {
+///     println!("Hello, world");
+/// }
+/// ```
+# fn foo() {}
+```
+
+The `no_run` attribute will compile your code, but not run it. This is
+important for examples such as "Here's how to retrieve a web page,"
+which you would want to ensure compiles, but might be run in a test
+environment that has no network access.