Document how to format code (#2375)

* Document how to format code

* Fix typo

* Fix formatting

* Ignore code snippet

Author: pvdrz

book/src/SUMMARY.md

@@ -22,6 +22,7 @@
     - [Preventing the Derivation of `Default`](./nodefault.md)
     - [Annotating types with `#[must-use]`](./must-use-types.md)
     - [Field visibility](./visibility.md)
+    - [Code formatting](./code-formatting.md)
 - [Generating Bindings to C++](./cpp.md)
 - [Generating Bindings to Objective-c](./objc.md)
 - [Using Unions](./using-unions.md)

book/src/code-formatting.md

@@ -0,0 +1,69 @@
+# Code Formatting
+
+`bindgen` uses `rustfmt` to format the emitted bindings. This section describes
+how to adjust the `rustfmt` behavior when being used from `bindgen`.
+
+## Passing a `rustfmt.toml` configuration file
+
+`rustfmt` should automatically use any `rustfmt.toml` file that is present in
+the directory from where `bindgen` will be run. If you want to use a
+configuration file that has a different name or that is in a different
+directory you can use the `--rustfmt-configuration-file` flag or the
+[`Builder::rustfmt_configuration_file`](https://docs.rs/bindgen/latest/bindgen/struct.Builder.html#method.rustfmt_configuration_file)
+method.
+
+## Using a nightly release of `rustfmt`
+
+If the `rustfmt` command does not correspond to a nightly release of `rustfmt`
+but you have `rustup` available, you can use `nightly` by following these
+steps:
+
+### When using `bindgen` as a CLI application
+
+Use `rustup run` to run `bindgen`:
+
+```bash
+$ rustup run nightly bindgen [ARGS]
+```
+
+### When using `bindgen` as a library
+
+Take the output of the following command:
+```bash
+$ rustup which rustfmt --toolchain=nightly
+```
+and pass it to
+[`Builder::with_rustfmt`](https://docs.rs/bindgen/latest/bindgen/struct.Builder.html#method.with_rustfmt):
+
+```rust,ignore
+use bindgen::Builder;
+use std::process::Command;
+
+fn main() {
+    let output = Command::new("rustup")
+        .args(["which", "rustfmt", "--toolchain", "nightly"])
+        .output()
+        .expect("Could not spawn `rustup` command");
+
+    assert!(
+        output.status.success(),
+        "Unsuccessful status code when running `rustup`: {:?}",
+        output
+    );
+
+    let rustfmt_path =
+        String::from_utf8(output.stdout).expect("The `rustfmt` path is not valid `utf-8`");
+
+    let bindings = Builder::default()
+        .header("path/to/input.h")
+        .with_rustfmt(rustfmt_path)
+        .generate()
+        .expect("Could not generate bindings");
+
+    bindings
+        .write_to_file("path/to/output.rs")
+        .expect("Could not write bindings");
+}
+```
+
+These two methods also apply to any other toolchain available in your system.

book/src/faq.md

@@ -108,3 +108,19 @@ $ bindgen <input_headers> -- --target=armv7a-none-eabi
 ```
 If you are using `bindgen` as a library, you should call
 `builder.clang_arg("--target=armv7a-none-eabi")` on your `builder`.
+
+### How can I normalize `#[doc]` attributes?
+
+`bindgen` emits all the documentation using `#[doc]` attributes by default. If
+you want to use the more user-friendly `///` syntax, you have to create a
+`rustfmt.toml` file with the following contents:
+
+```toml
+normalize_doc_attributes = true
+```
+
+Then, you have set up bindgen so it passes this file to `rustfmt`. Given that
+the `normalize_doc_attributes` option is
+[unstable](https://github.com/rust-lang/rustfmt/issues/3351), you also have to
+set up bindgen to use a `nightly` release of `rustfmt`. Please check the [code
+formatting](./code-formatting.md) section for further information.