Add guidelines about writing documentation

src/SUMMARY.md

@@ -48,6 +48,8 @@
 - [Documentation](./documentation/summary.md)
     - [doc alias policy](./documentation/doc-alias-policy.md)
     - [safety comments policy](./documentation/safety-comments.md)
+    - [how to write documentation](./how-to-write-documentation.md)
+    - [reviewing doc changes](./reviewing-doc-changes.md)
 
 ---
 

src/documentation/how-to-write-documentation.md

@@ -0,0 +1,149 @@
+# How to write documentation
+
+This document explains how to write documentation for the std/core public APIs.
+
+Let's start with some general information:
+
+### Contractions
+
+It is common in English to have contractions such as "don't" or "can't". Do not
+use these in documentation. Always write their "full form":
+
+ * "do not" instead of "don't"
+ * "cannot" instead of "can't"
+ * "it would" instead of "it'd"
+ * "it will" instead of "it'll"
+ * "it is"/"it has" instead of "it's"
+ * "you are" instead of "you're"
+ * "they are" instead of "they're"
+ * etc
+
+The only exception to this rule is "let's" as it is specific/known/common enough.
+
+The reason is simply to make the reading simpler for as many people as possible.
+
+### When to use inline code blocks
+
+Whenever you are talking about a type or anything code related, it should be in a
+inline code block. As a reminder, a inline code block is created with backticks
+(\`). For example:
+
+
+```text
+This a `Vec` and it has a method `push` which you can call by doing `Vec::push`.
+```
+
+### When to use intra-doc links
+
+Intra-doc links (you can see the full explanations for the feature
+[here](https://doc.rust-lang.org/rustdoc/write-documentation/linking-to-items-by-name.html))
+should be used as much as possible whenever a type is mentioned.
+
+Little note: when you are documenting an item, no need to link to it. So if you
+write documentation for `String::push_str` method, no need to link to the method
+`push_str` or to the `String` type.
+
+If you have cases like `Vec<String>`, you need to use intra-doc links on both
+`Vec` and `String` as well. It would look like this:
+
+```text
+This is a [`Vec`]`<`[`String`]`>`.
+```
+
+Extra explanations: since both `Vec` and `String` are in codeblocks, `<` and `>`
+should as well, otherwise it would render badly.
+
+### Code blocks
+
+With rustdoc, code blocks are tested (because they are treated as Rust code
+blocks by default). It allows us to know if the documentation is up to date. As
+such, please avoid using `ignore` as much as possible on code blocks! If you
+want as a language other than Rust, simply set it in the code block tags:
+
+    ```text
+    This is not rust code!
+    ```
+
+Some special cases:
+ * If the code example cannot be run (when documenting a I/O item for example),
+   use `no_run`.
+ * If it is expected to fail, use `should_panic`.
+ * If it is expected to fail compilation (which be quite rare!), use `compile_fail`.
+
+You can find more information about code blocks
+[here](https://doc.rust-lang.org/rustdoc/write-documentation/documentation-tests.html).
+
+## How to write documentation for a module
+
+A module is supposed to contain "similar" items. As such, its documentation is
+supposed to give an overview and eventually **a base** to understand what the
+items it contains are doing.
+
+You can take a look at the
+[f32 module](https://doc.rust-lang.org/nightly/std/f32/index.html) or at the
+[fmt module](https://doc.rust-lang.org/nightly/std/fmt/index.html) to see
+good examples.
+
+## How to write documentation for functions/methods
+
+The basic format of each documented methods/functions should roughly look like this:
+
+```text
+[explanations]
+
+[example(s)]
+```
+
+### Explanations
+
+By `explanations` we mean that the text should explain what the method and what
+each of its arguments are for. Let's take this method for example:
+
+```rust
+pub fn concat_str(&self, s: &str) -> String {
+    if s.is_empty() {
+        panic!("empty concat string");
+    }
+    format!("{}{}", self.string, s)
+}
+```
+
+The explanation should look like this:
+
+```text
+Returns a new [`String`] which contains `&self` content with `s` added at the end.
+```
+
+### Panic?
+
+If the function/method can panic in certain circumstances, it must to be
+mentioned! This explanation needs to be prepended by a `Panics` title:
+
+```text
+# Panics
+
+`concat_str` panics if `s` is empty.
+```
+
+### Examples
+
+As for the examples, they have to show the usage of the function/method. Just
+like the `panic` section, they need to be prepended by a `Examples` title.
+
+It is better if you use `assert*!` macros at the end to ensure that the example
+is working as expected. It also allows the readers to understand more easily
+what the function is doing (or returning).
+
+    # Examples
+
+    ```
+    let s = MyType::new("hello ");
+    assert_eq!("hello Georges", s.concat_str("Georges").as_str());
+    ```
+
+## How to write documentation for other items
+
+It is mostly the same as for methods and functions except that the examples
+are (strongly) recommended and not mandatory.
+
+A good example often shows how to create the item.

src/documentation/reviewing-doc-changes.md

@@ -0,0 +1,24 @@
+# Reviewing doc changes
+
+Most of the time, it is mostly reviewing that the documentation isn't wrong
+in any way and that it follows the
+[how to write documentation](./how-to-write-documentation.md) guideline.
+
+There is however something where we need to be extra careful: stability
+guarantees.
+
+## Stability guarantees
+
+First, short explanation about what a stability guarantee is: a statement in
+the document which explains what the item is doing in a precise case. For
+example:
+
+ * Showing precisely how a function on floats handles `NaN`.
+ * Saying that a sort method has a particular running-time bound.
+
+So if a doc change updates/adds/removes a stability guarantee, it has to be
+**very** carefully handled and needs to go through the
+[libs API team FCP](https://rustc-dev-guide.rust-lang.org/stabilization_guide.html?highlight=fcp#fcp).
+
+It can be circumvented by adding a `# Current Implementation` section
+[like done here](https://github.com/rust-lang/rust/blob/4a8d2e3856c0c75c71998b6c85937203839b946d/library/alloc/src/slice.rs#L250).

src/documentation/summary.md

@@ -1,2 +1,4 @@
 - [doc alias policy](./doc-alias-policy.md)
 - [safety comments policy](./safety-comments.md)
+- [how to write documentation](./how-to-write-documentation.md)
+- [reviewing doc changes](./reviewing-doc-changes.md)