Format all Markdown files with dprint (#1157)

This is the result of running `dprint fmt` after removing `src/` from
the list of excluded directories.

This also reformats the Rust code: we might want to tweak this a bit in
the future since some of the changes removes the hand-formatting. Of
course, this formatting can be seen as a mis-feature, so maybe this is
good overall.

Thanks to mdbook-i18n-helpers 0.2, the POT file is nearly unchanged
after this, meaning that all existing translations remain valid! A few
messages were changed because of stray whitespace characters:

     msgid ""
     "Slices always borrow from another object. In this example, `a` has to remain "
    -"'alive' (in scope) for at least as long as our slice. "
    +"'alive' (in scope) for at least as long as our slice."
     msgstr ""

The formatting is enforced in CI and we will have to see how annoying
this is in practice for the many contributors. If it becomes annoying,
we should look into fixing dprint/check#11 so that `dprint` can annotate
the lines that need fixing directly, then I think we can consider more
strict formatting checks.

I added more customization to `rustfmt.toml`. This is to better emulate
the dense style used in the course:

- `max_width = 85` allows lines to take up the full width available in
our code blocks (when taking margins and the line numbers into account).
- `wrap_comments = true` ensures that we don't show very long comments
in the code examples. I edited some comments to shorten them and avoid
unnecessary line breaks — please trim other unnecessarily long comments
when you see them! Remember we're writing code for slides 😄
- `use_small_heuristics = "Max"` allows for things like struct literals
and if-statements to take up the full line width configured above.

The formatting settings apply to all our Rust code right now — I think
we could improve this with https://github.com/dprint/dprint/issues/711
which lets us add per-directory `dprint` configuration files. However,
the `inherit: true` setting is not yet implemented (as far as I can
tell), so a nested configuration file will have to copy most or all of
the top-level file.

src/index.md

@@ -4,53 +4,52 @@
 [![GitHub contributors](https://img.shields.io/github/contributors/google/comprehensive-rust?style=flat-square)](https://github.com/google/comprehensive-rust/graphs/contributors)
 [![GitHub stars](https://img.shields.io/github/stars/google/comprehensive-rust?style=flat-square)](https://github.com/google/comprehensive-rust/stargazers)
 
-This is a free Rust course developed by the Android team at Google. The course covers
-the full spectrum of Rust, from basic syntax to advanced topics like generics
-and error handling.
+This is a free Rust course developed by the Android team at Google. The course
+covers the full spectrum of Rust, from basic syntax to advanced topics like
+generics and error handling.
 
 > The latest version of the course can be found at
-> <https://google.github.io/comprehensive-rust/>. If you are reading
-> somewhere else, please check there for updates.
+> <https://google.github.io/comprehensive-rust/>. If you are reading somewhere
+> else, please check there for updates.
 
 The goal of the course is to teach you Rust. We assume you don't know anything
 about Rust and hope to:
 
-* Give you a comprehensive understanding of the Rust syntax and language.
-* Enable you to modify existing programs and write new programs in Rust.
-* Show you common Rust idioms.
+- Give you a comprehensive understanding of the Rust syntax and language.
+- Enable you to modify existing programs and write new programs in Rust.
+- Show you common Rust idioms.
 
 We call the first four course days Rust Fundamentals.
 
 Building on this, you're invited to dive into one or more specialized topics:
 
-* [Android](android.md): a half-day course on using Rust for Android platform
+- [Android](android.md): a half-day course on using Rust for Android platform
   development (AOSP). This includes interoperability with C, C++, and Java.
-* [Chromium](chromium.md): a half-day course on using Rust within Chromium
-  based browsers. This includes interoperability with C++ and how to include
+- [Chromium](chromium.md): a half-day course on using Rust within Chromium based
+  browsers. This includes interoperability with C++ and how to include
   third-party crates in Chromium.
-* [Bare-metal](bare-metal.md): a whole-day class on using Rust for bare-metal
+- [Bare-metal](bare-metal.md): a whole-day class on using Rust for bare-metal
   (embedded) development. Both microcontrollers and application processors are
   covered.
-* [Concurrency](concurrency.md): a whole-day class on concurrency in Rust. We
+- [Concurrency](concurrency.md): a whole-day class on concurrency in Rust. We
   cover both classical concurrency (preemptively scheduling using threads and
-  mutexes) and async/await concurrency (cooperative multitasking using
-  futures).
-
+  mutexes) and async/await concurrency (cooperative multitasking using futures).
 
 ## Non-Goals
 
 Rust is a large language and we won't be able to cover all of it in a few days.
 Some non-goals of this course are:
 
-* Learning how to develop macros: please see [Chapter 19.5 in the Rust
-  Book](https://doc.rust-lang.org/book/ch19-06-macros.html) and [Rust by
-  Example](https://doc.rust-lang.org/rust-by-example/macros.html) instead.
+- Learning how to develop macros: please see
+  [Chapter 19.5 in the Rust Book](https://doc.rust-lang.org/book/ch19-06-macros.html)
+  and [Rust by Example](https://doc.rust-lang.org/rust-by-example/macros.html)
+  instead.
 
 ## Assumptions
 
-The course assumes that you already know how to program. Rust is a statically-typed
-language and we will sometimes make comparisons with C and C++ to better
-explain or contrast the Rust approach.
+The course assumes that you already know how to program. Rust is a
+statically-typed language and we will sometimes make comparisons with C and C++
+to better explain or contrast the Rust approach.
 
 If you know how to program in a dynamically-typed language such as Python or
 JavaScript, then you will be able to follow along just fine too.