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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.
45 lines
1.6 KiB
Markdown
45 lines
1.6 KiB
Markdown
## Using cxx in Chromium
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In Chromium, we define an independent `#[cxx::bridge] mod` for each leaf-node
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where we want to use Rust. You'd typically have one for each
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`rust_static_library`. Just add
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```gn
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cxx_bindings = [ "my_rust_file.rs" ]
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# list of files containing #[cxx::bridge], not all source files
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allow_unsafe = true
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```
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to your existing `rust_static_library` target alongside `crate_root` and
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`sources`.
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C++ headers will be generated at a sensible location, so you can just
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```cpp
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#include "ui/base/my_rust_file.rs.h"
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```
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You will find some utility functions in `//base` to convert to/from Chromium C++
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types to CXX Rust types --- for example [`SpanToRustSlice`][0].
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<details>
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Students may ask --- why do we still need `allow_unsafe = true`?
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The broad answer is that no C/C++ code is "safe" by the normal Rust standards.
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Calling back and forth to C/C++ from Rust may do arbitrary things to memory, and
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compromise the safety of Rust's own data layouts. Presence of _too many_
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`unsafe` keywords in C/C++ interop can harm the signal-to-noise ratio of such a
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keyword, and is [controversial][1], but strictly, bringing any foreign code into
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a Rust binary can cause unexpected behavior from Rust's perspective.
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The narrow answer lies in the diagram at the top of [this page][2] --- behind
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the scenes, CXX generates Rust `unsafe` and `extern "C"` functions just like we
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did manually in the previous section.
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</details>
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[0]: https://source.chromium.org/chromium/chromium/src/+/main:base/containers/span_rust.h;l=21
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[1]: https://steveklabnik.com/writing/the-cxx-debate
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[2]: ../interoperability-with-cpp.md
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