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book: document splitting a project into crates #763

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book: document splitting a project into crates #763

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15 changes: 15 additions & 0 deletions book/src/concepts/build_systems.md
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Expand Up @@ -28,3 +28,18 @@ When using QML with CXX-Qt [QML modules](https://doc.qt.io/qt-6/qtqml-writing-a-
This allows for attributes such as `#[qml_element]` to register the QObject with the QML type system without any C++ code.

See [`QmlModule` documentation](https://docs.rs/cxx-qt-build/latest/cxx_qt_build/struct.QmlModule.html) for more details.

## Splitting a project into multiple crates

As your project grows, it can be helpful to organize your code into multiple Rust crates. If your `main` function is
in Rust ([Cargo is your only build system](../getting-started/4-cargo-executable.md)), simply add the crates as
dependencies of the top level binary crate in its Cargo.toml file.

If your `main` function is in C++, you can only link one staticlib Rust crate into C++, otherwise linking
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don't know if referring to the CXX section on linking between C++ and Rust is useful too? https://cxx.rs/build/other.html#linking-the-c-and-rust-together

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I think linking that would be too much information which isn't very relevant for the audience. Though that was helpful to realize extern crate is more appropriate than pub use

would fail with duplicate symbol errors from multiple Rust runtimes. So, create one top level staticlib crate to link
into the C++ application. Specify your other crates as normal Rust library (rlib) dependencies
in the staticlib crate's Cargo.toml. You must reference the symbols of the Rust dependencies within the staticlib crate;
if you don't need those symbols in Rust code, you can add `extern crate crate_name;` statements in the staticlib's lib.rs file.
Refer to the [meta_project example](https://github.com/KDAB/cxx-qt/blob/main/examples/meta_project) for how to set this up.
Note that this requires Rust compiler features that were [stabilized](https://github.com/rust-lang/rust/pull/113301)
in Rust 1.74.