Initial Rust support

[ojeda]

Currently we have several lines of work:

• Integrating with the kernel tree and build system (Nick's & mine, both uploaded as branches, based on rustc; and another one I have been working on, based on cargo).
• Bindings and the first bits of functionality (Alex's & Geoffrey's, based on cargo).

This patch effectively merges the work we have been doing and integrates it in the latest mainline kernel tree.

This does not mean anything needs to stay as-is, but this gives us a working, common base to work and experiment upon. Hopefully, it will also attract external people to join!

As a summary, I added:

• cargo integration with the kernel Makefiles:

  • Virtual cargo workspace to have a single lock file and to share deps between cargo jobs.
  • Picks the same optimization level as configured for C.
  • Verbose output on V=1.
  • A cargoclean target to clean all the Rust-related artifacts.

• Initial support for built-in modules (i.e. Y as well as M):

  • It is a hack, we need to implement a few things (see the TODOs), but it is enough to start playing with things that depend on MODULE.
  • Passes --cfg module to built-in modules to be able to compile conditionally inside Rust.
  • Increased KSYM_NAME_LEN length to avoid warnings due to Rust long mangled symbols.

• Rust infrastructure in a new top level folder rust/:

  • A kernel package which contains the sources from Alex & Geoffrey's work plus some changes:
    • Adapted build.rs.
    • Removed the THIS_MODULE emulation until it is implemented.
    • Removed Makefile logic and the code that cfg-depended on kernel version (no need in mainline).
    • Moved the helpers to be triggered via normal compilation, renamed them under rust_* and exported via EXPORT_SYMBOL instead.
    • Added a prelude.
  • A shlex package which serves as an example of an "inline" dependency (i.e. package checked out copy to avoid the network)

• The example driver was setup at drivers/char/rust_example/.

• Misc

  • The beginning of Documentation/rust/ with a quick start guide.
  • MAINTAINERS entry.
  • SPDXs for all files.

Other notes that aren't in TODOs:

• We could minimize the network requirements (use bindgen binary, use more inline dependencies...), but it is not clear it would be a good idea for the moment due to core/alloc/compiler-builtins.

• The intention of rust/ is to have a place to put extra dependencies and split the kernel package into several in the future if it grows. It could resemble the usual kernel tree structure.

• With several drivers being built-in, cargo recompiles kernel and triggers duplicate symbol errors when merging thin archives. We need to make it only compile kernel once.

• When the above works, then make's -j calling concurrent cargos (e.g. several drivers at the same time) should be OK, I think. According to Fix running Cargo concurrently rust-lang/cargo#2486 it shouldn't miscompile anything, but if they start locking on each other we will have make jobs waiting that could have been doing something else.

Signed-off-by: Miguel Ojeda miguel.ojeda.sandonis@gmail.com