cxx: implement slice support for nostd

[twilfredo]

OVERVIEW:

This patch adds experimental support for us to use slices in a nostd/freestanding environment, aim is to run c++ <=> cxx-rs <=> rust on a baremental target environment and use slices, with the intention of supporting more features later.

CFG

In the application Cargo.toml file, the following feature cfgs are required:

[dependencies]
cxx = { ... , default-features = false }

[build-dependencies]
cxx-build = { ... , default-features = false }

Then build with:

RUSTFLAGS='--cfg cxx_experimental_no_alloc' cargo build

Build Notes

and in the particular .cpp file you may need to declare the following

void __assert_func (const char *__a, int, const char *__b, const char *__c) {
	while (true) {}
}

void *__gxx_personality_v0;

FUNCTIONALITY:

Currently, this only supports slices (outside of trivial features).

TESTING:

Tested by compiling:

• cargo test (run existing tests)

• cxx/demo/ running it.

• compiling with a arm toolchain setup with cmake/corrosion and running the binary on QEMU arm with basic logic assertions [1].

Current testing has been done in the direction of C++ -> Rust with a simple callback test to C++. A
simple test setup can be seen here [2].

TODO:

• Get features such as Results<Ok, Err> working.

• When we build for the none std case (no default-features), instead of creating a symlink to the original cxx.h file, we copy it over and define the macro to disable stdlib dependencies. Perhaps there's a better way to do this?

NOTES:

By default, all the standard features are enabled gaurded by the #ifndef CXXBRIDGE1_RUST_STD, so this shoudn't break anything.

[1] https://github.com/twilfredo/qemu_misc/tree/master/bm_arm
[2] https://github.com/twilfredo/qemu_misc/blob/master/bm_arm/main.cpp

Signed-off-by: Wilfred Mallawa [email protected]

[twilfredo]

@dtolnay ping! what are your thoughts on this?

[twilfredo]

ping!

[GregoryTravis]

I would love to use this.