Commits on Nov 3, 2022

1. cxx: implement slice support for nostd …

   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.
   
   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
   ```
   
   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 <wilfred.mallawa@wdc.com>

   

   twilfredo committed Nov 3, 2022
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