#ifdef support

[adetaylor]

It's my intention that we can support Rust compile-time feature enablement based on C++ macros.

Where users are using our build.rs support in https://github.com/google/autocxx/tree/main/gen/build this should really be pretty trivial by printing cargo:rustc-cfg lines to standard out as we're generating the C++ bindings.

I haven't thought about the exact syntax but obviously the goal is to allow something like #[cfg(cxx_ifdef = ENABLE_FEATURE)] which can disable a block of code in Rust.

I also haven't thought about #if but that should be achievable in a similar fashion. Probably.

This should be simple for the build.rs case but we need to figure out:

• How to do this for the non-Cargo case (https://github.com/google/autocxx/tree/main/gen/cmd) in a way that's easy to integrate into third party build systems. We probably need to write all the extra rustc arguments to a file which build systems can use as input to a subsequent rustc command line.
• How on earth to do this for the integration tests where we just don't launch another rustc instance at all. I don't have a plan there.

I'm also concerned that the sheer number of #define symbols are likely to overwhelm the rustc command line so we may need to be selective.

[adetaylor]

Pull request #74 removes a file, preprocessor_parse_callbacks.rs, which had a lot of the code required to make this work. We should reinstate parts of that file when fixing this issue.

[adetaylor]

Related Chromium issue: https://bugs.chromium.org/p/chromium/issues/detail?id=1298039

[adetaylor]

Solution for non-cargo platforms: emit a file of --cfg flags which can be pulled into the downstream rustc build using @file.txt. This is proven to work in gn, so long as the filename ends in .rs. We do this already for --cfg flags produced by https://source.chromium.org/chromium/chromium/src/+/main:build/rust/cargo_crate.gni