Experimental support for enum variant names and discriminant values retrieved from C++

[dtolnay]

This PR adds an optional feature based on https://github.com/dtolnay/clang-ast to extract information about shared enums (variant names, discriminant values, repr type) from a serialized Clang AST of the associated C++ headers.

The user-facing change is:

  #[cxx::bridge]
  mod ffi {
      extern "C++" {
          include!("path/to/ServiceManager.h");
      }

      ...

-     #[repr(i32)]
      enum ServiceState {
-         DISABLED = 0,
-         ENABLED = 1,
-         PRODUCTION = 2,
-         QA_SERVICE = 4,
-         IN_REPAIR = 8,
+         #![variants_from_header]
      }
  }

Wiring up an AST dump in a Buck-based build looks something like:

def rust_library(
        name,
        env = {},
        cxx_bridge = None,
        cxx_deps = None,
        **kwargs):
    if cxx_bridge:
        rust_cxx_bridge(
            name = "%s@bridge" % name,
            src = cxx_bridge,
            deps = cxx_deps,
        )

        clang_ast(
            name = "%s@ast" % name,
            header = ":%s@bridge=generated.h" % name,
            deps = cpp_deps,
        )

        env["CXX_CLANG_AST"] = "$(location :%s@ast)" % name

    native.rust_library(
        name = name,
        env = env,
        **kwargs,
    )

where clang_ast() is a simple bzl macro implemented as a genrule that runs clang++ -Xclang -ast-dump=json -fsyntax-only $(location {header}) -I$(location {deps}) -x c++, and rust_cxx_bridge is like tools/buck/rust_cxx_bridge.bzl.

This is experimental for now but opens up some interesting possibilities, such as:

• Rustfix-compatible autofix suggestion if you've written an incompatible signature (i.e. as opposed to a C++ type error later in the generated code)
• Automatically using type Kind = Trivial for extern "C++" types which have a trivial definition, thereby making them available by value in Rust