Merge pull request #1306 from google/more-cpp-version-notes-tests

Further documentation on C++ versions.

book/src/building.md

@@ -58,3 +58,14 @@ flowchart TB
 ```
 
 This interop inevitably involves lots of fiddly small functions. It's likely to perform far better if you can achieve cross-language link-time-optimization (LTO). [This issue](https://github.com/dtolnay/cxx/issues/371) may give some useful hints - see also all the build-related help in [the cxx manual](https://cxx.rs/) which all applies here too.
+
+## C++ versions and other compiler command-line flags
+
+The code generated by cxx and autocxx requires C++ 14, so it's not possible to use an earlier version of C++ than that.
+
+To use a later version, you need to:
+
+* Build the generated code with a later C++ version, for example using the clang argument `-std=c++17`. If you're using autocxx's cargo support, then you would do this by calling methods on the returned `cc::Build` object, for instance [`flag_if_supported`](https://docs.rs/cc/latest/cc/struct.Build.html#method.flag_if_supported).
+* _Also_ give similar directives to the C++ parsing which happens _within_ autocxx (specifically, by autocxx's version of bindgen). To do that, use [`Builder::extra_clang_args`](https://docs.rs/autocxx-engine/latest/autocxx_engine/struct.Builder.html#method.extra_clang_args).
+
+The same applies with the command-line `autocxx_gen` support - you'll need to pass such extra compiler options to `autocxx_gen` and also use them when building the generated C++ code.