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Replace confusing note in cpp.md on "RVO" #2327

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Oct 31, 2022
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Replace confusing note in cpp.md on "RVO" #2327

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11 changes: 7 additions & 4 deletions book/src/cpp.md
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Expand Up @@ -72,7 +72,10 @@ cannot translate into Rust:
[the tracking issue for exceptions](https://github.com/rust-lang/rust-bindgen/issues/1208)
for more details.

* Return value optimization. C++ compilers will in certain circumstances optimize functions
returning a struct type value to instead take an extra hidden argument that refers
to the return value struct. `bindgen` cannot necessarily know about this optimization and
thus at present `bindgen`-interfaces for these kinds of functions are invalid.
* Many C++ specific aspects of calling conventions. For example in the Itanium abi types that are
"[non trivial for the purposes of calls](https://itanium-cxx-abi.github.io/cxx-abi/abi.html#non-trivial)"
should be passed by pointer, even if they are otherwise eligable to be passed in a register.
Similarly in both the Itanium and MSVC ABIs such types are returned by "hidden parameter", much like
large structs in C that would not fit into a register. This also applies to types with any base classes
in the MSVC ABI (see [x64 calling convention](https://learn.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-170#return-values)).
Because bindgen does not know about these rules generated interfaces using such types are currently invalid.