`test_take_as_pod_with_is_relocatable` fails with wrappers forced

[adetaylor]

With the branch in #1250, test_take_as_pod_with_is_relocatable fails:

RUST_LOG=autocxx_engine=info AUTOCXX_FORCE_WRAPPER_GENERATION=1  cargo test test_take_as_pod_with_is_relocatable -- --nocapture
running: "c++" "-O1" "-ffunction-sections" "-fdata-sections" "-fPIC" "-arch" "arm64" "-I" "/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv" "-I" "/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv/target/include" "-I" "/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv" "-Wall" "-Wextra" "-std=c++14" "-Wall" "-Werror" "-o" "/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv/target/cxx/gen0.o" "-c" "/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv/target/cxx/gen0.cxx"
cargo:warning=In file included from /var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv/target/cxx/gen0.cxx:2:
cargo:warning=/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv/target/include/autocxxgen_ffi.h:45:90: error: call to implicitly-deleted copy constructor of 'Bob'
cargo:warning=inline uint32_t take_bob_autocxx_wrapper_0x7ae9fb9c423cc5ae(Bob arg0)  { return take_bob(arg0); }
cargo:warning=                                                                                         ^~~~
cargo:warning=/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv/input.h:9:12: note: copy constructor is implicitly deleted because 'Bob' has a user-declared move constructor
cargo:warning=    inline Bob(Bob&& other_bob) { a = other_bob.a; b = other_bob.b; }
cargo:warning=           ^
cargo:warning=/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv/input.h:12:23: note: passing argument to parameter 'a' here
cargo:warning=uint32_t take_bob(Bob a);
cargo:warning=                      ^
cargo:warning=1 error generated.
exit status: 1
thread 'integration_test::test_take_as_pod_with_is_relocatable' panicked at 'called `Result::unwrap()` on an `Err` value: CppBuild(Error { kind: ToolExecError, message: "Command \"c++\" \"-O1\" \"-ffunction-sections\" \"-fdata-sections\" \"-fPIC\" \"-arch\" \"arm64\" \"-I\" \"/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv\" \"-I\" \"/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv/target/include\" \"-I\" \"/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv\" \"-Wall\" \"-Wextra\" \"-std=c++14\" \"-Wall\" \"-Werror\" \"-o\" \"/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv/target/cxx/gen0.o\" \"-c\" \"/var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp3eWnBv/target/cxx/gen0.cxx\" with args \"c++\" did not execute successfully (status code exit status: 1)." })', integration-tests/tests/integration_test.rs:529:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
test integration_test::test_take_as_pod_with_is_relocatable ... FAILED

[adetaylor]

It's not quite clear what to do about this.

If Bob is declared as non-POD, we generate:

inline uint32_t take_bob_autocxx_wrapper_0xfd37c8511aecb2b1(Bob* arg0)  { return take_bob(std::move(*arg0)); }

If Bob is declared as POD, we generate:

inline uint32_t take_bob_autocxx_wrapper_0x7ae9fb9c423cc5ae(Bob arg0)  { return take_bob(arg0); }

which is reasonable for any type which is truly trivially relocatable. This type has a non-trivial move constructor but specifies using IsRelocatable = std::true_type; which is how we can use it with generate_pod!

Still, passing by value (without std::move) is preferable for truly POD types.

At code generation time, we can't know whether a type has using IsRelocatable = std::true_type; as opposed to a trivial move constructor.

So, our options are:

• Some kind of RAII template voodoo where we generate the same code each time, but it boils down to move-by-value where possible, or std::move otherwise.
• Some additional directive for folks to teach autocxx that a type always requires std::move.