[v24.3.7] Rust: push_u8_u64_alignment test fails on i686 target (Ubuntu 22.04, CentOS 9)

[danielrh]

I noticed, when building an app for a 32 bit target, that some rustc flatbuffers would fail to validate, when a uint64 was serialized by a i686 machine.

I reproduced this by checking out the flatbuffers and running the test suite, and noticed that one of the test cases specifically about uint64 alignment failed on this target:

rustup target add i686-unknown-linux-gnu
cd flatbuffers32/tests/rust_usage_test
cargo test --target=i686-unknown-linux-gnu
cargo test --test integration_test --target=i686-unknown-linux-gnu push_u8_u64_alignment
warning: unused import: `MapReaderIndexer`
  --> /home/danielrh/dev/flatbuffers32/rust/flexbuffers/src/reader/mod.rs:29:26
   |
29 | pub use map::{MapReader, MapReaderIndexer};
   |                          ^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

warning: `flexbuffers` (lib) generated 1 warning (run `cargo fix --lib -p flexbuffers` to apply 1 suggestion)
warning: variable `Message` should have a snake case name
  --> tests/../../rust_namer_test/rust_namer_test/game_message_wrapper_generated.rs:51:9
   |
51 |     let Message = match self.Message_type() {
   |         ^^^^^^^ help: convert the identifier to snake case: `message`
   |
   = note: `#[warn(non_snake_case)]` on by default

warning: method `Message_type` should have a snake case name
  --> tests/../../rust_namer_test/rust_namer_test/game_message_wrapper_generated.rs:76:10
   |
76 |   pub fn Message_type(&self) -> GameMessage {
   |          ^^^^^^^^^^^^ help: convert the identifier to snake case: `message_type`

warning: method `Message` should have a snake case name
  --> tests/../../rust_namer_test/rust_namer_test/game_message_wrapper_generated.rs:83:10
   |
83 |   pub fn Message(&self) -> Option<flatbuffers::Table<'a>> {
   |          ^^^^^^^ help: convert the identifier to snake case: `message`

warning: structure field `Message_type` should have a snake case name
   --> tests/../../rust_namer_test/rust_namer_test/game_message_wrapper_generated.rs:156:9
    |
156 |     pub Message_type: GameMessage,
    |         ^^^^^^^^^^^^ help: convert the identifier to snake case: `message_type`

warning: structure field `Message` should have a snake case name
   --> tests/../../rust_namer_test/rust_namer_test/game_message_wrapper_generated.rs:157:9
    |
157 | ...ub Message: Option<flatbuffers::WIPOffset<flatbuffers::UnionWIPOffse...
    |       ^^^^^^^ help: convert the identifier to snake case: `message`

warning: method `add_Message_type` should have a snake case name
   --> tests/../../rust_namer_test/rust_namer_test/game_message_wrapper_generated.rs:175:10
    |
175 |   pub fn add_Message_type(&mut self, Message_type: GameMessage) {
    |          ^^^^^^^^^^^^^^^^ help: convert the identifier to snake case: `add_message_type`

warning: variable `Message_type` should have a snake case name
   --> tests/../../rust_namer_test/rust_namer_test/game_message_wrapper_generated.rs:175:38
    |
175 |   pub fn add_Message_type(&mut self, Message_type: GameMessage) {
    |                                      ^^^^^^^^^^^^ help: convert the identifier to snake case: `message_type`

warning: method `add_Message` should have a snake case name
   --> tests/../../rust_namer_test/rust_namer_test/game_message_wrapper_generated.rs:179:10
    |
179 |   pub fn add_Message(&mut self, Message: flatbuffers::WIPOffset<flatbuf...
    |          ^^^^^^^^^^^ help: convert the identifier to snake case: `add_message`

warning: variable `Message` should have a snake case name
   --> tests/../../rust_namer_test/rust_namer_test/game_message_wrapper_generated.rs:179:33
    |
179 | ...f, Message: flatbuffers::WIPOffset<flatbuffers::UnionWIPOffset>) {
    |       ^^^^^^^ help: convert the identifier to snake case: `message`

warning: structure field `Message` should have a snake case name
   --> tests/../../rust_namer_test/rust_namer_test/game_message_wrapper_generated.rs:234:7
    |
234 |   pub Message: GameMessageT,
    |       ^^^^^^^ help: convert the identifier to snake case: `message`

warning: variable `Message_type` should have a snake case name
   --> tests/../../rust_namer_test/rust_namer_test/game_message_wrapper_generated.rs:248:9
    |
248 |     let Message_type = self.Message.game_message_type();
    |         ^^^^^^^^^^^^ help: convert the identifier to snake case: `message_type`

warning: variable `Message` should have a snake case name
   --> tests/../../rust_namer_test/rust_namer_test/game_message_wrapper_generated.rs:249:9
    |
249 |     let Message = self.Message.pack(_fbb);
    |         ^^^^^^^ help: convert the identifier to snake case: `message`

warning: `rust_usage_test` (test "integration_test") generated 12 warnings
    Finished test [unoptimized + debuginfo] target(s) in 0.03s
     Running tests/integration_test.rs (target/i686-unknown-linux-gnu/debug/deps/integration_test-7966cfe9a55a34cc)

running 1 test
test flatbuffers_tests::push_impls::push_u8_u64_alignment ... FAILED

failures:

---- flatbuffers_tests::push_impls::push_u8_u64_alignment stdout ----
thread 'flatbuffers_tests::push_impls::push_u8_u64_alignment' panicked at tests/integration_test.rs:2360:9:
assertion `left == right` failed
  left: [2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1]
 right: [2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    flatbuffers_tests::push_impls::push_u8_u64_alignment

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 261 filtered out; finished in 0.00s

error: test failed, to rerun pass `--test integration_test`
danielrh@vectron:~/dev/flatbuffers32/tests/rust_usage_test$ 

This was the same issue that was leading to failures with my rust application on a 32 bit target; the 32 bit rust target failed to pad the 64 bit value to align with 64 bits. Perhaps it had to do with usage of platform-specific core::mem::{align_of, size_of}; that may be defined differently on architectures with more relaxed requirements than the flatbuffer serialization format.