Description
In my code I read a vec with std::fs::read(path) from a file and get the element at index 5 before further processing with the affected byte then being passed on by value to a function. When there are no changes to the ownership of said byte the variable will always be zero. (Workarounds like black_box. .to_owned() or pass by reference work but I still think it should work without since it can lead to unexpected behaviour).
I tried this code:
let data = std::fs::read(path)?;
let byte = data[5];
Ok(MyStruct::new(data, byte)) pub fn new(data: Vec<u8>, byte: u8) -> MyStruct {
MyStruct {data, index_counter: 0, byte}
}I expected to see this happen: Value of data[5] should be 1 or depending on data some other value (but in this case data at index 5 is 1)
Instead, this happened: byte has value 0, as soon as some change to the ownership happens (either through dbg! or the other methods detailed above, or black_box is used the value of byte is 1)
Meta
Found in stable, but after testing seems to be present in beta and nightly too.
rustc --version --verbose:
rustc 1.87.0 (17067e9ac 2025-05-09)
binary: rustc
commit-hash: 17067e9ac6d7ecb70e50f92c1944e545188d2359
commit-date: 2025-05-09
host: x86_64-unknown-linux-gnu
release: 1.87.0
LLVM version: 20.1.1
Backtrace
RUST_BACKTRACE=1 cargo build
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s