I found this funny.

The context is as explained by @laund@hachyderm.io

the issue is that you can’t return from inside a closure, since the closure might be called later/elsewhere

and this post was the asnwer to the question by @antonok@fosstodon.org

you got me curious what the record for the longest ? operator chain on crates.io is

Original post: https://fosstodon.org/users/antonok/statuses/111134824451525448

  • BB_C@programming.dev
    211·
    1 year ago

    Is everyone genuinely liking this!

    This is, IMHO, not a good style.

    Isn’t something like this much clearer?

    // Add `as_cstr()` to `NixPath` trait first

let some_or_null_cstr = |v| v.map(NixPath::as_cstr)
  .unwrap_or(Ok(std::ptr::null()));

// `Option::or_null_cstr()` for `OptionᐸTᐳ`
// where `T:  NixPath` would make this even better
let source_cstr = some_or_null_cstr(＆source)?;
let target_cstr = target.as_cstr()?;
let fs_type_cstr = some_or_null_cstr(＆fs_type)?;
let data_cstr = some_or_null_cstr(＆data)?;
let res = unsafe { .. };

    Edit: using alternative chars to circumvent broken Lemmy sanitization.

    • realharo@lemm.ee
      1·
      1 year ago

      I think the issue with this is that the code (https://docs.rs/nix/0.27.1/src/nix/lib.rs.html#297) allocates a fixed-size buffer on the stack in order to add a terminating zero to the end of the path copied into it. So it just gives you a reference into that buffer, which can’t outlive the function call.

      They do also have a with_nix_path_allocating function (https://docs.rs/nix/0.27.1/src/nix/lib.rs.html#332) that just gives you a CString that owns its buffer on the heap, so there must be some reason why they went this design. Maybe premature optimization? Maybe it actually makes a difference? 🤔

      They could have just returned the buffer via some wrapper that owns it and has the as_cstr function on it, but that would have resulted in a copy, so I’m not sure if it would have still achieved what they are trying to achieve here. I wonder if they ran some benchmarks on all this stuff, or they’re just writing what they think will be fast.

      • orangeboats@lemmy.world
        1·
        1 year ago

        so there must be some reason why they went this design.

        Some applications have a hard zero-alloc requirement.

        • realharo@lemm.ee
          1·
          1 year ago

          But that’s not the case here, seeing as they have

          if self.len() >= MAX_STACK_ALLOCATION {
    return with_nix_path_allocating(self, f);
}

          in the code of with_nix_path. And I think they still could’ve made it return the value instead of calling the passed in function, by using something like

          enum NixPathValue {
    Short(MaybeUninitᐸ[u8; 1024]>, usize),
    Long(CString)
}

impl NixPathValue {
    fn as_c_str(＆self) -> ＆CStr {
        // ...

impl NixPath for [u8] {
    fn to_nix_path(＆self) -> ResultᐸNixPathValue> {
        // return Short(buf, self.len()) for short paths, and perform all checks here,
        // so that NixPathValue.as_c_str can then use CStr::from_bytes_with_nul_unchecked

          But I don’t know what performance implications that would have, and whether the difference would matter at all. Would there be an unnecessary copy? Would the compiler optimize it out? etc.

          Also, from a maintainability standpoint, the context through which the library authors need to manually ensure all the unsafe code is used correctly would be slightly larger.

          As a user of a library, I would still prefer all that over the nesting.