thread: implements available_concurrency on haiku

[devnexen]

No description provided.

[rust-highfive]

r? @joshtriplett

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[nagisa]

Seems reasonable to me.

@bors r+

[bors]

📌 Commit 5d4048b has been approved by nagisa

[nielx]

This change does not compile and is incorrect.

• mem::zeroed() and get_system_info() is unsafe, see compiler error message below.
• system_info() returns a status_t indicating the error code or B_OK. It does not set errno, thus calling io::Error::last_os_error() will give a previous unrelated error code.

@devnexen are you okay to fix this?

error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
   --> library/std/src/sys/unix/thread.rs:342:48
    |
342 |             let mut sinfo: libc::system_info = crate::mem::zeroed();
    |                                                ^^^^^^^^^^^^^^^^^^^^ call to unsafe function
    |
    = note: consult the function's documentation for information on how to avoid undefined behavior

error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
   --> library/std/src/sys/unix/thread.rs:343:23
    |
343 |             let res = libc::get_system_info(&mut sinfo);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
    |
    = note: consult the function's documentation for information on how to avoid undefined behavior

[nagisa]

Oh well. I'm open to having this reverted until a fixed version is submitted, too.

[devnexen]

I pushed a PR #89462