Repository: rust
Issue: Valgrind leak check reports a "possibly lost" leak on `std::thread::current()`
Labels: C-bug,T-libs,A-thread

### Code

I tried this code:

```rust
fn main() {
    let _ = std::thread::current();
}
```

...under `valgrind`.

I expected to see this happen: no leaks

Instead, this happened:

```
==2008936== Memcheck, a memory error detector
==2008936== Copyright (C) 2002-2024, and GNU GPL'd, by Julian Seward et al.
==2008936== Using Valgrind-3.24.0 and LibVEX; rerun with -h for copyright info
==2008936== Command: target/debug/cringe
==2008936== 
==2008936== 
==2008936== HEAP SUMMARY:
==2008936==     in use at exit: 48 bytes in 1 blocks
==2008936==   total heap usage: 8 allocs, 7 frees, 2,120 bytes allocated
==2008936== 
==2008936== 48 bytes in 1 blocks are possibly lost in loss record 1 of 1
==2008936==    at 0x48447A8: malloc (vg_replace_malloc.c:446)
==2008936==    by 0x138B77: alloc (alloc.rs:96)
==2008936==    by 0x138B77: alloc_impl (alloc.rs:192)
==2008936==    by 0x138B77: allocate (alloc.rs:254)
==2008936==    by 0x138B77: {closure#0}<std::thread::Inner> (sync.rs:484)
==2008936==    by 0x138B77: allocate_for_layout<core::mem::maybe_uninit::MaybeUninit<std::thread::Inner>, alloc::sync::{impl#14}::new_uninit::{closure_env#0}<std::thread::Inner>, fn(*mut u8) -> *mut alloc::sync::ArcInner<core::mem::maybe_uninit::MaybeUninit<std::thread::Inner>>> (sync.rs:1948)
==2008936==    by 0x138B77: new_uninit<std::thread::Inner> (sync.rs:482)
==2008936==    by 0x138B77: std::thread::Thread::new (mod.rs:1429)
==2008936==    by 0x138909: std::thread::current::init_current (current.rs:227)
==2008936==    by 0x11D516: cringe::main (main.rs:2)
==2008936==    by 0x11D44A: core::ops::function::FnOnce::call_once (function.rs:250)
==2008936==    by 0x11D3CD: std::sys::backtrace::__rust_begin_short_backtrace (backtrace.rs:152)
==2008936==    by 0x11D3A0: std::rt::lang_start::{{closure}} (rt.rs:194)
==2008936==    by 0x13852F: call_once<(), (dyn core::ops::function::Fn<(), Output=i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe)> (function.rs:284)
==2008936==    by 0x13852F: do_call<&(dyn core::ops::function::Fn<(), Output=i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe), i32> (panicking.rs:587)
==2008936==    by 0x13852F: try<i32, &(dyn core::ops::function::Fn<(), Output=i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe)> (panicking.rs:550)
==2008936==    by 0x13852F: catch_unwind<&(dyn core::ops::function::Fn<(), Output=i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe), i32> (panic.rs:358)
==2008936==    by 0x13852F: {closure#0} (rt.rs:163)
==2008936==    by 0x13852F: do_call<std::rt::lang_start_internal::{closure_env#0}, isize> (panicking.rs:587)
==2008936==    by 0x13852F: try<isize, std::rt::lang_start_internal::{closure_env#0}> (panicking.rs:550)
==2008936==    by 0x13852F: catch_unwind<std::rt::lang_start_internal::{closure_env#0}, isize> (panic.rs:358)
==2008936==    by 0x13852F: std::rt::lang_start_internal (rt.rs:159)
==2008936==    by 0x11D386: std::rt::lang_start (rt.rs:193)
==2008936==    by 0x11D54D: main (in /home/purplesyringa/cringe/target/debug/cringe)
==2008936== 
==2008936== LEAK SUMMARY:
==2008936==    definitely lost: 0 bytes in 0 blocks
==2008936==    indirectly lost: 0 bytes in 0 blocks
==2008936==      possibly lost: 48 bytes in 1 blocks
==2008936==    still reachable: 0 bytes in 0 blocks
==2008936==         suppressed: 0 bytes in 0 blocks
==2008936== 
==2008936== For lists of detected and suppressed errors, rerun with: -s
==2008936== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
```

This is basically https://github.com/rust-lang/rust/issues/133574 again. Calling this a regression might be stretching it a bit too far, but the consensus on that issue seemed to be that we should fix this if we can.

### Version it worked on

It most recently worked on: Rust 1.85.0-beta.2, as far as I'm aware

### Version with regression

`rustc --version --verbose`:
```
rustc 1.86.0-nightly (419b3e2d3 2025-01-15)
binary: rustc
commit-hash: 419b3e2d3e350822550eee0e82eeded4d324d584
commit-date: 2025-01-15
host: x86_64-unknown-linux-gnu
release: 1.86.0-nightly
LLVM version: 19.1.6
```