Auto merge of #126523 - joboet:the_great_big_tls_refactor, r=Mark-Simulacrum

std: refactor the TLS implementation

As discovered by Mara in #110897, our TLS implementation is a total mess. In the past months, I have simplified the actual macros and their expansions, but the majority of the complexity comes from the platform-specific support code needed to create keys and register destructors. In keeping with #117276, I have therefore moved all of the `thread_local_key`/`thread_local_dtor` modules to the `thread_local` module in `sys` and merged them into a new structure, so that future porters of `std` can simply mix-and-match the existing code instead of having to copy the same (bad) implementation everywhere. The new structure should become obvious when looking at `sys/thread_local/mod.rs`.

Unfortunately, the documentation changes associated with the refactoring have made this PR rather large. That said, this contains no functional changes except for two small ones:
* the key-based destructor fallback now, by virtue of sharing the implementation used by macOS and others, stores its list in a `#[thread_local]` static instead of in the key, eliminating one indirection layer and drastically simplifying its code.
* I've switched over ZKVM (tier 3) to use the same implementation as WebAssembly, as the implementation was just a way worse version of that

Please let me know if I can make this easier to review! I know these large PRs aren't optimal, but I couldn't think of any good intermediate steps.

`@rustbot` label +A-thread-locals

3 files changed, 0 insertions, 66 deletions

diff --git a/library/std/src/sys/pal/solid/mod.rs b/library/std/src/sys/pal/solid/mod.rs
index 3f6ff37903a..9a7741ddda7 100644
--- a/library/std/src/sys/pal/solid/mod.rs
+++ b/library/std/src/sys/pal/solid/mod.rs
@@ -33,8 +33,6 @@ pub mod pipe;
 pub mod process;
 pub mod stdio;
 pub use self::itron::thread;
-pub mod thread_local_dtor;
-pub mod thread_local_key;
 pub use self::itron::thread_parking;
 pub mod time;
 
diff --git a/library/std/src/sys/pal/solid/thread_local_dtor.rs b/library/std/src/sys/pal/solid/thread_local_dtor.rs
deleted file mode 100644
index 26918a4fcb0..00000000000
--- a/library/std/src/sys/pal/solid/thread_local_dtor.rs
+++ /dev/null
@@ -1,43 +0,0 @@
-#![cfg(target_thread_local)]
-#![unstable(feature = "thread_local_internals", issue = "none")]
-
-// Simplify dtor registration by using a list of destructors.
-
-use super::{abi, itron::task};
-use crate::cell::{Cell, RefCell};
-
-#[thread_local]
-static REGISTERED: Cell<bool> = Cell::new(false);
-
-#[thread_local]
-static DTORS: RefCell<Vec<(*mut u8, unsafe extern "C" fn(*mut u8))>> = RefCell::new(Vec::new());
-
-pub unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern "C" fn(*mut u8)) {
-    if !REGISTERED.get() {
-        let tid = task::current_task_id_aborting();
-        // Register `tls_dtor` to make sure the TLS destructors are called
-        // for tasks created by other means than `std::thread`
-        unsafe { abi::SOLID_TLS_AddDestructor(tid as i32, tls_dtor) };
-        REGISTERED.set(true);
-    }
-
-    match DTORS.try_borrow_mut() {
-        Ok(mut dtors) => dtors.push((t, dtor)),
-        Err(_) => rtabort!("global allocator may not use TLS"),
-    }
-}
-
-pub unsafe fn run_dtors() {
-    let mut list = DTORS.take();
-    while !list.is_empty() {
-        for (ptr, dtor) in list {
-            unsafe { dtor(ptr) };
-        }
-
-        list = DTORS.take();
-    }
-}
-
-unsafe extern "C" fn tls_dtor(_unused: *mut u8) {
-    unsafe { run_dtors() };
-}
diff --git a/library/std/src/sys/pal/solid/thread_local_key.rs b/library/std/src/sys/pal/solid/thread_local_key.rs
deleted file mode 100644
index b37bf999698..00000000000
--- a/library/std/src/sys/pal/solid/thread_local_key.rs
+++ /dev/null
@@ -1,21 +0,0 @@
-pub type Key = usize;
-
-#[inline]
-pub unsafe fn create(_dtor: Option<unsafe extern "C" fn(*mut u8)>) -> Key {
-    panic!("should not be used on the solid target");
-}
-
-#[inline]
-pub unsafe fn set(_key: Key, _value: *mut u8) {
-    panic!("should not be used on the solid target");
-}
-
-#[inline]
-pub unsafe fn get(_key: Key) -> *mut u8 {
-    panic!("should not be used on the solid target");
-}
-
-#[inline]
-pub unsafe fn destroy(_key: Key) {
-    panic!("should not be used on the solid target");
-}