Rollup merge of #102589 - RalfJung:scoped-threads-dangling, r=m-ou-se

scoped threads: pass closure through MaybeUninit to avoid invalid dangling references

The `main` function defined here looks roughly like this, if it were written as a more explicit stand-alone function:
```rust
// Not showing all the `'lifetime` tracking, the point is that
// this closure might live shorter than `thread`.
fn thread(control: ..., closure: impl FnOnce() + 'lifetime) {
    closure();
    control.signal_done();
    // A lot of time can pass here.
}
```
Note that `thread` continues to run even after `signal_done`! Now consider what happens if the `closure` captures a reference of lifetime `'lifetime`:
- The type of `closure` is a struct (the implicit unnameable closure type) with a `&'lifetime mut T` field. References passed to a function are marked with `dereferenceable`, which is LLVM speak for *this reference will remain live for the entire duration of this function*.
- The closure runs, `signal_done` runs. Then -- potentially -- this thread gets scheduled away and the main thread runs, seeing the signal and returning to the user. Now `'lifetime` ends and the memory the reference points to might be deallocated.
- Now we have UB! The reference that as passed to `thread` with the promise of remaining live for the entire duration of the function, actually got deallocated while the function still runs. Oops.

Long-term I think we should be able to use `ManuallyDrop` to fix this without `unsafe`, or maybe a new `MaybeDangling` type. I am working on an RFC for that. But in the mean time it'd be nice to fix this so that Miri with `-Zmiri-retag-fields` (which is needed for "full enforcement" of all the LLVM flags we generate) stops erroring on scoped threads.

Fixes #101983
r? `@m-ou-se`

library/std/src/thread/mod.rs

@@ -499,13 +499,40 @@ impl Builder {
         let output_capture = crate::io::set_output_capture(None);
         crate::io::set_output_capture(output_capture.clone());
 
+        // Pass `f` in `MaybeUninit` because actually that closure might *run longer than the lifetime of `F`*.
+        // See <https://github.com/rust-lang/rust/issues/101983> for more details.
+        // To prevent leaks we use a wrapper that drops its contents.
+        #[repr(transparent)]
+        struct MaybeDangling<T>(mem::MaybeUninit<T>);
+        impl<T> MaybeDangling<T> {
+            fn new(x: T) -> Self {
+                MaybeDangling(mem::MaybeUninit::new(x))
+            }
+            fn into_inner(self) -> T {
+                // SAFETY: we are always initiailized.
+                let ret = unsafe { self.0.assume_init_read() };
+                // Make sure we don't drop.
+                mem::forget(self);
+                ret
+            }
+        }
+        impl<T> Drop for MaybeDangling<T> {
+            fn drop(&mut self) {
+                // SAFETY: we are always initiailized.
+                unsafe { self.0.assume_init_drop() };
+            }
+        }
+
+        let f = MaybeDangling::new(f);
         let main = move || {
             if let Some(name) = their_thread.cname() {
                 imp::Thread::set_name(name);
             }
 
             crate::io::set_output_capture(output_capture);
 
+            // SAFETY: we constructed `f` initialized.
+            let f = f.into_inner();
             // SAFETY: the stack guard passed is the one for the current thread.
             // This means the current thread's stack and the new thread's stack
             // are properly set and protected from each other.
@@ -518,6 +545,12 @@ impl Builder {
             // same `JoinInner` as this closure meaning the mutation will be
             // safe (not modify it and affect a value far away).
             unsafe { *their_packet.result.get() = Some(try_result) };
+            // Here `their_packet` gets dropped, and if this is the last `Arc` for that packet that
+            // will call `decrement_num_running_threads` and therefore signal that this thread is
+            // done.
+            drop(their_packet);
+            // Here, the lifetime `'a` and even `'scope` can end. `main` keeps running for a bit
+            // after that before returning itself.
         };
 
         if let Some(scope_data) = &my_packet.scope {