Rollup merge of #143859 - orlp:thread-into-raw-align, r=jhpratt
Guarantee 8 bytes of alignment in Thread::into_raw
When using `AtomicPtr` for synchronization it's incredibly useful when you've got a couple bits you can stuff metadata in. By guaranteeing that `Thread`'s `Inner` struct is aligned to 8 bytes everyone can use the bottom 3 bits to signal other things, such as a critical section, etc.
This guarantee is thus very useful and costs us nothing.
diff --git a/library/std/src/thread/mod.rs b/library/std/src/thread/mod.rs
index 6075173..8cd1e01 100644
--- a/library/std/src/thread/mod.rs
+++ b/library/std/src/thread/mod.rs
@@ -1399,6 +1399,11 @@ pub(crate) fn with_current_name<F, R>(f: F) -> R
}
/// The internal representation of a `Thread` handle
+///
+/// We explicitly set the alignment for our guarantee in Thread::into_raw. This
+/// allows applications to stuff extra metadata bits into the alignment, which
+/// can be rather useful when working with atomics.
+#[repr(align(8))]
struct Inner {
name: Option<ThreadNameString>,
id: ThreadId,
@@ -1582,7 +1587,8 @@ pub fn name(&self) -> Option<&str> {
/// Consumes the `Thread`, returning a raw pointer.
///
/// To avoid a memory leak the pointer must be converted
- /// back into a `Thread` using [`Thread::from_raw`].
+ /// back into a `Thread` using [`Thread::from_raw`]. The pointer is
+ /// guaranteed to be aligned to at least 8 bytes.
///
/// # Examples
///
