Rollup merge of #94644 - m-ou-se:scoped-threads-drop-soundness, r=joshtriplett

Fix soundness issue in scoped threads.

This was discovered in https://github.com/rust-lang/rust/pull/94559#discussion_r820116323

The `scope()` function returns when all threads are finished, but I accidentally considered a thread 'finished' before dropping their panic payload or ignored return value.

So if a thread returned (or panics with) something that in its `Drop` implementation still uses borrowed stuff, it goes wrong.

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=2a1f19ac4676cdabe43e24e536ff9358

library/std/src/lib.rs

@@ -364,6 +364,11 @@ extern crate std as realstd;
 #[macro_use]
 mod macros;
 
+// The runtime entry point and a few unstable public functions used by the
+// compiler
+#[macro_use]
+pub mod rt;
+
 // The Rust prelude
 pub mod prelude;
 
@@ -548,11 +553,6 @@ pub mod arch {
 #[stable(feature = "simd_x86", since = "1.27.0")]
 pub use std_detect::is_x86_feature_detected;
 
-// The runtime entry point and a few unstable public functions used by the
-// compiler
-#[macro_use]
-pub mod rt;
-
 // Platform-abstraction modules
 mod sys;
 mod sys_common;