rollup merge of #23638: pnkfelix/fsk-reject-specialized-drops

Reject specialized Drop impls.

See Issue #8142 for discussion.

This makes it illegal for a Drop impl to be more specialized than the original item.

So for example, all of the following are now rejected (when they would have been blindly accepted before):

```rust
struct S<A> { ... };
impl Drop for S<i8> { ... } // error: specialized to concrete type

struct T<'a> { ... };
impl Drop for T<'static> { ... } // error: specialized to concrete region

struct U<A> { ... };
impl<A:Clone> Drop for U<A> { ... } // error: added extra type requirement

struct V<'a,'b>;
impl<'a,'b:a> Drop for V<'a,'b> { ... } // error: added extra region requirement
```

Due to examples like the above, this is a [breaking-change].

(The fix is to either remove the specialization from the `Drop` impl, or to transcribe the requirements into the struct/enum definition; examples of both are shown in the PR's fixed to `libstd`.)

----

This is likely to be the last thing blocking the removal of the `#[unsafe_destructor]` attribute.

Fix #8142
Fix #23584

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libstd/thread/mod.rs b/src/libstd/thread/mod.rs
index 57baeb1fb74..27b50fc9aaa 100644
--- a/src/libstd/thread/mod.rs
+++ b/src/libstd/thread/mod.rs
@@ -698,7 +698,7 @@ impl Drop for JoinHandle {
 /// permission.
 #[must_use = "thread will be immediately joined if `JoinGuard` is not used"]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub struct JoinGuard<'a, T: 'a> {
+pub struct JoinGuard<'a, T: Send + 'a> {
     inner: JoinInner<T>,
     _marker: PhantomData<&'a T>,
 }