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, 3 insertions, 3 deletions

diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index c9bbc0d74cd..b5d16d29272 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -321,7 +321,7 @@ impl<T: Send + Sync + Clone> Arc<T> {
 
 #[unsafe_destructor]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Sync + Send> Drop for Arc<T> {
+impl<T> Drop for Arc<T> {
     /// Drops the `Arc<T>`.
     ///
     /// This will decrement the strong reference count. If the strong reference
@@ -388,7 +388,7 @@ impl<T: Sync + Send> Drop for Arc<T> {
 
 #[unstable(feature = "alloc",
            reason = "Weak pointers may not belong in this module.")]
-impl<T: Sync + Send> Weak<T> {
+impl<T> Weak<T> {
     /// Upgrades a weak reference to a strong reference.
     ///
     /// Upgrades the `Weak<T>` reference to an `Arc<T>`, if possible.
@@ -454,7 +454,7 @@ impl<T: Sync + Send> Clone for Weak<T> {
 
 #[unsafe_destructor]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Sync + Send> Drop for Weak<T> {
+impl<T> Drop for Weak<T> {
     /// Drops the `Weak<T>`.
     ///
     /// This will decrement the weak reference count.