Auto merge of #114965 - benschulz:mpsc-drop, r=dtolnay

Remove Drop impl of mpsc Receiver and (Sync)Sender

This change removes the empty `Drop` implementations for `mpsc::Receiver`, `mpsc::Sender` and `mpsc::SyncSender`. These implementations do not specify `#[may_dangle]`, so by removing them we make `mpsc` types play nice with drop check.

This was previously attempted in [#105243](#105243 (comment)) but then [abandoned due to a test failure](#105243 (comment)). I've aligned the test with those for `Mutex` and `RwLock`.

library/std/src/sync/mpsc/mod.rs

@@ -626,11 +626,6 @@ impl<T> Clone for Sender<T> {
     }
 }
 
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Drop for Sender<T> {
-    fn drop(&mut self) {}
-}
-
 #[stable(feature = "mpsc_debug", since = "1.8.0")]
 impl<T> fmt::Debug for Sender<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -755,11 +750,6 @@ impl<T> Clone for SyncSender<T> {
     }
 }
 
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Drop for SyncSender<T> {
-    fn drop(&mut self) {}
-}
-
 #[stable(feature = "mpsc_debug", since = "1.8.0")]
 impl<T> fmt::Debug for SyncSender<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -1096,11 +1086,6 @@ impl<T> IntoIterator for Receiver<T> {
     }
 }
 
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Drop for Receiver<T> {
-    fn drop(&mut self) {}
-}
-
 #[stable(feature = "mpsc_debug", since = "1.8.0")]
 impl<T> fmt::Debug for Receiver<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

tests/ui/span/send-is-not-static-std-sync.rs

@@ -46,7 +46,7 @@ fn channel() {
         tx.send(&z).unwrap();
     }
     //~^^ ERROR `z` does not live long enough
-    // (channels lack #[may_dangle], thus their dtors are implicit uses of `z`)
+    tx.use_ref(); // (channel drop glue does not use `z` => needs explicit use)
 }
 
 fn main() {}

tests/ui/span/send-is-not-static-std-sync.stderr

@@ -75,11 +75,9 @@ LL |         tx.send(&z).unwrap();
    |                 ^^ borrowed value does not live long enough
 LL |     }
    |     - `z` dropped here while still borrowed
-...
-LL | }
-   | - borrow might be used here, when `tx` is dropped and runs the `Drop` code for type `Sender`
-   |
-   = note: values in a scope are dropped in the opposite order they are defined
+LL |
+LL |     tx.use_ref(); // (channel drop glue does not use `z` => needs explicit use)
+   |     -- borrow later used here
 
 error: aborting due to 6 previous errors