Auto merge of #26727 - remram44:coerceunsized-weak, r=eddyb

This is a simple addition, shouldn't change behavior.

Fixes #26704

I don't know if the coercion for `Rc` is tested, if it is this probably needs the same test with `Weak`.

src/liballoc/arc.rs

@@ -145,6 +145,8 @@ pub struct Weak<T: ?Sized> {
 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> { }
 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> { }
 
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

src/liballoc/rc.rs

@@ -734,6 +734,8 @@ pub struct Weak<T: ?Sized> {
 impl<T: ?Sized> !marker::Send for Weak<T> {}
 impl<T: ?Sized> !marker::Sync for Weak<T> {}
 
+impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
+
 #[unstable(feature = "rc_weak",
            reason = "Weak pointers may not belong in this module.")]
 impl<T: ?Sized> Weak<T> {

src/test/run-pass/dst-coerce-rc.rs

@@ -10,10 +10,10 @@
 
 // Test a very simple custom DST coercion.
 
-#![feature(core)]
+#![feature(core, rc_weak)]
 
 use std::cell::RefCell;
-use std::rc::Rc;
+use std::rc::{Rc, Weak};
 
 trait Baz {
     fn get(&self) -> i32;
@@ -36,9 +36,13 @@ fn main() {
     let b: Rc<Baz> = a.clone();
     assert_eq!(b.get(), 42);
 
+    let c: Weak<i32> = a.downgrade();
+    let d: Weak<Baz> = c.clone();
+
     let _c = b.clone();
 
     let a: Rc<RefCell<i32>> = Rc::new(RefCell::new(42));
     let b: Rc<RefCell<Baz>> = a.clone();
     assert_eq!(b.borrow().get(), 42);
+    let c: Weak<RefCell<Baz>> = a.downgrade();
 }