Rollup merge of #131857 - WaffleLapkin:dyn-drop-principal-3, r=compiler-errors

Allow dropping dyn principal

Revival of #126660, which was a revival of #114679. Fixes #126313.

Allows dropping principal when coercing trait objects, e.g. `dyn Debug + Send` -> `dyn Send`.

cc `@compiler-errors` `@Jules-Bertholet`
r? `@lcnr`

Author: matthiaskrgr

tests/pass/dyn-upcast.rs

@@ -9,6 +9,7 @@ fn main() {
     struct_();
     replace_vptr();
     vtable_nop_cast();
+    drop_principal();
 }
 
 fn vtable_nop_cast() {
@@ -430,3 +431,53 @@ fn replace_vptr() {
     let s = S(42);
     invoke_outer(&s);
 }
+
+fn drop_principal() {
+    use std::{alloc::Layout, any::Any};
+
+    const fn yeet_principal(x: Box<dyn Any + Send>) -> Box<dyn Send> {
+        x
+    }
+
+    trait Bar: Send + Sync {}
+
+    impl<T: Send + Sync> Bar for T {}
+
+    const fn yeet_principal_2(x: Box<dyn Bar>) -> Box<dyn Send> {
+        x
+    }
+
+    struct CallMe<F: FnOnce()>(Option<F>);
+
+    impl<F: FnOnce()> CallMe<F> {
+        fn new(f: F) -> Self {
+            CallMe(Some(f))
+        }
+    }
+
+    impl<F: FnOnce()> Drop for CallMe<F> {
+        fn drop(&mut self) {
+            (self.0.take().unwrap())();
+        }
+    }
+
+    fn goodbye() {
+        println!("goodbye");
+    }
+
+    let x = Box::new(CallMe::new(goodbye)) as Box<dyn Any + Send>;
+    let x_layout = Layout::for_value(&*x);
+    let y = yeet_principal(x);
+    let y_layout = Layout::for_value(&*y);
+    assert_eq!(x_layout, y_layout);
+    println!("before");
+    drop(y);
+
+    let x = Box::new(CallMe::new(goodbye)) as Box<dyn Bar>;
+    let x_layout = Layout::for_value(&*x);
+    let y = yeet_principal_2(x);
+    let y_layout = Layout::for_value(&*y);
+    assert_eq!(x_layout, y_layout);
+    println!("before");
+    drop(y);
+}

tests/pass/dyn-upcast.stdout

@@ -0,0 +1,4 @@
+before
+goodbye
+before
+goodbye