Rollup merge of #134367 - WaffleLapkin:trait_upcasting_as_a_treat, r=compiler-errors

Stabilize `feature(trait_upcasting)`

This feature was "done" for a while now, I think it's finally time to stabilize it! Stabilization report: rust-lang/rust#134367 (comment).
cc reference PR: rust-lang/reference#1622.

Closes #65991 (tracking issue), closes #89460 (the lint is no longer future incompat).

r? compiler-errors

Author: matthiaskrgr

Diff for: src/lib.rs

@@ -1,3 +1,4 @@
+#![cfg_attr(bootstrap, feature(trait_upcasting))]
 #![feature(rustc_private)]
 #![feature(cell_update)]
 #![feature(float_gamma)]
@@ -9,7 +10,6 @@
 #![feature(yeet_expr)]
 #![feature(nonzero_ops)]
 #![feature(let_chains)]
-#![feature(trait_upcasting)]
 #![feature(strict_overflow_ops)]
 #![feature(pointer_is_aligned_to)]
 #![feature(unqualified_local_imports)]

Diff for: tests/fail/dyn-upcast-trait-mismatch.rs

@@ -1,9 +1,6 @@
 // Validation stops this too early.
 //@compile-flags: -Zmiri-disable-validation
 
-#![feature(trait_upcasting)]
-#![allow(incomplete_features)]
-
 trait Foo: PartialEq<i32> + std::fmt::Debug + Send + Sync {
     #[allow(dead_code)]
     fn a(&self) -> i32 {

Diff for: tests/pass/box-custom-alloc.rs

@@ -1,7 +1,6 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
-#![allow(incomplete_features)] // for trait upcasting
-#![feature(allocator_api, trait_upcasting)]
+#![feature(allocator_api)]
 
 use std::alloc::{AllocError, Allocator, Layout};
 use std::cell::Cell;

Diff for: tests/pass/dyn-upcast.rs

@@ -1,6 +1,3 @@
-#![feature(trait_upcasting)]
-#![allow(incomplete_features)]
-
 use std::fmt;
 
 fn main() {