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: https://github.com/rust-lang/rust/pull/134367#issuecomment-2545839841.
cc reference PR: https://github.com/rust-lang/reference/pull/1622.

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

r? compiler-errors

5 files changed, 2 insertions, 35 deletions

diff --git a/src/doc/unstable-book/src/language-features/trait-upcasting.md b/src/doc/unstable-book/src/language-features/trait-upcasting.md
deleted file mode 100644
index a5f99cc86f2..00000000000
--- a/src/doc/unstable-book/src/language-features/trait-upcasting.md
+++ /dev/null
@@ -1,26 +0,0 @@
-# `trait_upcasting`
-
-The tracking issue for this feature is: [#65991]
-
-[#65991]: https://github.com/rust-lang/rust/issues/65991
-
-------------------------
-
-The `trait_upcasting` feature adds support for trait upcasting coercion. This allows a
-trait object of type `dyn Bar` to be cast to a trait object of type `dyn Foo`
-so long as `Bar: Foo`.
-
-```rust,edition2018
-#![feature(trait_upcasting)]
-
-trait Foo {}
-
-trait Bar: Foo {}
-
-impl Foo for i32 {}
-
-impl<T: Foo + ?Sized> Bar for T {}
-
-let bar: &dyn Bar = &123;
-let foo: &dyn Foo = bar;
-```
diff --git a/src/tools/miri/src/lib.rs b/src/tools/miri/src/lib.rs
index 45054c37c40..a717d8ccf28 100644
--- a/src/tools/miri/src/lib.rs
+++ b/src/tools/miri/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 --git a/src/tools/miri/tests/fail/dyn-upcast-trait-mismatch.rs b/src/tools/miri/tests/fail/dyn-upcast-trait-mismatch.rs
index f450e7e652c..1fd791a91f0 100644
--- a/src/tools/miri/tests/fail/dyn-upcast-trait-mismatch.rs
+++ b/src/tools/miri/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 --git a/src/tools/miri/tests/pass/box-custom-alloc.rs b/src/tools/miri/tests/pass/box-custom-alloc.rs
index 71ce019187c..f0614313e50 100644
--- a/src/tools/miri/tests/pass/box-custom-alloc.rs
+++ b/src/tools/miri/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 --git a/src/tools/miri/tests/pass/dyn-upcast.rs b/src/tools/miri/tests/pass/dyn-upcast.rs
index f100c4d6a86..6f8adc09640 100644
--- a/src/tools/miri/tests/pass/dyn-upcast.rs
+++ b/src/tools/miri/tests/pass/dyn-upcast.rs
@@ -1,6 +1,3 @@
-#![feature(trait_upcasting)]
-#![allow(incomplete_features)]
-
 use std::fmt;
 
 fn main() {