Revert stabilization of trait_upcasting feature

compiler/rustc_feature/src/accepted.rs

@@ -339,9 +339,6 @@ declare_features! (
     /// Allows `#[track_caller]` to be used which provides
     /// accurate caller location reporting during panic (RFC 2091).
     (accepted, track_caller, "1.46.0", Some(47809)),
-    /// Allows dyn upcasting trait objects via supertraits.
-    /// Dyn upcasting is casting, e.g., `dyn Foo -> dyn Bar` where `Foo: Bar`.
-    (accepted, trait_upcasting, "1.76.0", Some(65991)),
     /// Allows #[repr(transparent)] on univariant enums (RFC 2645).
     (accepted, transparent_enums, "1.42.0", Some(60405)),
     /// Allows indexing tuples.

compiler/rustc_feature/src/unstable.rs

@@ -584,6 +584,9 @@ declare_features! (
     (unstable, thread_local, "1.0.0", Some(29594)),
     /// Allows defining `trait X = A + B;` alias items.
     (unstable, trait_alias, "1.24.0", Some(41517)),
+    /// Allows dyn upcasting trait objects via supertraits.
+    /// Dyn upcasting is casting, e.g., `dyn Foo -> dyn Bar` where `Foo: Bar`.
+    (unstable, trait_upcasting, "1.56.0", Some(65991)),
     /// Allows for transmuting between arrays with sizes that contain generic consts.
     (unstable, transmute_generic_consts, "1.70.0", Some(109929)),
     /// Allows #[repr(transparent)] on unions (RFC 2645).

compiler/rustc_hir_typeck/src/coercion.rs

@@ -625,6 +625,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         )];
 
         let mut has_unsized_tuple_coercion = false;
+        let mut has_trait_upcasting_coercion = None;
 
         // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
         // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
@@ -692,6 +693,13 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                     // these here and emit a feature error if coercion doesn't fail
                     // due to another reason.
                     match impl_source {
+                        traits::ImplSource::Builtin(
+                            BuiltinImplSource::TraitUpcasting { .. },
+                            _,
+                        ) => {
+                            has_trait_upcasting_coercion =
+                                Some((trait_pred.self_ty(), trait_pred.trait_ref.args.type_at(1)));
+                        }
                         traits::ImplSource::Builtin(BuiltinImplSource::TupleUnsizing, _) => {
                             has_unsized_tuple_coercion = true;
                         }
@@ -702,6 +710,21 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             }
         }
 
+        if let Some((sub, sup)) = has_trait_upcasting_coercion
+            && !self.tcx().features().trait_upcasting
+        {
+            // Renders better when we erase regions, since they're not really the point here.
+            let (sub, sup) = self.tcx.erase_regions((sub, sup));
+            let mut err = feature_err(
+                &self.tcx.sess,
+                sym::trait_upcasting,
+                self.cause.span,
+                format!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"),
+            );
+            err.note(format!("required when coercing `{source}` into `{target}`"));
+            err.emit();
+        }
+
         if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
             feature_err(
                 &self.tcx.sess,

compiler/rustc_lint/src/deref_into_dyn_supertrait.rs

@@ -5,13 +5,17 @@ use crate::{
 
 use rustc_hir as hir;
 use rustc_middle::ty;
+use rustc_session::lint::FutureIncompatibilityReason;
 use rustc_span::sym;
 use rustc_trait_selection::traits::supertraits;
 
 declare_lint! {
     /// The `deref_into_dyn_supertrait` lint is output whenever there is a use of the
     /// `Deref` implementation with a `dyn SuperTrait` type as `Output`.
     ///
+    /// These implementations will become shadowed when the `trait_upcasting` feature is stabilized.
+    /// The `deref` functions will no longer be called implicitly, so there might be behavior change.
+    ///
     /// ### Example
     ///
     /// ```rust,compile_fail
@@ -40,10 +44,15 @@ declare_lint! {
     ///
     /// ### Explanation
     ///
-    /// The implicit dyn upcasting coercion take priority over those `Deref` impls.
+    /// The dyn upcasting coercion feature adds new coercion rules, taking priority
+    /// over certain other coercion rules, which will cause some behavior change.
     pub DEREF_INTO_DYN_SUPERTRAIT,
     Warn,
-    "`Deref` implementation usage with a supertrait trait object for output are shadow by implicit coercion",
+    "`Deref` implementation usage with a supertrait trait object for output might be shadowed in the future",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseSemanticsChange,
+        reference: "issue #89460 <https://github.com/rust-lang/rust/issues/89460>",
+    };
 }
 
 declare_lint_pass!(DerefIntoDynSupertrait => [DEREF_INTO_DYN_SUPERTRAIT]);

compiler/rustc_lint/src/lib.rs

@@ -35,6 +35,7 @@
 #![feature(iter_intersperse)]
 #![feature(iter_order_by)]
 #![feature(let_chains)]
+#![cfg_attr(not(bootstrap), feature(trait_upcasting))]
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(rustc_attrs)]

compiler/rustc_lint/src/lints.rs

@@ -532,7 +532,6 @@ pub enum BuiltinSpecialModuleNameUsed {
 // deref_into_dyn_supertrait.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_supertrait_as_deref_target)]
-#[help]
 pub struct SupertraitAsDerefTarget<'a> {
     pub self_ty: Ty<'a>,
     pub supertrait_principal: PolyExistentialTraitRef<'a>,

compiler/rustc_middle/src/lib.rs

@@ -49,6 +49,7 @@
 #![feature(associated_type_bounds)]
 #![feature(rustc_attrs)]
 #![feature(control_flow_enum)]
+#![cfg_attr(not(bootstrap), feature(trait_upcasting))]
 #![feature(trusted_step)]
 #![feature(try_blocks)]
 #![feature(try_reserve_kind)]

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -9,10 +9,13 @@
 use hir::def_id::DefId;
 use hir::LangItem;
 use rustc_hir as hir;
+use rustc_infer::traits::ObligationCause;
 use rustc_infer::traits::{Obligation, PolyTraitObligation, SelectionError};
 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
 
+use crate::traits;
+use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::util;
 
 use super::BuiltinImplConditions;
@@ -723,6 +726,45 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         })
     }
 
+    /// Temporary migration for #89190
+    fn need_migrate_deref_output_trait_object(
+        &mut self,
+        ty: Ty<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        cause: &ObligationCause<'tcx>,
+    ) -> Option<ty::PolyExistentialTraitRef<'tcx>> {
+        let tcx = self.tcx();
+        if tcx.features().trait_upcasting {
+            return None;
+        }
+
+        // <ty as Deref>
+        let trait_ref = ty::TraitRef::new(tcx, tcx.lang_items().deref_trait()?, [ty]);
+
+        let obligation =
+            traits::Obligation::new(tcx, cause.clone(), param_env, ty::Binder::dummy(trait_ref));
+        if !self.infcx.predicate_may_hold(&obligation) {
+            return None;
+        }
+
+        self.infcx.probe(|_| {
+            let ty = traits::normalize_projection_type(
+                self,
+                param_env,
+                ty::AliasTy::new(tcx, tcx.lang_items().deref_target()?, trait_ref.args),
+                cause.clone(),
+                0,
+                // We're *intentionally* throwing these away,
+                // since we don't actually use them.
+                &mut vec![],
+            )
+            .ty()
+            .unwrap();
+
+            if let ty::Dynamic(data, ..) = ty.kind() { data.principal() } else { None }
+        })
+    }
+
     /// Searches for unsizing that might apply to `obligation`.
     fn assemble_candidates_for_unsizing(
         &mut self,
@@ -780,6 +822,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         let principal_a = a_data.principal().unwrap();
                         let target_trait_did = principal_def_id_b.unwrap();
                         let source_trait_ref = principal_a.with_self_ty(self.tcx(), source);
+                        if let Some(deref_trait_ref) = self.need_migrate_deref_output_trait_object(
+                            source,
+                            obligation.param_env,
+                            &obligation.cause,
+                        ) {
+                            if deref_trait_ref.def_id() == target_trait_did {
+                                return;
+                            }
+                        }
 
                         for (idx, upcast_trait_ref) in
                             util::supertraits(self.tcx(), source_trait_ref).enumerate()

library/core/tests/lib.rs

@@ -113,6 +113,7 @@
 #![feature(slice_flatten)]
 #![feature(error_generic_member_access)]
 #![feature(error_in_core)]
+#![cfg_attr(not(bootstrap), feature(trait_upcasting))]
 #![feature(utf8_chunks)]
 #![feature(is_ascii_octdigit)]
 #![feature(get_many_mut)]

src/doc/unstable-book/src/language-features/trait-upcasting.md

@@ -0,0 +1,27 @@
+# `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)]
+#![allow(incomplete_features)]
+
+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;
+```

src/tools/miri/src/lib.rs

@@ -10,7 +10,7 @@
 #![feature(nonzero_ops)]
 #![feature(let_chains)]
 #![feature(lint_reasons)]
-#![feature(int_roundings)]
+#![cfg_attr(not(bootstrap), feature(trait_upcasting))]
 // Configure clippy and other lints
 #![allow(
     clippy::collapsible_else_if,

src/tools/miri/tests/fail/dyn-upcast-trait-mismatch.rs

@@ -1,3 +1,6 @@
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
 trait Foo: PartialEq<i32> + std::fmt::Debug + Send + Sync {
     fn a(&self) -> i32 {
         10

src/tools/miri/tests/pass/box-custom-alloc.rs

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

src/tools/miri/tests/pass/dyn-upcast.rs

@@ -1,3 +1,6 @@
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
 fn main() {
     basic();
     diamond();

tests/ui/codegen/issue-99551.rs

@@ -1,4 +1,5 @@
 // build-pass
+#![feature(trait_upcasting)]
 
 pub trait A {}
 pub trait B {}

tests/ui/dyn-star/no-unsize-coerce-dyn-trait.rs

@@ -1,4 +1,4 @@
-#![feature(dyn_star)]
+#![feature(dyn_star, trait_upcasting)]
 //~^ WARN the feature `dyn_star` is incomplete and may not be safe to use and/or cause compiler crashes
 
 trait A: B {}

tests/ui/dyn-star/no-unsize-coerce-dyn-trait.stderr

@@ -1,7 +1,7 @@
 warning: the feature `dyn_star` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/no-unsize-coerce-dyn-trait.rs:1:12
    |
-LL | #![feature(dyn_star)]
+LL | #![feature(dyn_star, trait_upcasting)]
    |            ^^^^^^^^
    |
    = note: see issue #102425 <https://github.com/rust-lang/rust/issues/102425> for more information

tests/ui/dyn-star/upcast.rs

@@ -1,6 +1,6 @@
 // known-bug: #104800
 
-#![feature(dyn_star)]
+#![feature(dyn_star, trait_upcasting)]
 
 trait Foo: Bar {
     fn hello(&self);

tests/ui/dyn-star/upcast.stderr

@@ -1,7 +1,7 @@
 warning: the feature `dyn_star` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/upcast.rs:3:12
    |
-LL | #![feature(dyn_star)]
+LL | #![feature(dyn_star, trait_upcasting)]
    |            ^^^^^^^^
    |
    = note: see issue #102425 <https://github.com/rust-lang/rust/issues/102425> for more information

tests/ui/feature-gates/feature-gate-trait_upcasting.rs

@@ -0,0 +1,13 @@
+trait Foo {}
+
+trait Bar: Foo {}
+
+impl Foo for () {}
+
+impl Bar for () {}
+
+fn main() {
+    let bar: &dyn Bar = &();
+    let foo: &dyn Foo = bar;
+    //~^ ERROR trait upcasting coercion is experimental [E0658]
+}

tests/ui/feature-gates/feature-gate-trait_upcasting.stderr

@@ -0,0 +1,14 @@
+error[E0658]: cannot cast `dyn Bar` to `dyn Foo`, trait upcasting coercion is experimental
+  --> $DIR/feature-gate-trait_upcasting.rs:11:25
+   |
+LL |     let foo: &dyn Foo = bar;
+   |                         ^^^
+   |
+   = note: see issue #65991 <https://github.com/rust-lang/rust/issues/65991> for more information
+   = help: add `#![feature(trait_upcasting)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = note: required when coercing `&dyn Bar` into `&dyn Foo`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/traits/next-solver/normalize-unsize-rhs.rs

@@ -1,5 +1,6 @@
 // compile-flags: -Znext-solver
 // check-pass
+#![feature(trait_upcasting)]
 
 trait A {}
 trait B: A {}

tests/ui/traits/next-solver/trait-upcast-lhs-needs-normalization.rs

@@ -1,5 +1,6 @@
 // check-pass
 // compile-flags: -Znext-solver
+#![feature(trait_upcasting)]
 
 pub trait A {}
 pub trait B: A {}

tests/ui/traits/next-solver/upcast-right-substs.rs

@@ -1,5 +1,6 @@
 // compile-flags: -Znext-solver
 // check-pass
+#![feature(trait_upcasting)]
 
 trait Foo: Bar<i32> + Bar<u32> {}
 

tests/ui/traits/trait-upcasting/alias-where-clause-isnt-supertrait.rs

@@ -1,3 +1,4 @@
+#![feature(trait_upcasting)]
 #![feature(trait_alias)]
 
 // Although we *elaborate* `T: Alias` to `i32: B`, we should

tests/ui/traits/trait-upcasting/alias-where-clause-isnt-supertrait.stderr

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/alias-where-clause-isnt-supertrait.rs:26:5
+  --> $DIR/alias-where-clause-isnt-supertrait.rs:27:5
    |
 LL | fn test(x: &dyn C) -> &dyn B {
    |                       ------ expected `&dyn B` because of return type

tests/ui/traits/trait-upcasting/basic.rs

@@ -1,5 +1,7 @@
 // run-pass
 
+#![feature(trait_upcasting)]
+
 trait Foo: PartialEq<i32> + std::fmt::Debug + Send + Sync {
     fn a(&self) -> i32 {
         10

tests/ui/traits/trait-upcasting/correct-supertrait-substitution.rs

@@ -1,4 +1,5 @@
 // run-pass
+#![feature(trait_upcasting)]
 
 trait Foo<T: Default + ToString>: Bar<i32> + Bar<T> {}
 trait Bar<T: Default + ToString> {