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

8 files changed, 19 insertions, 106 deletions

diff --git a/compiler/rustc_feature/src/accepted.rs b/compiler/rustc_feature/src/accepted.rs
index 6c431086414..822cf4982b5 100644
--- a/compiler/rustc_feature/src/accepted.rs
+++ b/compiler/rustc_feature/src/accepted.rs
@@ -400,6 +400,9 @@ 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, "CURRENT_RUSTC_VERSION", Some(65991)),
     /// Allows #[repr(transparent)] on univariant enums (RFC 2645).
     (accepted, transparent_enums, "1.42.0", Some(60405)),
     /// Allows indexing tuples.
diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
index 094962b265b..3a2e810dc6a 100644
--- a/compiler/rustc_feature/src/unstable.rs
+++ b/compiler/rustc_feature/src/unstable.rs
@@ -634,9 +634,6 @@ 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).
diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
index 83acc7dd6af..56cc9b6d551 100644
--- a/compiler/rustc_hir_typeck/src/coercion.rs
+++ b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -605,7 +605,6 @@ 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
@@ -690,13 +689,6 @@ 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;
                         }
@@ -707,21 +699,6 @@ 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,
diff --git a/compiler/rustc_lint/src/deref_into_dyn_supertrait.rs b/compiler/rustc_lint/src/deref_into_dyn_supertrait.rs
index 657642e093c..181f44fb4de 100644
--- a/compiler/rustc_lint/src/deref_into_dyn_supertrait.rs
+++ b/compiler/rustc_lint/src/deref_into_dyn_supertrait.rs
@@ -1,6 +1,5 @@
 use rustc_hir::{self as hir, LangItem};
 use rustc_middle::ty;
-use rustc_session::lint::FutureIncompatibilityReason;
 use rustc_session::{declare_lint, declare_lint_pass};
 use rustc_span::sym;
 use rustc_trait_selection::traits::supertraits;
@@ -9,11 +8,12 @@ use crate::lints::{SupertraitAsDerefTarget, SupertraitAsDerefTargetLabel};
 use crate::{LateContext, LateLintPass, LintContext};
 
 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`.
+    /// The `deref_into_dyn_supertrait` lint is emitted whenever there is a `Deref` implementation
+    /// for `dyn SubTrait` with a `dyn SuperTrait` type as the `Output` type.
     ///
-    /// 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.
+    /// These implementations are "shadowed" by trait upcasting (stabilized since
+    /// CURRENT_RUSTC_VERSION). The `deref` functions is no longer called implicitly, which might
+    /// change behavior compared to previous rustc versions.
     ///
     /// ### Example
     ///
@@ -43,15 +43,14 @@ declare_lint! {
     ///
     /// ### Explanation
     ///
-    /// The dyn upcasting coercion feature adds new coercion rules, taking priority
-    /// over certain other coercion rules, which will cause some behavior change.
+    /// The trait upcasting coercion added a new coercion rule, taking priority over certain other
+    /// coercion rules, which causes some behavior change compared to older `rustc` versions.
+    ///
+    /// `deref` can be still called explicitly, it just isn't called as part of a deref coercion
+    /// (since trait upcasting coercion takes priority).
     pub DEREF_INTO_DYN_SUPERTRAIT,
-    Warn,
-    "`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>",
-    };
+    Allow,
+    "`Deref` implementation with a supertrait trait object for output is shadowed by trait upcasting",
 }
 
 declare_lint_pass!(DerefIntoDynSupertrait => [DEREF_INTO_DYN_SUPERTRAIT]);
diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index fd6c17735c5..3e97f4c86ba 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -21,6 +21,7 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
+#![cfg_attr(bootstrap, feature(trait_upcasting))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(array_windows)]
@@ -32,7 +33,6 @@
 #![feature(let_chains)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
-#![feature(trait_upcasting)]
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
diff --git a/compiler/rustc_middle/src/lib.rs b/compiler/rustc_middle/src/lib.rs
index 7a6d329fd47..95128a5d903 100644
--- a/compiler/rustc_middle/src/lib.rs
+++ b/compiler/rustc_middle/src/lib.rs
@@ -29,6 +29,7 @@
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::potential_query_instability)]
 #![allow(rustc::untranslatable_diagnostic)]
+#![cfg_attr(bootstrap, feature(trait_upcasting))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(allocator_api)]
@@ -56,7 +57,6 @@
 #![feature(ptr_alignment_type)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
-#![feature(trait_upcasting)]
 #![feature(trusted_len)]
 #![feature(try_blocks)]
 #![feature(try_trait_v2)]
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index 13a6744c2e9..e495bdbf782 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -12,9 +12,7 @@ use hir::LangItem;
 use hir::def_id::DefId;
 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
 use rustc_hir as hir;
-use rustc_infer::traits::{
-    Obligation, ObligationCause, PolyTraitObligation, PredicateObligations, SelectionError,
-};
+use rustc_infer::traits::{Obligation, PolyTraitObligation, SelectionError};
 use rustc_middle::ty::fast_reject::DeepRejectCtxt;
 use rustc_middle::ty::{self, Ty, TypeVisitableExt, TypingMode};
 use rustc_middle::{bug, span_bug};
@@ -23,8 +21,6 @@ use tracing::{debug, instrument, trace};
 
 use super::SelectionCandidate::*;
 use super::{BuiltinImplConditions, SelectionCandidateSet, SelectionContext, TraitObligationStack};
-use crate::traits;
-use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::util;
 
 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
@@ -904,54 +900,6 @@ 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>> {
-        // Don't drop any candidates in intercrate mode, as it's incomplete.
-        // (Not that it matters, since `Unsize` is not a stable trait.)
-        //
-        // FIXME(@lcnr): This should probably only trigger during analysis,
-        // disabling candidates during codegen is also questionable.
-        if let TypingMode::Coherence = self.infcx.typing_mode() {
-            return None;
-        }
-
-        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_ty(
-                self,
-                param_env,
-                ty::AliasTy::new_from_args(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 PredicateObligations::new(),
-            )
-            .as_type()
-            .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,
@@ -1019,15 +967,6 @@ 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()
diff --git a/compiler/rustc_type_ir/src/solve/mod.rs b/compiler/rustc_type_ir/src/solve/mod.rs
index c06004d4d0f..a562b751d8a 100644
--- a/compiler/rustc_type_ir/src/solve/mod.rs
+++ b/compiler/rustc_type_ir/src/solve/mod.rs
@@ -179,9 +179,7 @@ pub enum BuiltinImplSource {
     Object(usize),
     /// A built-in implementation of `Upcast` for trait objects to other trait objects.
     ///
-    /// This can be removed when `feature(dyn_upcasting)` is stabilized, since we only
-    /// use it to detect when upcasting traits in hir typeck. The index is only used
-    /// for winnowing.
+    /// The index is only used for winnowing.
     TraitUpcasting(usize),
     /// Unsizing a tuple like `(A, B, ..., X)` to `(A, B, ..., Y)` if `X` unsizes to `Y`.
     ///