Auto merge of rust-lang#118133 - Urgau:stabilize_trait_upcasting, r=WaffleLapkin

Stabilize RFC3324 dyn upcasting coercion

This PR stabilize the `trait_upcasting` feature, aka rust-lang/rfcs#3324.

The FCP was completed here: rust-lang#65991 (comment).

~~And also remove the `deref_into_dyn_supertrait` lint which is now handled by dyn upcasting coercion.~~

Heavily inspired by rust-lang#101718
Fixes rust-lang#65991

Author: bors

compiler/rustc_feature/src/accepted.rs

@@ -338,6 +338,9 @@ 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), None),
+    /// 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), None),
     /// Allows #[repr(transparent)] on univariant enums (RFC 2645).
     (accepted, transparent_enums, "1.42.0", Some(60405), None),
     /// Allows indexing tuples.

compiler/rustc_feature/src/unstable.rs

@@ -562,9 +562,6 @@ declare_features! (
     (unstable, thread_local, "1.0.0", Some(29594), None),
     /// Allows defining `trait X = A + B;` alias items.
     (unstable, trait_alias, "1.24.0", Some(41517), None),
-    /// 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), None),
     /// Allows for transmuting between arrays with sizes that contain generic consts.
     (unstable, transmute_generic_consts, "1.70.0", Some(109929), None),
     /// Allows #[repr(transparent)] on unions (RFC 2645).

compiler/rustc_hir_typeck/src/coercion.rs

@@ -626,7 +626,6 @@ 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
@@ -694,13 +693,6 @@ 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;
                         }
@@ -711,21 +703,6 @@ 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.parse_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.parse_sess,

compiler/rustc_lint/messages.ftl

@@ -490,8 +490,9 @@ lint_requested_level = requested on the command line with `{$level} {$lint_name}
 
 lint_span_use_eq_ctxt = use `.eq_ctxt()` instead of `.ctxt() == .ctxt()`
 
-lint_supertrait_as_deref_target = `{$t}` implements `Deref` with supertrait `{$target_principal}` as target
-    .label = target type is set here
+lint_supertrait_as_deref_target = this `Deref` implementation is covered by an implicit supertrait coercion
+    .help = consider removing this implementation or replacing it with a method instead
+    .label = target type is a supertrait of `{$t}`
 
 lint_suspicious_double_ref_clone =
     using `.clone()` on a double reference, which returns `{$ty}` instead of cloning the inner type

compiler/rustc_lint/src/deref_into_dyn_supertrait.rs

@@ -5,17 +5,13 @@ 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
@@ -44,15 +40,10 @@ 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 implicit dyn upcasting coercion take priority over those `Deref` impls.
     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>",
-    };
+    "`Deref` implementation usage with a supertrait trait object for output are shadow by implicit coercion",
 }
 
 declare_lint_pass!(DerefIntoDynSupertrait => [DEREF_INTO_DYN_SUPERTRAIT]);
@@ -90,7 +81,7 @@ impl<'tcx> LateLintPass<'tcx> for DerefIntoDynSupertrait {
             cx.emit_spanned_lint(
                 DEREF_INTO_DYN_SUPERTRAIT,
                 tcx.def_span(item.owner_id.def_id),
-                SupertraitAsDerefTarget { t, target_principal, label },
+                SupertraitAsDerefTarget { t, label },
             );
         }
     }

compiler/rustc_lint/src/lints.rs

@@ -10,9 +10,7 @@ use rustc_errors::{
 };
 use rustc_hir::def_id::DefId;
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
-use rustc_middle::ty::{
-    inhabitedness::InhabitedPredicate, Clause, PolyExistentialTraitRef, Ty, TyCtxt,
-};
+use rustc_middle::ty::{inhabitedness::InhabitedPredicate, Clause, Ty, TyCtxt};
 use rustc_session::parse::ParseSess;
 use rustc_span::{edition::Edition, sym, symbol::Ident, Span, Symbol};
 
@@ -556,9 +554,9 @@ pub enum BuiltinSpecialModuleNameUsed {
 // deref_into_dyn_supertrait.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_supertrait_as_deref_target)]
+#[help]
 pub struct SupertraitAsDerefTarget<'a> {
     pub t: Ty<'a>,
-    pub target_principal: PolyExistentialTraitRef<'a>,
     #[subdiagnostic]
     pub label: Option<SupertraitAsDerefTargetLabel>,
 }

compiler/rustc_middle/src/lib.rs

@@ -49,7 +49,7 @@
 #![feature(associated_type_bounds)]
 #![feature(rustc_attrs)]
 #![feature(control_flow_enum)]
-#![feature(trait_upcasting)]
+#![cfg_attr(bootstrap, feature(trait_upcasting))]
 #![feature(trusted_step)]
 #![feature(try_blocks)]
 #![feature(try_reserve_kind)]

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

@@ -9,13 +9,10 @@
 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;
@@ -690,45 +687,6 @@ 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>> {
-        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,
@@ -786,15 +744,6 @@ 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()

library/core/tests/lib.rs

@@ -111,7 +111,7 @@
 #![feature(slice_flatten)]
 #![feature(error_generic_member_access)]
 #![feature(error_in_core)]
-#![feature(trait_upcasting)]
+#![cfg_attr(bootstrap, feature(trait_upcasting))]
 #![feature(utf8_chunks)]
 #![feature(is_ascii_octdigit)]
 #![feature(get_many_mut)]