Add `deref_into_dyn_supertrait` lint.

2 files changed, 80 insertions, 1 deletions

diff --git a/compiler/rustc_trait_selection/Cargo.toml b/compiler/rustc_trait_selection/Cargo.toml
index 9a9737362c6..27cd5c2acac 100644
--- a/compiler/rustc_trait_selection/Cargo.toml
+++ b/compiler/rustc_trait_selection/Cargo.toml
@@ -17,6 +17,7 @@ rustc_errors = { path = "../rustc_errors" }
 rustc_hir = { path = "../rustc_hir" }
 rustc_index = { path = "../rustc_index" }
 rustc_infer = { path = "../rustc_infer" }
+rustc_lint_defs = { path = "../rustc_lint_defs" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
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 719412492f6..f3706aa6e71 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -6,12 +6,17 @@
 //!
 //! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
 use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use rustc_infer::traits::TraitEngine;
 use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
+use rustc_lint_defs::builtin::DEREF_INTO_DYN_SUPERTRAIT;
 use rustc_middle::ty::print::with_no_trimmed_paths;
-use rustc_middle::ty::{self, Ty, TypeFoldable};
+use rustc_middle::ty::{self, ToPredicate, Ty, TypeFoldable, WithConstness};
 use rustc_target::spec::abi::Abi;
 
+use crate::traits;
 use crate::traits::coherence::Conflict;
+use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::{util, SelectionResult};
 use crate::traits::{Overflow, Unimplemented};
 
@@ -672,6 +677,55 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         })
     }
 
+    /// Temporary migration for #89190
+    fn need_migrate_deref_output_trait_object(
+        &mut self,
+        ty: Ty<'tcx>,
+        cause: &traits::ObligationCause<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> Option<(Ty<'tcx>, DefId)> {
+        let tcx = self.tcx();
+        if tcx.features().trait_upcasting {
+            return None;
+        }
+
+        // <ty as Deref>
+        let trait_ref = ty::TraitRef {
+            def_id: tcx.lang_items().deref_trait()?,
+            substs: tcx.mk_substs_trait(ty, &[]),
+        };
+
+        let obligation = traits::Obligation::new(
+            cause.clone(),
+            param_env,
+            ty::Binder::dummy(trait_ref).without_const().to_predicate(tcx),
+        );
+        if !self.infcx.predicate_may_hold(&obligation) {
+            return None;
+        }
+
+        let mut fulfillcx = traits::FulfillmentContext::new_in_snapshot();
+        let normalized_ty = fulfillcx.normalize_projection_type(
+            &self.infcx,
+            param_env,
+            ty::ProjectionTy {
+                item_def_id: tcx.lang_items().deref_target()?,
+                substs: trait_ref.substs,
+            },
+            cause.clone(),
+        );
+
+        let data = if let ty::Dynamic(ref data, ..) = normalized_ty.kind() {
+            data
+        } else {
+            return None;
+        };
+
+        let def_id = data.principal_def_id()?;
+
+        return Some((normalized_ty, def_id));
+    }
+
     /// Searches for unsizing that might apply to `obligation`.
     fn assemble_candidates_for_unsizing(
         &mut self,
@@ -732,6 +786,30 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         let principal_a = data_a.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_output_ty, deref_output_trait_did)) = self
+                            .need_migrate_deref_output_trait_object(
+                                source,
+                                &obligation.cause,
+                                obligation.param_env,
+                            )
+                        {
+                            if deref_output_trait_did == target_trait_did {
+                                self.tcx().struct_span_lint_hir(
+                                    DEREF_INTO_DYN_SUPERTRAIT,
+                                    obligation.cause.body_id,
+                                    obligation.cause.span,
+                                    |lint| {
+                                        lint.build(&format!(
+                                            "`{}` implements `Deref` with supertrait `{}` as output",
+                                            source,
+                                            deref_output_ty
+                                        )).emit();
+                                    },
+                                );
+                                return;
+                            }
+                        }
+
                         for (idx, upcast_trait_ref) in
                             util::supertraits(self.tcx(), source_trait_ref).enumerate()
                         {