Rollup merge of #104597 - compiler-errors:need_migrate_deref_output_trait_object-msg, r=eholk

Probe + better error messsage for `need_migrate_deref_output_trait_object`

1. Use `InferCtxt::probe` in `need_migrate_deref_output_trait_object` -- that normalization *could* technically do type inference as a side-effect, and this is a lint, so it should have no side-effects.
2. Return the trait-ref so we format the error message correctly. See the UI test change -- `(dyn A + 'static)` is not a trait.

3 files changed, 29 insertions, 34 deletions

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 8183f34bb3c..57680669b26 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -8,7 +8,6 @@
 use hir::LangItem;
 use rustc_errors::DelayDm;
 use rustc_hir as hir;
-use rustc_hir::def_id::DefId;
 use rustc_infer::traits::ObligationCause;
 use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
 use rustc_lint_defs::builtin::DEREF_INTO_DYN_SUPERTRAIT;
@@ -707,7 +706,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         ty: Ty<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         cause: &ObligationCause<'tcx>,
-    ) -> Option<(Ty<'tcx>, DefId)> {
+    ) -> Option<ty::PolyExistentialTraitRef<'tcx>> {
         let tcx = self.tcx();
         if tcx.features().trait_upcasting {
             return None;
@@ -729,27 +728,25 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return None;
         }
 
-        let ty = traits::normalize_projection_type(
-            self,
-            param_env,
-            ty::ProjectionTy {
-                item_def_id: tcx.lang_items().deref_target()?,
-                substs: trait_ref.substs,
-            },
-            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() {
-            Some((ty, data.principal_def_id()?))
-        } else {
-            None
-        }
+        self.infcx.probe(|_| {
+            let ty = traits::normalize_projection_type(
+                self,
+                param_env,
+                ty::ProjectionTy {
+                    item_def_id: tcx.lang_items().deref_target()?,
+                    substs: trait_ref.substs,
+                },
+                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`.
@@ -811,21 +808,19 @@ 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.param_env,
-                                &obligation.cause,
-                            )
-                        {
-                            if deref_output_trait_did == target_trait_did {
+                        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 {
                                 self.tcx().struct_span_lint_hir(
                                     DEREF_INTO_DYN_SUPERTRAIT,
                                     obligation.cause.body_id,
                                     obligation.cause.span,
                                     DelayDm(|| format!(
                                         "`{}` implements `Deref` with supertrait `{}` as output",
-                                        source, deref_output_ty
+                                        source, deref_trait_ref
                                     )),
                                     |lint| lint,
                                 );
diff --git a/src/test/ui/traits/trait-upcasting/migrate-lint-deny.rs b/src/test/ui/traits/trait-upcasting/migrate-lint-deny.rs
index c6725101858..5f5a2574bab 100644
--- a/src/test/ui/traits/trait-upcasting/migrate-lint-deny.rs
+++ b/src/test/ui/traits/trait-upcasting/migrate-lint-deny.rs
@@ -18,7 +18,7 @@ fn take_a(_: &dyn A) {}
 
 fn whoops(b: &dyn B) {
     take_a(b)
-    //~^ ERROR `dyn B` implements `Deref` with supertrait `(dyn A + 'static)` as output
+    //~^ ERROR `dyn B` implements `Deref` with supertrait `A` as output
     //~^^ WARN this was previously accepted by the compiler but is being phased out;
 }
 
diff --git a/src/test/ui/traits/trait-upcasting/migrate-lint-deny.stderr b/src/test/ui/traits/trait-upcasting/migrate-lint-deny.stderr
index 6c359b69836..41381a3ffd1 100644
--- a/src/test/ui/traits/trait-upcasting/migrate-lint-deny.stderr
+++ b/src/test/ui/traits/trait-upcasting/migrate-lint-deny.stderr
@@ -1,4 +1,4 @@
-error: `dyn B` implements `Deref` with supertrait `(dyn A + 'static)` as output
+error: `dyn B` implements `Deref` with supertrait `A` as output
   --> $DIR/migrate-lint-deny.rs:20:12
    |
 LL |     take_a(b)