1 files changed, 92 insertions, 0 deletions
diff --git a/compiler/rustc_lint/src/deref_into_dyn_supertrait.rs b/compiler/rustc_lint/src/deref_into_dyn_supertrait.rs
new file mode 100644
index 000000000..1d29a234a
--- /dev/null
+++ b/compiler/rustc_lint/src/deref_into_dyn_supertrait.rs
@@ -0,0 +1,92 @@
+use crate::{LateContext, LateLintPass, LintContext};
+
+use rustc_errors::DelayDm;
+use rustc_hir as hir;
+use rustc_middle::{traits::util::supertraits, ty};
+use rustc_span::sym;
+
+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
+    /// #![deny(deref_into_dyn_supertrait)]
+    /// #![allow(dead_code)]
+    ///
+    /// use core::ops::Deref;
+    ///
+    /// trait A {}
+    /// trait B: A {}
+    /// impl<'a> Deref for dyn 'a + B {
+    ///     type Target = dyn A;
+    ///     fn deref(&self) -> &Self::Target {
+    ///         todo!()
+    ///     }
+    /// }
+    ///
+    /// fn take_a(_: &dyn A) { }
+    ///
+    /// fn take_b(b: &dyn B) {
+    ///     take_a(b);
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// 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 might be shadowed in the future",
+    @future_incompatible = FutureIncompatibleInfo {
+        reference: "issue #89460 <https://github.com/rust-lang/rust/issues/89460>",
+    };
+}
+
+declare_lint_pass!(DerefIntoDynSupertrait => [DEREF_INTO_DYN_SUPERTRAIT]);
+
+impl<'tcx> LateLintPass<'tcx> for DerefIntoDynSupertrait {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
+        // `Deref` is being implemented for `t`
+        if let hir::ItemKind::Impl(impl_) = item.kind
+            && let Some(trait_) = &impl_.of_trait
+            && let t = cx.tcx.type_of(item.owner_id)
+            && let opt_did @ Some(did) = trait_.trait_def_id()
+            && opt_did == cx.tcx.lang_items().deref_trait()
+            // `t` is `dyn t_principal`
+            && let ty::Dynamic(data, _, ty::Dyn) = t.kind()
+            && let Some(t_principal) = data.principal()
+            // `<T as Deref>::Target` is `dyn target_principal`
+            && let Some(target) = cx.get_associated_type(t, did, "Target")
+            && let ty::Dynamic(data, _, ty::Dyn) = target.kind()
+            && let Some(target_principal) = data.principal()
+            // `target_principal` is a supertrait of `t_principal`
+            && supertraits(cx.tcx, t_principal.with_self_ty(cx.tcx, cx.tcx.types.trait_object_dummy_self))
+                .any(|sup| sup.map_bound(|x| ty::ExistentialTraitRef::erase_self_ty(cx.tcx, x)) == target_principal)
+        {
+            cx.struct_span_lint(
+                DEREF_INTO_DYN_SUPERTRAIT,
+                cx.tcx.def_span(item.owner_id.def_id),
+                DelayDm(|| {
+                    format!(
+                        "`{t}` implements `Deref` with supertrait `{target_principal}` as target"
+                    )
+                }),
+                |lint| {
+                    if let Some(target_span) = impl_.items.iter().find_map(|i| (i.ident.name == sym::Target).then_some(i.span)) {
+                        lint.span_label(target_span, "target type is set here");
+                    }
+
+                    lint
+                },
+            )
+        }
+    }
+}