Rollup merge of #118146 - compiler-errors:deref-into-dyn-regions, r=lcnr

Rework supertrait lint once again

I accidentally pushed the wrong commits because I totally didn't check I was on the right computer when updating #118026.
Sorry, this should address all the nits in #118026.

r? lcnr

compiler/rustc_lint/messages.ftl

@@ -491,8 +491,9 @@ 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 = this `Deref` implementation is covered by an implicit supertrait coercion
+    .label = `{$self_ty}` implements `Deref<Target = dyn {$target_principal}>` which conflicts with supertrait `{$supertrait_principal}`
+    .label2 = target type is a supertrait of `{$self_ty}`
     .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

@@ -53,35 +53,43 @@ impl<'tcx> LateLintPass<'tcx> for DerefIntoDynSupertrait {
         let tcx = cx.tcx;
         // `Deref` is being implemented for `t`
         if let hir::ItemKind::Impl(impl_) = item.kind
+            // the trait is a `Deref` implementation
             && let Some(trait_) = &impl_.of_trait
-            && let t = tcx.type_of(item.owner_id).instantiate_identity()
-            && let opt_did @ Some(did) = trait_.trait_def_id()
-            && opt_did == tcx.lang_items().deref_trait()
-            // `t` is `dyn t_principal`
-            && let ty::Dynamic(data, _, ty::Dyn) = t.kind()
-            && let Some(t_principal) = data.principal()
+            && let Some(did) = trait_.trait_def_id()
+            && Some(did) == tcx.lang_items().deref_trait()
+            // the self type is `dyn t_principal`
+            && let self_ty = tcx.type_of(item.owner_id).instantiate_identity()
+            && let ty::Dynamic(data, _, ty::Dyn) = self_ty.kind()
+            && let Some(self_principal) = data.principal()
             // `<T as Deref>::Target` is `dyn target_principal`
-            && let Some(target) = cx.get_associated_type(t, did, "Target")
+            && let Some(target) = cx.get_associated_type(self_ty, 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(tcx, t_principal.with_self_ty(tcx, tcx.types.trait_object_dummy_self))
-                .any(|sup| {
-                    tcx.erase_regions(
-                        sup.map_bound(|x| ty::ExistentialTraitRef::erase_self_ty(tcx, x)),
-                    ) == tcx.erase_regions(target_principal)
-                })
+            && let Some(supertrait_principal) = supertraits(tcx, self_principal.with_self_ty(tcx, self_ty))
+                .find(|supertrait| supertrait.def_id() == target_principal.def_id())
         {
-            let t = tcx.erase_regions(t);
-            let label = impl_
+            // erase regions in self type for better diagnostic presentation
+            let (self_ty, target_principal, supertrait_principal) =
+                tcx.erase_regions((self_ty, target_principal, supertrait_principal));
+            let label2 = impl_
                 .items
                 .iter()
                 .find_map(|i| (i.ident.name == sym::Target).then_some(i.span))
                 .map(|label| SupertraitAsDerefTargetLabel { label });
+            let span = tcx.def_span(item.owner_id.def_id);
             cx.emit_spanned_lint(
                 DEREF_INTO_DYN_SUPERTRAIT,
-                tcx.def_span(item.owner_id.def_id),
-                SupertraitAsDerefTarget { t, label },
+                span,
+                SupertraitAsDerefTarget {
+                    self_ty,
+                    supertrait_principal: supertrait_principal.map_bound(|trait_ref| {
+                        ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
+                    }),
+                    target_principal,
+                    label: span,
+                    label2,
+                },
             );
         }
     }

compiler/rustc_lint/src/lints.rs

@@ -10,7 +10,9 @@ use rustc_errors::{
 };
 use rustc_hir::def_id::DefId;
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
-use rustc_middle::ty::{inhabitedness::InhabitedPredicate, Clause, Ty, TyCtxt};
+use rustc_middle::ty::{
+    inhabitedness::InhabitedPredicate, Clause, PolyExistentialTraitRef, Ty, TyCtxt,
+};
 use rustc_session::parse::ParseSess;
 use rustc_span::{edition::Edition, sym, symbol::Ident, Span, Symbol};
 
@@ -556,13 +558,17 @@ pub enum BuiltinSpecialModuleNameUsed {
 #[diag(lint_supertrait_as_deref_target)]
 #[help]
 pub struct SupertraitAsDerefTarget<'a> {
-    pub t: Ty<'a>,
+    pub self_ty: Ty<'a>,
+    pub supertrait_principal: PolyExistentialTraitRef<'a>,
+    pub target_principal: PolyExistentialTraitRef<'a>,
+    #[label]
+    pub label: Span,
     #[subdiagnostic]
-    pub label: Option<SupertraitAsDerefTargetLabel>,
+    pub label2: Option<SupertraitAsDerefTargetLabel>,
 }
 
 #[derive(Subdiagnostic)]
-#[label(lint_label)]
+#[label(lint_label2)]
 pub struct SupertraitAsDerefTargetLabel {
     #[primary_span]
     pub label: Span,