Remove redundant flags from lower_ty_common that can be inferred from the HIR

compiler/rustc_hir_analysis/src/collect.rs

@@ -392,7 +392,7 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
     }
 
     fn re_infer(&self, span: Span, reason: RegionInferReason<'_>) -> ty::Region<'tcx> {
-        if let RegionInferReason::BorrowedObjectLifetimeDefault = reason {
+        if let RegionInferReason::ObjectLifetimeDefault = reason {
             let e = struct_span_code_err!(
                 self.dcx(),
                 span,

compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs

@@ -15,11 +15,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     ///
     /// *Bare* trait object types are ones that aren't preceded by the keyword `dyn`.
     /// In edition 2021 and onward we emit a hard error for them.
-    pub(super) fn prohibit_or_lint_bare_trait_object_ty(
-        &self,
-        self_ty: &hir::Ty<'_>,
-        in_path: bool,
-    ) {
+    pub(super) fn prohibit_or_lint_bare_trait_object_ty(&self, self_ty: &hir::Ty<'_>) {
         let tcx = self.tcx();
 
         let hir::TyKind::TraitObject([poly_trait_ref, ..], _, TraitObjectSyntax::None) =
@@ -28,6 +24,21 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             return;
         };
 
+        let in_path = match tcx.parent_hir_node(self_ty.hir_id) {
+            hir::Node::Ty(hir::Ty {
+                kind: hir::TyKind::Path(hir::QPath::TypeRelative(qself, _)),
+                ..
+            })
+            | hir::Node::Expr(hir::Expr {
+                kind: hir::ExprKind::Path(hir::QPath::TypeRelative(qself, _)),
+                ..
+            })
+            | hir::Node::Pat(hir::Pat {
+                kind: hir::PatKind::Path(hir::QPath::TypeRelative(qself, _)),
+                ..
+            }) if qself.hir_id == self_ty.hir_id => true,
+            _ => false,
+        };
         let needs_bracket = in_path
             && !tcx
                 .sess

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -85,10 +85,9 @@ pub enum PredicateFilter {
 
 #[derive(Debug)]
 pub enum RegionInferReason<'a> {
-    /// Lifetime on a trait object behind a reference.
-    /// This allows inferring information from the reference.
-    BorrowedObjectLifetimeDefault,
-    /// A trait object's lifetime.
+    /// Lifetime on a trait object that is spelled explicitly, e.g. `+ 'a` or `+ '_`.
+    ExplicitObjectLifetime,
+    /// A trait object's lifetime when it is elided, e.g. `dyn Any`.
     ObjectLifetimeDefault,
     /// Generic lifetime parameter
     Param(&'a ty::GenericParamDef),
@@ -1999,16 +1998,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         }
     }
 
-    /// Lower a type from the HIR to our internal notion of a type.
-    pub fn lower_ty(&self, hir_ty: &hir::Ty<'tcx>) -> Ty<'tcx> {
-        self.lower_ty_common(hir_ty, false, false)
-    }
-
-    /// Lower a type inside of a path from the HIR to our internal notion of a type.
-    pub fn lower_ty_in_path(&self, hir_ty: &hir::Ty<'tcx>) -> Ty<'tcx> {
-        self.lower_ty_common(hir_ty, false, true)
-    }
-
     fn lower_delegation_ty(&self, idx: hir::InferDelegationKind) -> Ty<'tcx> {
         let delegation_sig = self.tcx().inherit_sig_for_delegation_item(self.item_def_id());
         match idx {
@@ -2026,7 +2015,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     /// 2. `in_path`: Whether the type appears inside of a path.
     ///    Used to provide correct diagnostics for bare trait object types.
     #[instrument(level = "debug", skip(self), ret)]
-    fn lower_ty_common(&self, hir_ty: &hir::Ty<'tcx>, borrowed: bool, in_path: bool) -> Ty<'tcx> {
+    pub fn lower_ty(&self, hir_ty: &hir::Ty<'tcx>) -> Ty<'tcx> {
         let tcx = self.tcx();
 
         let result_ty = match &hir_ty.kind {
@@ -2036,7 +2025,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             hir::TyKind::Ref(region, mt) => {
                 let r = self.lower_lifetime(region, RegionInferReason::Reference);
                 debug!(?r);
-                let t = self.lower_ty_common(mt.ty, true, false);
+                let t = self.lower_ty(mt.ty);
                 Ty::new_ref(tcx, r, t, mt.mutbl)
             }
             hir::TyKind::Never => tcx.types.never,
@@ -2065,20 +2054,13 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 )
             }
             hir::TyKind::TraitObject(bounds, lifetime, repr) => {
-                self.prohibit_or_lint_bare_trait_object_ty(hir_ty, in_path);
+                self.prohibit_or_lint_bare_trait_object_ty(hir_ty);
 
                 let repr = match repr {
                     TraitObjectSyntax::Dyn | TraitObjectSyntax::None => ty::Dyn,
                     TraitObjectSyntax::DynStar => ty::DynStar,
                 };
-                self.lower_trait_object_ty(
-                    hir_ty.span,
-                    hir_ty.hir_id,
-                    bounds,
-                    lifetime,
-                    borrowed,
-                    repr,
-                )
+                self.lower_trait_object_ty(hir_ty.span, hir_ty.hir_id, bounds, lifetime, repr)
             }
             hir::TyKind::Path(hir::QPath::Resolved(maybe_qself, path)) => {
                 debug!(?maybe_qself, ?path);
@@ -2106,7 +2088,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             }
             hir::TyKind::Path(hir::QPath::TypeRelative(qself, segment)) => {
                 debug!(?qself, ?segment);
-                let ty = self.lower_ty_common(qself, false, true);
+                let ty = self.lower_ty(qself);
                 self.lower_assoc_path(hir_ty.hir_id, hir_ty.span, ty, qself, segment, false)
                     .map(|(ty, _, _)| ty)
                     .unwrap_or_else(|guar| Ty::new_error(tcx, guar))

compiler/rustc_hir_analysis/src/hir_ty_lowering/object_safety.rs

@@ -30,7 +30,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         hir_id: hir::HirId,
         hir_trait_bounds: &[(hir::PolyTraitRef<'tcx>, hir::TraitBoundModifier)],
         lifetime: &hir::Lifetime,
-        borrowed: bool,
         representation: DynKind,
     ) -> Ty<'tcx> {
         let tcx = self.tcx();
@@ -325,22 +324,32 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         v.dedup();
         let existential_predicates = tcx.mk_poly_existential_predicates(&v);
 
-        // Use explicitly-specified region bound.
+        // Use explicitly-specified region bound, unless the bound is missing.
         let region_bound = if !lifetime.is_elided() {
-            self.lower_lifetime(lifetime, RegionInferReason::ObjectLifetimeDefault)
+            self.lower_lifetime(lifetime, RegionInferReason::ExplicitObjectLifetime)
         } else {
             self.compute_object_lifetime_bound(span, existential_predicates).unwrap_or_else(|| {
+                // Curiously, we prefer object lifetime default for `+ '_`...
                 if tcx.named_bound_var(lifetime.hir_id).is_some() {
-                    self.lower_lifetime(lifetime, RegionInferReason::ObjectLifetimeDefault)
+                    self.lower_lifetime(lifetime, RegionInferReason::ExplicitObjectLifetime)
                 } else {
-                    self.re_infer(
-                        span,
-                        if borrowed {
-                            RegionInferReason::ObjectLifetimeDefault
+                    let reason =
+                        if let hir::LifetimeName::ImplicitObjectLifetimeDefault = lifetime.res {
+                            if let hir::Node::Ty(hir::Ty {
+                                kind: hir::TyKind::Ref(parent_lifetime, _),
+                                ..
+                            }) = tcx.parent_hir_node(hir_id)
+                                && tcx.named_bound_var(parent_lifetime.hir_id).is_none()
+                            {
+                                // Parent lifetime must have failed to resolve. Don't emit a redundant error.
+                                RegionInferReason::ExplicitObjectLifetime
+                            } else {
+                                RegionInferReason::ObjectLifetimeDefault
+                            }
                         } else {
-                            RegionInferReason::BorrowedObjectLifetimeDefault
-                        },
-                    )
+                            RegionInferReason::ExplicitObjectLifetime
+                        };
+                    self.re_infer(span, reason)
                 }
             })
         };

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -798,7 +798,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 // to be object-safe.
                 // We manually call `register_wf_obligation` in the success path
                 // below.
-                let ty = self.lowerer().lower_ty_in_path(qself);
+                let ty = self.lowerer().lower_ty(qself);
                 (LoweredTy::from_raw(self, span, ty), qself, segment)
             }
             QPath::LangItem(..) => {

tests/ui/dyn-keyword/suggest-dyn-on-bare-trait-in-pat.rs

@@ -0,0 +1,14 @@
+//@ edition: 2021
+
+trait Trait {}
+
+impl dyn Trait {
+    const CONST: () = ();
+}
+
+fn main() {
+    match () {
+        Trait::CONST => {}
+        //~^ ERROR trait objects must include the `dyn` keyword
+    }
+}

tests/ui/dyn-keyword/suggest-dyn-on-bare-trait-in-pat.stderr

@@ -0,0 +1,14 @@
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/suggest-dyn-on-bare-trait-in-pat.rs:11:9
+   |
+LL |         Trait::CONST => {}
+   |         ^^^^^
+   |
+help: add `dyn` keyword before this trait
+   |
+LL |         <dyn Trait>::CONST => {}
+   |         ++++      +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0782`.