Record binder for bare trait object in LifetimeCollectVisitor

compiler/rustc_ast_lowering/src/lifetime_collector.rs

@@ -1,7 +1,7 @@
 use super::ResolverAstLoweringExt;
 use rustc_ast::visit::{self, BoundKind, LifetimeCtxt, Visitor};
 use rustc_ast::{GenericBounds, Lifetime, NodeId, PathSegment, PolyTraitRef, Ty, TyKind};
-use rustc_hir::def::LifetimeRes;
+use rustc_hir::def::{DefKind, LifetimeRes, Res};
 use rustc_middle::span_bug;
 use rustc_middle::ty::ResolverAstLowering;
 use rustc_span::symbol::{kw, Ident};
@@ -77,7 +77,20 @@ impl<'ast> Visitor<'ast> for LifetimeCollectVisitor<'ast> {
     }
 
     fn visit_ty(&mut self, t: &'ast Ty) {
-        match t.kind {
+        match &t.kind {
+            TyKind::Path(None, _) => {
+                // We can sometimes encounter bare trait objects
+                // which are represented in AST as paths.
+                if let Some(partial_res) = self.resolver.get_partial_res(t.id)
+                    && let Some(Res::Def(DefKind::Trait | DefKind::TraitAlias, _)) = partial_res.full_res()
+                {
+                    self.current_binders.push(t.id);
+                    visit::walk_ty(self, t);
+                    self.current_binders.pop();
+                } else {
+                    visit::walk_ty(self, t);
+                }
+            }
             TyKind::BareFn(_) => {
                 self.current_binders.push(t.id);
                 visit::walk_ty(self, t);

tests/ui/impl-trait/fresh-lifetime-from-bare-trait-obj-114664.rs

@@ -0,0 +1,22 @@
+// edition:2015
+// check-pass
+// issue: 114664
+
+fn ice() -> impl AsRef<Fn(&())> {
+    //~^ WARN trait objects without an explicit `dyn` are deprecated
+    //~| WARN trait objects without an explicit `dyn` are deprecated
+    //~| WARN trait objects without an explicit `dyn` are deprecated
+    //~| WARN this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+    //~| WARN this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+    //~| WARN this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+    Foo
+}
+
+struct Foo;
+impl AsRef<dyn Fn(&())> for Foo {
+    fn as_ref(&self) -> &(dyn for<'a> Fn(&'a ()) + 'static) {
+        todo!()
+    }
+}
+
+pub fn main() {}

tests/ui/impl-trait/fresh-lifetime-from-bare-trait-obj-114664.stderr

@@ -0,0 +1,42 @@
+warning: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/fresh-lifetime-from-bare-trait-obj-114664.rs:5:24
+   |
+LL | fn ice() -> impl AsRef<Fn(&())> {
+   |                        ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
+   = note: `#[warn(bare_trait_objects)]` on by default
+help: use `dyn`
+   |
+LL | fn ice() -> impl AsRef<dyn Fn(&())> {
+   |                        +++
+
+warning: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/fresh-lifetime-from-bare-trait-obj-114664.rs:5:24
+   |
+LL | fn ice() -> impl AsRef<Fn(&())> {
+   |                        ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
+help: use `dyn`
+   |
+LL | fn ice() -> impl AsRef<dyn Fn(&())> {
+   |                        +++
+
+warning: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/fresh-lifetime-from-bare-trait-obj-114664.rs:5:24
+   |
+LL | fn ice() -> impl AsRef<Fn(&())> {
+   |                        ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
+help: use `dyn`
+   |
+LL | fn ice() -> impl AsRef<dyn Fn(&())> {
+   |                        +++
+
+warning: 3 warnings emitted
+