Enforce non-lifetime-binders in supertrait preds are not object safe

compiler/rustc_middle/src/traits/mod.rs

@@ -897,6 +897,9 @@ pub enum ObjectSafetyViolation {
     /// (e.g., `trait Foo : Bar<Self>`).
     SupertraitSelf(SmallVec<[Span; 1]>),
 
+    // Supertrait has a non-lifetime `for<T>` binder.
+    SupertraitNonLifetimeBinder(SmallVec<[Span; 1]>),
+
     /// Method has something illegal.
     Method(Symbol, MethodViolationCode, Span),
 
@@ -919,6 +922,9 @@ impl ObjectSafetyViolation {
                         .into()
                 }
             }
+            ObjectSafetyViolation::SupertraitNonLifetimeBinder(_) => {
+                format!("where clause cannot reference non-lifetime `for<...>` variables").into()
+            }
             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod(_), _) => {
                 format!("associated function `{}` has no `self` parameter", name).into()
             }
@@ -969,7 +975,9 @@ impl ObjectSafetyViolation {
 
     pub fn solution(&self, err: &mut Diagnostic) {
         match self {
-            ObjectSafetyViolation::SizedSelf(_) | ObjectSafetyViolation::SupertraitSelf(_) => {}
+            ObjectSafetyViolation::SizedSelf(_)
+            | ObjectSafetyViolation::SupertraitSelf(_)
+            | ObjectSafetyViolation::SupertraitNonLifetimeBinder(..) => {}
             ObjectSafetyViolation::Method(
                 name,
                 MethodViolationCode::StaticMethod(Some((add_self_sugg, make_sized_sugg))),
@@ -1023,7 +1031,8 @@ impl ObjectSafetyViolation {
         // diagnostics use a `note` instead of a `span_label`.
         match self {
             ObjectSafetyViolation::SupertraitSelf(spans)
-            | ObjectSafetyViolation::SizedSelf(spans) => spans.clone(),
+            | ObjectSafetyViolation::SizedSelf(spans)
+            | ObjectSafetyViolation::SupertraitNonLifetimeBinder(spans) => spans.clone(),
             ObjectSafetyViolation::AssocConst(_, span)
             | ObjectSafetyViolation::GAT(_, span)
             | ObjectSafetyViolation::Method(_, _, span)

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -17,10 +17,10 @@ use rustc_errors::{DelayDm, FatalError, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::subst::{GenericArg, InternalSubsts};
-use rustc_middle::ty::ToPredicate;
 use rustc_middle::ty::{
     self, EarlyBinder, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor,
 };
+use rustc_middle::ty::{ToPredicate, TypeVisitableExt};
 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
 use rustc_span::symbol::Symbol;
 use rustc_span::Span;
@@ -139,6 +139,10 @@ fn object_safety_violations_for_trait(
     if !spans.is_empty() {
         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
     }
+    let spans = super_predicates_have_non_lifetime_binders(tcx, trait_def_id);
+    if !spans.is_empty() {
+        violations.push(ObjectSafetyViolation::SupertraitNonLifetimeBinder(spans));
+    }
 
     violations.extend(
         tcx.associated_items(trait_def_id)
@@ -348,6 +352,21 @@ fn predicate_references_self<'tcx>(
     }
 }
 
+fn super_predicates_have_non_lifetime_binders(
+    tcx: TyCtxt<'_>,
+    trait_def_id: DefId,
+) -> SmallVec<[Span; 1]> {
+    // If non_lifetime_binders is disabled, then exit early
+    if !tcx.features().non_lifetime_binders {
+        return SmallVec::new();
+    }
+    tcx.super_predicates_of(trait_def_id)
+        .predicates
+        .iter()
+        .filter_map(|(pred, span)| pred.has_non_region_late_bound().then_some(*span))
+        .collect()
+}
+
 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
     generics_require_sized_self(tcx, trait_def_id)
 }

tests/ui/traits/non_lifetime_binders/supertrait-object-safety.rs

@@ -0,0 +1,24 @@
+#![feature(non_lifetime_binders)]
+//~^ WARN the feature `non_lifetime_binders` is incomplete
+
+trait Foo: for<T> Bar<T> {}
+
+trait Bar<T: ?Sized> {
+    fn method(&self) {}
+}
+
+fn needs_bar(x: &(impl Bar<i32> + ?Sized)) {
+    x.method();
+}
+
+impl Foo for () {}
+
+impl<T: ?Sized> Bar<T> for () {}
+
+fn main() {
+    let x: &dyn Foo = &();
+    //~^ ERROR the trait `Foo` cannot be made into an object
+    //~| ERROR the trait `Foo` cannot be made into an object
+    needs_bar(x);
+    //~^ ERROR the trait `Foo` cannot be made into an object
+}

tests/ui/traits/non_lifetime_binders/supertrait-object-safety.stderr

@@ -0,0 +1,56 @@
+warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/supertrait-object-safety.rs:1:12
+   |
+LL | #![feature(non_lifetime_binders)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0038]: the trait `Foo` cannot be made into an object
+  --> $DIR/supertrait-object-safety.rs:19:23
+   |
+LL |     let x: &dyn Foo = &();
+   |                       ^^^ `Foo` cannot be made into an object
+   |
+note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+  --> $DIR/supertrait-object-safety.rs:4:12
+   |
+LL | trait Foo: for<T> Bar<T> {}
+   |       ---  ^^^^^^^^^^^^^ ...because where clause cannot reference non-lifetime `for<...>` variables
+   |       |
+   |       this trait cannot be made into an object...
+   = note: required for `&()` to implement `CoerceUnsized<&dyn Foo>`
+   = note: required by cast to type `&dyn Foo`
+
+error[E0038]: the trait `Foo` cannot be made into an object
+  --> $DIR/supertrait-object-safety.rs:19:12
+   |
+LL |     let x: &dyn Foo = &();
+   |            ^^^^^^^^ `Foo` cannot be made into an object
+   |
+note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+  --> $DIR/supertrait-object-safety.rs:4:12
+   |
+LL | trait Foo: for<T> Bar<T> {}
+   |       ---  ^^^^^^^^^^^^^ ...because where clause cannot reference non-lifetime `for<...>` variables
+   |       |
+   |       this trait cannot be made into an object...
+
+error[E0038]: the trait `Foo` cannot be made into an object
+  --> $DIR/supertrait-object-safety.rs:22:5
+   |
+LL |     needs_bar(x);
+   |     ^^^^^^^^^ `Foo` cannot be made into an object
+   |
+note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+  --> $DIR/supertrait-object-safety.rs:4:12
+   |
+LL | trait Foo: for<T> Bar<T> {}
+   |       ---  ^^^^^^^^^^^^^ ...because where clause cannot reference non-lifetime `for<...>` variables
+   |       |
+   |       this trait cannot be made into an object...
+
+error: aborting due to 3 previous errors; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0038`.