Check elaborated projections from dyn don't mention unconstrained late bound lifetimes

Author: compiler-errors

compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs

@@ -174,6 +174,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                             // Include projections defined on supertraits.
                             projection_bounds.push((pred, span));
                         }
+
+                        self.check_elaborated_projection_mentions_input_lifetimes(pred, span);
                     }
                     _ => (),
                 }
@@ -360,6 +362,54 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         Ty::new_dynamic(tcx, existential_predicates, region_bound, representation)
     }
+
+    /// Check that elaborating the principal of a trait ref doesn't lead to projections
+    /// that are unconstrained. This can happen because an otherwise unconstrained
+    /// *type variable* can be substituted with a type that has late-bound regions. See
+    /// `elaborated-predicates-unconstrained-late-bound.rs` for a test.
+    fn check_elaborated_projection_mentions_input_lifetimes(
+        &self,
+        pred: ty::PolyProjectionPredicate<'tcx>,
+        span: Span,
+    ) {
+        let tcx = self.tcx();
+
+        // Find any late-bound regions declared in `ty` that are not
+        // declared in the trait-ref or assoc_item. These are not well-formed.
+        //
+        // Example:
+        //
+        //     for<'a> <T as Iterator>::Item = &'a str // <-- 'a is bad
+        //     for<'a> <T as FnMut<(&'a u32,)>>::Output = &'a str // <-- 'a is ok
+        let late_bound_in_projection_term =
+            tcx.collect_constrained_late_bound_regions(pred.map_bound(|pred| pred.projection_term));
+        let late_bound_in_term =
+            tcx.collect_referenced_late_bound_regions(pred.map_bound(|pred| pred.term));
+        debug!(?late_bound_in_projection_term);
+        debug!(?late_bound_in_term);
+
+        // FIXME: point at the type params that don't have appropriate lifetimes:
+        // struct S1<F: for<'a> Fn(&i32, &i32) -> &'a i32>(F);
+        //                         ----  ----     ^^^^^^^
+        // NOTE(associated_const_equality): This error should be impossible to trigger
+        //                                  with associated const equality constraints.
+        self.validate_late_bound_regions(
+            late_bound_in_projection_term,
+            late_bound_in_term,
+            |br_name| {
+                let item_name = tcx.item_name(pred.projection_def_id());
+                struct_span_code_err!(
+                    self.dcx(),
+                    span,
+                    E0582,
+                    "binding for associated type `{}` references {}, \
+                             which does not appear in the trait input types",
+                    item_name,
+                    br_name
+                )
+            },
+        );
+    }
 }
 
 fn replace_dummy_self_with_error<'tcx, T: TypeFoldable<TyCtxt<'tcx>>>(

tests/ui/traits/object/elaborated-predicates-unconstrained-late-bound.rs

@@ -0,0 +1,24 @@
+// Make sure that when elaborating the principal of a dyn trait for projection predicates
+//  we don't end up in a situation where we have an unconstrained late-bound lifetime in
+// the output of a projection.
+
+// Fix for <https://github.com/rust-lang/rust/issues/130347>.
+
+trait A<T>: B<T = T> {}
+
+trait B {
+    type T;
+}
+
+struct Erase<T: ?Sized + B>(T::T);
+
+fn main() {
+    let x = {
+        let x = String::from("hello");
+
+        Erase::<dyn for<'a> A<&'a _>>(x.as_str())
+        //~^ ERROR binding for associated type `T` references lifetime `'a`, which does not appear in the trait input types
+    };
+
+    dbg!(x.0);
+}

tests/ui/traits/object/elaborated-predicates-unconstrained-late-bound.stderr

@@ -0,0 +1,9 @@
+error[E0582]: binding for associated type `T` references lifetime `'a`, which does not appear in the trait input types
+  --> $DIR/elaborated-predicates-unconstrained-late-bound.rs:19:21
+   |
+LL |         Erase::<dyn for<'a> A<&'a _>>(x.as_str())
+   |                     ^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0582`.

tests/ui/traits/object/pretty.rs

@@ -13,7 +13,7 @@ trait SuperGeneric<'a> {
 }
 trait AnyGeneric<'a>: SuperGeneric<'a> {}
 trait FixedGeneric1<'a>: SuperGeneric<'a, Assoc2 = &'a u8> {}
-trait FixedGeneric2<'a>: Super<Assoc = &'a u8> {}
+// trait FixedGeneric2<'a>: Super<Assoc = &'a u8> {} // Unsound!
 trait FixedHrtb: for<'a> SuperGeneric<'a, Assoc2 = &'a u8> {}
 trait AnyDifferentBinders: for<'a> SuperGeneric<'a, Assoc2 = &'a u8> + Super {}
 trait FixedDifferentBinders: for<'a> SuperGeneric<'a, Assoc2 = &'a u8> + Super<Assoc = u8> {}
@@ -32,7 +32,7 @@ fn dyn_fixed_static(x: &dyn FixedStatic) { x } //~ERROR mismatched types
 fn dyn_super_generic(x: &dyn for<'a> SuperGeneric<'a, Assoc2 = &'a u8>) { x } //~ERROR mismatched types
 fn dyn_any_generic(x: &dyn for<'a> AnyGeneric<'a, Assoc2 = &'a u8>) { x } //~ERROR mismatched types
 fn dyn_fixed_generic1(x: &dyn for<'a> FixedGeneric1<'a>) { x } //~ERROR mismatched types
-fn dyn_fixed_generic2(x: &dyn for<'a> FixedGeneric2<'a>) { x } //~ERROR mismatched types
+// fn dyn_fixed_generic2(x: &dyn for<'a> FixedGeneric2<'a>) { x } // Unsound!
 fn dyn_fixed_generic_multi(x: &dyn for<'a> FixedGeneric1<'a, Assoc2 = &u8>) { x } //~ERROR mismatched types
 fn dyn_fixed_hrtb(x: &dyn FixedHrtb) { x } //~ERROR mismatched types
 fn dyn_any_different_binders(x: &dyn AnyDifferentBinders<Assoc = u8>) { x } //~ERROR mismatched types

tests/ui/traits/object/pretty.stderr

@@ -106,17 +106,6 @@ LL | fn dyn_fixed_generic1(x: &dyn for<'a> FixedGeneric1<'a>) { x }
    = note: expected unit type `()`
               found reference `&dyn for<'a> FixedGeneric1<'a>`
 
-error[E0308]: mismatched types
-  --> $DIR/pretty.rs:35:60
-   |
-LL | fn dyn_fixed_generic2(x: &dyn for<'a> FixedGeneric2<'a>) { x }
-   |                                                         -  ^ expected `()`, found `&dyn FixedGeneric2<'a>`
-   |                                                         |
-   |                                                         help: try adding a return type: `-> &dyn for<'a> FixedGeneric2<'a>`
-   |
-   = note: expected unit type `()`
-              found reference `&dyn for<'a> FixedGeneric2<'a>`
-
 error[E0308]: mismatched types
   --> $DIR/pretty.rs:36:79
    |
@@ -172,6 +161,6 @@ LL | fn dyn_has_gat(x: &dyn HasGat<u8, Assoc<bool> = ()>) { x }
    = note: expected unit type `()`
               found reference `&dyn HasGat<u8, Assoc<bool> = ()>`
 
-error: aborting due to 15 previous errors; 1 warning emitted
+error: aborting due to 14 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0308`.