Ensure that associated types for trait objects satisfy their bounds

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -9,7 +9,7 @@
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir::lang_items::LangItem;
 use rustc_index::bit_set::GrowableBitSet;
-use rustc_infer::infer::InferOk;
+use rustc_infer::infer::{self, InferOk};
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, Subst, SubstsRef};
 use rustc_middle::ty::{self, Ty};
 use rustc_middle::ty::{ToPolyTraitRef, ToPredicate, WithConstness};
@@ -342,21 +342,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> ImplSourceObjectData<'tcx, PredicateObligation<'tcx>> {
         debug!("confirm_object_candidate({:?})", obligation);
 
-        // FIXME(nmatsakis) skipping binder here seems wrong -- we should
-        // probably flatten the binder from the obligation and the binder
-        // from the object. Have to try to make a broken test case that
-        // results.
-        let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
-        let poly_trait_ref = match self_ty.kind() {
-            ty::Dynamic(data, ..) => data
-                .principal()
-                .unwrap_or_else(|| {
-                    span_bug!(obligation.cause.span, "object candidate with no principal")
-                })
-                .with_self_ty(self.tcx(), self_ty),
+        let self_ty = self.infcx.replace_bound_vars_with_placeholders(&obligation.self_ty());
+        let data = match self_ty.kind() {
+            ty::Dynamic(data, ..) => data,
             _ => span_bug!(obligation.cause.span, "object candidate with non-object"),
         };
 
+        let poly_trait_ref = data
+            .principal()
+            .unwrap_or_else(|| {
+                span_bug!(obligation.cause.span, "object candidate with no principal")
+            })
+            .with_self_ty(self.tcx(), self_ty);
+
         let mut upcast_trait_ref = None;
         let mut nested = vec![];
         let vtable_base;
@@ -388,6 +386,46 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             vtable_base = nonmatching.map(|t| super::util::count_own_vtable_entries(tcx, t)).sum();
         }
 
+        for bound in data.skip_binder() {
+            match bound {
+                ty::ExistentialPredicate::Projection(projection) => {
+                    // This maybe belongs in wf, but that can't (doesn't) handle
+                    // higher-ranked things.
+                    // Prevent, e.g., `dyn Iterator<Item = str>`.
+                    // FIXME(generic_associated_types): We need some way to
+                    // ensure that for `dyn for<'a> X<Item<'a> = &'a ()>` the
+                    // bound holds for all `'a`.
+                    let (infer_projection, _) = self.infcx.replace_bound_vars_with_fresh_vars(
+                        obligation.cause.span,
+                        infer::HigherRankedType,
+                        &ty::Binder::bind(projection),
+                    );
+                    let substs: Vec<_> =
+                        iter::once(self_ty.into()).chain(infer_projection.substs).collect();
+                    let bounds =
+                        self.tcx().item_bounds(projection.item_def_id).iter().map(|bound| {
+                            // In the example above, `bound` is `<Self as Iterator>::Item: Sized`
+                            // `subst_bound` is `str: Sized`.
+                            let subst_bound = util::subst_assoc_item_bound(
+                                self.tcx(),
+                                bound,
+                                infer_projection.ty,
+                                &substs,
+                            );
+                            Obligation::new(
+                                obligation.cause.clone(),
+                                obligation.param_env.clone(),
+                                subst_bound,
+                            )
+                        });
+                    debug!("confirm_object_candidate: adding bounds: {:?}", bounds);
+                    nested.extend(bounds);
+                }
+                ty::ExistentialPredicate::Trait(_) | ty::ExistentialPredicate::AutoTrait(_) => {}
+            }
+        }
+
+        debug!("confirm_object_candidate: nested: {:?}", nested);
         ImplSourceObjectData { upcast_trait_ref: upcast_trait_ref.unwrap(), vtable_base, nested }
     }
 

src/test/ui/issues/issue-41139.rs

@@ -3,6 +3,8 @@ trait Trait {}
 fn get_function<'a>() -> &'a dyn Fn() -> dyn Trait { panic!("") }
 
 fn main() {
+    // This isn't great. The issue here is that `dyn Trait` is not sized, so
+    // `dyn Fn() -> dyn Trait` is not well-formed.
     let t : &dyn Trait = &get_function()();
-    //~^ ERROR cannot move a value of type dyn Trait
+    //~^ ERROR expected function, found `&dyn std::ops::Fn() -> (dyn Trait + 'static)`
 }

src/test/ui/issues/issue-41139.stderr

@@ -1,9 +1,14 @@
-error[E0161]: cannot move a value of type dyn Trait: the size of dyn Trait cannot be statically determined
-  --> $DIR/issue-41139.rs:6:27
+error[E0618]: expected function, found `&dyn std::ops::Fn() -> (dyn Trait + 'static)`
+  --> $DIR/issue-41139.rs:8:27
    |
+LL | fn get_function<'a>() -> &'a dyn Fn() -> dyn Trait { panic!("") }
+   | ----------------------------------------------------------------- `get_function` defined here returns `&dyn std::ops::Fn() -> (dyn Trait + 'static)`
+...
 LL |     let t : &dyn Trait = &get_function()();
-   |                           ^^^^^^^^^^^^^^^^
+   |                           ^^^^^^^^^^^^^^--
+   |                           |
+   |                           call expression requires function
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0161`.
+For more information about this error, try `rustc --explain E0618`.

src/test/ui/traits/check-trait-object-bounds-1.rs

@@ -0,0 +1,14 @@
+// Check that we validate associated type bounds for trait objects
+
+trait X {
+    type Y: Clone;
+}
+
+fn f<T: X + ?Sized>() {
+    None::<T::Y>.clone();
+}
+
+fn main() {
+    f::<dyn X<Y = str>>();
+    //~^ ERROR the trait bound `str: std::clone::Clone` is not satisfied
+}

src/test/ui/traits/check-trait-object-bounds-1.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the trait bound `str: std::clone::Clone` is not satisfied
+  --> $DIR/check-trait-object-bounds-1.rs:12:5
+   |
+LL | fn f<T: X + ?Sized>() {
+   |         - required by this bound in `f`
+...
+LL |     f::<dyn X<Y = str>>();
+   |     ^^^^^^^^^^^^^^^^^^^ the trait `std::clone::Clone` is not implemented for `str`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/traits/check-trait-object-bounds-2-ok.rs

@@ -0,0 +1,15 @@
+// Make sure that we're handling bound lifetimes correctly when validating trait
+// bounds.
+// run-pass
+
+trait X<'a> {
+    type F: FnOnce(&i32) -> &'a i32;
+}
+
+fn f<T: for<'r> X<'r> + ?Sized>() {
+    None::<T::F>.map(|f| f(&0));
+}
+
+fn main() {
+    f::<dyn for<'x> X<'x, F = fn(&i32) -> &'x i32>>();
+}

src/test/ui/traits/check-trait-object-bounds-2.rs

@@ -0,0 +1,15 @@
+// Check that we validate associated type bounds for trait objects when they
+// have bound lifetimes
+
+trait X<'a> {
+    type F: FnOnce(&i32) -> &'a i32;
+}
+
+fn f<T: for<'r> X<'r> + ?Sized>() {
+    None::<T::F>.map(|f| f(&0));
+}
+
+fn main() {
+    f::<dyn for<'x> X<'x, F = i32>>();
+    //~^ expected a `std::ops::FnOnce<(&i32,)>` closure, found `i32`
+}

src/test/ui/traits/check-trait-object-bounds-2.stderr

@@ -0,0 +1,14 @@
+error[E0277]: expected a `std::ops::FnOnce<(&i32,)>` closure, found `i32`
+  --> $DIR/check-trait-object-bounds-2.rs:13:5
+   |
+LL | fn f<T: for<'r> X<'r> + ?Sized>() {
+   |         ------------- required by this bound in `f`
+...
+LL |     f::<dyn for<'x> X<'x, F = i32>>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an `FnOnce<(&i32,)>` closure, found `i32`
+   |
+   = help: the trait `for<'r> std::ops::FnOnce<(&'r i32,)>` is not implemented for `i32`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/traits/check-trait-object-bounds-3.rs

@@ -0,0 +1,21 @@
+// Check that we validate associated type bounds for trait objects
+
+trait X<'a> {
+    type Y: Into<&'static str> + From<&'a str>;
+}
+
+fn f<'a, T: X<'a> + ?Sized>(s: &'a str) -> &'static str {
+    T::Y::from(s).into()
+}
+
+pub fn main() {
+    let z;
+    {
+        let s = String::from("abcdef");
+        z = f::<dyn X<Y = &str>>(&s);
+        //~^ ERROR `s` does not live long enough
+    }
+
+    println!("{}", z)
+}
+

src/test/ui/traits/check-trait-object-bounds-3.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the trait bound `str: std::clone::Clone` is not satisfied
+  --> $DIR/check-trait-object-bounds-3.rs:12:5
+   |
+LL | fn f<T: X + ?Sized>() {
+   |         - required by this bound in `f`
+...
+LL |     f::<dyn X<Y = str>>();
+   |     ^^^^^^^^^^^^^^^^^^^ the trait `std::clone::Clone` is not implemented for `str`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.