Rollup merge of rust-lang#51104 - zackmdavis:dynamo, r=nikomatsakis

add `dyn ` to display of dynamic (trait) types

~~I'm not sure we want the `dyn` in the E0277 "trait bound [...] is not satisfied" messages ("bound" sounds like a different thing in contrast to the names of specific trait-object types like `Box<dyn Trait>`), but I'm finding the code I would need to change that hard to follow—the [display object seems to](https://github.com/rust-lang/rust/blob/f0805a4421449bd6fe3096d63820fbebe2bfcd1d/src/librustc/traits/error_reporting.rs#L600) be a [`Predicate::Trait`](https://github.com/rust-lang/rust/blob/f0805a4421449bd6fe3096d63820fbebe2bfcd1d/src/librustc/ty/mod.rs#L962) variant, whose [`Display` implementation](https://github.com/rust-lang/rust/blob/f0805a4421449bd6fe3096d63820fbebe2bfcd1d/src/librustc/util/ppaux.rs#L1309) calls `.print` on its `PolyTraitPredicate` member, [which is a type alias](https://github.com/rust-lang/rust/blob/f0805a4421449bd6fe3096d63820fbebe2bfcd1d/src/librustc/ty/mod.rs#L1112) for `ty::Binder<TraitPredicate<'tcx>>`, whose [`Display` implementation](https://github.com/rust-lang/rust/blob/f0805a4421449bd6fe3096d63820fbebe2bfcd1d/src/librustc/util/ppaux.rs#L975-L985) ... _&c._— so maybe it's time to pull-request this and see what reviewers think.~~

 Resolves rust-lang#49277 (?).

r? @nikomatsakis

Author: pietroalbini

src/librustc/util/ppaux.rs

@@ -1062,10 +1062,14 @@ define_print! {
                 TyParam(ref param_ty) => write!(f, "{}", param_ty),
                 TyAdt(def, substs) => cx.parameterized(f, substs, def.did, &[]),
                 TyDynamic(data, r) => {
-                    data.print(f, cx)?;
                     let r = r.print_to_string(cx);
                     if !r.is_empty() {
-                        write!(f, " + {}", r)
+                        write!(f, "(")?;
+                    }
+                    write!(f, "dyn ")?;
+                    data.print(f, cx)?;
+                    if !r.is_empty() {
+                        write!(f, " + {})", r)
                     } else {
                         Ok(())
                     }

src/test/compile-fail/cross-borrow-trait.rs

@@ -18,6 +18,6 @@ impl Trait for Foo {}
 pub fn main() {
     let x: Box<Trait> = Box::new(Foo);
     let _y: &Trait = x; //~ ERROR E0308
-                        //~| expected type `&Trait`
-                        //~| found type `std::boxed::Box<Trait>`
+                        //~| expected type `&dyn Trait`
+                        //~| found type `std::boxed::Box<dyn Trait>`
 }

src/test/compile-fail/destructure-trait-ref.rs

@@ -33,23 +33,23 @@ fn main() {
     let &&x = &&(&1isize as &T);
 
     // n == m
-    let &x = &1isize as &T;      //~ ERROR type `&T` cannot be dereferenced
-    let &&x = &(&1isize as &T);  //~ ERROR type `&T` cannot be dereferenced
-    let box x = box 1isize as Box<T>; //~ ERROR type `std::boxed::Box<T>` cannot be dereferenced
+    let &x = &1isize as &T;      //~ ERROR type `&dyn T` cannot be dereferenced
+    let &&x = &(&1isize as &T);  //~ ERROR type `&dyn T` cannot be dereferenced
+    let box x = box 1isize as Box<T>; //~ ERROR type `std::boxed::Box<dyn T>` cannot be dereferenced
 
     // n > m
     let &&x = &1isize as &T;
     //~^ ERROR mismatched types
-    //~| expected type `T`
+    //~| expected type `dyn T`
     //~| found type `&_`
     //~| expected trait T, found reference
     let &&&x = &(&1isize as &T);
     //~^ ERROR mismatched types
-    //~| expected type `T`
+    //~| expected type `dyn T`
     //~| found type `&_`
     //~| expected trait T, found reference
     let box box x = box 1isize as Box<T>;
     //~^ ERROR mismatched types
-    //~| expected type `T`
+    //~| expected type `dyn T`
     //~| found type `std::boxed::Box<_>`
 }

src/test/compile-fail/dst-bad-assign-3.rs

@@ -42,7 +42,7 @@ pub fn main() {
     let z: Box<ToBar> = Box::new(Bar1 {f: 36});
     f5.2 = Bar1 {f: 36};
     //~^ ERROR mismatched types
-    //~| expected type `ToBar`
+    //~| expected type `dyn ToBar`
     //~| found type `Bar1`
     //~| expected trait ToBar, found struct `Bar1`
     //~| ERROR the size for value values of type

src/test/compile-fail/dst-bad-assign.rs

@@ -44,7 +44,7 @@ pub fn main() {
     let z: Box<ToBar> = Box::new(Bar1 {f: 36});
     f5.ptr = Bar1 {f: 36};
     //~^ ERROR mismatched types
-    //~| expected type `ToBar`
+    //~| expected type `dyn ToBar`
     //~| found type `Bar1`
     //~| expected trait ToBar, found struct `Bar1`
     //~| ERROR the size for value values of type

src/test/compile-fail/fn-trait-formatting.rs

@@ -16,15 +16,15 @@ fn main() {
     let _: () = (box |_: isize| {}) as Box<FnOnce(isize)>;
     //~^ ERROR mismatched types
     //~| expected type `()`
-    //~| found type `std::boxed::Box<std::ops::FnOnce(isize)>`
+    //~| found type `std::boxed::Box<dyn std::ops::FnOnce(isize)>`
     let _: () = (box |_: isize, isize| {}) as Box<Fn(isize, isize)>;
     //~^ ERROR mismatched types
     //~| expected type `()`
-    //~| found type `std::boxed::Box<std::ops::Fn(isize, isize)>`
+    //~| found type `std::boxed::Box<dyn std::ops::Fn(isize, isize)>`
     let _: () = (box || -> isize { unimplemented!() }) as Box<FnMut() -> isize>;
     //~^ ERROR mismatched types
     //~| expected type `()`
-    //~| found type `std::boxed::Box<std::ops::FnMut() -> isize>`
+    //~| found type `std::boxed::Box<dyn std::ops::FnMut() -> isize>`
 
     needs_fn(1);
     //~^ ERROR : std::ops::Fn<(isize,)>`

src/test/compile-fail/issue-13033.rs

@@ -17,8 +17,8 @@ struct Baz;
 impl Foo for Baz {
     fn bar(&mut self, other: &Foo) {}
     //~^ ERROR method `bar` has an incompatible type for trait
-    //~| expected type `fn(&mut Baz, &mut Foo)`
-    //~| found type `fn(&mut Baz, &Foo)`
+    //~| expected type `fn(&mut Baz, &mut dyn Foo)`
+    //~| found type `fn(&mut Baz, &dyn Foo)`
 }
 
 fn main() {}

src/test/compile-fail/issue-20939.rs

@@ -11,6 +11,6 @@
 trait Foo {}
 
 impl<'a> Foo for Foo+'a {}
-//~^ ERROR the object type `Foo + 'a` automatically implements the trait `Foo`
+//~^ ERROR the object type `(dyn Foo + 'a)` automatically implements the trait `Foo`
 
 fn main() {}

src/test/compile-fail/issue-32963.rs

@@ -17,5 +17,5 @@ fn size_of_copy<T: Copy+?Sized>() -> usize { mem::size_of::<T>() }
 fn main() {
     size_of_copy::<Misc+Copy>();
     //~^ ERROR only auto traits can be used as additional traits in a trait object
-    //~| ERROR the trait bound `Misc: std::marker::Copy` is not satisfied
+    //~| ERROR the trait bound `dyn Misc: std::marker::Copy` is not satisfied
 }

src/test/compile-fail/issue-41139.rs

@@ -14,5 +14,5 @@ fn get_function<'a>() -> &'a Fn() -> Trait { panic!("") }
 
 fn main() {
     let t : &Trait = &get_function()();
-    //~^ ERROR cannot move a value of type Trait + 'static
+    //~^ ERROR cannot move a value of type (dyn Trait + 'static)
 }

src/test/compile-fail/issue-5153.rs

@@ -18,5 +18,5 @@ impl Foo for isize {
 
 fn main() {
     (&5isize as &Foo).foo();
-    //~^ ERROR: no method named `foo` found for type `&Foo` in the current scope
+    //~^ ERROR: no method named `foo` found for type `&dyn Foo` in the current scope
 }

src/test/compile-fail/kindck-send-object.rs

@@ -20,12 +20,12 @@ trait Message : Send { }
 
 fn object_ref_with_static_bound_not_ok() {
     assert_send::<&'static (Dummy+'static)>();
-    //~^ ERROR `Dummy + 'static` cannot be shared between threads safely [E0277]
+    //~^ ERROR `(dyn Dummy + 'static)` cannot be shared between threads safely [E0277]
 }
 
 fn box_object_with_no_bound_not_ok<'a>() {
     assert_send::<Box<Dummy>>();
-    //~^ ERROR `Dummy` cannot be sent between threads safely
+    //~^ ERROR `dyn Dummy` cannot be sent between threads safely
 }
 
 fn object_with_send_bound_ok() {

src/test/compile-fail/kindck-send-object1.rs

@@ -18,7 +18,7 @@ trait Dummy { }
 // careful with object types, who knows what they close over...
 fn test51<'a>() {
     assert_send::<&'a Dummy>();
-    //~^ ERROR `Dummy + 'a` cannot be shared between threads safely [E0277]
+    //~^ ERROR `(dyn Dummy + 'a)` cannot be shared between threads safely [E0277]
 }
 fn test52<'a>() {
     assert_send::<&'a (Dummy+Sync)>();
@@ -37,7 +37,7 @@ fn test61() {
 // them not ok
 fn test_71<'a>() {
     assert_send::<Box<Dummy+'a>>();
-    //~^ ERROR `Dummy + 'a` cannot be sent between threads safely
+    //~^ ERROR `(dyn Dummy + 'a)` cannot be sent between threads safely
 }
 
 fn main() { }

src/test/compile-fail/kindck-send-object2.rs

@@ -15,12 +15,12 @@ trait Dummy { }
 
 fn test50() {
     assert_send::<&'static Dummy>();
-    //~^ ERROR `Dummy + 'static` cannot be shared between threads safely [E0277]
+    //~^ ERROR `(dyn Dummy + 'static)` cannot be shared between threads safely [E0277]
 }
 
 fn test53() {
     assert_send::<Box<Dummy>>();
-    //~^ ERROR `Dummy` cannot be sent between threads safely
+    //~^ ERROR `dyn Dummy` cannot be sent between threads safely
 }
 
 // ...unless they are properly bounded

src/test/compile-fail/map-types.rs

@@ -25,5 +25,5 @@ fn main() {
     let x: Box<HashMap<isize, isize>> = box HashMap::new();
     let x: Box<Map<isize, isize>> = x;
     let y: Box<Map<usize, isize>> = Box::new(x);
-    //~^ ERROR `std::boxed::Box<Map<isize, isize>>: Map<usize, isize>` is not satisfied
+    //~^ ERROR `std::boxed::Box<dyn Map<isize, isize>>: Map<usize, isize>` is not satisfied
 }

src/test/compile-fail/non-interger-atomic.rs

@@ -61,22 +61,22 @@ pub unsafe fn test_Foo_cxchg(p: &mut Foo, v: Foo) {
 
 pub unsafe fn test_Bar_load(p: &mut Bar, v: Bar) {
     intrinsics::atomic_load(p);
-    //~^ ERROR expected basic integer type, found `&std::ops::Fn()`
+    //~^ ERROR expected basic integer type, found `&dyn std::ops::Fn()`
 }
 
 pub unsafe fn test_Bar_store(p: &mut Bar, v: Bar) {
     intrinsics::atomic_store(p, v);
-    //~^ ERROR expected basic integer type, found `&std::ops::Fn()`
+    //~^ ERROR expected basic integer type, found `&dyn std::ops::Fn()`
 }
 
 pub unsafe fn test_Bar_xchg(p: &mut Bar, v: Bar) {
     intrinsics::atomic_xchg(p, v);
-    //~^ ERROR expected basic integer type, found `&std::ops::Fn()`
+    //~^ ERROR expected basic integer type, found `&dyn std::ops::Fn()`
 }
 
 pub unsafe fn test_Bar_cxchg(p: &mut Bar, v: Bar) {
     intrinsics::atomic_cxchg(p, v, v);
-    //~^ ERROR expected basic integer type, found `&std::ops::Fn()`
+    //~^ ERROR expected basic integer type, found `&dyn std::ops::Fn()`
 }
 
 pub unsafe fn test_Quux_load(p: &mut Quux, v: Quux) {

src/test/compile-fail/object-does-not-impl-trait.rs

@@ -14,5 +14,5 @@
 trait Foo {}
 fn take_foo<F:Foo>(f: F) {}
 fn take_object(f: Box<Foo>) { take_foo(f); }
-//~^ ERROR `std::boxed::Box<Foo>: Foo` is not satisfied
+//~^ ERROR `std::boxed::Box<dyn Foo>: Foo` is not satisfied
 fn main() {}

src/test/compile-fail/object-safety-by-value-self-use.rs

@@ -22,7 +22,7 @@ trait Baz {
 }
 
 fn use_bar(t: Box<Bar>) {
-    t.bar() //~ ERROR cannot move a value of type Bar
+    t.bar() //~ ERROR cannot move a value of type (dyn Bar + 'static)
 }
 
 fn main() { }

src/test/compile-fail/privacy/associated-item-privacy-type-binding.rs

@@ -19,19 +19,19 @@ mod priv_trait {
 
     pub macro mac1() {
         let _: Box<PubTr<AssocTy = u8>>;
-        //~^ ERROR type `priv_trait::PubTr<AssocTy=u8> + '<empty>` is private
-        //~| ERROR type `priv_trait::PubTr<AssocTy=u8> + '<empty>` is private
+        //~^ ERROR type `(dyn priv_trait::PubTr<AssocTy=u8> + '<empty>)` is private
+        //~| ERROR type `(dyn priv_trait::PubTr<AssocTy=u8> + '<empty>)` is private
         type InSignatureTy2 = Box<PubTr<AssocTy = u8>>;
-        //~^ ERROR type `priv_trait::PubTr<AssocTy=u8> + 'static` is private
+        //~^ ERROR type `(dyn priv_trait::PubTr<AssocTy=u8> + 'static)` is private
         trait InSignatureTr2: PubTr<AssocTy = u8> {}
         //~^ ERROR trait `priv_trait::PrivTr` is private
     }
     pub macro mac2() {
         let _: Box<PrivTr<AssocTy = u8>>;
-        //~^ ERROR type `priv_trait::PrivTr<AssocTy=u8> + '<empty>` is private
-        //~| ERROR type `priv_trait::PrivTr<AssocTy=u8> + '<empty>` is private
+        //~^ ERROR type `(dyn priv_trait::PrivTr<AssocTy=u8> + '<empty>)` is private
+        //~| ERROR type `(dyn priv_trait::PrivTr<AssocTy=u8> + '<empty>)` is private
         type InSignatureTy1 = Box<PrivTr<AssocTy = u8>>;
-        //~^ ERROR type `priv_trait::PrivTr<AssocTy=u8> + 'static` is private
+        //~^ ERROR type `(dyn priv_trait::PrivTr<AssocTy=u8> + 'static)` is private
         trait InSignatureTr1: PrivTr<AssocTy = u8> {}
         //~^ ERROR trait `priv_trait::PrivTr` is private
     }

src/test/compile-fail/private-inferred-type.rs

@@ -129,7 +129,7 @@ fn main() {
     m::leak_anon2(); //~ ERROR type `m::Priv` is private
     m::leak_anon3(); //~ ERROR type `m::Priv` is private
 
-    m::leak_dyn1(); //~ ERROR type `m::Trait + 'static` is private
+    m::leak_dyn1(); //~ ERROR type `(dyn m::Trait + 'static)` is private
     m::leak_dyn2(); //~ ERROR type `m::Priv` is private
     m::leak_dyn3(); //~ ERROR type `m::Priv` is private
 

src/test/compile-fail/trait-item-privacy.rs

@@ -110,9 +110,9 @@ fn check_assoc_const() {
     // A, B, C are resolved as inherent items, their traits don't need to be in scope
     C::A; //~ ERROR associated constant `A` is private
           //~^ ERROR the trait `assoc_const::C` cannot be made into an object
-          //~| ERROR the trait bound `assoc_const::C: assoc_const::A` is not satisfied
+          //~| ERROR the trait bound `dyn assoc_const::C: assoc_const::A` is not satisfied
     C::B; // ERROR the trait `assoc_const::C` cannot be made into an object
-          //~^ ERROR the trait bound `assoc_const::C: assoc_const::B` is not satisfied
+          //~^ ERROR the trait bound `dyn assoc_const::C: assoc_const::B` is not satisfied
     C::C; // OK
 }
 

src/test/compile-fail/traits-repeated-supertrait-ambig.rs

@@ -33,15 +33,15 @@ impl CompareTo<u64> for i64 {
 impl CompareToInts for i64 { }
 
 fn with_obj(c: &CompareToInts) -> bool {
-    c.same_as(22) //~ ERROR `CompareToInts: CompareTo<i32>` is not satisfied
+    c.same_as(22) //~ ERROR `dyn CompareToInts: CompareTo<i32>` is not satisfied
 }
 
 fn with_trait<C:CompareToInts>(c: &C) -> bool {
     c.same_as(22) //~ ERROR `C: CompareTo<i32>` is not satisfied
 }
 
 fn with_ufcs1<C:CompareToInts>(c: &C) -> bool {
-    CompareToInts::same_as(c, 22) //~ ERROR `CompareToInts: CompareTo<i32>` is not satisfied
+    CompareToInts::same_as(c, 22) //~ ERROR `dyn CompareToInts: CompareTo<i32>` is not satisfied
 }
 
 fn with_ufcs2<C:CompareToInts>(c: &C) -> bool {

src/test/compile-fail/trivial_casts.rs

@@ -59,25 +59,25 @@ pub fn main() {
 
     // unsize trait
     let x: &Bar = &Bar;
-    let _ = x as &Foo; //~ERROR trivial cast: `&Bar` as `&Foo`
-    let _ = x as *const Foo; //~ERROR trivial cast: `&Bar` as `*const Foo`
+    let _ = x as &Foo; //~ERROR trivial cast: `&Bar` as `&dyn Foo`
+    let _ = x as *const Foo; //~ERROR trivial cast: `&Bar` as `*const dyn Foo`
     let _: &Foo = x;
     let _: *const Foo = x;
 
     let x: &mut Bar = &mut Bar;
-    let _ = x as &mut Foo; //~ERROR trivial cast: `&mut Bar` as `&mut Foo`
-    let _ = x as *mut Foo; //~ERROR trivial cast: `&mut Bar` as `*mut Foo`
+    let _ = x as &mut Foo; //~ERROR trivial cast: `&mut Bar` as `&mut dyn Foo`
+    let _ = x as *mut Foo; //~ERROR trivial cast: `&mut Bar` as `*mut dyn Foo`
     let _: &mut Foo = x;
     let _: *mut Foo = x;
 
     let x: Box<Bar> = Box::new(Bar);
-    let _ = x as Box<Foo>; //~ERROR trivial cast: `std::boxed::Box<Bar>` as `std::boxed::Box<Foo>`
+    let _ = x as Box<Foo>; //~ERROR `std::boxed::Box<Bar>` as `std::boxed::Box<dyn Foo>`
     let x: Box<Bar> = Box::new(Bar);
     let _: Box<Foo> = x;
 
     // functions
     fn baz(_x: i32) {}
-    let _ = &baz as &Fn(i32); //~ERROR trivial cast: `&fn(i32) {main::baz}` as `&std::ops::Fn(i32)`
+    let _ = &baz as &Fn(i32); //~ERROR `&fn(i32) {main::baz}` as `&dyn std::ops::Fn(i32)`
     let _: &Fn(i32) = &baz;
     let x = |_x: i32| {};
     let _ = &x as &Fn(i32); //~ERROR trivial cast

src/test/compile-fail/type-mismatch-same-crate-name.rs

@@ -33,7 +33,7 @@ fn main() {
         //~^ ERROR mismatched types
         //~| Perhaps two different versions of crate `crate_a1`
         //~| expected trait `main::a::Bar`
-        //~| expected type `std::boxed::Box<main::a::Bar + 'static>`
-        //~| found type `std::boxed::Box<main::a::Bar>`
+        //~| expected type `std::boxed::Box<(dyn main::a::Bar + 'static)>`
+        //~| found type `std::boxed::Box<dyn main::a::Bar>`
     }
 }

src/test/compile-fail/type-parameter-defaults-referencing-Self-ppaux.rs

@@ -23,5 +23,5 @@ fn main() {
     let x: i32 = 5;
     let y = x as MyAdd<i32>;
     //~^ ERROR E0038
-    //~| ERROR cast to unsized type: `i32` as `MyAdd<i32>`
+    //~| ERROR cast to unsized type: `i32` as `dyn MyAdd<i32>`
 }

src/test/run-pass/issue-21058.rs

@@ -26,5 +26,5 @@ fn main() {
         std::intrinsics::type_name::<NT>(),
         // DST
         std::intrinsics::type_name::<DST>()
-    )}, ("[u8]", "str", "std::marker::Send", "NT", "DST"));
+    )}, ("[u8]", "str", "dyn std::marker::Send", "NT", "DST"));
 }

src/test/ui/anonymous-higher-ranked-lifetime.stderr

@@ -74,7 +74,7 @@ error[E0631]: type mismatch in closure arguments
 LL |     g1(|_: (), _: ()| {}); //~ ERROR type mismatch
    |     ^^ -------------- found signature of `fn((), ()) -> _`
    |     |
-   |     expected signature of `for<'r> fn(&'r (), std::boxed::Box<for<'s> std::ops::Fn(&'s ()) + 'static>) -> _`
+   |     expected signature of `for<'r> fn(&'r (), std::boxed::Box<(dyn for<'s> std::ops::Fn(&'s ()) + 'static)>) -> _`
    |
 note: required by `g1`
   --> $DIR/anonymous-higher-ranked-lifetime.rs:33:1
@@ -102,7 +102,7 @@ error[E0631]: type mismatch in closure arguments
 LL |     g3(|_: (), _: ()| {}); //~ ERROR type mismatch
    |     ^^ -------------- found signature of `fn((), ()) -> _`
    |     |
-   |     expected signature of `for<'s> fn(&'s (), std::boxed::Box<for<'r> std::ops::Fn(&'r ()) + 'static>) -> _`
+   |     expected signature of `for<'s> fn(&'s (), std::boxed::Box<(dyn for<'r> std::ops::Fn(&'r ()) + 'static)>) -> _`
    |
 note: required by `g3`
   --> $DIR/anonymous-higher-ranked-lifetime.rs:35:1
@@ -130,7 +130,7 @@ error[E0631]: type mismatch in closure arguments
 LL |     h1(|_: (), _: (), _: (), _: ()| {}); //~ ERROR type mismatch
    |     ^^ ---------------------------- found signature of `fn((), (), (), ()) -> _`
    |     |
-   |     expected signature of `for<'r, 's> fn(&'r (), std::boxed::Box<for<'t0> std::ops::Fn(&'t0 ()) + 'static>, &'s (), for<'t0, 't1> fn(&'t0 (), &'t1 ())) -> _`
+   |     expected signature of `for<'r, 's> fn(&'r (), std::boxed::Box<(dyn for<'t0> std::ops::Fn(&'t0 ()) + 'static)>, &'s (), for<'t0, 't1> fn(&'t0 (), &'t1 ())) -> _`
    |
 note: required by `h1`
   --> $DIR/anonymous-higher-ranked-lifetime.rs:39:1
@@ -144,7 +144,7 @@ error[E0631]: type mismatch in closure arguments
 LL |     h2(|_: (), _: (), _: (), _: ()| {}); //~ ERROR type mismatch
    |     ^^ ---------------------------- found signature of `fn((), (), (), ()) -> _`
    |     |
-   |     expected signature of `for<'r, 't0> fn(&'r (), std::boxed::Box<for<'s> std::ops::Fn(&'s ()) + 'static>, &'t0 (), for<'s, 't1> fn(&'s (), &'t1 ())) -> _`
+   |     expected signature of `for<'r, 't0> fn(&'r (), std::boxed::Box<(dyn for<'s> std::ops::Fn(&'s ()) + 'static)>, &'t0 (), for<'s, 't1> fn(&'s (), &'t1 ())) -> _`
    |
 note: required by `h2`
   --> $DIR/anonymous-higher-ranked-lifetime.rs:40:1

src/test/ui/arbitrary-self-types-not-object-safe.stderr

@@ -13,7 +13,7 @@ LL |     let x = Box::new(5usize) as Box<Foo>;
    |             ^^^^^^^^^^^^^^^^ the trait `Foo` cannot be made into an object
    |
    = note: method `foo` has a non-standard `self` type
-   = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<std::boxed::Box<Foo>>` for `std::boxed::Box<usize>`
+   = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<std::boxed::Box<dyn Foo>>` for `std::boxed::Box<usize>`
 
 error: aborting due to 2 previous errors