Add the following suggestions:

```
error[E0782]: trait objects must include the `dyn` keyword
  --> $DIR/not-on-bare-trait-2021.rs:11:11
   |
LL | fn bar(x: Foo) -> Foo {
   |           ^^^
   |
help: use a generic type parameter, constrained by the trait `Foo`
   |
LL | fn bar<T: Foo>(x: T) -> Foo {
   |       ++++++++    ~
help: you can also use `impl Foo`, but users won't be able to specify the type paramer when calling the `fn`, having to rely exclusively on type inference
   |
LL | fn bar(x: impl Foo) -> Foo {
   |           ++++
help: alternatively, use a trait object to accept any type that implements `Foo`, accessing its methods at runtime using dynamic dispatch
   |
LL | fn bar(x: &dyn Foo) -> Foo {
   |           ++++

error[E0782]: trait objects must include the `dyn` keyword
  --> $DIR/not-on-bare-trait-2021.rs:11:19
   |
LL | fn bar(x: Foo) -> Foo {
   |                   ^^^
   |
help: use `impl Foo` to return an opaque type, as long as you return a single underlying type
   |
LL | fn bar(x: Foo) -> impl Foo {
   |                   ++++
help: alternatively, you can return an owned trait object
   |
LL | fn bar(x: Foo) -> Box<dyn Foo> {
   |                   +++++++    +
```

Note the parentheses in the last suggestion:

```
error[E0277]: the size for values of type `(dyn Foo + Send + 'static)` cannot be known at compilation time
  --> $DIR/not-on-bare-trait.rs:7:8
   |
LL | fn foo(_x: Foo + Send) {
   |        ^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `(dyn Foo + Send + 'static)`
   = help: unsized fn params are gated as an unstable feature
help: you can use `impl Trait` as the argument type
   |
LL | fn foo(_x: impl Foo + Send) {
   |            ++++
help: function arguments must have a statically known size, borrowed types always have a known size
   |
LL | fn foo(_x: &(Foo + Send)) {
   |            ++          +
```

…tType`

This gets us more accurate suggestions.

Fix rust-lang#119525.