Tweak suggestions for bare trait used as a type

[estebank]

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> {
   |                   +++++++    +

Fix #119525:

error[E0038]: the trait `Ord` cannot be made into an object
  --> $DIR/bare-trait-dont-suggest-dyn.rs:3:33
   |
LL | fn ord_prefer_dot(s: String) -> Ord {
   |                                 ^^^ `Ord` 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>
  --> $SRC_DIR/core/src/cmp.rs:LL:COL
   |
   = note: the trait cannot be made into an object because it uses `Self` as a type parameter
  ::: $SRC_DIR/core/src/cmp.rs:LL:COL
   |
   = note: the trait cannot be made into an object because it uses `Self` as a type parameter
help: consider using an opaque type instead
   |
LL | fn ord_prefer_dot(s: String) -> impl Ord {
   |                                 ++++

[rustbot]

r? @davidtwco

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[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[davidtwco]

@bors r+

[bors]

📌 Commit 698dfc3 has been approved by davidtwco

It is now in the queue for this repository.