Detect use of unnecessary `dyn` in function parameter

What it does

Scan for all uses of a function, and check if the &dyn T can be replaced by &impl T.

Advantage

• Avoid the overhead introduced by dynamic dispatch

Drawbacks

Code bloat and compile overhead.

Example

// A private trait that takes a dyn reference.
trait Trait {
  fn f(writer: &mut dyn std::io::Write);
}

// Using that trait with concrete types.
fn use_trait(obj: impl Trait) {
  obj.f(&mut Vec::new());
}

Could be written as:

trait Trait {
  fn f(writer: &mut impl std::io::Write);
}

fn use_trait(obj: impl Trait) {
  obj.f(&mut Vec::new());
}

No implementation file or test is named. Start by reviewing how rust-clippy registers lints and how existing lints inspect function parameters and their uses. Done means detecting eligible &dyn T parameters that can become &impl T, while accounting for the stated code-bloat and compile-overhead drawbacks.