Using Deref in traits

[pitdicker]

Currently the guidelines say "Only smart pointers implement Deref and DerefMut (C-DEREF)".

In this PR in rand we are considering using this trick:

impl<R: RngCore + ?Sized, T: DerefMut<Target = R>> RngCore for T { ... }

With the comment

This isn't abusing Deref for inheritance like the anti-pattern you listed; it's implementing a trait for anything Deref-able to a type implementing the trait.

Would this be a good exception to the rule?

[dtolnay]

That RngCore impl does not implement Deref or DerefMut for anything that is not a smart pointer, so the C-DEREF guideline has no bearing. But as a downside you should consider that adding this impl would make RngCore-related error messages much worse. Blanket impls (meaning for T where T is a type parameter) are pretty unusual and should be used sparingly. All in all I would be mildly against adding this impl.

+ use std::ops::DerefMut;

  trait RngCore {}

+ impl<R: RngCore + ?Sized, T: DerefMut<Target = R>> RngCore for T {}

  fn assert_rng_core<T: RngCore>() {}

  fn main() {
      assert_rng_core::<String>();
  }

Error message without impl

error[E0277]: the trait bound `std::string::String: RngCore` is not satisfied
  --> src/main.rs:10:5
   |
10 |     assert_rng_core::<String>();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `RngCore` is not implemented for `std::string::String`
   |
   = note: required by `assert_rng_core`

Error message after adding impl

error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
  --> src/main.rs:10:5
   |
10 |     assert_rng_core::<String>();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ `str` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `str`
   = note: required because of the requirements on the impl of `RngCore` for `str`
   = note: required because of the requirements on the impl of `RngCore` for `std::string::String`
   = note: required by `assert_rng_core`

error[E0277]: the trait bound `str: std::ops::DerefMut` is not satisfied
  --> src/main.rs:10:5
   |
10 |     assert_rng_core::<String>();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::ops::DerefMut` is not implemented for `str`
   |
   = note: required because of the requirements on the impl of `RngCore` for `str`
   = note: required because of the requirements on the impl of `RngCore` for `std::string::String`
   = note: required by `assert_rng_core`

[dtolnay]

rust-random/rand#291