Bound for-deref types with Deref/DerefMut for better error messages

[dhardy]

Implements #27. @nsunderland1 thoughts? (And thanks for the suggestions.)

TODO: changelog and release

[nsunderland1]

error[E0277]: the trait bound `MyWrapper<T>: Deref` is not satisfied
 --> src/lib.rs:255:1
  |
7 | #[autoimpl(for<T: trait + ?Sized> &T, &mut T, Box<T>, MyWrapper<T>)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Deref` is not implemented for `MyWrapper<T>`
  |
note: required by a bound in `TargetImplsDeref`
 --> src/lib.rs:255:1
  |
7 | #[autoimpl(for<T: trait + ?Sized> &T, &mut T, Box<T>, MyWrapper<T>)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `TargetImplsDeref`
  = note: this error originates in the attribute macro `autoimpl` (in Nightly builds, run with -Z macro-backtrace for more info)

This looks great, and thanks for the super fast turnaround on the implementation!

I might be just blind, but I don't see a doctest anywhere in the codebase for custom deref types in autoimpl-on-trait. Would that be a useful addition?

[dhardy]

Good point about documenting explicit Target for Deref.

This diagnostic should work for DerefMut too:

error[E0277]: the trait bound `MyGreatType<T>: DerefMut` is not satisfied
 --> tests/error_for_deref.rs:6:1
  |
6 | #[autoimpl(for<T: trait + ?Sized> MyGreatType<T>)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `DerefMut` is not implemented for `MyGreatType<T>`
  |
note: required by a bound in `TargetMustImplDeref`
 --> tests/error_for_deref.rs:6:1
  |
6 | #[autoimpl(for<T: trait + ?Sized> MyGreatType<T>)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `TargetMustImplDeref`
  = note: this error originates in the attribute macro `autoimpl` (in Nightly builds, run with -Z macro-backtrace for more info)

And wrong Target:

error[E0271]: type mismatch resolving `<MyGreatType<T> as Deref>::Target == T`
 --> tests/error_for_deref.rs:6:1
  |
6 | #[autoimpl(for<T: trait + ?Sized> MyGreatType<T>)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected type parameter `T`, found struct `Box`
  |
  = note: expected type parameter `T`
                     found struct `Box<T>`
note: required by a bound in `TargetMustImplDeref`
 --> tests/error_for_deref.rs:6:1
  |
6 | #[autoimpl(for<T: trait + ?Sized> MyGreatType<T>)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `TargetMustImplDeref`
  = note: this error originates in the attribute macro `autoimpl` (in Nightly builds, run with -Z macro-backtrace for more info)

Admittedly, the latter error may not be crystal clear, especially where autoimpl is used to impl Deref as in:

#[autoimpl(Deref using self.0)]
struct MyGreatType<T: ?Sized>(Box<T>);

Correct version:

#[autoimpl(Deref<Target = T> using self.0)]
struct MyGreatType<T: ?Sized>(Box<T>);

I should probably try adding tests involving the error message, but I'm too lazy.