Suggestion does not compile: use self with associated types

[lopopolo]

$ cargo clippy -V
clippy 0.0.212 (fc96aa03 2019-05-04)

I have two traits that have the same associated types that are shaped like From and TryFrom. I provide a default TryFrom implementation for types that implement From. Clippy suggests I modify the code below to replace the To::From and To::To references with Self::From and Self::To, which causes a recursive overflow.

pub trait FromMrb<T> {
    type From;
    type To;

    fn from_mrb(interp: &Mrb, value: T) -> Self;
}

pub trait TryFromMrb<T>
where
    Self: Sized,
{
    type From;
    type To;

    unsafe fn try_from_mrb(interp: &Mrb, value: T) -> Result<Self, Error<Self::From, Self::To>>;
}

impl<From, To> TryFromMrb<From> for To
where
    To: FromMrb<From>,
{
    type From = To::From;
    type To = To::To;

    unsafe fn try_from_mrb(interp: &Mrb, value: From) -> Result<Self, Error<Self::From, Self::To>> {
        Ok(FromMrb::from_mrb(interp, value))
    }
}

error: unnecessary structure name repetition
  --> mruby/src/convert.rs:50:17
   |
50 |     type From = To::From;
   |                 ^^ help: use the applicable keyword: `Self`
   |
note: lint level defined here
  --> mruby/src/lib.rs:1:22
   |
1  | #![deny(clippy::all, clippy::pedantic)]
   |                      ^^^^^^^^^^^^^^^^
   = note: #[deny(clippy::use_self)] implied by #[deny(clippy::pedantic)]
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#use_self

error: unnecessary structure name repetition
  --> mruby/src/convert.rs:51:15
   |
51 |     type To = To::To;
   |               ^^ help: use the applicable keyword: `Self`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#use_self

[lopopolo]

This is a more minimal and self-contained example:

#![deny(clippy::all, clippy::pedantic)]

pub struct Error<From, To> {
    _from: From,
    _too: To,
}

pub trait From<T> {
    type From;
    type To;

    fn from(value: T) -> Self;
}

pub trait TryFrom<T>
where
    Self: Sized,
{
    type From;
    type To;

    fn try_from(value: T) -> Result<Self, Error<Self::From, Self::To>>;
}

impl<F, T> TryFrom<F> for T
where
    T: From<F>,
{
    type From = T::From;
    type To = T::To;

    fn try_from(value: F) -> Result<Self, Error<Self::From, Self::To>> {
        Ok(From::from(value))
    }
}

impl From<bool> for i64 {
    type From = bool;
    type To = Self;

    fn from(value: bool) -> Self {
        if value {
            100
        } else {
            0
        }
    }
}

fn main() {}

Following clippy's suggestions results in this compilation error:

$ cargo clippy
    Checking clippy-use-self v0.1.0 (/Users/lopopolo/Desktop/clippy-use-self)
error[E0275]: overflow evaluating the requirement `<T as TryFrom<F>>::To`
  --> src/main.rs:25:12
   |
25 | impl<F, T> TryFrom<F> for T
   |            ^^^^^^^^^^

error[E0275]: overflow evaluating the requirement `<T as TryFrom<F>>::From`
  --> src/main.rs:29:5
   |
29 |     type From = Self::From;
   |     ^^^^^^^^^^^^^^^^^^^^^^^

error[E0275]: overflow evaluating the requirement `<T as TryFrom<F>>::To`
  --> src/main.rs:30:5
   |
30 |     type To = Self::To;
   |     ^^^^^^^^^^^^^^^^^^^

error[E0275]: overflow evaluating the requirement `<T as TryFrom<F>>::From`
  --> src/main.rs:32:5
   |
32 | /     fn try_from(value: F) -> Result<Self, Error<Self::From, Self::To>> {
33 | |         Ok(From::from(value))
34 | |     }
   | |_____^

error: aborting due to 4 previous errors

For more information about this error, try `rustc --explain E0275`.
error: Could not compile `clippy-use-self`.

To learn more, run the command again with --verbose.

[lopopolo]

It seems the way to make the compiler and clippy happy is to do this:

impl<F, T> TryFrom<F> for T
where
    T: From<F>,
{
    type From = <Self as From<F>>::From;
    type To = <Self as From<F>>::To;

    fn try_from(value: F) -> Result<Self, Error<Self::From, Self::To>> {
        Ok(From::from(value))
    }
}

[repi]

Really like the use_self lint, and this seems to be the last remaining issue that until it can be generally used which would be lovely once resolved.

[Y-Nak]

@rustbot claim

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