Suggestion does not compile: use self with when implementing `From` conversion for the `Box<dyn Trait>`.

[alekseysidorov]

clippy 0.0.212 (082cfa7 2019-06-07)

Clippy incorrectly suggests to use Self when implementing From conversion for the Box<dyn Trait> type.

#![deny(clippy::pedantic)]

trait Foo: 'static {}

struct Bar;

impl Foo for Bar {}

impl<T: Foo> From<T> for Box<dyn Foo> {
    fn from(t: T) -> Self {
        Box::new(t)
    }
}

Results the following error:

unnecessary structure name repetition
#[deny(clippy::use_self)] implied by #[deny(clippy::pedantic)]
for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#use_selfclippy(use_self)
lib.rs(1, 9): lint level defined here

But code with this suggestion does not compile:

#![deny(clippy::pedantic)]

trait Foo: 'static {}

struct Bar;

impl Foo for Bar {}

impl<T: Foo> From<T> for Box<dyn Foo> {
    fn from(t: T) -> Self {
        Self::new(t)
    }
}

no function or associated item named `new` found for type `std::boxed::Box<(dyn Foo + 'static)>` in the current scope
the method `new` exists but the following trait bounds were not satisfied:
`dyn Foo : std::marker::Sized`

[flip1995]

This needs a check if the dyn Trait is Sized. More precisely: This situation, where the method of a type must have a bound, but the type itself not, can only happen with the special ?Sized bound. This means, on functions, this lint has to check if the argument needs the Sized bound and if it is satisfied.

This may not be that easy to implement, so I would suggest to add this to the Known Problems section of the lint, until someone gets to fix this.

---

good-first-issue: Just document this.
E-medium: Fixing this issue.

I don't think it's a problem with the bounds. This looks like another instance of use_self not checking generics. In the example, it's actually Box::<T>::new(t) and in the suggestion it's Box::<dyn Foo>::new(t).