Extension traits should not be used as trait bounds

[Darksonn]

What it does

It detects when an extension trait is used as a trait bound. The base trait should be used instead. For example:

async fn takes_async_read<R: AsyncReadExt>(read: R) { ... }

In this case, it should suggest writing R: AsyncRead instead.

To define "extension trait", one possibility is any trait of the following form:

trait MyTraitExt: MyTrait { .. }

impl<T: MyTrait + ?Sized> MyTraitExt for T {}

Lint Name

extension-trait-as-trait-bound

Category

style

Advantage

• It is conceptually incorrect.

Drawbacks

Somewhat unclear how "extension trait" should be defined. For example, should + ?Sized be required for it to be an extension trait?

It's also unclear if there are cases where this would be a false positive.

Example

async fn takes_async_read<R: AsyncReadExt>(read: R) { ... }

Could be written as:

async fn takes_async_read<R: AsyncRead>(read: R) { ... }

[lukaslueg]

It's not clear to me why a trait bound like this should be considered bad practice. AsyncReadExt: AsyncRead, and takes_async_read may require both.

[Darksonn]

Due to the blanket impl, R: AsyncRead implies R: AsyncReadExt, so they are equivalent mechanically. I suppose "bad style" would be a better category.

Here's another example:

fn takes_iterator<I: itertools::Itertools>(iter: I) { .. }

However, people tend to do this more often with AsyncRead.

[lukaslueg]

Ah, now I see your point. As any R that is R: AsyncReadExt has to be R: AsyncRead (due to pub trait AsyncReadExt: AsyncRead), and as all R: AsyncRead + ?Sized are AsyncReadExt due to impl<R: AsyncRead + ?Sized> AsyncReadExt for R, a trait bound R: AsyncReadExt is never more powerful than a R: AsyncRead.

I guess one could make a list of common(?) traits which have this property. Solving the general case is probably quite hard.

[Darksonn]

I've only ever seen people make the mistake with traits called StreamExt, AsyncReadExt, and AsyncWriteExt. (Though note that all of those names have multiple instances of them in different crates.)