Box::from(&str) -> Box<Error> regression

[diwic]

The following code compiles fine on the stable channel but fails on the beta and nigthly channels:

fn test() -> Box<std::error::Error> { Box::from("test") }
fn main() { test(); }

The error message is:

error: the trait `std::error::Error` is not implemented for the type `&str`

[bluss]

cc @brson

If the result type is Box<Error + Send + Sync> instead, it continues to compile. I'm not immediately sure why.

[bluss]

Is #29580 related?

[diwic]

Side note - I discovered this when running cargo test on my dbus crate, as now one of the examples fails to build. If the crater is run on all crates on crates.io and it includes a cargo test, this would have been discovered by crater as a root regression?

[brson]

This has bit me. Crater had to be modified to not try to box strings as errors.

[aturon]

@bluss That suggests that there's some bad interplay between the coercion that would throw away the extra constraints and the dispatch for From (which is indeed tied to Box<Error + Send + Sync + 'a>). Not sure why it was working before, honestly.

[brson]

I'll try to bisect.

[brson]

If the crater is run on all crates on crates.io and it includes a cargo test, this would have been discovered by crater as a root regression?

Yes.

[brson]

So this is working as intended, right? @bluss why doesn't one need to write the type with the lifetime in Box<Error + Send + Sync + 'static>? Is 'static the default?

[bluss]

Yep, 'static is the default with Box. For &'a Trait it's &'a (Trait + 'a) instead.

[brson]

I started bisecting last week to find the patch that made this change, but it's slow going.

[diwic]

Might be solved by #30509 ?

[bluss]

Yes, it compiles on nightly now. The regression to beta still remains.

[brson]

This has been ported to beta. Closing.