Tracking issue for object-unsafe non-principal fragments

[arielb1]

This is the summary issue for the OBJECT_UNSAFE_FRAGMENT
future-compatibility warning and other related errors. The goal of
this page is describe why this change was made and how you can fix
code that is affected by it. It also provides a place to ask questions
or register a complaint if you feel the change should not be made. For
more information on the policy around future-compatibility warnings,
see our breaking change policy guidelines.

What is the warning for?

Trait object types (for example fmt::Debug + Send + 'a) are defined as a set of bounds, called
fragments. For soundness, all trait fragments are required to be object-safe.

The current implementation of rustc implements object types as being comprised of a principal trait and a set of non-principal fragments. Versions of rustc prior to 1.10 did not check that the traits in the non-principal fragments are safe. This was fixed in PR #33138. As a result, types like Error + Sized were allowed, causing confusion and ICEs.

As these types do not make much sense, they must not be used. Removing the offending fragment should generally fix the problem.

When will this warning become a hard error?

At the beginning of each 6-week release cycle, the Rust compiler team
will review the set of outstanding future compatibility warnings and
nominate some of them for Final Comment Period. Toward the end of
the cycle, we will review any comments and make a final determination
whether to convert the warning into a hard error or remove it
entirely.

[nikomatsakis]

cc @rust-lang/compiler -- @arielb1 has a PR to turn this into a hard-error (#34982). I started a crater run but am generally in favor, although it's not quite per policy (that is, it's not the start of a release cycle, etc). We don't seem to have that policy running in "full gear" though. Same applies to #33242, so for now let's discuss there (though it's conceivable that the impact is different).