INCOHERENT_AUTO_TRAIT_OBJECTS future-compatibility warning

[alexreg]

This is the summary issue for the INCOHERENT_AUTO_TRAIT_OBJECTS
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?

Up to now there has been a bug in the de-duplication of auto traits in trait object bounds, such that the principal trait is not taken into account, meaning that impls of the same trait for, e.g., dyn Send and dyn Send + Send, are considered to be coherent, when in fact they should not be.

See #57057 for more details.

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.

[alexreg]

CC @nikomatsakis @Centril @arielb1

[alexreg]

This is superseded by #56481. Closing.