Apply c95bbb59a9b2 to 6.12.y

Hi Greg, Sasha,

Please consider applying commit c95bbb59a9b2 ("rust: enable arbitrary_self_types and remove `Receiver`") to 6.12.y.

It is meant to support the upcoming Rust 1.84.0 compiler (to be released in a month), since 6.12 LTS is the first stable kernel that supports a minimum Rust version, thus users may use newer compilers. Older LTSs do not need it, for that reason.

It applies almost cleanly (there is a simple conflict).

Thanks!

Cheers, Miguel