Make `rustc_on_unimplemented` std-agnostic for `alloc::rc`#115210

bors merged 1 commit intorust-lang:masterrust-lang/rust:masterfrom

DogPawHat:std-agnostic-rustc_on_unimplemented-for-alloc-rcDogPawHat/rust:std-agnostic-rustc_on_unimplemented-for-alloc-rcCopy head branch name to clipboard