I'm curious about the "change channels to be mpmc" one. Why isn't it backwards-compatible? About `std::env::set_var()`, there was a proposal for a `#[deprecate

I'm curious about the "change channels to be mpmc" one. Why isn't it backwards-compatible? About `std::env::set_var()`, there was a proposal for a `#[deprecated_safe]` attribute that triggers a warning if the function is used without an unsafe block, and eventually make that a hard error in future edition.