Rust merges the never type stabilization, changing never type fallback to `!` on all editions and making Infallible an alias

Sources: pull request, tracking issue
Summary: The merged pull request stabilizes the never type, !, changes never type fallback from () to ! on all editions, and makes Infallible an alias for !. The pull request states that the fallback change is a breaking change, and the reviewer marked it rollup=iffy causes breakage. This is a merge into the development branch, so no stable release carries it yet.
Why it matters: Code that relies on fallback resolving to () changes meaning, so crates depending on the old behaviour need checking before the change reaches a stable release.
Follow-up: Which stable Rust release carries the stabilization, and what the fallback change breaks in practice across published crates.