Tracking issue for release notes of #149543: Reduce unreachable-code churn after todo!() #157767

# Language
- The `todo!()` macro will suppress the `unreachable_code` lint, so you no longer get a pile of warnings about unreachability for code in progress; instead, you'll get a single `todo_macro_calls` lint on the `todo!()` call itself. [Reduce `unreachable_code` churn after `todo!()`](https://github.com/rust-lang/rust/pull/149543)

## Better handling of `todo!()`

When you're working on code, you can use the `todo!()` macro (since Rust 1.40) to document that you haven't written a piece of code yet. This acts as a runtime panic if actually reached, but unlike `panic!()` or `unreachable!()`, it specifically documents that the code is in progress. You can then compile and test the rest of the code.

However, because `todo!()` is an unconditional panic, previous versions of Rust would warn that any code occurring after it is unreachable. This can produce a lot of noise about the code you're currently working on.

Now, the `todo!()` macro will suppress the `unreachable_code` lint, so you no longer get a pile of warnings about unreachability for code in progress. Instead, you'll get a single `todo_macro_calls` lint on the `todo!()` call itself. You can control these lints independently.