remove unnecessary trait bound

[lcnr]

This is necessary to continue to compile in future Rust versions as this previously only compiled due to an underspecified part of the type system. It will stop compiling once rust-lang/rust#119820 lands.

The where bound F: MakeWriter<'a> is weaker than the required bound of BunyanFormattingLayer which requires for<'a> F: MakeWriter<'a>, i.e. F implements MakeWriter<'a> for all lifetimes 'a, not just the lifetime 'a used when calling the function. This bound does hold because of an implementation of MakeWriter for all types which implement Fn() -> W. However, we generally prefer where-bounds over impls when proving trait bounds, causing us to get a higher-ranked region error.

This error was previously detected more eagerly, causing the compiler to discard this where-bound, proving the requirement by using the impl instead. This now stops being the case, causing your code to error.

Please ask for clarification if you would like further information. I apologize for the inconvenience.

[lcnr]

We ended up reverting the change to the compiler in rust-lang/rust#127568 for now, causing this change to no longer be necessary.