First of all, it’s not actually valid syntax because I believe Rust doesn’t have negative trait bounds. But it’s a trait bound, so it’s (part) of a type signature. The type here is generic over both lifetimes and types (I think). My vague understanding is that for<‘a, T: ?Sized> means “in any lifetime, any type that is a member of ‘?Sized’”. And then the rest of the signature goes on to say (in made-up syntax) “does not implement DerefMut”. I think ?Sized means “size not known at compile time.”

I think I originally had “piece of a type signature”, but I didn’t actually want to figure out which piece, so I fudged it. The point is well taken, I oughtn’t pretend I have a Rust parser in my head :)