One minor thing: this is arguably a breaking change, since foo(_ = 4) can compile today.
TBH, I'm quite enamoured, theoretically, by concatenative programming. I do really like avoiding useless variable names and having things in dataflow order. And in some places, following those principles can be great for Rust -- see .await, for example.
But it's unclear to me how far along these lines Rust should go. For example, a pipelining operator comes up here regularly: When will Rust adopt the syntax of pipline operator "|>" in ocaml
What would a big change in this direction look like? Is it worth churning the idiomatic way of doing things? Dunno.