> What is the correct procedure for this?

in general to work on something that will get "into the rust compiler" it needs at least an RFC and then a Major Change Proposal [0]

in practice it starts with asking on zulip [1]

[0] https://forge.rust-lang.org/compiler/proposals-and-stabiliza...

[1] https://forge.rust-lang.org/how-to-start-contributing.html