You want to take a look at the RFC process. You could start with the feature we're discussing right now - Implicit arguments in format strings (https://rust-lang.github.io/rfcs/2795-format-args-implicit-i...). Take a look, it's very readable.

There's discussion in the corresponding Github issue of the RFCs repo (https://github.com/rust-lang/rfcs). If the folks who work in that area are supportive, the RFC will be merged. After that someone needs to implement the feature.

Take a look at the Readme in the RFCs repo for more details.