Advanced Rust users, what is the most valuable skill to become more productive with the language?

This is an open ended question. I am curious to know about any language features, crates, ideas around architecting code, etc that really changed how you write Rust programs. Hoping to get answers like "learning the ins and outs of how the module system works" to "learning functional programming". Basically, where would you encourage someone to develop their understanding of the language if they already had the fundamentals down and wanted to get more out of the language?