It's pretty much unheard of, but nearly every open source project would benefit far more from a week of watching potential contributors trying to get up to speed than it would from making sure the project roadmap is delivered a week sooner.
That's my claim for projects in general, and what you're asking for re Rust is a little different, but the approach can applied there, too. Focus on everything from navigating the project website and other docs, to interactions with the tools that make up the "ecosystem", to programmer expectations that could shape the language (with the latter actually being the least important).
Don't trust users to self-report. Don't overestimate your ability to make a combined diagnosis and prescription, and don't underestimate how much silent resignation is killing your adoption rate. Observe and work off real notes.