It’s a Mozilla-ism that’s been around for years - Bugzilla has a review flag that gets set to ? when asking for review on a patch, which gets shorted in conversation to r?, where the response is usually r+, or equivalently r=me (which evolved from the convention of r=[reviewer's name] being added to the commit message). I’m pretty sure this was imported into the Rust project ecosystem by people like pcwalton, who spent a lot of time in the Firefox development world before moving full time to Rust.
You know, in case you were wondering about the etymology of the construct, rather than the actual meaning in PRs today…