The source of my assertion comes from repeated conversations with the Servo developers, who have written many hundreds of thousands of lines of Rust in a project whose goal is to be twice as fast as modern web browsers (in other words, they are comparing themselves to programs written in C++, and speed matters). They regularly profile in pursuit of tracking down inefficiencies, and bounds checking has never been even a blip on their radar. Quotes from the exchange I've had just now with pcwalton: "I haven't done rigorous benchmarking but I have never seen it in instruction level profiles [...] I'd rather just spend my time shipping software that uses fast bounds checks and prove it that way :) [...] my point is simply that the delta between idiomatic Rust code that uses iterators and no bounds checks and the idiomatic Rust code that uses iterators and bounds checks is incredibly small since bounds checks are so rare to begin with" (http://logs.glob.uno/?c=mozilla%23servo#c271807)

As for actual benchmarks, I think they would actually be quite easy to produce. There's exactly one line in the Rust stdlib that provides bounds checking for indexing, right here: https://github.com/rust-lang/rust/blob/master/src/libcore/sl... . All you would need to do is take out that line and compile Servo with your modified stdlib and compare the results of running the built-in benchmarks. I may just do this myself as a blog post. :)