I don't know about that specific measurement, but memory usage of the Rust compiler has not doubled in general. If it did, we'd notice immediately, as the script crate in Servo would probably OOM.

I wouldn't be surprised if it was due to paralellism, e.g., compiling multiple code units in parallel or even better parallelism at the Cargo level. My stat is just the maximum memory usage reported at any point in time. But that's just a guess. ripgrep itself uses more memory when using parallelism, just because of having more buffers.

Not sure if it’s the case here, but decreasing time complexity can often require a corresponding increase in space complexity.