In the video (at 35:39), they show the future plans and the plan is to start using bits of Servo code in Gecko. So, they will replace small parts of C++ code with Rust. http://youtu.be/7q9vIMXSTzc?t=35m39s

One plan was to first replace easy parts, like image decoders (bmp/ico formats). https://github.com/servo/servo/wiki/Mozlandia-Rust-In-Gecko#...

It won't be a one big replacement, more like a gradual take over.