[Perz1val]

Rust does not produce magic in the assembly code. It does not even produce faster assembly code. Rust toolchain on it's own does not even produce assembly code. It just passes that to LLVM that does THE ENTIRE optimization. Without LLVM (written in C++) doing the heavy lifting, Rust is probably slower than V8 (written in C++) running JavaScript. There's no technical marver in Servo compared to Ladybird. I don't understand the yapping how a language makes projects good/bad, despite it being proven completely false again and again. The appeal is in independence and politics around the project.

[jcranmer]

The express purpose of building Servo in the first place was to experiment with ways to parallelize the layout algorithm. The advantage of Rust is that it is a language which enables the compiler to better enforce some of the rules that need to be followed to write correct thread-safe code. Note that Mozilla had previously tried--more than once--to do the same thing in C++ for Gecko, and both attempts had failed.

As for the rest of your comment... I believe Rust now has MIR-based optimizations, so it's no longer the case that "THE ENTIRE optimization" is due to LLVM. But it's a non-sequitur to say that Rust would be slower without LLVM. Rust doesn't do many optimizations on its own, because it's quite frankly a lot of boring tedium to implement an entire optimizing compiler stack, and if you've got a library you can use to do that, why not? If no such library were available, Rust would merely be implementing all of those optimizations itself, much as V8 implements those optimizations itself.