The site's going up and down. Here's Conclusion :

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Are compilation times a problem with Rust? Yes.

Are build times as bad with Rust as with C++? Yes.

Looking at my hypotheses, I was wrong on all counts:

The Rust port had more lines than the C++ version, not fewer.

For full builds, compared to Rust, C++ builds took about the same amount of time (17k SLOC) or took less time (100k+ SLOC), not longer.

For incremental builds, compared to C++, Rust builds were sometimes shorter and sometimes longer (17k SLOC) or much longer (100k+ SLOC), not always longer.