A key theme of the Rust 2019 roadmap is maturity. This covers a variety of topics, but a crucial one is compile times. For example, the roadmap itself has the following as the first main theme for the compiler team.

Improving “core strength” by lowering raw compilation times and also generating better code (which in turn can help with compilation times)

The roadmap explainer post has a “polish” section that has the following as the first example.

Compile times and IDE support

I previously wrote about one period of improvement in Rust compiler speed. How are things going in 2019?

Speed improvements in 2019

The following image shows changes in time taken to compile the standard benchmarks used on the Rust performance tracker. It compares the compiler from 2019-01-01 with the compiler from 2019-07-24 (the most recent data at the time of writing).

These are the wall-time results for 29 benchmarks. There are three different build kinds measured for each one: a debug build, an optimized build, and a check build (which detects errors but doesn’t generate code). For each build kind there is a mix of incremental and non-incremental runs done. The numbers for the individual runs aren’t shown here but you can see them if you view the results directly on the site and click around. The “avg” column shows the average change for those runs. The “min” and “max” columns show the minimum and maximum changes among those same runs.

The table has 261 numbers. The thing to take away is that 258 of them are negative, representing a decrease in compile time. Most of the “avg” values are in the range -20% to -40%. The “min” values (representing the best time reduction for each build kind) range from -12.4% to -51.3%. Even the “max” values (representing the worst time reduction for each build kind) are mostly better than -10%. These are pleasing results.

speed improvements since late 2017

What happens if we look further back? The image below compares the compiler from 2017-11-12 (the earliest date for which I could get data from the site) against the compiler from 2019-07-24, a period of just over 20 months.

These are the wall-time results for only 18 benchmarks, because the benchmark suite was smaller in late 2017. Check builds were also not measured then. You can view the results directly on the site.

My initial thought from looking at the “avg” results was “the compiler is twice as fast” but closer inspection shows that’s not quite true; the average “avg” result is 42%. (I know that averaging averages is statistically dubious, I did it just to get a rough feel.) Overall, the results are significantly better than those for 2019: the “avg” values range from -19.9% to -61.3%, and the “min” values are mostly better than -60%.

(And don’t forget that time reduction percentages can be misleading when they get large. A 50% time reduction means the compiler is twice as fast; a 75% time reduction means the compiler is four times as fast; a 90% time reduction means the compiler is ten times as fast.)

All this is good news. The Rust compiler has long had a reputation for being slow. I still wouldn’t describe it as fast, but it is clearly a lot faster than it used to be. Many thanks to all those who made this happen, and I would be happy to hear from anyone who wants to help continue the trend!

Thanks to theZcuber for a Reddit post that was the starting point for this article.