"Two recent compiler changes should speed up compiles in debug mode. We don’t have any specific numbers to commit to with these changes, but as always, compile times are very important to us, and we’re continuing to work on improving them."
Can't imagine why compile times haven't been good in the past with this level of detailed analysis.
To be clear, I wrote the announcement, but don't work on compiler optimizations. Those that do tend to have a better idea than me.
But beyond that, it's ultimately about what you promise. Say something sped up compilation times 2x for most people, but slowed them 10x in a corner case. If you promise the 2x, the people for who it slowed down will be (rightfully) pretty mad.
None of these changes are as drastic as that, but still.
There's a difference between having numbers and having specific numbers to commit to… If you want raw numbers, some are tracked at http://perf.rust-lang.org
Also, incremental compilation is making very good progress. More fuzzy numbers: I'm currently working on a mid-size Rust project, where the app crate has around 10k lines. `env CARGO_INCREMENTAL=1 cargo run --release` takes 2-3 seconds depending on what I changed.
Not all improvements give you specific numbers. We track a lot of performance data, but some changes may give an extremely broad range of improvements so you can't say much about the numbers.
Can't imagine why compile times haven't been good in the past with this level of detailed analysis.