In bug 1437627, I turned on incremental compilation for Rust for local
developer opt builds as the default behavior. Debug builds should be
using incremental compilation already, and automation builds continue
to *not* use incremental compilation, due to environmental
considerations that would make incremental builds unprofitable.
If you use local builds for performance analysis purposes, you should
set --enable-release or set RUSTC_OPT_LEVEL to 2 or 3 in your
mozconfig. This condition was reached after looking through the
discussion at https://lists.mozilla.org/pipermail/dev-platform/2017-October/020324.html
when the default Rust optimization level was changed to -O1 from -O2.
Discussion in that thread suggested that -O2 or -O3 for Rust
compilation was more suitable for performance analysis than the
default of -O1. (Our release builds use -O2 for Rust code.)
If you have issues or suggestions on how to improve, please file
blocking bugs for bug 1437627 and CC me.