Right now, cargo build compiles build.rs in debug mode, while cargo build --release compiles build.rs in release mode. However build.rs scripts can potentially be very computationally intensive, and I would like the option to run my build.rs script in release mode even when doing a debug compile of the rest of my crate.

As a motivating example, nikomatsakis/lalrpop is a parser generator that does source generation during the custom build step. Just the build.rs step takes 6m30s when compiling in debug, and 33s when compiling in release for a complex grammar.