Cargo: `cargo bench` builds build dependencies in release mode

[bluss]

Build deps are unrelated to the performance or benchmarking, so they don't need to be rebuilt, and don't need to be in release mode.

[bluss]

Example: crate depends on rustc_version which in turn depends on semver. Semver is built in release mode for benchmarks.

[sfackler]

I believe this is a more general issue of the release profile building build-dependencies in release mode, of which bench is one case. It's particularly annoying with things like syntex_syntax that _already_ take a long time.

[alexcrichton]

I don't think there's a clear answer here of what Cargo _should_ be doing. Most build dependencies probably don't need to be optimized, but that doesn't mean that _none_ need to be optimized. It's unlikely that the optimization of the final artifacts should be related to the optimization of the build scripts

[dtolnay]

It's unlikely that the optimization of the final artifacts should be related to the optimization of the build scripts

In fact I would consider it a bug if the optimization of the final artifacts were related to the optimization of the build scripts. I agree with @bluss and @sfackler that build-dependencies never need to be optimized.

[alexcrichton]

Unfortunately others actually feel the opposite, where build dependencies like lalrpop should _always_ be optimized as otherwise their execution is super slow.

This is basically what I was just thinking before, however, where I don't think there's a clear answer for what Cargo should do. There should likely be some form of configuration option at least for this, however (in one form or another)

[lilith]

I would love to drop 160 seconds from my release build time by letting Clippy build without optimizations. Clippy is fast enough, it's just slow to compile. And build caching is a bit flaky for me.

[kornelski]

I've ran into this too. I use bindgen at build time, and it takes ages to compile it release mode.

An option to have separate profiles for build dependencies (similar to #1359) would be fantastic.