Commits on Jul 17, 2020

1. Build host dependencies with opt-level 0 by default …

   This commit updates Cargo's build of host dependencies to build them
   with optimization level 0 by default instead of matching the profile of
   the final binary.
   
   Since Cargo's inception build dependencies have, by default, been built
   in a profile that largely matches the profile of the final target
   artifact. Build dependencies, however, rarely actually need to be
   optimized and are often executing very small tasks, which means that
   optimizing them often wastes a lot of build time. A great example of
   this is procedural macros where `syn` and friends are pretty heavyweight
   to optimize, and the amount of Rust code they're parsing is typically
   quite small, so the time spent optimizing rarely comes as a benefit.
   
   The goal of this PR is to improve build times on average in the
   community by not spending time optimizing build dependencies (build
   scripts, procedural macros, and their transitive dependencies). The PR
   will not be a universal win for everyone, however. There's some
   situations where your build time may actually increase:
   
   * In some cases build scripts and procedural macros can take quite a
     long time to run!
   * Cargo may not build dependencies more than once if they're shared with
     the main build. This only applies to builds without `--target` where
     the same crate is used in the final binary as in a build script.
   
   In these cases, however, the `build-override` profile has existed for
   some time know and allows giving a knob to tweak this behavior. For
   example to get back the previous build behavior of Cargo you would
   specify, in `Cargo.toml`:
   
       [profile.release.build-override]
       opt-level = 3
   
   or you can configure this via the environment:
   
       export CARGO_PROFILE_RELEASE_BUILD_OVERRIDE_OPT_LEVEL=3
   
   There are two notable features we would like to add in the future which
   would make the impact of a change like this smaller, but they're not
   implemented at this time (nor do we have concrete plans to implement
   them). First we would like crates to have a way of specifying they
   should be optimized by default, despite default profile options. Often
   crates, like lalrpop historically, have abysmal performance in debug
   mode and almost always (even in debug builds) want to be built in
   release mode. The second feature is that ideally crate authors would be
   able to tell Cargo to minimize the number of crates built, unifying
   profiles where possible to avoid double-compiling crates.
   
   At this time though the Cargo team feels that the benefit of changing
   the defaults is well worth this change. Neither today nor directly after
   this change will be a perfect world, but it's hoped that this change
   makes things less bad!

   

   alexcrichton committed Jul 17, 2020
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