now allows you to choose your build targets

Mar. 15, 2020 · Jynn Nelson on behalf of the team

Recently, added a feature that allows crates to opt-out of building on all targets. If you don't need to build on all targets, you can enable this feature to reduce your build times.

What does the feature do?

By default, builds all crates published to for every tier one target. However, most crates have the same content on all targets. Of the platform-dependent crates, almost all target a single platform, and do not need to be built on other targets. For example, winapi only has documentation on the x86_64-pc-windows-msvc and i686-pc-windows-msvc targets, and is blank on all others.

This feature allows you to request building only on specific targets. For example, winapi could opt into only building windows targets by putting the following in its Cargo.toml:

# This also sets the default target to `x86_64-pc-windows-msvc`
targets = ["x86_64-pc-windows-msvc", "i686-pc-windows-msvc"]

If you only need a single target, it's even simpler:

# This sets the default target to `x86_64-unknown-linux-gnu`
# and only builds that target
targets = ["x86_64-unknown-linux-gnu"]

See the documentation for more details about how to opt-in.

How does this help my crate?

Instead of building for every tier-one target, you can build for only a single target, reducing your documentation build times by a factor of 6. This can especially help large crates or projects with many crates that take several hours to document.

How does this help

Building all crates from can take a long time! Building fewer targets will allow us to reduce wait times for every crate. Additionally, this will decrease the growth of our storage costs, improving the sustainability of the project.

Possible future changes

We're considering turning this on by default in the future; i.e. only building for one target unless multiple targets are specifically requested. However, we don't want to break anyone's documentation, so we're making this feature opt-in while we decide the migration strategy.

This change will also make it easier for to build for targets that are not tier one, such as embedded targets.

How can I learn more?

You can learn more about the change in the issue proposing it and the PR with the implementation. Details on building non-tier-one targets are also available in the issue requesting the feature.

More information on targets and what it means to be a tier-one target is available in the platform support page.