Module cargo::core::resolver::features

Expand description

Feature resolver.

This is a new feature resolver that runs independently of the main dependency resolver. It is enabled when the user specifies resolver = "2" in Cargo.toml.

One of its key characteristics is that it can avoid unifying features for shared dependencies in some situations. See FeatureOpts for the different behaviors that can be enabled. If no extra options are enabled, then it should behave exactly the same as the dependency resolver’s feature resolution. This can be verified by setting the __CARGO_FORCE_NEW_FEATURES=compare environment variable and running Cargo’s test suite (or building other projects), and checking if it panics. Note: the features2 tests will fail because they intentionally compare the old vs new behavior, so forcing the old behavior will naturally fail the tests.

The preferred way to engage this new resolver is via resolve_ws_with_opts.

This does not replace feature resolution in the dependency resolver, but instead acts as a second pass which can narrow the features selected in the dependency resolver. The dependency resolver still needs to do its own feature resolution in order to avoid selecting optional dependencies that are never enabled. The dependency resolver could, in theory, just assume all optional dependencies on all packages are enabled (and remove all knowledge of features), but that could introduce new requirements that might change old behavior or cause conflicts. Maybe some day in the future we could experiment with that, but it seems unlikely to work or be all that helpful.

There are many assumptions made about the dependency resolver. This feature resolver assumes validation has already been done on the feature maps, and doesn’t do any validation itself. It assumes dev-dependencies within a dependency have been removed. There are probably other assumptions that I am forgetting.