Pulling over my reddit comment

> Providing a mechanism to manually disable individual default features when specifying a dependency

We want this on the Cargo team; somebody just needs to do the design work, see https://github.com/rust-lang/cargo/issues/3126

Note that there is a related problem of feature evolution. Can you take existing functionality and move it behind a feature? No because that would be a breaking change for `default-features = false`. On the surface, it sounds like disabling default features works around that and we can remove `default-features = false` in an Edition but that is in the dependents edition and doesn't control the dependencies edition. We need something else to go with this.

> Providing a less verbose way for libraries to expose the features of their direct dependencies to other packages that depend on them directly

> Providing a way to disable features from transitive dependencies

I'm tying these two together because I wonder how well the same solution would work for this: maybe not everything should be a feature but a ["global"](https://internals.rust-lang.org/t/pre-rfc-mutually-excusive-...). Our idea for solving mutually exclusive features is to provide a key-value pair that a package defines and sets a default on and then the final application can override it. Maybe we'll eventually allow toolkits / sdks to also override but that can run into unification issues and is a lower priority.

Otherwise, I think we'd need to dig into exact use cases to make sure we have a good understanding of what kinds of features in what situations that libraries need to re-export before figuring out what design is appropriate for making large scale feature management manageable.

> "Zero-config" features that allow enabling/disabling code in a library without the author having to manually define it

We have `hints.mostly-unused` (https://doc.rust-lang.org/cargo/reference/unstable.html#prof...) which defers parts of the compilation process for most of a package until we know whether it is needed. Currently, it is a mixed bag. Explicit features still provide noticeable benefits. It is mostly for packages like `windows-sys` and `aws-sdk`.