RFC: Give users control over feature unification

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+- Feature Name: `feature-unification`
+- Start Date: 2024-09-11
+- RFC PR: [rust-lang/rfcs#3692](https://github.com/rust-lang/rfcs/pull/3692)
+- Rust Issue: [rust-lang/rust#3692](https://github.com/rust-lang/rust/issues/3692)
+
+# Summary
+[summary]: #summary
+
+Allow users to control feature unification.
+
+Related issues:
+- [#5210: Resolve feature and optional dependencies for workspace as a whole](https://github.com/rust-lang/cargo/issues/5210)
+- [#4463: Feature selection in workspace depends on the set of packages compiled](https://github.com/rust-lang/cargo/issues/4463)
+- [#8157: --bin B resolves features differently than -p B in a workspace](https://github.com/rust-lang/cargo/issues/8157)
+- [#13844: The cargo build --bins re-builds binaries again after cargo build --all-targets](https://github.com/rust-lang/cargo/issues/13844)
+
+# Motivation
+[motivation]: #motivation
+
+Today, when Cargo is building, features in dependencies are enabled based on the set of packages selected to build.
+This is an attempt to balance
+- Build speed: we should reuse builds between packages within the same invocation
+- Ability to verify features for a given package
+
+This isn't always ideal.
+
+If a user is building an application, they may be jumping around the application's components which are packages within the workspace.
+The final artifact is the same but Cargo will select different features depending on which package they are currently building,
+causing build churn for the same set of dependencies that, in the end, will only be used with the same set of features.
+The "cargo-workspace-hack" is a pattern that has existed for years
+(e.g. [`rustc-workspace-hack`](https://crates.io/crates/rustc-workspace-hack))
+where users have all workspace members that depend on a generated package that depends on direct-dependencies in the workspace along with their features.
+Tools like [`cargo-hakari`](https://crates.io/crates/cargo-hakari) automate this process.
+To allow others to pull in a package depending on a workspace-hack package as a git dependency, you then need to publish the workspace-hack as an empty package with no dependencies
+and then locally patch in the real instance of it.
+
+This also makes testing of features more difficult because a user can't just run `cargo check --workspace` to verify that the correct set of features are enabled.
+This has led to the rise of tools like [cargo-hack](https://crates.io/crates/cargo-hack) which de-unify packages.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+We'll add two new modes to feature unification:
+
+**Unify features across the workspace, independent of the selected packages**
+
+This would be built-in support for "cargo-workspace-hack".
+
+This would require effectively changing from
+1. Resolve dependencies
+2. Filter dependencies down for current target and selected packages
+3. Resolve features
+
+To
+1. Resolve dependencies
+2. Filter dependencies down for current target
+3. Resolve features
+4. Filter for selected packages
+
+**Features will be evaluated for each package in isolation**
+
+This will require building duplicate copies of build units when there are disjoint sets of features.
+
+For example, this could be implemented as either
+- Loop over the packages, resolving, and then run a build plan for that package
+- Resolve for each package and generate everything into the same build plan
+
+This is not prescriptive of the implementation but to illustrate what the feature does.
+The initial implementation may be sub-optimal.
+Likely, the implementation could be improved over time.
+
+**Note:** these features do not need to be stabilized together.
+
+##### `resolver.feature-unification`
+
+*(update to [Configuration](https://doc.rust-lang.org/cargo/reference/config.html))*
+
+* Type: string
+* Default: "selected"
+* Environment: `CARGO_RESOLVER_FEATURE_UNIFICATION`
+
+Specify which packages participate in [feature unification](https://doc.rust-lang.org/cargo/reference/features.html#feature-unification).
+
+* `selected`: merge dependency features from all package specified for the current build
+* `workspace`: merge dependency features across all workspace members, regardless of which packages are specified for the current build
+* `package`: dependency features are only considered on a package-by-package basis, preferring duplicate builds of dependencies when different sets of feature are activated by the packages.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+This increases entropy within Cargo and the universe at large.
+
+If someone enables mutually exclusive features in different packages, then `workspace` unification will fail.
+Officially, features are supposed to be additive, making mutually exclusive features officially unsupported.
+Instead, effort should be put towards [official mutually exclusive globals](https://internals.rust-lang.org/t/pre-rfc-mutually-excusive-global-features/19618).
+
+Some features cannot be enabled in some packages, like a `no_std` package not wanting `std` features.
+These workspaces will not be able to use `workspace` unification.
+For now, unifying for the `"workspace"` is primarily targeted at single-application workspaces.
+The other config fields can always be used instead.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+This is done in the config instead of the manifest:
+- As this can change from run to run, this covers more use cases.
+- As this fits easily into the `resolver` table, there is less design work.
+
+We could extend this with configuration to exclude packages for the various use cases mentioned.
+Supporting excludes adds environment/project configuration complexity as well as implementation complexity.
+
+# Prior art
+[prior-art]: #prior-art
+
+[`cargo-hakari`](https://crates.io/crates/cargo-hakari) is a "cargo-workspace-hack" generator that builds a graph off of `cargo metadata` and re-implements feature unification.
+
+[cargo-hack](https://crates.io/crates/cargo-hack) can run each selected package in a separate `cargo` invocation to prevent unification.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+- How to name the config field to not block the future possibilities
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+### Support in manifests
+
+Add a related field to manifests that the config can override.
+
+### Dependency version unification
+
+Unlike feature unification, dependency versions are always unified across the
+entire workspace, making `Cargo.lock` the same regardless of which package you
+select or how you build.
+
+This can mask minimal-version bugs.
+If a version-req is lower than it needs, `-Zminimal-versions` won't resolve down to that to show the problem if another version req in the workspace is higher.
+We have `-Zdirect-minimal-versions` which will error if workspace members do not have the lowest version reqs of all of the workspace but that is brittle.
+
+If you have a workspace with multiple MSRVs, you can't verify your MSRV if you
+set a high-MSRV package's version req for a dependency that invalidates the
+MSRV-requirements of a low-MSRV package.
+
+We could offer an opt-in to per-package `Cargo.lock` files.  For builds, this
+could be implemented similar to `resolver.feature-unification = "package"`.
+
+This could run into problems with
+- `cargo update` being workspace-focused
+- third-party updating tools
+
+As for the MSRV-case, this would only help if you develop with the latest
+versions locally and then have a job that resolves down to your MSRVs.
+
+### Unify features in other settings
+
+[`workspace.resolver = "2"`](https://doc.rust-lang.org/cargo/reference/resolver.html#features) removed unification from the following scenarios
+- Cross-platform target unification
+- `build-dependencies` / `dependencies` unification
+- `dev-dependencies` / `dependencies` unification unless a dev target is enabled
+
+Depending on how we design this, the solution might be good enough to
+re-evaluate
+[build-target features](https://github.com/rust-lang/rfcs/pull/3374) as we
+could offer a way for users to opt-out of build-target unification.
+
+Like with `resolver.incompatible-rust-version`, a solution for this would override the defaults of `workspace.resolver`.
+
+`cargo hakari` gives control over `build-dependencies` / `dependencies` unification with
+[`unify-target-host`](https://docs.rs/cargo-hakari/latest/cargo_hakari/config/index.html#unify-target-host):
+- [`none`](https://docs.rs/hakari/0.17.4/hakari/enum.UnifyTargetHost.html#variant.None): Perform no unification across the target and host feature sets.
+  - The same as `resolver = "2"`
+- [`unify-if-both`](https://docs.rs/hakari/0.17.4/hakari/enum.UnifyTargetHost.html#variant.UnifyIfBoth): Perform unification across target and host feature sets, but only if a dependency is built on both the target and the host.
+- [`replicate-target-on-host`](https://docs.rs/hakari/0.17.4/hakari/enum.UnifyTargetHost.html#variant.ReplicateTargetOnHost): Perform unification across target and host feature sets, and also replicate all target-only lines to the host.
+- [`auto`](https://docs.rs/hakari/0.17.4/hakari/enum.UnifyTargetHost.html#variant.Auto) (default): select `replicate-target-on-host` if a workspace member may be built for the host (used as a proc-macro or build-dependency)
+
+`unify-target-host` might be somewhat related to [`-Ztarget-applies-to-host`](https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#target-applies-to-host)
+
+For Oxide `unify-target-host` reduced build units from 1900 to 1500, dramatically improving compile times, see https://github.com/oxidecomputer/omicron/pull/4535
+If integrated into cargo, there would no longer be a use case for the current maintainer of `cargo-hakari` to continue maintenance.
+
+If we supported `dev-dependencies` / `dependencies` like `resolver = "1"`, it
+could help with cases like `cargo miri` where through `dev-dependencies` a
+`libc` feature is enabled. preventing reuse of builds between `cargo build` and
+`cargo test` for local development.
+
+In helping this case, we should make clear that this can also break people
+- `fail` injects failures into your production code, only wanting it enabled for tests
+- Tests generally enabled `std` on dependencies for `no_std` packages
+- We were told of use cases around private keys where `Clone` is only provided when testing but not for production to help catch the leaking of secrets