Feature Flag Management

Feature flag management — stochastic-rs

The workspace has 9 sub-crates, several of which carry optional dependencies (openblas, cuda, metal, python, yahoo, ai, hotpath, viz). Without discipline, cargo check --all-features explodes with "feature X needed but not propagated" or, worse, an unflagged path silently compiles a Box<dyn Fn(f64) -> f64> that panics at runtime.

This SKILL documents the conventions that prevent the §4.1 feature-flag trap (rc.0 release-blocker: a dep: proxy feature that compiled in single-crate isolation but blew up under --all-features).

1. Three patterns, three uses

1.1 dep: proxy syntax (preferred for optional dependencies)

# In stochastic-rs-quant/Cargo.toml:
[dependencies]
yahoo_finance_api = { workspace = true, optional = true }

[features]
yahoo = ["dep:yahoo_finance_api"]

Why: the dep: prefix tells Cargo that "yahoo" is a feature only, and never a transitive enable. Without dep:, cargo auto-creates a feature named yahoo_finance_api whenever the crate has an optional dep of that name, and any sub-crate that lists yahoo_finance_api as a plain dep silently activates it. That's the exact failure mode we hit in rc.0.

1.2 Workspace propagation for cross-crate features

When the umbrella crate (stochastic-rs/Cargo.toml) needs to expose a feature that lives in a sub-crate, propagate via the <sub-crate>/<feature> syntax:

# In stochastic-rs/Cargo.toml (umbrella):
[features]
ai = ["stochastic-rs-ai/python", "stochastic-rs-ai/openblas"]
gpu = ["stochastic-rs-stochastic/gpu", "stochastic-rs-distributions/gpu"]
openblas = [
  "stochastic-rs-stats/openblas",
  "stochastic-rs-quant/openblas",
  "stochastic-rs-stochastic/openblas",
  "stochastic-rs-distributions/openblas",
]

Why: cargo does not auto-enable sub-crate features when the umbrella exposes a same-named one. You must list every sub-crate that needs it explicitly. Forgetting a sub-crate is the most common cause of cargo --all-features failures (a path that's gated on openblas in two crates but only on default in a third compiles inconsistently).

1.3 Internal feature gates (sub-crate scoped)

For a sub-crate's own internal feature (not exposed via the umbrella):

// stochastic-rs-stats/src/openblas_estimators.rs
#![cfg(feature = "openblas")]   // module-level guard

// or, at item level:
#[cfg(feature = "openblas")]
pub fn linear_regression_lapack(...) -> ... { ... }

#[cfg(not(feature = "openblas"))]
pub fn linear_regression_lapack(...) -> ... {
  panic!("linear_regression_lapack requires the 'openblas' feature");
}

The #[cfg(not(feature))] no-op variant is sometimes useful for keeping the public API stable, but the dev-rules feedback memo discourages "make absent paths panic"; prefer plain #[cfg(feature)] gating without a fallback so callers see a compile-time error.

2. Mandatory pre-publish checks

Before any version bump (see release-checklist SKILL):

# 2.1 Default-features baseline
cargo check --workspace --no-default-features

# 2.2 Most-common combinations
cargo check --workspace --features openblas
cargo check --workspace --features ai
cargo check --workspace --features python
cargo check --workspace --features cuda

# 2.3 The thermonuclear all-features path — catches the §4.1 trap
cargo check --workspace --all-features

# 2.4 Tests on the openblas path (most user-facing)
cargo test --workspace --exclude stochastic-rs-py --features openblas

If cargo check --workspace --all-features fails but the per-feature checks pass, the failure is almost always a feature-flag interaction: two features that work alone but conflict together. Common causes:

• Two optional deps both expose a Send-bound impl on a third type; --all-features makes both visible and the orphan rule rejects.
• A #[cfg(any(feature = "x", feature = "y"))] gate where the body references a type re-exported by only one of the two.
• A test that uses #[cfg(feature = "x")] but indirectly depends on a helper gated #[cfg(feature = "y")].

3. Adding a new feature — checklist

To add feature foo (e.g. a new optional GPU backend, a new dataset loader, a new distribution backend):

1. Decide scope. Is foo a sub-crate-only feature, or does it need to bubble up to the umbrella?

2. Sub-crate Cargo.toml:

   [dependencies]
   foo_dep = { workspace = true, optional = true }
   
   [features]
   foo = ["dep:foo_dep"]
   

3. If the umbrella needs to expose it:

   # stochastic-rs/Cargo.toml
   [features]
   foo = ["stochastic-rs-X/foo", "stochastic-rs-Y/foo"]
   

4. Source-side gating:

   • Module-level: #![cfg(feature = "foo")] at the top of src/foo.rs.
   • Re-exports: gate the pub use line.
   • Tests: #[cfg(feature = "foo")] on the test or test module.
   • Benches: in Cargo.toml, [[bench]] required-features = ["foo"].

5. CLAUDE.md update: if foo exposes a new public surface (new trait, new type, new module path), document it in the relevant sub-crate's CLAUDE.md — the prelude listing or the "Key traits" block.

6. Run the full feature matrix (section 2) to confirm --all-features is still clean.

4. Anti-patterns

• Do not use bare optional = true without a dep: proxy feature. The auto-feature-named-after-the-dep behaviour is the rc.0 trap.
• Do not propagate features in both directions (umbrella → sub-crate AND sub-crate → umbrella via reexport). Pick one. The umbrella's [features] block is the canonical surface.
• Do not put runtime panics behind #[cfg(not(feature))]. Make the path a compile error so users know at build time.
• Do not add a new optional dependency without also adding a feature flag for it. The crate's default build size matters.

5. Diagnosing failures

6. Reference: feature flags currently in the workspace

Related SKILLs

• release-checklist — uses this SKILL's matrix as a release gate.
• add-gpu-sampler, add-jump-process — invoke when the new module is feature-gated.

Source: rust-dd/stochastic-rs — distributed by TomeVault.