[WIP] crossbeam-epoch: expose loom support via feature flag

[tobz]

This PR exposes the loom_crossbeam feature flag to enable internal loom usage, rather than relying on cfg-based settings.

The usage of --cfg flags is cumbersome as it forces recompilation of all crates given that RUSTFLAGS has changed.

[taiki-e]

I understand the motivation, but the cargo feature needs to be additive and I don't think the loom feature meets that requirement.

[tobz]

I'm happy to stick with the --cfg based approach if the feature approach isn't workable, but can you explain a bit more?

From my perspective, the feature flag would enable pulling in loom itself as well as flipping the codepaths from std types to loom types. Is there something else that I'm missing?

[taiki-e]

can you explain a bit more?

Feature flags are silently shared across the dependencies graph.
For example, if I create a crate for the test that always depends on the loom feature and I run cargo test with --all/--workspace, the loom feature will be enabled for all crates. This will break tests on all other crates.
See also rust-lang/cargo#4328, rust-lang/rfcs#1841, rust-lang-nursery/lazy-static.rs#150, serde-rs/serde#918, https://rust-lang.github.io/api-guidelines/naming.html#feature-names-are-free-of-placeholder-words-c-feature

the feature flag would enable pulling in loom itself as well as flipping the codepaths from std types to loom types.

This also applies when using cfg, right?

[tobz]

Feature flags are silently shared across the dependencies graph.
For example, if I create a crate for the test that always depends on the loom feature and I run cargo test with --all/--workspace, the loom feature will be enabled for all crates. This will break tests on all other crates.
See also rust-lang/cargo#4328, rust-lang/rfcs#1841, rust-lang-nursery/lazy-static.rs#150, serde-rs/serde#918, https://rust-lang.github.io/api-guidelines/print.html#feature-names-are-free-of-placeholder-words-c-feature

I see what you mean now, and this makes sense. Appreciate the links. 👍🏻